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Variations in Surface Soil Organic Carbon at the Duckabush River Delta, Washington

Item

Title (dcterms:title)

Eng Variations in Surface Soil Organic Carbon at the Duckabush River Delta, Washington

Date (dcterms:date)

2013

Creator (dcterms:creator)

Eng Masello, Daniel M

Subject (dcterms:subject)

Eng Environmental Studies

extracted text (extracttext:extracted_text)

VARIATIONS IN SURFACE SOIL ORGANIC CARBON AT
THE DUCKABUSH RIVER DELTA, WASHINGTON

by
Daniel M. Masello

A Thesis
Submitted in partial fulfillment
of the requirements for the degree
Master of Environmental Studies
The Evergreen State College
June 10, 2013

© 2013 by Daniel M. Masello. All rights reserved.

This Thesis for the Master of Environmental Studies Degree
by
Daniel M. Masello

has been approved for
The Evergreen State College
by

_________________________
Dr. Erin Ellis
Member of the Faculty

_________________________
Date

ABSTRACT
Variation in Surface Soil Organic Carbon at the
Duckabush River Delta, Washington
Daniel M. Masello
Intertidal wetland soils are estimated to sequester carbon at a rate that greatly
exceeds those of other terrestrial ecosystems (Pidgeon, 2009). Despite this, data
regarding intertidal wetland area and soil carbon content and density are scarce. Puget
Sound intertidal wetlands have been greatly diminished from their historical extent, and
sea level rise threatens those that remain. Further, there are currently no estimates of
carbon content, density, or stocks of Puget Sound intertidal wetland soils. This study
examined the organic carbon content and density of the top 30 cm of soil at the
Duckabush River Delta, located on the Hood Canal in the Puget Sound, Washington.
Carbon content (percent organic carbon), density (grams of carbon per cm3), and soil
texture (percent sand, silt and clay in sediment) were measured at varying salinities and
elevations gradients. The study area was divided into three zones by elevation (1: low, 2:
middle, 3: high), which was based on apparent groupings of the non-linear distribution of
soil bulk density plotted against elevation. Carbon content was significantly different in
all zones, with 2 > 3 > 1. Carbon density was not significantly different between zones 2
and 3, but both were significantly higher than zone 1. The increase in carbon density
with elevation between the zones was driven by significantly greater soil bulk density in
3 than 2, as well as the silt-dominated soil of zones 2 and 3, which was positively
correlated with carbon content in this study (r2=0.57). Estimates of surface soil organic
carbon stocks (Mg/ha) were 52.9, 93.9, and 106.3 for zone 1, 2, and 3, respectively.
These stocks are similar to estimates of soil organic carbon stocks in other terrestrial
systems (e.g. 96.2 in Temperate Forests, 127. 4 in Tundra (Pidgeon, 2009)). However,
taking into account the extremely high sequestration rate previously reported for
intertidal wetland soils, it is logical to conclude that surface soils at the Duckabush River
Delta and other intertidal wetlands are high-value ecosystems in the effort to mitigate
climate change. Long-term research examining sequestration rates at the Duckabush
Delta (as well as soil organic carbon content, density, and sequestration rates at other
Puget Sound intertidal wetlands) would be useful supporting or dismissing this claim.

Table of Contents

Introduction ......................................................................................................................... 1
1. Literature Review............................................................................................................ 4
1.1. Intertidal wetlands and the global carbon reservoir ............................................... 4
1.2. The Development of Intertidal Wetlands .................................................................. 8
1.3. Productivity .............................................................................................................. 9
1.4. Soil texture and carbon .......................................................................................... 10
1.5. Carbon and elevation ............................................................................................. 12
1.6. Intertidal wetlands and methane ............................................................................ 13
1.7. Soil sampling .......................................................................................................... 15
1.8. Impacts of anthropogenic change on intertidal wetlands ...................................... 16
2. Manuscript .................................................................................................................... 21
2.1. Site description ....................................................................................................... 21
2.11. Physical setting ................................................................................................. 21
2.12. Pre-European contact....................................................................................... 23
2.13. Post-European contact ..................................................................................... 24
2.2. Methods .................................................................................................................. 26
2.21. Transect Design ................................................................................................ 26
2.22. Elevation ........................................................................................................... 26
2.23. Salinity .............................................................................................................. 28
2.24. Soil cores .......................................................................................................... 28
2.25. Soil bulk density ................................................................................................ 29
2.26. Soil texture composition ................................................................................... 30
2.27. Organic carbon content and density ................................................................ 31
2.28. Estimating carbon stocks .................................................................................. 33
2.29. Statistical analyses ........................................................................................... 34
2.3. Results .................................................................................................................... 35
2.31. Soil bulk density ................................................................................................ 35
2.32. Elevation and salinity gradients ....................................................................... 37
2.33. Organic carbon content and density ................................................................ 37
iv

2.34. Soil texture composition ................................................................................... 39
2.4. Discussion .............................................................................................................. 43
2.41. Relationships between organic carbon, soil texture, and soil bulk density...... 43
2.42. Organic matter, organic carbon, and carbon stock density ............................. 49
2.43. Variations in organic carbon and estimating carbon stocks at the Duckabush
River Delta ................................................................................................................. 51
2.44. Climate change, sea level rise, and “coastal squeeze” .................................... 53
3. Conserving and restoring Puget Sound intertidal wetlands .......................................... 56
3.1. Why it matters......................................................................................................... 56
3.2. Restoring the Duckabush River Delta .................................................................... 57
3.3 Restoring the areal extent of Puget Sound intertidal wetlands ............................... 59
Conclusion ........................................................................................................................ 61
References………………………………………………………………………………. 62

v

List of Figures
Map 2.1 Aerial view of the Duckabush River Delta and its location within Washington
state……………………………………………………………………..………………..22
Map 2.2 Detail of the study site and sample locations…………………………………..27
Figure 2.1 Soil bulk density against elevation…………………………………………..36
Figure 2.2 Analysis of variance of soil bulk density between different zones………….36
Figure 2.3 Organic matter content against bulk density………………………………...36
Figure 2.4 Salinity in parts per thousand against elevation in meters……….…………..37
Figure 2.5 Organic matter content against elevation……………………………………38
Figure 2.6 Organic carbon content against elevation……………………………………38
Figure 2.7 Analysis of variance of organic matter content between zones……………...39
Figure 2.8 Analysis of variance of organic carbon content between zones……………..39
Figure 2.9 Organic carbon density against elevation……………………………………39
Figure 2.10 Analysis of variance of organic carbon density between different zones....39
Figure 2.11 Sand content against elevation……………………………………………..40
Figure 2.12 Analysis of variance of sand content between different zones……………..40
Figure 2.13 Silt content against elevation……………………………………………….41
Figure 2.14 Analysis of variance of silt content between different zones………………41
Figure 2.15 Clay content against elevation……………………………………………...41
Figure 2.16 Analysis of variance of clay content between different zones……………..41
Figure 2.17 Organic carbon content against silt content………………………………...42
Figure 2.18 Organic carbon content against sand content………………………………42
Figure 2.19 Organic carbon content against clay content……………………………….42
Figure 2.20 Organic carbon density against sand content………………………………42
vi

Figure 2.21 Organic carbon density against silt content………………………………...43
Figure 2.22 Organic carbon density against clay content……………………………….43
Figure 2.23 “Coastal Squeeze”………………………………………………………….55

vii

List of Tables
Table 1.1 Carbon stocks and long-term accumulation of carbon in various ecosystems...6
Table 1.2 Current methane emmisions from natural sources……………………………14
Table 2.1 Mean carbon stock density in various regions of the world…………………..50
Table 2.2 Comparison carbon density at salt marsh sites in the NE Pacific……………50
Table 2.3 Estimates of total carbon stocks, carbon per hectare, and carbon densities of
different zones throughout the entire Duckabush River Delta…………………………...51
Table 2.4 Estimated carbon density and sequestration rates comparing the Duckabush
river Delta to other terrestrial systems…………………………………………………...52

viii

Acknowledgements
I would like to thank Dr. Erin Ellis first and foremost, my dedicated reader who
stayed constantly engaged and challenged me to produce the best study I could. I would
also like to thank Dr. Carri LeRoy and Dr. Dylan Fischer, both of whom were invaluable
sources of methodological advice and insight. Kaile Adney and the Evergreen Lab Stores
staff were tremendously helpful in executing the field and laboratory portions of this
study. Finally, I would like to thanks my wonderful fiancée, Kiersten Boehm for helping
me in the field, tolerating me at home, and supporting me throughout my studies.

ix

Introduction
As global temperatures rise and consensus builds that anthropogenic climate
change is the culprit, it becomes more and more valuable to understand how greenhouse
gasses are allocated globally in the terrestrial, oceanic, and atmospheric reservoirs. The
terrestrial carbon reservoir is perhaps the most complex of the three carbon reservoirs,
due to the diversity of environments and ecosystems that fall under the umbrella of the
greater terrestrial system (Crooks et al., 2010; Chmura, 2009).
Coastal ecosystems, at the interface of the terrestrial and oceanic world, account
for a small fraction of the earth’s area, but may account for a disproportionately large
portion of organic carbon stored in their soils (Pidgeon, 2009; Hussein et al., 2004).
They account for 1% or less of global terrestrial ecosystems, but constant accretion of
sediment in these systems over millennia results in huge stores of buried carbon
(Pidgeon, 2009) (Table 1.1). Not only do coastal ecosystems, including intertidal
wetlands, store large amounts of carbon, but they emit negligible methane (CH4), a potent
greenhouse gas (Crooks et al., 2010; Chmura et al., 2003; Saarnio et al., 2009; Bartlett et
al., 1987). There is debate in the literature, but many sources assert that intertidal
wetlands also emit negligible nitrous oxide (N2O), another potent greenhouse gas
(Chmura et al., 2003). Unfortunately, the global extent of these systems, including
intertidal wetlands, is currently unknown, due to their ever-shifting geomorphology and a
legacy of land-use change along coasts all over the world.
As these coastal ecosystems are developed, their ability to continuously sequester
carbon is also lost, thus contributing to atmospheric carbon dioxide (CO2) levels. Sea
level rise, which has been accelerated by climate change, will result in coastal ecosystems
1

becoming trapped between rising seas and inland development, further diminishing
carbon accumulation and thus becoming another positive feedback to anthropogenic CO2
emissions (Hopkinson et al., 2012; Hussein et al., 2004).
The value of these ecosystems in the face of climate change cannot be
overemphasized. This study seeks to examine whether claims made in previous research
from other parts of the world regarding the high capacity of intertidal wetland soils to
accumulate large carbon stocks, and subsequently mitigate a fraction of anthropogenic
CO2, holds true in an unstudied location by measuring carbon content and density, and
estimating surface soil organic carbon stocks of an intertidal wetland located in the Puget
Sound, Washington State. Data regarding carbon in intertidal wetland soil in this region
are absent from the literature, so this study will be this first to look at soil carbon density,
content and stocks in this unique region. The Puget Sound is unique because it is
historically rich in intertidal wetland area and diversity, relative to the global extent of
intertidal wetlands. The historical extent of intertidal wetlands has been greatly
diminished, and opportunities for restoration and conservation abound in the region
(Collins & Sheikh, 2005; Correa, 2003). Furthermore, Puget Sound is more vulnerable to
climate-change-accelerated sea level rise than other coastal regions of Washington,
including the northern coast of the Olympic Peninsula along the Strait of San Juan de
Fuca and the west coast of the state along the Pacific Ocean, which are experiencing
tectonic uplift, largely negating the effects of local sea level rise (Mote et al., 2008).
Most similar research has taken place on the Atlantic and Gulf Coasts of North
America, and few studies have been done at all on the entire Pacific Coast, with none
being conducted in Washington State. Puget Sound shorelines and intertidal wetlands are

2

already greatly diminished from their historic extent, due mostly to land-use change, and
the combination of shoreline armoring and sea level rise. Those who support the
conservation and restoration of Puget Sound intertidal wetlands often focus on the value
for fish and wildlife, as well as public recreation. However, with baseline data of the
carbon content and density of an intertidal wetland, the value as greenhouse gas sinks
could further support the argument for conservation and restoration of these ecosystems.
For example, if it is demonstrated that high amounts of carbon are stored in these
ecosystems relative to other upland ecosystems in Washington State, one could conclude
that emphasis should be placed on preserving these ecosystems. Furthermore, because
sea level rise and and coastal development threaten the continued existence of coastal
ecosystems and intertidal wetlands around the world, this study examines carbon content
and density along an elevation gradient. In addition, other factors that can influence
carbon content, such as soil texture and salinity, were concurrently measured. Soil
organic carbon content is also known to correlate to physical soil texture. Correlating the
content of different soil particle size classes to elevation and carbon (content and density)
may provide insight into understanding the distribution of carbon in an intertidal wetland
using these physical characteristics as a proxy. As such, the goal of this study is to
provide an estimate of surface soil carbon stocks in a unique intertidal wetland system in
an understudied region, and to contribute to the larger goal of understanding the
mechanisms controlling carbon storage in the terrestrial carbon reservoir.

