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EXAMINATION OF BIVALVE SHELL DEGRADATION
FOR ALKALINITY REGENERATION PURPOSES
IN HOOD CANAL, WASHINGTON
by
Lisa Abdulghani
A Thesis
Submitted in partial fulfillment
of the requirements for the degree
Master of Environmental Studies
The Evergreen State College
June 2014
© 2014 by Lisa Abdulghani. All rights reserved.
This Thesis for the Master of Environmental Studies Degree
by
Lisa Abdulghani
has been approved for
The Evergreen State College
by
________________________
Dr. Erin Martin
Member of the Faculty
________________________
Date
ABSTRACT
EXAMINATION OF BIVALVE SHELL DEGRADATION FOR
ALKALINITY REGENERATION PURPOSES IN
HOOD CANAL, WASHINGTON
Lisa Abdulghani
Spreading shell material can buffer corrosive conditions by
providing alkalinity regeneration by dissolution of calcium carbonate
(CaCO3). This research explored how enhancing the seafloor with a
particular size or species of bivalve shell may influence different rates of
shell degradation in Hood Canal, Washington. The differences in
degradation, measured by changes in mass, was examined over an incubation
period of eight weeks among whole and crushed size types for three different
species: Crassostrea gigas, Ostrea lurida, and Mytilus galloprovincialis. All
shell treatments lost mass, while M. galloprovincialis shells degraded the
most mass, losing up to 2.78% ± 0.08% of its shell matter. Each species had a
significantly different rate of mass loss relative to the other species, whether
the shell was crushed (F2,87 = 37.39, p<0.0001) or whole (F2,70 = 18.74,
p<0.0001). For all species, whole shells displayed higher rates of SML than
crushed shells for each of the species examined: M. galloprovincialis (p=0.02),
O. lurida (p=0.003), and C. gigas (p=0.01). Through CaCO3 dissolution,
whole M. galloprovincialis and C. gigas shells may contribute the most g
CO32- every year to the seawater (133.3 ± 36.8 and 135.6 ± 18.6 respectively).
Both whole and crushed shells of M. galloprovincialis contribute the greatest
amount of organic matter among all the species through decomposition (11.5
± 3.5 and 9.5 ± 3.4 respectively). Conversely, whole and crushed shells of O.
lurida contributed the least amount of CO32- and organic matter among all
the species (52.7 ± 11.5 and 42.5 ± 25.6; 1.5 ± 0.4 and 1.4 ± 0.4 respectively).
Nonetheless, all shell treatments contributed a substantial amount of CO32-,
relative to organic matter, and are recommended for alkalinity regeneration
purposes.
Table of Contents
CHAPTER ONE: LITERATURE REVIEW ................................................... 1
Introduction ..................................................................................................... 1
The Causes and Effects of Ocean Acidification in Washington State ........... 3
Seawater Carbonate Chemistry ................................................................... 4
Water Circulation of Puget Sound and Hood Canal ................................... 7
Global Contributions to Ocean Acidification ............................................... 9
Local Contributions to Ocean Acidification ............................................... 11
Calcium Carbonate Mechanics ...................................................................... 13
Calcium Carbonate Saturation State ........................................................ 13
Biogenic Calcification................................................................................. 15
Shell Composition....................................................................................... 17
Responding to Ocean Acidification: Shell Recycling .................................... 19
Utilizing Shells as an Alkalinity Buffer .................................................... 19
Alkalinity Regeneration Performance in Shells ........................................ 20
Shell Budget ............................................................................................... 22
Shell-Recycling Programs .......................................................................... 25
CHAPTER TWO: MANUSCRIPT ................................................................. 28
Introduction ................................................................................................... 31
Materials and Methods .................................................................................. 35
Study Site ................................................................................................... 35
iv
Shell Collection and Preparation ............................................................... 36
Seawater Collection and Analysis.............................................................. 38
Organic Matter in Shells ............................................................................ 40
Statistical Analyses .................................................................................... 41
Results ............................................................................................................ 42
Seawater Chemistry ................................................................................... 42
Shell Mass and Degradation Rate ............................................................. 44
Organic Matter in Shells ............................................................................ 51
Contribution of Carbonate and Organic Matter to the Seawater ............. 54
Discussion ...................................................................................................... 55
Seawater Chemistry ................................................................................... 55
Shell Degradation: CaCO3 Dissolution and Organic Decomposition ....... 56
Future Considerations of Shell-Recycling Programs ................................ 59
CHAPTER THREE: General Conclusions and Discussion.................... 62
Introduction ................................................................................................... 62
Alkalinity Regeneration Performance .......................................................... 63
Disease and Invasive Species Management ................................................. 63
Ethical Considerations .................................................................................. 67
Responding to Ocean Acidification in Washington State............................. 68
Economic and Cultural Impacts of Ocean Acidification ............................... 69
Key Policy Changes to Address Ocean Acidification .................................... 71
Conclusion ...................................................................................................... 72
v
List of Figures
Figure 1. Bjerrum plot of the carbonate system in the ocean graphing the
equilibrium relationships............................................................................. 5
Figure 2. Map of the main bodies of water surrounding Washington State,
including Admiralty Inlet and the main Puget Sound Basins ................... 9
Figure 3. Map of the sites of interest in Southern Hood Canal ..................... 36
Figure 4. The shell incubation design ............................................................ 38
Figure 5. The temporal variation of air and surface seawater pCO2 in
Southern Hood Canal ................................................................................ 44
Figure 6. The shell mass loss throughout the eight-week study for the
crushed and whole size varieties of each species. ..................................... 48
Figure 7. The differences of mass loss rates (% wk-1) among the species for
each whole (A) and crushed (B) shell treatment ....................................... 49
Figure 8. The differences of shell mass loss rates (% wk-1) between the size
treatments of each species ........................................................................ 50
Figure 9. The percentage of organic matter in each shell treatment before
and after the experiment. .......................................................................... 52
Figure 10. The percentage of organic matter in each shell treatment over
time ............................................................................................................. 53
vi
List of Tables
Table 1. Carbonate chemistry and calcium carbonate saturation values in
Southern Hood Canal during February-April 2014 .. Error! Bookmark not
defined.
Table 2. The percentages of shell mass losses over time. These values are
shown in this table as averages with standard errors ............................. 45
Table 3. The relationships between time (covariate) and the percentage of
shell mass loss for each shell treatment .................................................. 46
Table 4. The differences in proportions of organic matter (%) for each shell
treatment before and after placing shells in the seawater ...................... 52
Table 5. The calculated annual contribution of carbonate and organic matter
(g) per kg of shell material ........................................................................ 54
vii
Acknowledgements
This research was supported by the faculty of The Evergreen State
College, notably: Dr. Erin Martin, Dr. Carri LeRoy, Jenna Nelson, Ladd
Rutherford, Kaile Adney, and the entire Science Support Center staff. In
particular, I would like to thank my advisor, Dr. Erin Martin, for being
incredibly supportive and inspiring throughout my time in the Master of
Environmental Studies Program.
Thank you to Mitch Redfern, for all the constant support as my field
assistant and partner. Thank you to Burke Hales and Joe Jennings at the
College of Earth, Ocean, and Atmospheric Sciences at Oregon State
University, for providing the water sample analyses, Brady Blake for the
input that drove me to understand different perspectives, Brian Allen for
guiding and motivating me through the first steps of this process, Andy
Suhrbier for taking me on field trips to learn about the instruments that
study ocean acidification, Rolin Christopherson for guiding me through the
permitting process, Dave DeAndre and Saleh Prohim at Taylor Shellfish for
donating fresh shells for this research, the Twanoh State Park rangers,
Charlie Korb and Brent, for providing research quarters during the rainy
months of February and March, Seattle Shellfish for lending me equipment
for this research, Wendi Ruef, Sylvia Musielewicz, and Gretchen Thuesen on
viii
the UW and NOAA teams for sharing their data, knowledge, and boat for the
pure sake of science and education.