3

1. Literature Review
1.1. Intertidal wetlands and the global carbon reservoir
In May of 2013, the atmospheric concentration of CO2 exceeded 400 ppm for the
first time in more than 2.5 million years (NOAA, 2013). Only two-and-a-half centuries
ago, the pre-industrial atmospheric concentration of CO2 was about 280 ppm (Lal, 2004;
NOAA, 2013). The consensus in the scientific community is that the anthropogenic
addition to atmospheric CO2 is a central driver behind climate change (Denman et al.,
2007; Chmura, 2009). Anthropogenic CO2 is released through fossil fuel combustion and
through land-use change (Crooks et al., 2010; Lal, 2004). It is believed that
anthropogenic CO2 can be partly mitigated by carbon sequestration in terrestrial
ecosystems, both in vegetation and in soils (Hopkinson et al., 2012; Hussein et al., 2004;
Phachomphon, 2008). Therefore, assessing the content, density, and stocks of carbon
stored in different types of soil is the first step in evaluating the potential of soils to
sequester carbon.
The amounts of carbon in oceanic and atmospheric reservoirs are better
understood relative to the amount of carbon stored in terrestrial reservoirs (Lal, 2004).
Studies of carbon stocks in terrestrial ecosystems have been most extensive in peatland,
freshwater wetlands, and large upland ecosystems (including temperate and tropical
systems) (Chmura et al., 2003). Coastal wetlands have received less attention relative to
these other terrestrial systems, despite the agreement in the literature that various types of
coastal ecosystems sequester carbon at a rate far exceeding their other terrestrial
counterparts (Crooks et al., 2010; Chmura, 2009; Li et al., 2010). The low attention paid
to coastal ecosystems may stem from the fact that they account for only a small fraction,
1% or less of the greater terrestrial system (Pidgeon, 2009). Estimates regarding the areal
4

extent and carbon stocks of coastal wetlands cover a wide range, due mostly to limited
data and differences in methodology between studies (Hopkinson et al., 2012).
Furthermore, estimating the area of coastal wetlands is complicated by sea level rise, and
the constantly-changing geomorphology of coastal wetlands (Pidgeon, 2009). Estimates
of the area of global coastal systems (including mangroves, intertidal marshes, and
seagrass beds) lie between 5.15x105 and 1.2x106 km2, with intertidal marshes accounting
for between 2x105 and 4x105 km2 (Hopkinson et al., 2012), or approximately 0.130.26% of Earth’s total land surface area. In the IUCN’s report, The Management of
Natural Coastal Carbon Sinks, it is stated that the total global area of tidal salt marshes is
simply “unknown,” with 0.22x1012 m2 currently reported (Pidgeon, 2009).
Synthesis studies of global carbon sequestration have come up with varied
estimates, but all agree that relatively huge amounts of carbon are sequestered per unit
area in coastal ecosystems and intertidal wetlands. Because intertidal wetlands must
continuously accrete vertically to stay above sea level, they are constantly sequestering
more and more carbon. Other terrestrial systems, on the other hand, have been shown to
approach a point of equilibrium in which carbon is no longer sequestered at a rate
exceeding respiration, because accretion does not occur at a comparable pace in these
other terrestrial ecosystems (Ellis, 2003; Crooks et al., 2010). Chmura et al. (2003)
estimated the average carbon sequestration rate of coastal wetlands at 210 grams of
carbon per square meter per year (g C m2/yr). Hopkinson et al. (2012) estimate the
sequestration rate at 57 ± 6 to 218 ± 24 g C m2/yr, based on sequestration rates estimated
in other studies of specific intertidal wetlands. These estimates clearly stand out when
compared to carbon sequestration rates of other, well-studied terrestrial systems, where

5

rates range from 0.2-20 g C m2/yr (Table 1.1). The variability in the rate of carbon
sequestration further supports the need to more broadly assess carbon stocks distributed
in different coastal wetland systems around the world.
Ecosystem
Type

Standing
carbon (g/m2)
Plants

Standing
carbon (g/m2)
Soil

Total global
area (*1012m2)

Global
Carbon
Stocks
(*1015g)
Plants

Global
Carbon
Stocks
(*1015g)
Soil

Annual rate of
carbon
accumulation in
sediment (g/m2)

Tropical forests
Temperate
forests
Boreal forests
Tropical
savannas and
grasslands
Temperate
grasslands and
shrublands
Deserts and
semi-deserts
Tundra
Croplands
Wetlands
Tidal salt
marshes

12,045
5,673

12,273
9,615

17.6
10.4

212
59

216
100

2.3-2.5
1.4-12.0

6,423
2,933

34,380
11,733

13.7
22.5

88
66

471
264

0.8-2.2

720

23,600

12.5

9

295

2.2

176

4,198

45.5

8

191

0.8

632
188
4,286

12,737
8,000
72,857

9.5
16
3.5
Unknown
(0.22
reported)
0.152
0.3

6
3
15

121
128
225

0.2-5.7

Mangroves
7,990
1.2
Seagrass
184
7,000
0.06
2.1
meadows
Kelp forests
120-720
na
0.02-0.4
0.009-0.02
na
Table 1.1 Carbon stocks and long-term accumulation of carbon in various ecosystems (Pidgeon, 2009).

20
210

139
83
na

Coastal wetlands span a broad range of ecosystem types, from mangrove to
marsh, and carbon sequestration dynamics are unique to each (Crooks et al,. 2010).
Understanding amount of carbon stored in each system is necessary to better assess the
distribution of carbon in the terrestrial reservoir. The Puget Sound, Washington, presents
an important opportunity to study the carbon content and density of a particular type of
coastal ecosystem, the intertidal wetland, composed of salt marsh and bare flats (Dethier,
1990). Studies of carbon in soils in intertidal wetlands are scarce, as most of these
studies have taken place on the North American Atlantic and Gulf coasts, European
coasts, and Chinese coasts (Zhou, et al., 2007; Bouchard & Lefeuvre, 2000; Andrews et

6

al., 2008). For a sense of perspective, a synthesis report of global coastal ecosystem
carbon density and sequestration rates, only six studies accounted for the entire west
coast of North America, while the same report included a total of 84 studies from the
Gulf and Atlantic coasts of North America (Chmura et al., 2003). Studies of soil carbon
content in the Puget Sound (and Pacific Northwest coast in general) are absent from the
literature.
Historically, the Puget Sound was estimated to have 29,500 hectares (ha) of
intertidal wetland (Collins & Sheikh, 2005), accounting for approximately 1.3% of the
Puget Sound lowland ecoregion (DellaSalla et al.,, 2013). Relative to the estimate of the
area of intertidal wetlands globally, the Puget Sound was historically a region rich in
intertidal wetland ecosystems, but due to land-use change and development, intertidal
wetlands now only account for approximately 0.23% of the Puget Sound lowland
ecoregion. By the late nineteenth century, approximately 38% of Puget Sound wetlands
had been converted to agricultural and urban land uses (Essington et al., 2011).
Currently, Puget Sound wetlands occupy only 17-19% of their historic extent, and the
median size of wetlands has decreased from approximately 0.93 hectares to 0.57 hectares
(Collins & Sheikh, 2005). This loss is the result of land-use change such as diking for
agriculture as well as shoreline modification and armoring for industry and development.
The lost intertidal wetland represents lost potential to sequester carbon (Lal, 2004;
Hopkinson et al., 2012). By better understanding the carbon content and density of an
existent Puget Sound intertidal wetland, natural resource managers and those working to
mitigate CO2 will be able to estimate potential additional carbon stored in the soil as a

7

result of restoring degraded and lost intertidal wetlands (Andrews et al., 2008; Chmura,
2009).
To better understand the objectives and results of this study, it helps to be familiar
with the biogeochemical properties of intertidal wetlands and past work done on this
topic. The following sections of this literature review are meant to provide background
in the biological, geomorphological, and chemical processes of intertidal wetlands.

1.2. The Development of Intertidal Wetlands
The development of intertidal wetlands is dependent on rates of sea level rise,
sediment supply, and the ability to accrete vertically and and move laterally (Hopkinson
et al., 2012; Hussein et al., 2004). Intertidal wetlands began to develop with the slowing
of sea level rise to a rate of about 5 mm/yr, about four to five thousand years ago
following the last glacial period (Hopkinson et al., 2012). When sea level rise slowed to
about 3.5 mm/yr, rapid expansion of intertidal wetlands occurred (Hopkinson et al.,
2012). Intertidal wetlands first colonized the tidal fringes of estuarine ecosystems and
then transgressed inland as sea level rise continued to flood higher into terrestrial
ecosystems (Hopkinson et al., 2012). As higher elevation terrestrial ecosystems
experienced tidal submergence, forest species died and eventually halophytes (salttolerant plants) came to dominate (Spohn & Giani, 2012; Hussein et al., 2004).
As intertidal wetlands transgressed inland, they also accreted vertically on pace
with the rate of sea level rise. Intertidal wetlands accrete vertically through sediment
accumulation of organic and inorganic material from a variety of sources. Upland
sediment is transported to intertidal wetlands by rivers and streams. Some of this
8

sediment remains in the intertidal wetland, some of the sediment is transported seaward
in the water beyond the intertidal wetland, and some upland sediment is washed back into
an intertidal wetland upon tidal inundation (Zhou et al., 2007). When vertical sediment
accretion surpasses rates of sea level rise, intertidal wetlands move seaward (laterally), in
a process known as progradation (Hopkinson et al., 2012). Above- and below-ground
halophyte biomass in intertidal wetlands leads to the addition of organic matter.
Furthermore, these marsh plants trap sediment and organic matter as tides inundate and
subside (Hussein et al., 2004). The soils of intertidal wetlands have continued to
accumulate for the last five thousand years as a result of constant sediment accretion (in
the absence of human intervention) (Crooks et al., 2010).