This research was partially funded by the Evergreen Foundation
Grant and was completed under the Right of Entry Permit No. 23-090919
and the Shellfish Transfer Permit No. 14-0144.
ix
CHAPTER ONE: LITERATURE REVIEW
Introduction
Ocean acidification (OA) is the prolonged reduction of seawater pH and is
threatening marine life globally (Buck & Folger, 2009; Cooley et al., 2009;
Doney et al., 2009; Fabry et al., 2008; Orr et al., 2005). OA is primarily
caused by the uptake of anthropogenic carbon dioxide (CO2) from the
atmosphere by the ocean (Feely et al., 2004; Orr et al., 2005; Zeebe, 2001),
but can also be influenced by local sources such as nutrient runoff,
eutrophication, and other natural phenomena (Abril et al., 2003; Borges and
Gypens, 2010; Feely et al., 2010). The need for action to prepare for OA is
great in the inland and coastal waters of Washington State, a region that is
especially vulnerable to synergistic causes of OA (Gazeau et al., 2007).
Probably the largest concern with OA is the magnitude and rapid pace
of its effects due to anthropogenic influences. It is estimated that surface
ocean pH has dropped slightly more than 0.1 pH units from 8.25 to 8.14 since
the beginning of the Industrial Revolution in 1751 (IPCC, 2013). It is
forecasted to decrease another 0.29 pH units (near 7.85) by 2100 (Jacobson,
2005). Although these decreases seem miniscule, the pH scale is logarithmic,
so each pH unit is a 10-fold change, where the change from 8.25 to 8.14
corresponds to a 26% increase in the hydrogen ion concentration ([H+])
(IPCC, 2013). These drastic changes in seawater chemistry are reducing the
1
concentration of the carbonate ion in seawater, and thus the level of calcium
carbonate saturation. Calcifying organisms are most vulnerable, as they
have difficulty maintaining their exoskeletons (Orr et al., 2005) under these
conditions. This is evident, as shellfish hatcheries have experienced losses in
oyster larvae since 2008 (Barton et al., 2012). Natural recruitment of other
bivalves has also decreased (Place et al., 2008) with ocean acidification being
the main culprit.
Shellfish provide many ecosystem services, including provisioning
services such as food and income; regulating services such as water quality
through the control of eutrophication, algal blooms, and hypoxia; supporting
services such as nutrient cycling that maintain ecosystem functions; and
cultural services such as spiritual, recreational, and social benefits (Cooley et
al., 2009; UNEP and Millenium Ecosystem Assessment Board, 2005). Ocean
acidification can disproportionately affect coastal ecosystems and the
communities that rely on them by negatively affecting bivalve molluscs and
the ecosystem services they provide (Borges and Gypens, 2010; Kelly et al.,
2011).
In this literature review, I will discuss how the waters in Washington
State are influenced by both global and local causes of OA. Next, I will
highlight the implications that OA encompasses, including ecological change
(Cooley et al., 2009) and alteration of marine-based resources (Cooley and
Doney, 2009). I will then explain why calcifying organisms are most at risk
2
of dissolution (Gazeau et al., 2007; Miller et al., 2009; Orr et al., 2005) and
describe the chemical mechanics behind this phenomenon. Lastly, this
literature review will explore how spreading shell material can buffer
seawaters from OA and how recent studies have contributed to the
understanding of shells’ capacity to impact alkalinity. This thesis will
explore the notion that the size or species of shell may influence different
rates of shell dissolution, which may help restoration organizations manage
their approach to shell recycling efforts.
The Causes and Effects of Ocean Acidification in Washington State
In the following sections, both natural and anthropogenic factors that
contribute to ocean acidification will be discussed. First, seawater carbonate
chemistry will be described to understand the chemical reactions behind
ocean acidification. Then, the study region will be discussed, with respect to
seawater circulation patterns. Lastly, the global and local contributions to
ocean acidification will be discussed further in depth, focusing on why this
study region is vulnerable to ocean acidification. These sections will setup
the foundations of why this research is needed to further understand the
causes and effects of ocean acidification.
3
Seawater Carbonate Chemistry
The oceanic carbonate system can influence and be influenced by pH —
the measurement of hydrogen ion concentration, [H+], in seawater through a
series of chemical reactions that occur at equilibrium. The carbonate system
comprises three inorganic species of carbon: carbonic acid (H2CO3)
bicarbonate, (HCO3-) and carbonate (CO32-). A forth form is carbon dioxide,
CO2 (aq) = aqueous carbon dioxide, which is chemically not separable from
H2CO3 (Zeebe, 2001). Throughout this literature review, the former H2CO3
(Eq. 1) will be used. The sum of the dissolved forms of H2CO3, HCO3-, and
CO32- is called the total dissolved inorganic carbon (DIC) (Eq. 2).
[H2CO3]* = [CO2 (aq)] + [H2CO3]
(1)
DIC = [H2CO3] + [HCO3-] + [CO32-]
(2)
where brackets refer to total stoichiometric concentrations and the asterisk
denotes a sum of two compounds.
When CO2 first dissolves into seawater, it immediately reacts with
water, H2O, and forms H2CO3 (Eq. 3). Because H2CO3 is a weak acid, it can
disassociate into H+ and HCO3-, which can further disassociate into H+ and
CO32-. This system is reversible and equilibrates depending on the
temperature, salinity, and pH of the seawater (Doney et al., 2009). Under
4
current oceanic conditions, the most abundant form of inorganic carbon is
HCO3- (91%), followed by CO32- (8%), and H2CO3 (1%) (Raven et al., 2005).
This is illustrated in Fig. 1 by the dotted line that crosses over the
concentration curves at pH = 8.1. Also indicated in Fig. 1 are the pH values
at which [H2CO3] = [HCO3] and [HCO3-] = [CO32-]. The circle and the
diamond indicate pK1* = 5.86 and pK2* = 8.92 as the equilibrium constants
(Zeebe, 2001), or when the stoichiometric concentrations of the products and
reactants are equal.
K0
K1
K2
+
H2O + CO2 ⇌ H2CO3 ⇌ HCO3 + H ⇌ CO32- + 2H+
(3)
where K0, K1 and K2 are equilibrium constants.
Figure 1. Bjerrum plot of the carbonate system in the ocean graphing the
equilibrium relationships (Zeebe, 2001).
5
Ocean acidification can cause significant shifts in ocean carbonate
chemistry by altering carbonate speciation. For instance, if the concentration
of H2CO3 increases in the water by intrusion of atmospheric CO2, H2CO3
would disassociate into HCO3- and H+ because HCO3- is the dominant species
of carbon in the seawater. Then, existing CO32- reacts with the H+ by forming
more HCO3-. The pH does not change rapidly because the carbonate system
is a natural buffer for seawater, but [CO32-] decreases appreciably (Zeebe,
2001), effectively reducing the buffering capacity, also known as alkalinity.
There are many definitions of alkalinity (Andersson et al., 2003; Morse
et al., 2007; Rounds, 2006; Wolf-Gladrow et al., Dickson, 2007; Zeebe, 2001),
as it is a concept of increasing complexity. Most basically, alkalinity is a
measure of the capacity for seawater to resist sudden changes in pH by
absorbing hydrogen ions using available bases such as bicarbonate and
carbonate. For example, carbonate can absorb two hydrogen ions before
turning back into H2CO3 because it has a double negative charge. Therefore,
having more carbonate ions will increase the seawater’s alkalinity. In the
ocean, there are more ions than just carbonate and bicarbonate that
contribute to the alkalinity. Total alkalinity (TA) is a measure of the
alkalinity caused by the presence of both carbonate species and noncarbonate species, including boric acid, and hydroxide. (Dickson et al., 2007)
defines TA as:
6
TA = [HCO3-] + 2[CO32-] + [B(OH)4=] + [OH-] +
[HPO42-] + 2[PO43-] + [H3SiO4-] + [NH3] +
[HS-] – [H+]F – [HSO4-] – [HF] – [H3PO4]
(4)
where [H+]F is the free concentration of hydrogen ions.
In order to understand how CO2 emissions are affecting the ocean, we
must understand the sea–air CO2 exchange. Due to thermodynamics,
equilibrium at the sea-air interface can be characterized as an equality of
partial pressures. For example, when the partial pressure of carbon dioxide
in the air is greater than that of seawater’s, the seawater draws in the
atmospheric CO2. The partial pressure of CO2 (pCO2) refers to the gas phase
that is in equilibrium with that of seawater (Zeebe, 2001).
DIC and pCO2, alkalinity, and pH constitute four measureable factors
in the carbonate system that can be determined analytically (Wolf-Gladrow et
al., 2007). The knowledge of any two of them allows us to calculate the
carbonate chemistry of a seawater sample. For this thesis, the DIC and pCO2
values were measured and entered into a computer-modeling program to
calculate pH, alkalinity, and the calcium carbonate saturation values.