1.3. Productivity
Favorable nutrient and water supply position intertidal wetlands as some of the
most productive ecosystems on earth (Spohn & Gianni, 2012; Chmura et al., 2003).
Tidal mixing and fluvially transported upland sediments provide important nutrients for
intertidal wetland plants, particularly high levels of nitrogen. (Hussein et al., 2004;
Hopkinson et al., 2012). Although intertidal wetland plants can tolerate high levels of
pore water salinity, the saline soils still cause physiological stress, which causes a greater
nitrogen demand, and in turn drives greater root production to obtain the limiting nutrient
resulting in high levels of biomass above and below ground (Chmura, 2009). These high
levels of subtidal, intertidal, and emergent primary producer organisms, result in
exceptionally high levels of primary production (Hopkinson et al., 2012). Duarte et al.,
(2005) estimate rates of gross primary production to range between 100 and 4,000 grams
of g C m2/yr. The high net primary productivity of these systems, coupled with the
9

contribution of tidal inundation, leads to high inputs of both autochthonous (on-site input)
and allochthonous (tidal input) organic matter (Spohn & Gianni, 2012; Chmura et al.,
2003). Fluvially transported sediments are also important sources of organic matter. The
relative importance of fluvial versus tidal sediment varies from one intertidal wetland to
another, depending on the quantity of these sediments relative to one another and the
relative abundance of organic matter between the two groups of sediments (Zedler &
Callaway, 2001).
Multiple factors contribute to the ability of intertidal wetlands and other coastal
wetlands to sequester such high levels of carbon. Tidal inundation events not only
contribute sediments and nutrients, but saturate the soil, creating anoxic conditions and
inhibiting aerobic decomposition (Chmura, 2009; Hussein et al., 2004; Zedler &
Callaway, 2001). In intertidal wetlands, anaerobic sediments store carbon by slowing the
decomposition of the in situ primary production, especially below-ground primary
production, which results in carbon-rich peat deposits (Zedler and Callaway, 2001).
Anoxic conditions impede decomposition in intertidal wetlands because heterotrophic
bacteria have a reduced energy yield per unit of substrate consumed under anoxic
conditions (Bastviken et al., 2004). Furthermore, oxygenase reactions which are required
to break down certain compounds, cannot occur in the absence of oxygen (Schink, 2005;
Bastviken et al., 2004).

1.4. Soil texture and carbon
These systems are not only productive because of the reasons discussed above.
Physical soil properties, particularly composition of different sized soil particles in the
10

sediment, influences the vegetation of an intertidal wetland, as well as the carbon content
and density. The very presence of above-ground vegetation diminishes the scouring
effect of tidal currents, which allows finer soil particles to accumulate, and subsequently
improves nutrient retention (Zedler & Callaway, 2001).
It is generally accepted that there is a positive correlation between smaller soil
particles, particularly clay content, and soil organic carbon preservation in soils (Krull et
al., 2001; Ladd et al., 1985). The physical protection of soil organic carbon is a function
of soil texture, specific surface area (SSA), and soil mineralogy (Krull et al., 2001). The
SSA of soil particles increases from large to small particles. Sand particles range in size
from 0.062 mm to 2.0 mm and have the lowest SSA. Silt particles range from 0.004 mm
to 0.062 mm and have greater SSA than sand. Clay particles are everything under 0.004
mm and have the highest SSA (USGS, 2013). Ransom et al. (1998) showed the impact
that small amounts of high SSA material can have on the total SSA of mineral particle
textures. In the study conducted by Ransom et al., the presence of 1% weight of highSSA clay (SSA of 100 m2/g) in 1.0mm diameter sand grains (SSA of 0.001 m2/g)
increases the total SSA of the particle mixture by three orders of magnitude (Ransom et
al., 1998; Krull et al., 2001). Therefore, it is clear that clay particles, because of the
extremely high SSA relative to larger soil particles, have the most significant surface area
to adsorb organic carbon (Krull et al., 2001.)
In addition to the fact that soil organic carbon content is positively correlated with
SSA, almost all organic carbon is found within pores between mineral grains in the form
of discrete particles, as molecules sorb onto the surface of minerals (Krull et al., 2001.)
Kilbertus (1980) and Van der Linden et al. (1989) demonstrated that the micro-organisms

11

responsible for the decomposition of organic matter are excluded from entering pores
below a certain size. As clay content increases, the proportion of total porosity in small
pores increases, resulting in the exclusion of biological decomposers, thus protecting
stores of organic carbon (Krull et al., 2001).
The properties of soil texture and carbon retention described above were
consistent with studies of carbon content and clay content in intertidal wetlands. Li et al.
(2010) studied microbial activity, carbon content, and soil texture in the Yangtze River
Estuary, China. Soil organic carbon content in sandy soil was significantly lower than
clay soil (Li et al., 2010). This is consistent with other studies that have demonstrated a
correlation between soil composition and particle size distribution and carbon
sequestration (Zhou et al., 2007). Specifically, soil with a higher clay content has a
greater ability to sequester more carbon (Ellis et al., 2003).

1.5. Carbon and elevation
The trend in previous studies from around the world suggests that concentration
of soil carbon generally increases with elevation within the intertidal wetland studied
(Spohn & Giani, 2012; Zhou et al., 2007;Li et al., 2010; Bouchard & Lefeuvre, 2000).
Bouchard and Lefeuvre (2000) found that net annual primary production is significantly
lower in low marsh than high and middle marsh. This is probably due to less frequent
inundation at the higher levels of the marsh, allowing for higher primary production.
Since primary productivity tends to positively correlate with elevation, there are greater
inputs of autochthonous organic matter at higher elevations within the intertidal wetland.
However, higher sites within the intertidal wetland must, at least occasionally, be
12

submerged by tidal inundation. This keeps the anaerobic conditions in place that decrease
the respiration of soil organic carbon (Li et al., 2010). Although these higher elevation
areas are less frequently inundated, they still receive and accumulate tidal detritus, and
the decreased tidal energy at these elevations, combined with vegetation, allows organic
matter to settle at these elevations instead of being scoured by tides. However, once the
conditions that define an intertidal wetland cease, such as primary productivity of
halophytes and tidal inundation leading to anaerobic conditions, upland soil organic
carbon stocks decline as increasing aerobic respiration releases more carbon (Mudd et al.,
2009).

1.6. Intertidal wetlands and methane
It is important to note that CO2 is not the only greenhouse gas of concern in the
discussion of climate change mitigation. While the atmospheric concentration of CO2
has been climbing steadily over our recent past, so too has the atmospheric concentration
of methane (CH4), from 700 ppb in 1750 to 1745 ppb in 1999 (Lal, 2004). Although
other terrestrial ecosystems, such as peatlands and freshwater wetlands, have
demonstrated their ability to sequester substantial amounts of carbon, there is a great deal
of debate as to whether these systems in fact act as positive inputs to the global warming
cycle because of their high levels of methane emissions (Table 1.2) (Sha et al., 2011;
Saarnio et al., 2009). Intertidal wetland soils are inundated by tides by definition, and
this frequent saturation of saline sea-water results in sulfate-rich soils. The sulfate-rich
soils of coastal wetland systems inhibit the microbial activity which produces methane
(Chmura, 2009).

13

Source

Northern wetlands/bogs
Tropical wetlands/swamps
Oceans, estuaries, and rivers
(Including intertidal
wetlands)
Lakes
Wild animals

Total Global Methane
emmisions Estimate (Tg
CH4/year)
42.7
127.6
9.1

Methane emission range
(Tg CH4/year)

30
8

10-50
2-15

24-72
81-206
2.3-15.6

Table 1.2 Current methane emmisions from natural sources (EPA, 2013).

Bartlett et al. (1987) studied methane efflux in an intertidal wetland in the
Chesapeake Bay, VA. Their results demonstrated a strong negative correlation between
methane efflux (g CH4 m2/yr) and soil salinity (ppt). According to their work, as sulfates
increase in concentration, so do sulfate-reducing bacteria and archaea. Some of these
sulfate-reducing micro-organisms can consume methane, and can therefore be a
controlling variable determining differences in methane concentrations along a salinity
gradient (Bartlett et al., 1987). A more recent study conducted by Saarnio et al. (2009)
compared methane efflux from different types of European wetlands, including various
freshwater peatlands, bogs, and marshes, and salt marsh. The results showed a marked
contrast in methane efflux from saltwater marsh versus freshwater systems. Freshwater
marsh effluxed 0.48 teragrams of methane per square kilometer per year (Tg CH4
km2/yr), while saltwater marsh effluxed a meager 0.01 Tg CH4 km2/yr (Saarnio et al.,
2009). These studies demonstrate that intertidal wetlands are negligible sources of CH4,
especially relative to freshwater wetlands. Other studies of greenhouse gas fluxes also
suggest that nitrous oxide efflux from intertidal wetlands and coastal wetlands are also
diminished by the presence of sulfates in the soil, although this is debated in the literature
(Chmura et al., 2003). The negligible efflux of CH4 adds to the capacity of intertidal
wetlands and coastal wetland systems to act as more powerful greenhouse gas sinks than
14

their freshwater counterparts, which in many cases act as sources (Chmura, 2009; Crooks
et al., 2010).
When considering questions of climate change mitigation, it is important to take
into account that CH4 has a much higher capacity to retain heat than CO2, making it a
more potent warming agent despite its low relative atmospheric concentration compared
to CO2 (Crooks et al., 2010). Pound for pound, CH4 is over twenty times more efficient at
trapping radiant heat than CO2 (EPA, 2013). Therefore, when climate change mitigation
opportunities through restoration and conservation are prioritized, the question whether a
site is a sink or source of all greenhouse gasses arises. Obviously, if climate change
mitigation is one of the objectives of a conservation or restoration scenario, it is
important that the site be a sink, not a source of greenhouse gasses. Since intertidal
wetlands sequester carbon with negligible output of methane, these are excellent areas to
protect and restore to mitigate climate change (Crooks et al., 2010).

1.7. Soil sampling
A common inconsistency between studies of soil carbon sequestration in coastal
systems and intertidal wetlands is the depth to which soil samples are gathered. As soil
depth increases, so does bulk density and the portion of minerals, and the amount of
carbon stored in deeper soils diminishes (Spohn & Giani, 2012). For this reason, a
significantly higher proportion of carbon is stored higher in the soil, and subsequently
sampling has stayed in shallower soil depths. Generally, these studies measure soil
carbon by depth increments as opposed to soil horizons. The trend in previous studies
has demonstrated that concentrations of carbon decrease with depth, but the depth to

15

which samples have been taken range from the top 20 cm (Santín et al., 2007) to 210 cm
deep (Hussein et al., 2004). Most studies have ranged between 30 and 60 cm sample
depths (Chmura et al., 2003; MacClellan, 2011).

1.8. Impacts of anthropogenic change on intertidal wetlands
So far the discussion has focused on the ability of coastal wetland ecosystems and
intertidal wetlands to store carbon. The ability of these systems to store carbon may
indeed mitigate a portion of anthropogenic atmospheric CO2 (Lal, 2004; Hussein et al.,
2004; Crooks et al., 2010; Hopkinson et al., 2012). However, these systems are highly
vulnerable to climate change (Hopkinson et al., 2012; Hussein et al., 2004; Chmura,
2009). Recall from earlier that intertidal wetlands must accrete vertically in sync with
sea level rise. Simultaneously with vertical accretion, intertidal wetlands must be
allowed to transgress inland (when sediment supplies are insufficient for progradation),
gaining elevation relative to sea level rise. Different intertidal wetlands have different
rates of sediment accretion, depending on tidal currents, wave energy, and suspended
sediment concentrations.
To minimize intertidal wetland loss, researchers suggest conserving adjacent
uplands for inland transgression (Stralberg et al., 2011). As intertidal wetlands accrete
ahead of sea level, carbon will continue to accumulate in intertidal wetland soils.
However, when the rate of sea level rise catches up with, and surpasses the rate of
sediment accretion, intertidal wetlands will essentially drown and represent unrealized
potential carbon storage (Mudd et al., 2009).

16

Estimates of the rate sea level rise for the last 100 years are 1-2 mm/yr, however
rates over the last decades have accelerated and are projected not to slow (Mudd et al.,
2009; Denman et al., 2007). The average global rate of sea level rise from 1961-2003
was 1.8 ± 0.5 mm/yr. Data from the Poseidon and Jason satellites suggested that rates of
sea level rise have accelerated, with a rate of 3.1 ± 0.7 mm/yr from 1993-2003 (Denman
et al., 2007). Based on the current science, sea level rise in Puget Sound is likely to
match global projections of sea level rise (Mote et al., 2008). The northwest coast of the
Olympic Peninsula will show little apparent sea level rise due to high rates of local
tectonic uplift, which exceed current rates of sea level rise (Mote et al., 2008). Data for
the central and southern Washington coast are scarce, but available data suggests that
tectonic uplifting is occurring in this region as well (Mote et al., 2008). Low-probability,
high-impact estimates of sea level rise in the Puget Sound are 55 cm by 2050 and 128 cm
by 2100. Low-probability, high-impact estimates of sea level rise are less for the
central/southern coast and Northwest Olympic Peninsula, at 45 cm and 35 cm by 2050,
and 108cm and 88 cm by 2100, respectively (Mote et al., 2008). Regardless of whether
these worst-case estimates are realized, Puget Sound is the marine area of Washington
where the effects of sea level rise will be most apparent. Because of shoreline
modification and the historic loss of intertidal wetlands in the Puget Sound, remaining
intertidal wetlands are in danger of being lost as sea levels continue to rise.
To add another dimension to this scenario, a substantial quantity of sediment is
delivered from upland sources to intertidal wetlands through fluvial transportation
(Zedler and Callaway, 2001). The seaward journey of sediment, however, has been
widely interrupted in systems throughout the world. This is certainly the case in the

17

watersheds that flow into Puget Sound. The culprit is land-use change, mostly in the
form of dams built for hydroelectric power (Hopkinson et al., 2012; Mudd et al., 2009).
The dams halt the seaward journey of upland sediment, allowing the sediment to descend
through the water column as the current velocity diminishes. Instead of being transported
to intertidal wetlands and beyond, the sediment settles on the floor of the reservoir behind
the dam. For example, in the case of Alder Lake, a hydroelectric reservoir on the upper
Nisqually River, sediment withheld by the dam has accumulated to 48 m deep (South
Sound Science Symposium, 2012). Withheld sediment diminishes an intertidal wetland’s
ability to accrete vertically and maintain the pace with sea level rise necessary to ensure
its continued existence (Mudd et al., 2009).
As sea levels rise, and vertical accretion is unable to keep pace because of
diminished sediment supply and climate change-induced acceleration of sea level rise,
intertidal wetlands will transgress inland to higher elevations. As in the case of dams,
sediment, and vertical accretion, anthropogenic drivers pose a major problem for the
inland transgression of intertidal wetlands as well. Shoreline armoring, agricultural
dikes, and other infrastructure designed to support and protect coastal industry and
development stand in between intertidal wetlands and the higher ground these systems
must retreat to in order to maintain themselves (Chmura, 2009; Andrews et al., 2008).
Where shoreline modifications exist prohibiting transgression, intertidal wetlands quite
literally have their back up against a wall, a phenomenon known as “coastal squeeze”
(Chmura, 2009).
For the reasons outlined above, it is important to continue studying these systems
around the world (Chmura et al., 2009). Puget Sound intertidal wetlands soil carbon has

18

not been thoroughly studied, and represents an opportunity to further our understanding
of the allocation of carbon in the terrestrial reservoir. This work will build on previous
studies by measuring soil carbon content and density in the top 30 cm of different
elevations within a Puget Sound intertidal wetland. Data from a Puget Sound intertidal
wetland is valuable because it contributes data from another unique location, and as noted
in previous studies, carbon content and density can vary widely between locations.
Furthermore, Puget Sound intertidal wetlands are under threat from a combination of
land-use change and sea level rise, so demonstrating the value of intertidal wetlands to
offset anthropogenic CO2 may strengthen the argument to restore and conserve Puget
Sound intertidal wetlands. The study will seek to further the body of knowledge on
relationships between soil carbon and intertidal wetland elevation, salinity, tidal
inundation frequency, and physical soil attributes such as grain size and presence of clay.
Results of this work support the theory that coastal wetland soil contains large stores of
carbon per unit area, and that carbon content and density levels will positively correlate
with higher elevation zones within the intertidal wetland.
The ability of intertidal wetlands and other coastal wetlands to sequester carbon,
combined with negligible methane emissions, support the theory that coastal wetlands are
valuable greenhouse gas sinks and make up a small but significant portion of the
terrestrial carbon reservoir (Crooks et al., 2010; Chmura et al., 2003; Hussein et al.,
2004). Although these systems may mitigate a portion of anthropogenic CO2, climate
change and subsequent sea level rise threaten their continued existence (Mudd et al.,
2009). In planning for restoration and conservation activities, natural resource managers
should consider the benefit these systems play as greenhouse gas sinks combined with the

19

fact that they are under threat. Losing coastal systems and intertidal wetlands would
represent a loss for the terrestrial carbon reservoir. It is therefore important to understand
these systems and their role in the greenhouse gas cycle, and to build upon the case for
their preservation where they still exist, and their restoration to where they once
belonged.
As we continue the plunge into the realm of prehistoric levels of atmospheric
CO2, increasing our understanding the global carbon cycle and all of its many
components becomes more crucial every day.

20

2. Manuscript
2.1. Site description
2.11. Physical setting
The Duckabush Watershed is one of four sub-watersheds (the Dosewallips,
Duckabush, Hamma Hamma and Skokomish) that drain the Dosewallips-Skokomish
Watershed of the eastern Olympic Peninsula, Washington state (Correa, 2003). The
Duckabush River originates in the vicinity of Mount Duckabush and Mount Steele in the
Olympic Mountains and flows generally eastward, draining approximately 20,207 ha
(Correa, 2003). The Olympic National Park contains 11,685 ha of the watershed, and an
additional 6,345 ha lie within the Olympic National Forest, together accounting for 89%
of the watershed (Hood Canal Coordinating Council, 2000). The remaining portion of
the watershed is mostly privately-held forest land, along with residential and park land
(Hood Canal Coordinating Council, 2000; Correa, 2003). There is no commercial or
industrial-zoned land in the entire Duckabush Watershed, and there are no dams along the
river (Hood Canal Coordinating Council, 2000). The mainstem of the Duckabush River is
approximately 39.43 km, with over fifty tributary streams for a total of 191.19 stream
kilometers in the watershed (Correa, 2003). The average annual discharge is average of
11.64 cubic meters per second, with bimodal hydrology resulting in winter and spring
peaks (Hood Canal Coordinating Council, 2000).
The Duckabush River terminates at its delta on the northwestern western shore of
the Hood Canal, the westernmost sub-basin of the Puget Sound. The delta is located at
approximately N 47°38’56”, W 122°56’05 (Map 2.1) (National Geodetic Survey, 2013).

21

Map 2.1 Aerial view of the Duckabush River Delta and it location within Washington state.

22

The Duckabush delta is approximately 60 ha. Tides at the delta range from extreme
highs of 4.11 m to extreme lows of -1.19 m (NOAA, 2013). All sample points in this
study were located in the eulittoral zone, between the mean high water (MHW) and mean
low water (MLW) of the intertidal zone (Dethier, 1990). MHW and MLW at the
Duckabush Delta are approximately 3.12 m and 0.82 m, respectively (Mojfeld, et al.,
2002; NOAA, 2013).
The delta is owned and maintained by the Washington Department of Fish and
Wildlife. It is open to the public for recreation, particularly shellfish gathering (WDFW,
2013).

2.12. Pre-European contact
Around 12,000 BP, after 2,000 years of glacial retreat and subsequent temperature
and climatic changes, conditions in the region became suitable for human habitation
(Mather et al., 2006). The post-glacial landscape was sparsely-vegetated, but within
approximately two millennia of glacial retreat, dense forests of Pseudotsuga menziesii
(Douglas Fir), Thuja plicata (Western Red Cedar), and Tsuga heterophylla (Western
Hemlock) came to dominate the landscape. Early human habitation in the region, 9,0005,000 BP, was characterized by upland site occupation, along river terraces, and in
temporary hunting camps. Evidence of task-specific, year-round, broad-based activities
date from about 4,200 BP. Permanent villages, which served as bases for other seasonal
activities, were established at this same time. Salmon was the primary food source for
the people of the region, supplemented by steelhead and cutthroat trout, shellfish, deer,
elk, roots, bulbs, and berries (Elmandorf & Kroeber, 1992).
23

The Duckabush River Delta is located within the traditional territory of the Twana
People, who occupied the entire Hood Canal Drainage and spoke a Salishan language
unintelligible to neighboring groups (Mather et al., 2006). The Twana peoples’ first
contact with Europeans came in 1792 when Captain George Vancouver explored the area
(Mather et al., 2006). The banks of the Duckabush River, close to the delta, was the site
of a Twana-speaking winter fishing village. The name “Duckabush” is the Anglicized
version of the village name, duxwyabu’s, which means, “place of crooked-jaw salmon,”
(Elmandorf & Kroeber, 1992).

2.13. Post-European contact
Washington Territory was established in 1853, the same year Euro-American
settlement began along the shores of the Hood Canal at the Duckabush and Dosewallips
River Deltas (Mather et al., 2006). In 1855, the Treaty of Point-No-Point compelled
many of the native people in the region to move south to the Skokomish reservation,
located on the lower Skokomish River in the southwest corner of Hood Canal (Elmandorf
& Kroeber, 1992).
Elwell Brinnon is considered to be the first Euro-American to settle in the region
permanently. Mr. Brinnon settled at the mouth of the Duckabush River and married a
Clallam woman named Kate, the sister of a local chief (Mather et al., 2006). In the
1860’s, Mr. Brinnon sold his claim to Thomas Pierce and moved to a new claim at the
mouth of the Dosewallips River, about 5 km to the north. The town located between the
Duckabush and Dosewallips Rivers still bears the name “Brinnon”. In 1859, Mr. Pierce

24

began logging the area by hand for the Washington Mill Company in Seabeck, on the
other side of Hood Canal (Mather et al., 2006).
The passage of the Homestead Act of 1862 spurred settlement in the Brinnon area
during the mid-1860’s. In the absence of roads, railroads, or even a dock, Brinnon
remained fairly isolated, 40 miles north or south to the nearest towns through rugged
terrain and dense woods. A road connecting Brinnon to Quilcene was finally built in
1896. Following the construction of the road, logging continued to be the central driver
of the local economy, progressing from hand-logging to ox teams, to horse teams, to
railroads, and finally to logging trucks. In 1938, the Olympic National Park was
established and logging ceased within its borders (Mather et al., 2006). Logging
practices in areas outside the park have continued throughout the region.
During the last century, the Duckabush River Delta has been intersected by State
Route 101, truncating approximately 5.26 ha of existing intertidal wetland habitat
(Correa, 2003) (Map 2.1). This truncation by SR 101 has resulted in intertidal wetland
habitat prograding eastward and seaward to the north of the mainstem of the Duckabush
River beyond historic boundaries. The mainstem of the Duckabush River is armored
with concrete bulkheads and riprap within the intertidal zone of the wetland. Armoring
along the mainstem, SR 101, and other residential armoring and development has
disrupted backshore sediment recruitment (Correa, 2003). Historic intertidal wetland
habitat exists to the south of the main stem of the Duckabush, but its extent has been
reduced from approximately 2.43 ha to 1.46 ha. Correa (2003) estimates (conservatively)
that approximately 9% of the shoreline at the Duckabush River Delta is armored.