Water Circulation of Puget Sound and Hood Canal
Puget Sound is a semi-closed estuary in Washington State with many
freshwater inputs and an oceanic passage at its northern end through the
Strait of Juan de Fuca (Fig. 2). It consists of interconnected basins separated
by sills. A shallow sill at Admiralty Inlet limits the exchange of seawater to
7
and from the Pacific Ocean (Ebbesmeyer and Barnes, 1980). Although tidal
currents and vertical mixing are strongest at Admiralty Inlet, there is little
water movement and strong vertical stratification below Admiralty Inlet
(Feely et al., 2010). This contributes to the sluggish circulation in the inlets
that branch from Admiralty Inlet, including Hood Canal.
Hood Canal is a deep, natural fjord forming one of the major basins of
Puget Sound in Washington State. In Hood Canal, the seawater circulates
slowly and has a residence time that varies from 64 to 121 days (Babson et
al., 2006; Warner et al., 2001). This residence time is long, compared to the
northern Whidbey Basin, whose residence time varies from 33 to 44 days
(Babson et al., 2006).
Circulation is driven by new water inputs, wind, and upwelling
currents. During winter months, circulation is gradual, due to the intrusion
of newer, denser ocean water (Warner et al., 2001). During summer months,
circulation is more rapid due to northerly winds that push the surface layer
in the main stem of Hood Canal northwards, resulting in upwelling of deeper
waters (Feely et al., 2010).
8
Strait of Juan de Fuca
Whidbey
Basin
Admiralty
Inlet
48°N
Central
Sound
Hood
Canal
Pacific
Ocean
South
Sound
47°N
Esri,
MapmyIndia,
©
0 HERE,
25 DeLorme,
50
100 Kilometers
OpenStreetMap contributors, and the GIS user
community
124°W
123°W
122°W
Figure 2. Map of the main bodies of water surrounding Washington State,
including Admiralty Inlet and the main Puget Sound Basins. The star
represents the study site in Southern Hood Canal.
Upwelling exposes deep water rich with CO2 and nutrients to the
surface. Because of the slow water circulation in Hood Canal during winter
conditions, the water stratifies vertically with cold, salty water in the depths.
Compared to the main basin of the Puget Sound, Hood Canal has a stronger
stratification that separates the upper and lower layers of the water column
(Warner et al., 2001). The lack of flushing in Hood Canal during winter
causes confinement of acidic waters, which makes it an excellent site for
studying the effect of ocean acidification on shell dissolution on the seafloor.
Global Contributions to Ocean Acidification
Rates of atmospheric CO2 emissions have increased exponentially since
the Industrial Revolution (Orr et al., 2005). Human activities cause a net
CO2 flux to the atmosphere from burning fossil fuels, cement production, and
9
land use change such as deforestation (Chapin et al., 2011; IPCC, 2013).
From 1750 to 2011, anthropogenic CO2 emissions have released 545
gigatonnes of carbon (Gt C) to the atmosphere. From these cumulative
anthropogenic CO2 emissions, 240 Gt C (~44%) have been stored in the
atmosphere, 150 Gt C (~27%) have been accumulated in natural terrestrial
ecosystems, and 155 Gt C (~28%) have been absorbed by the ocean (IPCC,
2013).
The total ocean uptake flux, including the anthropogenic CO2, is
estimated to be 2.7 ± 0.5 Gt C in 2011 (Quéré et al., 2013). This rate differs
among spatial and temporal values, though the general trend in the
literature confirms that seawater and atmospheric pCO2 correlate (Doney et
al., 2009) indicating that human emissions of CO2 have caused and will
further cause CO2 absorption by seawater due to air-sea CO2 exchange
(Cooley et al., 2009; Doney et al., 2009; Orr et al., 2005).
In Washington State, the other mechanism for the invasion of CO2 into
coastal waters is attributed to upwelling events. The change in seasonal
wind directions dictate the upwelling or downwelling currents that occur over
the oceanic continental shelf and the sills of Puget Sound (Feely et al., 2010).
Upwelling exposes water rich with CO2 from the deep water to the coast.
Globally upwelling water contains CO2 that has accumulated from past
anthropogenic additions, biological respiration, and physical-chemical
processes due to the ocean’s thermohaline circulation patterns (Chapin et al.,
10
2011; Orr et al., 2005).