25

Samples for this study were taken from within the historic habitat to the south of the
Duckabush River.

2.2. Methods
Field
2.21. Transect Design
Sample points were established with a grid sampling transect (Pennock, et al.,
2008). The transects were oriented north to south, because the elevation gradient is most
varied along this bearing, providing about a 2 m range of elevation. Other orientations
would require significantly longer transects to include a similar elevation gradient. Also,
deep channels in other areas of the delta presented logistical complications that could not
be overcome without watercraft. This grid transect captures the eulittoral zone, with
salinities ranging from mesohaline (5-18 ppt) to euhaline (30-40 ppt) in intertidal wetland
soils (Dethier, 1990). The grid was composed of three parallel transects, spaced 15 m
apart. Twenty samples were taken every 25 m on each of the three transects, for a total of
60 samples, and total transect length of 475 m (Map 2.2)

2.22. Elevation
Elevation was measured at each sample point with a TopCon G-7 auto level and
stadia rod. Base elevation was established from the National Geodetic Survey marker
PID – SY1194 (47°38’56” N, 122°56’05” W) with subsequent measurements taken from
the marker to the first sample point on the delta. The elevation of the marker is 6.70 m
above sea level (National Geodetic Survey, 2013). The auto level, mounted on a tripod,

26

Map 2.2 Detail of the study site and sample locations.

27

was placed over the marker. From the marker, elevation measurements were taken
proceeding towards the northernmost sample point of the west transect. Starting with the
known elevation at the first sample point, subsequent elevation measurements were taken
at each sample point. Before positioning the stadia rod, vegetation and debris was
cleared at each point so that elevation would be measured from the soil surface. The
stadia rod was then placed on the cleared soil surface, gently to avoid compaction, and
held vertically.

2.23. Salinity
Approximately 70-100 cm3 of soil from the sample point were placed in a paper
coffee filter. The soil for salinity tests was taken from approximately 30 cm depth. The
soil in the coffee filter was gently squeezed so as not to tear the filter. Water then
dripped onto an A366ATC salinity refractometer, which was held under the soil in the
coffee filter. Salinity readings were taken in parts per thousand (ppt).

2.24. Soil cores
Soil cores were extracted with a PVC electrical conduit pipe cut to 30 cm, which
was beveled for cutting on one end. Samples were taken to 30 cm because this was
consistent with the range of depths from previous studies (Ellis & Aherton, 2003;
MacClellan, 2011; Chmura et al., 2003); the top 30 cm captures the area of the soil
closest to the interface between atmospheric and terrestrial carbon reservoirs.
Furthermore, sampling to greater depths rapidly becomes more and more logistically
difficult. The inside diameter of the pipe was 5.2 cm, which resulted in cores with a
28

volume of 637 cm3. The pipe was placed on the bare soil, after it had been cleared of
vegetation and debris, and driven into the ground. The pipe containing the soil core was
excavated from the ground, and any soil protruding from the bottom of the pipe was cut
off with a knife. The core was removed, sealed in polyethylene bag, and refrigerated at a
temperature of 4-5 °C until analysis.

Laboratory
2.25. Soil bulk density
To obtain bulk density, samples were dried at 65 °C until a constant weight was
reached (Blake and Hartge, 1986). This took anywhere from 12-36 hours depending on
the water content of the sample. Once samples were oven-dry, the soil was passed
through a 2.0 mm sieve (#10 sieve) to remove any coarse organic matter, coarse sand,
gravel, and cobbles. The remaining soil was composed only of soil particles less than or
equal to 2.0 mm. The bulk density was calculated by the oven-dry mass of soil less than
or equal to 2.0 mm (MS) divided by the sample volume (VS), 637.37 cm3 (Blake and
Hartge, 1986):

(2.1) Soil bulk density (g/cm3) = MS (g) / VS (cm3)

Bulk density units are grams of soil ≤2.0 mm per cubic centimeter (g/cm3). All
subsequent analyses were conducted with soil that was ≤2.0 mm in size.

29

2.26. Soil texture composition

The hydrometer method was used to obtain soil texture measurements (Kroetch
and Wang, 2008; Gee and Bauder, 1986). Essentially, the hydrometer method creates a
liquid suspension of soil and, upon settling, the depths of different-sized particles are
measured. The hydrometer method procedure and calculations used in this study are
described by Gee and Bauder (1986).
Briefly, a dispersing solution was prepared containing 50 g of sodium
hexametaphosphate per liter of deionized water. Fifty to 100 g of oven-dried, sieved soil
(Sod) was added to a liter mason jar, to which 100 mL of the dispersing solution and 300
mL of deionized water were added. The jar was then shaken vigorously for one minute
and allowed to sit overnight for at least twelve hours, allowing the dispersing solution to
completely soak the soil and disperse the soil particles. The soil solutions were then
mixed with an electric mixer for five minutes, and then poured into 1,000 mL graduated
cylinders. Deionized water was added to the soil solution until a total volume of 1,000
mL was reached in the cylinders. Soil solutions were allowed to equilibrate to a
temperature of 22-23 °C. A plunger was then inserted into the cylinder and the solution
was vigorously mixed to create a suspension. As soon as the plunger was removed, a
timer was started and the hydrometer was gently lowered into the suspension. A reading
was taken at 40 seconds (R40 s), and again at 7 hours (R7 h). The procedure was repeated
for each soil sample. The hydrometer was calibrated by adding 100 mL of the dispersing
solution to a graduated cylinder. To this, 900 mL of deionized water was added for a
total volume of 1,000 mL. The hydrometer was lowered into the calibration solution and
30

the scale reading was taken from the hydrometer (RL). Content of sand, silt, and clay
were calculated as follows (Gee and Bauder, 1986):

(2.2) Sand % = 100 – [R40 s - RL] × [100 ÷ Sod]
(2.3) Clay % = [R7 h - RL] × [100 ÷ Sod]
(2.4) Silt % = 100 – [sand % + clay %]

2.27. Organic carbon content and density
To calculate organic carbon content of the soil samples, the loss-on-ignition (LOI)
method was used (Skjemstad and Baldock, 2008: Wright et al., 2007). LOI combusts any
organic matter in the soil, and the carbon content is calculated as a percent of the organic
matter present in the sample. LOI only combusts organic carbon, because inorganic
carbon requires a higher temperature to combust, closer to 825 °C (Wright et al., 2007).
Ceramic crucibles were precombusted by baking them for five hours at 550 °C. Once
cooled, the each crucible was placed on a scale and the mass was recorded. While each
crucible was on the scale, 100 mg of soil from a sample was added, and the total mass of
the crucible plus soil was recorded. The crucibles plus soil were covered with aluminum
foil and placed in the muffle furnace for five hours at 550 °C. The crucibles were
removed from the muffle furnace and placed in a desiccator for 20 minutes, and then
reweighed. The organic matter content was measured as the difference between the

31

oven-dry soil mass (SA) and the soil mass after combustion (SB), divided by the oven-dry
soil mass (Wright et al., 2007):

(2.5) OM % = 100 – [{SA - SB} ÷ SA]

Two conversion factors were used to estimate organic carbon content of the soil. It was
first calculated as 58% of the organic matter. This calculation was based on the
recommended conversion factor from the EPA (Schumacher, 2002), and used by Lal
(2004) in his study of terrestrial soil carbon stocks. The second conversion factor was
68%, based on the conversion factor used by MacClellan (2011) in a similar study of
Oregon estuaries. These two conversion factors provide a useful range for estimates of
soil carbon content. This study reports the average of this range, organic carbon at 63%
of organic matter:

(2.6) OC % = OM % × 0.63

Carbon density is measured as grams of organic carbon per cubic centimeter (g/cm3).
Carbon density was calculated by multiplying carbon content by soil bulk density (BD):

(2.7) OC density (g/cm3) = OC % × BD (g/cm3)

32

2.28. Estimating carbon stocks
Carbon stocks are the mass of carbon for a given surface area, unlike carbon
density which measures carbon mass for a given volume. There are a variety of metrics
used to convey carbon stocks, and for the purposes of consistency with carbon stock
estimates from other systems, this study estimates carbon stocks in megagrams per
hectare (Mg/ha). Since this study only sampled soil to 30 cm depth, this estimate only
represents carbon stocks in the top 30 cm of soil, excluding any organic matter >2.0mm.
To arrive at this estimate, organic carbon density (g/cm3) we multiplied by 30 cm. This
number represents the mass of carbon in one square centimeter (g/cm2), to a depth of 30
cm. To arrive at megagrams (Mg) of carbon per hectare, carbon g/cm2 were simply
multiplied by 100, since 1 g/cm2 = 100 Mg/ha.
This study used a GIS digital elevation model (DEM) of the Duckabush River
Delta to divide the delta into zones 1, 2, and 3, and to calculate the area of the different
zones (see “Statistical Analyses” section for zone designation). The area of each zone, in
hectares, was then multiplied by the mean carbon stock of each zone (Mg/ha), and all
three zones were added together for an estimate of organic carbon stocks in the top 30 cm
of the intertidal zone of the Duckabush River Delta.
Estimates of carbon stocks at the Duckabush River Delta do not include carbon
stored belowground in the form of coarse woody debris or root biomass. Because these
sources of carbon are not accounted for, and because carbon density was only estimated
to 30 cm depth, estimates of carbon stocks at this site should be considered conservative.

33

2.29. Statistical analyses
Bootstrap analysis was used to determine the distribution of salinity, elevation,
organic matter content, organic carbon content, organic carbon density, and soil texture
composition. The Shapiro-Wilkes Goodness-of-Fit test was applied to each distribution
to see if data was from the normal distribution and met the assumptions of regression and
analysis of variance tests. The relationships between elevation and salinity, organic
matter content, organic carbon content, organic carbon density, and soil texture
compositions were tested with regression analyses. Correlation analysis was used to
examine the relationship between salinity and organic matter content, organic carbon
content, organic carbon density, and soil texture compositions. The study site was also
divided into three zones. The three zones are based on observations during laboratory
analysis, in which three types of groups emerged: (1) a low-elevation, high- bulk density
zone (0.92-1.59 m), (2) a mid-elevation, low-bulk density zone (1.6-2.54 m), and (3) a
high-elevation, high-bulk density zone (2.55-2.81 m). These zone corresponded to field
observations, with regard to apparent soil bulk density, and differences in vegetation.
Most notably, zone 1 was sparsely vegetated with Fucus spp. (rockweed) or bare. Zones
2 and 3 were densely vegetated mostly with Distichlis spicata (seashore saltgrass). This
field work was conducted in February, so vegetation was mostly dormant and not
flowering, making thorough plant identification difficult. The different zones acted as
categorical variables by which to analyze significant differences of elevation, salinity,
organic matter content, organic carbon content, organic carbon density, and soil texture
composition through analysis of variance (ANOVA). Significant differences between
zones were assessed with Tukey’s Honest Significant Difference test.

34

2.3. Results
An analysis of the results indicated that surface soil organic carbon content is
driven largely by the increased proportion of smaller particles, particularly silt, in the
sediment. Organic carbon density was driven by both the carbon content of the sediment
as well as the soil bulk density of the sediment. Bulk density did not correlate linearly
with elevation; two distinct bulk densities among three distinct elevation ranges appeared
in the results. Samples with high bulk density and high silt content had the highest
carbon density. This is discussed in more detail below.

2.31. Soil bulk density
Soil bulk density values ranged from 0.19 to 0.95 g/cm3. The mean and median
bulk density was 0.57 and 0.56 g/cm3, respectively. The relationship between bulk
density and elevation was not linear (Figure 2.1); mean soil bulk density was significantly
lower in zone 2 at 0.39 g/cm3 than zones 1 or 3, which had virtually identical bulk density
values at 0.69 g/cm3 (F[2,57]=21.25, p<0.01) (Figure 2.2), despite nearly a meter of
elevation separating them. Organic matter and carbon content were significantly
correlated with soil bulk density (r2=0.56, p<0. 01) (Figure 2.3), but soil bulk density and
carbon density were not significantly correlated.

35

Soil Bulk Density
1
0.9

Soil (g/cm^3)

0.8
0.7
0.6
0.5

Transect 1

0.4

Transect 2

0.3

Transect 3

0.2
0.1
0
0

0.5

1

1.5

2

2.5

3

Elevation (m)

Figure 2.1 Soil bulk density against elevation.

Figure 2.2 Analysis of variance of soil
bulkdensity between different zones.
Different letters indicate significant
differences between zones. Error bars
represent one standard error from the mean.

Figure 2.3 Organic matter content against bulk density.

36

2.32. Elevation and salinity gradients
A strong negative relationship between salinity and elevation (r2=0.80) was
observed (Figure 2.4). Salinity values ranged from 12 ppt to 33 ppt, and elevation ranged
from 0.92 to 2.81 m. The mean salinity was 22.8 ppt. The mean elevation value was
1.87m and the median was 1.74m.

Figure 2.4 Salinity in parts per thousand against elevation in meters.

2.33. Organic carbon content and density
The organic matter content (and subsequently, organic carbon content) varied
widely with a maximum value of 28.27% at 2.04m elevation, and a minimum value of
2.04% at 1.19m. Organic matter and therefore carbon content were positively correlated
with elevation (r2=0.37, p<0.01) and negatively correlated with salinity (r2=0.46, p<0.01)
(Figures 2.5, 2.6). Organic carbon content ranged from 1.29% to 17.81%. This positive
correlation, however, does not capture the drop-off in organic matter content at some of
the high-elevation, high- bulk density sites of zone 3. Therefore, it is useful to look at for
differences between the three zones. Organic matter and organic carbon content varied
significantly between all three zones, with zone 1 < zone 3 < zone 2 (F[2, 57]=36.83,
p<0.01) (Figure 2.7, 2.8).

37

Estimates of organic carbon content (percent of sediment that is organic carbon)
were multiplied by soil bulk density (mass of soil in a given volume) for an estimate of
organic carbon density (g/cm3). Carbon density values ranged from 0.008 g/cm3 to 0.075
g/cm3. Regression analysis displayed a modest positive correlation between elevation
and organic carbon density (r2=0.38, p<0.01) (Figure 2.9) and negative correlation
between organic carbon density and salinity (r2=0.32, p<0.01). Mean organic carbon
density of zone 3 was greater than that of zone 2, 0.035 g/cm3 and 0.031 g/cm3, but the
difference was not statistically significant. Zones 2 and 3 both showed significantly
greater carbon density than zone 1, with a value of 0.018 g/cm3 (F[2,57]=20.35, p<0.01)
(Figure 2.10).

Figure 2.5 Organic matter content against elevation. Figure 2.6 Organic carbon content against elevation.

38

Figure 2.7 Analysis of variance of organic matter
content between different zones. Different letters
indicate significant differences between zones.
Error bars represent one standard error from the
mean.

Figure 2.8 Analysis of variance of organic carbon content
between different zones. Different letters indicate significant
differences between zones. Error bars represent on standard
error from the mean.

Figure 2.9 Organic carbon density against elevation. Figure 2.10 Analysis of variance of organic carbon
density between different zones. Different letters
indicate significant differences between zones.
Error bars represent one standard error from the mean.

2.34. Soil texture composition
Hydrometer analysis sheds light on relationships between soil texture composition
and other variables. Generally, sand and silt content displayed an inverse relationship,
with sandier soil at lower elevations and siltier soil at higher elevations. Sand content
ranged from 22% to 85%, and silt content ranged from 10% to 66%, with means of 49%

39

and 42%, respectively. The clay content was the least variant, with a mean of 9% and a
range between 5% and 16%.
Sand content displayed a significant negative correlation with elevation (r2=0.63,
p<0.01) (Figure 2.11). The mean sand content was significantly higher in zone 1
(67.62%), than zone 2 (35.61%) or zone 3 (35.04%). Although the mean sand content
was greater in the zone 2 than the zone 3, the difference was not significant
(F[2,57]=44.57, p<0.01) (Figure 2.12).

Figure 2.11 Sand content against elevation.

Figure 2.12 Analysis of variance of sand content between
different zones. Different letters indicate significant differences
between zones. Error bars represent one standard error from
mean.

The silt content displayed a significant positive correlation with elevation
(r2=0.64, p<0.01) (Figure 2.13). Mean silt content in zones 1, 2 and 3 were 24.38%,
54,71%, and 55.19%, respectively. As with sand content, silt content did not vary
significantly between zones 2 and 3, but both were significantly greater than zone 1
(F[2,57]=49.76, p<0.01)(Figure 2.14).

40

Figure 2.13 Silt content against elevation.

Figure 2.14 Analysis of variance of silt content between
different zones. Different letters indicate significant
differences between zones. Error bars represent one
standard error from the mean.

Correlation analysis between clay and elevation showed a slight positive
relationship, however the correlation coefficient was quite low (r2=0.14, p<0.01) (Figure
2.15). Clay did not vary significantly between any of the zones, with means of 8.00%,
9.68%, and 9.78% in zones 1, 2, and 3, respectively (F[2,57]=3.45, p=0.04) (Figure 2.16).

Figure 2.15 Clay content against elevation.

Figure 2.16 Analysis of variance of clay content
between different zones. Different letters indicate
significant differences between zones. Error bars
represent one standard error from the mean.

Sand, silt, and clay all displayed a significant relationship with organic matter
content and subsequently with organic carbon content. The relationship was strongest
between silt, positively correlated (r2=0.57, p<0.01), and sand, negatively correlated
41

(r2=0.60, p<0.01) and organic carbon content, and somewhat more modest between clay,
positively correlated (r2=0.32, p<0.01) and organic carbon content (Figure 2.17-2.19).
Organic carbon density also displayed significant relationships with all three soil
texture classes. As with organic matter and carbon content, sand was negatively
correlated with organic carbon density (r2=0.53, P,0.01), while silt and clay were both
positively correlated with carbon density (r2=0.51, p<0.01; r2=0.30, p<0.01 ) (Figure
2.20-2.22).

Figure 2.17 Organic carbon content against silt content. Figure 2.18 Organic carbon content against sand content.

Figure 2.19 Organic carbon content against clay content. Figure 2.20 Organic carbon density against sand content.

42

Figure 2.21 Organic carbon density against silt content.

Figure 2.22 Organic carbon density against clay content.

2.4. Discussion
2.41. Relationships between organic carbon, soil texture, and soil bulk density
The correlation results of this study support the hypothesis that as elevation
increases, so does the organic carbon content and density. The patterns from this study
are consistent with other studies of carbon distribution in intertidal wetlands. Spohn &
Gianni (2012) measured organic carbon stocks against an inundation frequency gradient
in intertidal wetlands of the north German coast. Because inundation frequency is a
proxy for elevation, their data is useful for comparison (Zedler & Callaway, 2001). The
results of their study showed that sites that were least frequently inundated had the
highest organic carbon density, while the most frequently inundated sites had the least
organic carbon density (Spohn & Gianni, 2012).
Analysis of variance between zones of this study show that other factors besides
elevation contribute to carbon content and density, most importantly the soil bulk density
and the silt and sand content of the sample. Zone 3 displayed the highest mean carbon
density value, 0.035 g/cm3, while zone 2’s mean value was only slightly lower, with a
density of 0.031 g/cm3, although these differences are not statistically significant (Figure
43

2.10). The soil of zone 2 exhibits significantly lower bulk density values relative to zone
3, while the organic carbon content of zone 2 is greater than zone 3. In the case of this
study, the significantly higher mean soil bulk density of zone 3 compensated for the
lower organic carbon content in this zone, allowing zone 3 to have the greatest mean
carbon density, despite having lower carbon content than zone 2.
This raises another question: if samples from zone 1 and zone 3 have about the
same mean soil bulk density (Figure 2.2), why does zone 3 have a significantly higher
value for carbon content and density than zone 1? In this case, soil texture composition
explains a large part of why zone 3’s carbon density is higher than zone 1’s. As
mentioned earlier, silt content was positively correlated with carbon content and density
(Figures 2.17, 2.21) Because silt has greater SSA than sand, and SSA has been
previously shown to positively correlate to soil carbon content, it follows that zones 1 and
3, with nearly identical soil bulk density, have significantly different mean carbon
content. Furthermore, zone 3 is vegetated salt marsh, while zone 1 contains soil sparsely
vegetated with Fucus spp., or bare sediment. Therefore, a logical conclusion is that
greater above and below-ground primary productivity at zone 3 drives the higher carbon
content of the soil. The very presence of above-ground biomass contributes to higher
carbon content in the following two ways. First, tidal organic matter input, in the form of
marine floral and faunal detritus, as well as coarse woody debris, is transported to higherelevation areas with the tide, and when the tide recedes, the dense mat of vegetation
snags and catches a great deal of this tidal organic matter input, keeping it in the higher
elevation zones, and (Zedler & Callaway, 1998). Further, tidal current energy could be
dissipated by dense vegetation, decreasing the scouring action and allowing silt to remain

44

in place and accumulate; in areas without vegetation, tidal energy scours silt and clay and
leaves mostly heavier, coarser soil particles, which retain much less organic carbon
(Zedler & Callaway, 1998). This immediate organic matter input, coupled with the
carbon retaining capacity of high-SSA soil results in zone 3 having the greatest mean
carbon density value of all three zones. Although zone 3 has the highest mean value of
carbon density, it is not significantly different than zone 2, which had a much greater
carbon content but much lower soil bulk density.
Li et al. (2010) looked at soil organic carbon content and density by sampling
along two transects at the Chongming Island in the Yangtze River Delta, China. Li et al.
divided the transects into four zone (high tidal, mid-tidal, low tidal, and bare flat). These
zones also serve as a proxy for elevation. Soil organic carbon density was significantly
lower in their “bare flat” zone than low, mid-, or high tidal zones. These results are
consistent with the results of this study, in which the low zone had significantly lower
organic carbon density. Furthermore, both the Li et al. study and this study showed no
significant difference of organic carbon density between zones above the lowest zone.
This does not come as a surprise; high inputs of autochthonous organic matter from onsite primary production of vascular plants, and allochthonous organic matter from tidal
and upland detritus that is deposited in higher tidal zones result in greater organic matter
content than in low tidal zones (Spohn & Gianni, 2012; Chmura et al., 2003).
J. Zhou et al. (2007) also conducted a study in the Yangtze estuary in China,
examining spatial variations in carbon. Their results for organic carbon content were
considerably lower than organic carbon content from the Duckabush River Delta,