Global contributions of CO2 may have a profound impact on pH and
CO32- availability. The increases of atmospheric CO2 — and consequently the
H2CO3 in the seawater —have a relatively small effect in waters with a high
pH and alkalinity. However, as more atmospheric CO2 is absorbed and
[CO32-] declines, the change in the H2CO3 and [H+] gets stronger as CO2 is
added, effectively lowering pH and CO32- levels (Orr et al., 2005; Zeebe, 2001).
Local Contributions to Ocean Acidification
In Puget Sound, global influences can account for 24-49% of the pH
decrease in the deep of waters of Hood Canal relative to pre-industrial levels
(Feely et al., 2010). However, recent studies demonstrate that local sources of
nutrients delivered by freshwater inputs, pollutants, soil erosion, water
circulation, biological processes can acidify coastal waters at substantially
higher rates than atmospheric carbon dioxide alone (Abril et al., 2003; Borges
and Gypens, 2010; Feely et al., 2010; Kelly et al., 2011). These impacts are
likely to be intensified when combined with other stressors in coastal
ecosystems, such as overfishing, habitat destruction, and temperature
increases (Kelly et al., 2011).
Estuaries within Puget Sound often experience eutrophication through
the over-abundance of nutrients in the water (Mackas and Harrison, 1997;
Washington State Blue Ribbon Panel on Ocean Acidification, 2012).
11
Eutrophication can be natural or human-caused, though the leading cause in
Puget Sound is attributed to agricultural nutrient runoff and insufficient
wastewater treatment (Feely et al., 2010; Washington State Blue Ribbon
Panel on Ocean Acidification, 2012). Eutrophication can cause
disproportionate plant and algae growth that can lead to hypoxic zones after
aerobic bacteria break down the organic material and deplete dissolved
oxygen levels. Waters then becomes supersaturated with respect to CO2 due
to aerobic respiration, which can lead to an increase in [H+] and decrease in
[CO32-] (Abril et al., 2003).
In estuaries, low salinity and high productivity due to riverine inputs
create conditions that can be corrosive to shells due to inputs of organic
matter that can fuel net respiration due to natural or anthropogenic
stimulated respiration processes in the estuary (Abril et al., 2003; Feely et
al., 2010; Kelly et al., 2011). Further, estuarine carbonate chemistry can be
variable and complex due to many biogeochemical processes driven by
production and respiration cycles, freshwater input, and atmospheric CO2
(Waldbusser et al., 2011) and it is unclear to what magnitude these local
impacts contribute to acidification.
12
Calcium Carbonate Mechanics
Calcium Carbonate Saturation State
The calcium carbonate (CaCO3) saturation state is the equilibrium
between the solid state and solution. One well-known and profound effect of
OA is the lowering of CaCO3 saturation states (Zeebe, 2001), which can
impact calcifying organisms negatively (Feely et al., 2010; Green et al., 2013,
2009). Because Ca2+ is such an abundant ion in seawater, the CaCO3
saturation state (Ω) in seawater is controlled by the amount of CO32available (Eq. 5). As mentioned before, when [H2CO3] increases in seawater,
[CO32-] is reduced because H+ binds to it, leading to decreased CO32availability and lower CaCO3 saturation states. As ocean pH falls, decreasing
levels of CO32- limits calcifying organisms to form their shells or skeletons
because of lower aragonite and calcite saturation states (Feely et al., 2010).
Ω calcium carbonate = [CO32-] [Ca2+] / K*sp
(5)
where K*sp is the solubility product, or equilibrium constant, of CaCO3.
The saturation horizon is the depth in the ocean that identifies the
boundary between super-saturation and under-saturation of CaCO3 (Orr et
al., 2005). As particles fall through the water column, respiration occurs, and
CO2 accumulates with depth, making it more acidic. Above the saturation
horizon, the saturation state (Ω) is greater than 1 and CaCO3 does not readily
13
dissolve into Ca2+ and CO32-. However, below the saturation horizon, the
saturation state (Ω) is less than 1, and CaCO3 will dissolve. When the
saturation state (Ω) equals 1, the reaction is in equilibrium (Eq. 6) and
CaCO3 is dissolving at the same rate that it is precipitating.