45

however the clear trend in their data showed that as elevation decreased, so did organic
carbon content (Zhou et al., 2007).
Consistent with other studies (Zhou et al., 2007; Li et al., 2010), carbon was
correlated with soil texture. Clay content and organic carbon content in the Yangtze
River Estuary displayed a strong significant positive relationship (r2=0.77, p<0.01) (Zhou
et al., 2007). At the Duckabush River Delta, the relationship between clay content and
organic carbon content was also significant, but less strong (r2=0.32, p<0.01)
(Figure2.19). The positive correlation between silt content and organic carbon content,
and the negative correlation between sand and organic carbon content, accounted for
more variation in carbon content (silt: r2=0.57 and sand: r2=0.60, p<0. 01) (Figures 2.17,
2.18). Likewise, organic carbon density was most strongly negatively correlated with
sand content at the Duckabush River Delta (r2=0.53, p<0.01), followed by a positive
correlation with silt content (r2=0.51, p<0.01), and least strongly correlated with clay
(r2=0.30, p<0.01) (Figures 2.20-2.22). Although the relationship between soil texture and
carbon density and content is not as strongly correlated as other studies, higher
percentage smaller particle sizes, clay and silt, were both positively correlated with
carbon content and density, while higher percentages of sand particles were negatively
correlated with both carbon content and carbon density. This is supported by the
literature, which agrees that organic carbon is positively correlated with specific surface
area, and specific surface area is negatively correlated with soil texture size (Krull et al.,
2001).
Organic carbon density at the Duckabush was driven largely by soil texture
composition as discussed above, but variations in soil bulk density also drove the organic

46

carbon density. Bearing in mind that soil texture composition is correlated to carbon
storage, it is important to think about how the different zones of the study site came to be.
The difference in soil bulk density between zones 2 and 3 is particularly interesting. Both
zones are composed of vary similar proportions of sand, silt and clay, but the soil of zone
3 is so much more dense than zone 2. One factor that may influence bulk density of the
surface soil is below-ground biomass. During the process of drying the soil samples and
passing them through a sieve, zone 2 consistently contained more of below-ground
biomass (plant roots) than zone 3. The removal of this biomass represented a larger
portion of the original mass of samples from zone 2 than zone 3. Zones 2 and 3 were
vegetated mostly by Distichlis spicata. Zone 3 did support some, which was absent from
lower elevation sites in zone 2. Zone 2 supported Salicornia virginica (Pickleweed),
while zone 3 did not. Because zones 2 and 3 were both dominated by Distichlis spicata,
it is difficult to say that plant differences in plant communities account for the difference
in carbon content. An in-depth study of plant communities and their relationship with
soil organic carbon would be useful to understanding these differences. Because zones 2
and 3 have approximately the same silt content, the soil bulk density difference between
these sites appears to be a stronger driver of carbon density, which appears to be driven
by below-ground biomass. However, given that there isn’t a strong compositional
difference between plant communities, it is difficult to say why the below-ground
biomass is appears greater in zone 2. Unfortunately, the below-ground biomass was not
measured in this study, because it was too difficult to separate that biomass from the soil
samples through the 2.0 mm sieve.

47

Another possible explanation for this difference in soil bulk density is elevation,
and subsequently tidal inundation. Because zone 2 is lower in elevation than zone 3, it
spends more time submerged, increasing the average water content of the soil relative to
zone 3. Since zone 3 spends less time saturated than zone 2, and its soil could be more
consolidated without the dispersing influence of water and tidal energy. A third possible
explanation for the increased bulk density may be compaction by elk. At the study site,
in zone 3, there was evidence of elk moving from the wooded hills down to the river in
the form of tracks, scat, and evidence of browsing. The pressure of a herd of elk may be
enough to cause significant compaction of the surface soil, resulting in a much higher
bulk density than zone 2. Dethier’s (1990) A Marine and Estuarine Habitat Classification
Systems for Washington State notes that eulittoral marsh, specifically including the
Duckabush River Delta, is frequently used by deer and elk, supporting the notion that elk
may indeed contribute to soil compaction.
This study has shown that at the Duckabush River Delta, carbon content and
density is positively correlated with silt content. Zones 2 and 3 had about the same silt
content, while zone 1 had only about half the silt content. Because zone 1 is the lowest
elevation zone, it contains the most saline water, spends the most time tidally submerged,
and subsequently supports the least vegetation. In the relative absence of vegetation,
zone 1 does not have the on-site input of organic matter that zones 2 and 3 have. Without
above ground biomass, zone 1is totally exposed to the full force of tidal energy, waves
and freshwater flows (Dethier, 1990), subsequently, organic matter and silt that settle in
zone 1 is likely to be scoured and moved by incoming tides into zones 2 and 3, or
removed by outgoing tides. Because of this scouring, a greater proportion of heavier soil

48

particles, as well as gravel and cobble are left. This results in zone 1 having high bulk
density but low organic carbon content and density.

2.42. Organic matter, organic carbon, and carbon stock density
The range of organic matter and carbon content in this study was consistent with
the range in MacClellan’s 2011 study of carbon content in Oregon tidal wetland soils.
The maximum organic matter content in MacClellan’s was 27.75% with a corresponding
organic carbon content of 18.87% (they assumed that 68% of the organic matter was
comprised of carbon). By comparison, this study produced a maximum organic matter
content of 28.27% and organic carbon content of 19.22% (calculated at 68% of organic
matter for the basis of comparison). Minimum organic matter and carbon content varied
more widely between the two studies, probably because the minimum carbon content of
this study came from a zone 1 sample with little or no vegetation and a very high sand
content. The MacClellan study minimum content was 9.28% organic matter and 6.31%
organic carbon, while the Duckabush minimum content was 2.04% organic matter and
1.39% organic carbon.
Chmura et al. (2003) estimated average soil carbon density globally in tidal, saline
wetland soils. They found the average soil organic carbon density of all sites to be 0.043
± 0.002 g/cm3, but this included mangroves as well as salt marshes. The global intertidal
wetland average carbon density was 0.039 ± 0.003 g/cm3. This estimate is somewhat
higher than the average carbon density at the Duckabush River Delta, which is 0.026 ±
0.002 g/cm3. However, average carbons density at the Duckabush River Delta excluding
the zone 1 was 0.033 g/cm3, which is more consistent with the finding of Chmura et al.

49

Chmura et al. also note that carbon stocks may increase at lower latitudes due to higher
productivity in warmer climates, and the Duckabush River Delta is at a higher latitude
than many of the sites included in the Chmura et al. study. Sites at similar latitudes to the
Duckabush River Delta included in the Chmura et al. study generally exhibited similar
density of carbon stocks, but there is noticeable variation in carbon stocks in different
regions of the world (Table 2.1).
Region
Gulf of Mexico
Northeastern
Atlantic
Mediterranean
Northeastern
Pacific*
Northwestern
Atlantic

Number of
Sites
27
12

Latitude Range

Longitude Range

28.4-30.4 °N
51.5-55.5 °N

84.2-96.8 °W
0.7-8.4 °E

Mean carbon
g/cm3
0.051
0.033

1
6

43.3 °N
32.5-48.9 °N

4.6 °E
117.1-125.5 °W

0.073
0.019

57

35.0-47.4 °N

63.2-76.4 °W

0.036

Table 2.1 Mean carbon stock density in various regions of the world (Chmura et al., 2003). *Northeastern Pacific does
not include results of Duckabush River delta study.

The six Northeastern Pacific sites included in the Chmura et al. study had a mean
carbon density of 0.019 g/cm3, which is less than half the density of the global mean
(Chmura et al., 2003). The carbon density found at the Duckabush River Delta is well
within the range of other sites studies in the Northeastern Pacific (Table 2.2).
Site
Tijuana Slough 1,
CA
Tijuana Slough 2,
CA
Tijuana Slough 3,
CA
Alviso, San
Francisco Bay, CA
Bird Island, San
Francisco Bay, CA
Duckabush River
Delta, WA
Uculet, BC

Latitude (°N)
32.5

Longitude (°W)
117.1

Carbon g/cm3
0.018

32.6

117.1

0.017

32.6

117.1

0.040

37.5

122.0

0.009

37.6

122.2

0.014

47.6

122.9

0.026

48.9

125.5

0.017

Table 2.2 Comparison carbon density at salt marsh sites in the NE Pacific (Chmura et al., 2003).

50

2.43. Variations in organic carbon and estimating carbon stocks at the Duckabush River
Delta
To estimate carbon stocks at the Duckabush River Delta, the area of each zone
throughout the entire delta was calculated using DEM and categorizing the zones by their
corresponding elevations. The total area of each zone was multiplied by megagrams of
organic carbon per hectare as described in the methods (Table 2.3). This study estimates
of total carbon stored in zones 1, 2 and 3 at the Duckabush River Delta to be

Zone

OC %

OC
density
(g/cm3)

OC
stock
(Mg/ha)

Area
(ha)

Total C
(Mg)

1

2.60

0.018

52.875

20.79

2

9.39

0.031

93.875

3

5.32

0.035

106.292

Total

1099.27

% of total
C found in
Duckabush
River
Delta
26.72

% of total
area of
Duckabush
River
Delta
40.45

19.19

1801.46

43.78

37.33

11.42

1213.86

29.50

22.22

50.75

4049.96

Table 2.3 Estimates of total carbon stocks, carbon per hectare, and carbon densities of different zones throughout the
entire Duckabush River Delta. All values represent carbon found within the top 30 cm of soil and exclude coarse
organic matter great than 2.0 mm. This estimate should be considered conservative.

approximately 4,050 Mg of carbon in the top 30 cm of soil, excluding coarse organic
matter.
Depth increments from other studies vary widely, from 20-210 cm, but the highest
concentrations of carbon was consistently found in the top 50 cm of soil. Unfortunately,
gathering soil samples from a depth greater than 30 cm would have become exponentially
more difficult to extract, due to logistical constraints of this study.
51

Furthermore, it is unclear in other studies how soil bulk density is measured (what
size of soil particles are excluded by sieving). The depth increment and soil bulk density
measurement variations between studies may represent a source of error in comparing
estimates. In higher elevation zones at the Duckabush River Delta, organic-rich soil may
be several meters deep as a result of years of accretion. Assuming this is the case, the
reported carbon stocks per unit area are very low.
For the sake of comparison, consider other terrestrial system carbon stocks (Table
2.4). The conservative estimates of carbon stocks (Mg/ha) at the Duckabush River Delta
may at first glance appear comparable to ecosystems such as temperate forests and
tundra, and perhaps may seem weak compared to freshwater wetlands. However,
estimates from the Duckabush River Delta only include the top 30 cm of soil and exclude
any coarse organic matter. Because intertidal wetland soils, including at the Duckabush
Delta, are constantly accreting and sequestering carbon at a rate far exceeding other
terrestrial systems (Table 2.4), significantly more carbon is stored at the Duckabush Delta
than these informal estimates indicate. Furthermore, the estimates of carbon stocks in
other terrestrial systems listed in Table 2.4 include the entire soil column, not just the top
30 cm.
Biome
C density (Mg/ha)
C sequestration (g/m2y1)
105
0.2-5.7
Tundra
343
0.8-2.2
Boreal/Taiga
96
1.4-12.0
Temperate
123
2.3-2.5
Tropical
723
20.0
Wetlands
53-106
Unknown
Duckabush River Delta*
Table 2.4 Estimated carbon density and sequestration rates comparing the Duckabush river Delta to other terrestrial
systems. *The C density range for the Duckabush river delta only accounts for the top 30cm of soil and does not
include buried organic material >2.0 mm diameter. (Data sourced from: Lal, 2005; Pidgeon,2009).

52

2.44. Climate change, sea level rise, and “coastal squeeze”
In addition to the ability of intertidal wetlands to sequester carbon, intertidal
wetlands produce negligible methane. Gail Chmura (2009) summarizes the significance
of intertidal wetland soil organic carbon thus:
When one considers feedbacks to climate, each molecule of carbon dioxide
sequestered in soils of tidal salt marshes and their tropical equivalents, mangrove
swamps, probably has greater value than that stored in any other natural
ecosystem, due to lack of production of other greenhouse gases.

It is important to convey this information to people in decision-making positions where
the opportunity to conserve or restore intertidal wetlands is a real possibility. The general
public may easily grasp the value that intertidal wetlands and coastal systems have for
fish and wildlife habitat, recreation, etc., but communicating the ability of intertidal
wetlands and coastal wetlands to mitigate anthropogenic CO2 emissions is the real
challenge for the scientific community to convey to the public.
One of the main challenges for intertidal wetlands and coastal wetlands to
mitigate climate change is the history of land-use change and reclamation associated with
intertidal wetlands. As mentioned earlier, anthropogenic CO2 is a key driver of global
warming and climate change (IPCC, 2007). Fossil fuel combustion is most frequently
associated with anthropogenic CO2, but land-use change has made significant
contributions to the atmospheric reservoir of anthropogenic CO2 (Crooks et al., 2010;
Hopkinson et al., 2012). In the Puget Sound, where the Duckabush River Delta is
located, opportunities to restore intertidal wetlands abound, because less than 20% of
historical intertidal wetlands are left. Since it is well-documented that intertidal wetland
soils sequester carbon at a high rate (Chmura et al., 2003; Pidgeon, 2009; Hopkinson et

53

al., 2012) restoring the areal extent of regional intertidal wetlands could cause more
carbon to be locally sequestered. However, it will be important to first study the carbon
stocks of degraded intertidal wetlands in Puget Sound before estimating how much
additional carbon could be locally sequestered due to restoration. In addition, carbon
content and density should be studied in sites that are undergoing restoration, such as the
Nisqually River Delta or Stillaguamish River Delta. This would provide valuable insight
into degraded and restored wetland carbon dynamics relative to reference conditions,
such as at the Duckabush River Delta. Furthermore, in degraded intertidal wetlands
where saline water has been excluded due to diking and shoreline armoring, it is likely
that the presence of sulfate-reducing bacteria that consume methane is diminished or
absent (Bartlett et al., 1987; Zedler & Callaway, 2001). Returning saline influence to
these soils may subsequently inhibit local methane flux, further mitigating a fraction of
local greenhouse gas emissions (Crooks et al., 2010). If Puget Sound intertidal wetlands
are restored, they may indeed mitigate past land-use change that led to anthropogenic
inputs of greenhouse gasses, but further studies regarding the points made above are
needed to support this claim.
Unfortunately, in the Puget Sound, as well as other coastal areas, shorelines have
been developed and armored, prohibiting the inland transgression of intertidal wetlands,
which is necessary to ensure the survival of intertidal wetlands in the face of sea level
rise. Although intertidal wetlands grow by vertically accreting sediment, if the rate of
sediment accretion is insufficient to keep pace with sea level rise, intertidal wetlands will
move vertically by transgressing inland and upland, rather than by accreting sediment.
However, in areas where shorelines have been armored and developed, intertidal

54

wetlands have nowhere to transgress to, and are trapped between and armored shoreline
and rising seas. Eventually, rising sea levels will outpace vertical accretion, an intertidal
wetlands will drown (Figure 2.23). This phenomenon is called “coastal squeeze”
(Chmura, 2009). As these systems drown carbon sequestration will slow and eventually
halt in the absences of primary production and sedimentation.
Opportunities to remove shoreline armoring and other shoreline modifications
should be seriously considered; this represents an opportunity to increase local carbon
storage and offset anthropogenic CO2. Cost-benefit analyses of restoring and protecting
intertidal wetlands and other coastal systems should be undertaken to compare the costs
and benefits of greenhouse gas mitigation by restoring these systems versus other
strategies.

Figure 2.23 “Coastal Squeeze” (Chmura, 2009).

55

3. Conserving and restoring Puget Sound intertidal wetlands
This study has demonstrated that soil organic carbon density is positively
correlated with elevation, and negatively correlated with increasing soil texture size. Soil
carbon density at the Duckabush River Delta is slightly higher than the mean carbon
density of sites in the Northeastern Pacific, and slightly lower than the global mean
carbon density of salt marshes. More studies in the region would help shed light on
whether or not the carbon content and density estimated for this site is consistent with
that of intertidal wetlands in the Puget Sound. The next step with this information is to
apply it to a broader range of environmental issues, and further our understanding the role
of intertidal wetlands in the global carbon reservoir and greenhouse gas cycle.

3.1. Why it matters
The global extent of intertidal wetlands and coastal ecosystems is diminishing,
and is substantially reduced in Puget Sound (Hopkinson et al., 2010, Collins & Sheikh,
2005). As this study has discussed, conserving and restoring intertidal wetlands may
have benefits regarding carbon storage and their ability to act as greenhouse gas sinks. In
addition to greenhouse gas benefits, intertidal wetlands offer many other important
ecosystem services. In the Puget Sound, intertidal wetlands provide complex habitat and
abundant food sources, which in turn supports a complex and diverse array of terrestrial
and marine organisms (Dethier, 1990). Insects emerging from intertidal wetlands are
consumed by fish at high tides, and tidal detritus is consumed and filtered by benthic
invertebrates, including mollusks, crustaceans, and annelids. These fish and invertebrates

56

are subsequently consumed by larger fish and invertebrates, mammals, waterfowl,
shorebirds, and a huge array of other birds (Dethier, 1990).
In the Puget Sound, the plight of the salmon receives more attention than the
problems faced by any other fauna in the region. All salmon species spend at least part of
their lives in intertidal wetlands. Intertidal wetlands are particularly important in the life
history of Oncorhynchus tshawytscha and Oncorhynchus keta (Chinook and Chum
salmon). Many genetically-distinct runs of Puget Sound Chinook and Chum salmon are
endangered, and the disappearance of intertidal habitat, critical for juveniles of these
species, has played a role in the shrinking salmon populations.
Conserving and restoring intertidal wetlands is critical for so many environmental
reasons, and they are all intertwined. Even the least-disturbed intertidal wetlands in
Puget Sound, such as the Duckabush River Delta, have been undergone anthropogenic
alteration, presenting opportunities for restoration and conservation.

3.2. Restoring the Duckabush River Delta
Ginna Correa’s 2003 report, which assessed habitat for salmon and steelhead in
the Dosewallips-Skokomish Watershed, made several important restoration
recommendations. The recommendations in the report are chiefly aimed at salmon and
steelhead habitat improvement, but the benefits of restoring salmon and steelhead habitat
also benefit the rest of the intertidal wetland community, as well as the biogeochemical
processes that take place at the delta, including the storage of carbon and other
greenhouse gasses. At the Duckabush River Delta, approximately 9% (conservatively) of
the shoreline is armored (Correa, 2003). Most of the armoring at the delta is along SR
57

101, as well as a WDFW parking lot that was formerly part of the intertidal zone. Not
only does shoreline armoring prohibit natural processes and diminish the extent critical
fish and wildlife habitat, but it provides a location for invasive species to take hold and
spread. Armoring at the Duckabush River Delta is dominated by the highly-invasive
Cytisus scoparius (Scotch Broom). Correa recommends removing armoring and
associated invasive species wherever possible, and replanting the area with native
species.
The Puget Sound Nearshore Ecosystem Restoration Project (PNSERP) has
designed a restoration plan at the Duckabush River Delta. The key elements of the plan
include the removal of 640 m of the present SR 101 and its associated armoring. SR 101
would be reconstructed further upstream from where it currently exists, and would
include a 335 m elevated roadway, which would allow for restored tidal connection and
restoration of backshore sediment recruitment (PNSERP, 2013). Another 21 m section of
armored roadway in the northwest part of the delta would be replaced with a bridge,
restoring the intertidal zone to its historic extent. The PNSERP estimates that this design
would add 15 ha of restored intertidal wetland.
This would have obvious benefits for fish and wildlife, including juvenile
Chinook and Chum Salmon (both of which are endangered in the Duckabush River), but
would have the potential to sequester additional carbon as well. If the project proposed
by the PSNERP restores 15 ha of intertidal wetland, a “back-of-the-envelope” calculation
based on the carbon stocks of zones 2 and 3 (which would correspond to the area to be
restored) could account for an addition of approximately 1,485 Mg of carbon stored at the
Duckabush River Delta. Again, this is an informal estimate, and many other factors may

58

alter the rate of carbon sequestration and subsequently carbon stocks in the restored area
(MacClellan, 2011; Santín et al., 2007).

3.3. Restoring the areal extent of Puget Sound intertidal wetlands
Only 17-19% of the historical extent of Puget Sound intertidal wetlands still exist
(Collins & Sheikh, 2005). Intertidal wetlands at the deltas of Olympic Peninsula rivers,
including the Duckabush, historically accounted for only 1% of Puget Sound Intertidal
wetlands, but now account for 5% due to drastic losses of intertidal wetlands in other,
more developed areas of Puget Sound (Collins & Sheikh, 2005). This historic extent of
some of the largest intertidal wetland complexes on the Skagit, Stillaguamish, and
Samish rivers has been greatly diminished, and intertidal wetlands have been almost
totally eliminated in some rivers including the Green, Lummi, Puyallup, and Duwamish
(Collins & Sheikh, 2005).
It is unlikely that the historic extent of Puget Sound intertidal wetlands will ever
be restored, mostly due to the massive infrastructure, development and human population
in many parts of the Puget Sound. Therefore, conserving and protecting the remaining
intertidal wetlands of the Puget Sound is very important. The challenge in restoring
many intertidal wetlands is simply that they no longer exist, having been diked, armored,
and developed to a point that restoration is not an option. Some of the best opportunities
to restore the historic extent of Puget Sound intertidal wetlands lie in degraded, but still
existent, intertidal wetlands (PSNERP, 2013).
The PSNERP has outlined a set of general restoration objectives for Puget Sound
nearshore ecosystems. The objectives are: (1) restore the size and quality of large river
59

deltas and the nearshore processes the deltas support; (2) restore the number and quality
of coastal embayments; (3) restore the size and quality of beaches and bluffs; (4) increase
understanding of natural process restoration in order to improve effectiveness of
restoration actions.
Achieving the last objective will be the most important in guiding effective
restoration for intertidal wetlands and other nearshore ecosystems of the Puget Sound.
Communicating new and better understandings of ecosystem services and functions to
the public should be an additional objective in restoring Puget Sound intertidal wetlands
and nearshore ecosystems. Broader public understanding will strengthen public support,
funding, and restoration and conservation efforts. For a task as great as restoring the
intertidal wetlands of the Puget Sound, it is absolutely necessary to work with the public
as much as possible, because ultimately, the people of the Puget Sound region will enjoy
the benefits of restoration first-hand.

60

Conclusion
As the results of this study demonstrated, carbon content and density are
positively correlated with elevation within the intertidal wetland, driven largely by
variations in soil texture composition and soil bulk density. Further research in the Puget
Sound Region should consider accretion (or erosion) rates against rates of sea level rise to
estimate local sequestration rates. Future research should compare the Duckabush River
Delta to other intertidal systems in the Puget Sound to examine variation between
systems locally. Other studies have investigated differences between carbon stored in
natural intertidal wetlands, anthropogenically altered and degraded intertidal wetlands,
and restored intertidal wetlands. The Puget Sound is home to intertidal wetlands that run
the gamut of anthropogenic alteration, and comparisons between these different wetlands
would be useful in understanding the potential to store carbon through restoration, and
the amount of carbon that has been lost due to land-use change.
Much of the world’s coastal systems, and certainly much of the Puget Sound’s
intertidal areas have already been lost, and much of what remains is trapped between
development and rising sea levels. As sea levels rise, higher elevation areas will be more
frequently inundated by tides, diminishing the ability to store carbon. As the this study
shows, combined with previous studies’ estimates of sequestration rates, intertidal
wetlands, including the Duckabush River Delta, store huge amounts of carbon (relative to
their size) and thus offset anthropogenic CO2. Therefore, in the face of climate change, it
is important to protect these systems where they exist, and restore them wherever it is
feasible.

61

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