CO32- + Ca2+ ⇌ CaCO3
(6)
The carbonate compensation depth occurs at a depth in the ocean
where production is exceeded by dissolution. Surface seawater is
supersaturated with respect to calcite and aragonite (Morse et al., 2007).
However, CaCO3 can dissolve in waters supersaturated with CaCO3
(Waldbusser et al., 2011), demonstrating that the carbonate compensation
depth can be correlated — but not directly dependent upon — certain values
of the saturation horizon.
Both dissolution and precipitation of CaCO3 can occur depending on
carbonate saturation levels. Precipitation and dissolution are quantified by
their rates. Solubility quantifies the dynamic equilibrium state achieved
when the rate of dissolution equals the rate of precipitation. The typical unit
for dissolution is mol/s, compared to the unit for solubility as mol/kg (Chang,
2013).
14
Biogenic Calcification
Biogenic calcification is the biological ability to precipitate CaCO3 from
seawater (Eq. 6), most often in the phase of calcite or aragonite (Doney et al.,
2009; Morse et al., 2007). This thesis will be examining the shells from
bivalve molluscs, which are made up of mostly mineral calcium carbonate,
CaCO3, an important component of marine sediments and biological
organisms.
CO32- + Ca2+ ⇌ CaCO3
(6)
Different organisms from all three domains of life are able to produce
minerals that serve a variety of functions. In fact, living organisms utilize
more than sixty different minerals, including amorphous minerals, inorganic
crystals, and organic crystals (Addadi and Weiner, 1992). Calcium minerals
represent about 50% of all known biogenic mineralization, reflecting
calcium’s abundance in the ocean as well as its versatile use in cells (Addadi
and Weiner, 1992).
Aragonite and calcite are both mineral forms of CaCO3, though they
differ in the positions of their CO32- (Addadi and Weiner, 1992), which
influences the crystalline structure and solubility constant (Morse et al.,
2007). Aragonite is also both denser and more soluble than calcite (Morse et
al., 2007). This chemical and anatomical difference between aragonite and
15
calcite may have powerful biological implications, as changes in seawater
chemistry may affect aragonite-calcifiers more negatively than calcitecalcifiers (Morse et al., 2007).
Aragonite plays an important role in the calcifying life stages of
pteropods, corals, and larval oysters (Barton et al., 2012). Larval oysters
precipitate aragonite to form their initial larval shell. Following settlement,
they then form their shell out of calcite. They seem to be particularly
susceptible to changes in seawater chemistry when the larval shell is formed
from the more-soluble aragonite (Barton et al., 2012; Beniash et al., 2010;
Kurihara et al., 2007).
In one study, early development in juvenile oysters slowed when
exposed to acidified conditions (pH = 7.4) (Kurihara et al., 2007). In situ
experiments in surface waters validate previous lab-based experiments that
highlighted decreased calcification rates in juvenile Pacific oysters (C. gigas)
(Barton et al., 2012). In another study, shells of the larval Mediterranean
mussel (Mytilus galloprovincialis) dissolved due to the expenditure of more
energy needed to maintain calcifying functions in very acidic waters
(Kurihara, 2008). These studies have alluded that lower calcium carbonate
saturation states are due to decreasing pH and [CO32-].
16
Shell Composition
Although bivalve shells are mostly made of CaCO3, there are
additional components within it that need to be considered when examining
shell degradation, including organic matter and other inorganic elements.
The inorganic and organic matter in shells was examined throughout the
literature to understand if shell mass loss can be attributed mineral
dissolution or organic decomposition.
According to a study on the chemical-mechanical characteristics of
oyster shell, CaCO3 accounts for 96% of the inorganic material in its mineral
phase of calcite (Yoon et al., 2003). The remaining mass was composed of
seven other minerals, including silica, magnesia, and sodium oxide in trivial
amounts (Yoon et al., 2003). Therefore, mineral dissolution can be
principally attributed to dissolution of CaCO3.
Depending on the proportion of organic matter of the shell, it can slow
mineral dissolution, thus potentially making shell degradation less favorable
for alkalinity regeneration purposes. The organic matter found in bivalve
shells can be described as an organic protein matrix that is enclosed around
the CaCO3 crystals (Simkiss, 1965; Weiner & Hood, 1975). The proportion of
organic matter proportions in bivalve shells can depend on species and life
stage (Barros et al., 2013; Barton et al., 2012; Glover and Kidwell, 1993;
Goulletquer and Wolowicz, 1989; Kvenvolden et al., 1980; Waldbusser et al.,
2011; Weiner and Hood, 1975). As such, the protein proportion in shell can
17
vary from 0.1 to 10% of shell weight between different species of bivalves
(Almeida et al., 1998). (Goulletquer and Wolowicz, 1989) measured the
organic material in clam shells and found that the average percentage of
shell organic matter in the shell varied from 2.38% for Cardium edule and
2.34% for Cardium glaugum to 2.80% for Ruditapes phillippinarum. The
proportion of organic matter in bivalve shells is understudied and little is
known on this actual figure.
Within the organic matrix of the bivalve shell, there is a mixture of
glycoproteins, mucopolysaccharides, lipids and amino acids, with differences
that depend on the species (Weiner and Hood, 1975). (Simkiss, 1965)
compared organic matter of the California mussel (Mytilus californius), the
Eastern oyster (Crossostrea gigas), and a snail (Australorbis globratus) and
found that the protein in molluscan shells resembled each other closely
compared to the snail shell, indicating that bivalve shells have a generally
similar organic matrix compared to other taxa. More research is needed to
analyze the proportions of organic matter in different species of shells to
examine if organic decomposition may be the mechanism for shell
degradation rather than mineral dissolution.
18
Responding to Ocean Acidification: Shell Recycling
Utilizing Shells as an Alkalinity Buffer
Calcium carbonate plays an important role in regulating carbon
sequestration by the oceans. However, the rate of anthropogenic CO2
additions is outpacing the ocean’s ability to restore oceanic pH and carbonate
chemistry. By putting shells back into targeted areas, local waters may be
able adjust to the abnormal amounts of CO2 that is invading the waters.
Recognizing the risks of ocean acidification to Washington, Governor
Christine Gregoire created the Washington State Blue Ribbon Panel on
Ocean Acidification to chart a course for addressing the causes and
consequences of acidification. In a collaborative effort to strategically
respond to the effects of OA, the Panel created both short and long-term
goals. One possible short-term strategy to combat locally intensified
acidification is to return shell material to coastal habitats where shellfish are
present. Spreading shell material (CaCO3) can buffer corrosive conditions by
increasing seawater alkalinity (Washington State Blue Ribbon Panel on
Ocean Acidification, 2012). The dissolution of CaCO3 provides alkalinity
regeneration by buffering weak acids, such as H2CO3 (Abril et al., 2003;
Morse et al., 2007; Waldbusser et al., 2013). Conversely, the physical
removal of shell from the system also would result in the loss of a substrate
that can contribute to alkalinity through dissolution.
19
In addition to alkalinity regeneration, shells can also be used as
important substrate for oyster reef restoration (Brumbaugh and Coen, 2009).
Physical habitat restoration for shellfish most often involves placing fresh,
weathered, or fossilized dredged shell directly on the bottom of the seafloor.
Shells act as a suitable habitat for shellfish larvae to settle, grow, and die
(Brumbaugh and Coen, 2009). Enhancing an area with shells may increase
shellfish populations as well as restoring the shell resource.
Shell recycling can also provide ecological benefits. Shells form
complex structures that provide refuge or hard substrate for other species of
marine plants and animals to inhabit, enhancing biodiversity (Dumbauld et
al., 2009; Gutiérrez et al., 2003). On the Pacific Coast, shells of the Pacific
oyster placed at high densities in the intertidal zone provide excellent habitat
for juvenile Dungeness crabs (Ruesink et al., 2006).
Alkalinity Regeneration Performance in Shells
Studies in the primary literature have tested if shell recycling can
indeed provide alkalinity regeneration by shell dissolution. Most studies took
place in the Chesapeake Bay, an area that has been overharvested and
polluted heavily. One study added crushed shells of the hard shell clam
(Mercenaria mercenaria) to a mudflat before seeding it with M. mercenaria
juveniles. The addition of shell material caused the CaCO3 saturation state
to increase from Ω = 0.25 to Ω = 0.53 (Green et al., 2009), which is a small,
20
yet effective change which increased the number of live clams almost threefold in two weeks, suggesting that settling clam larvae respond greatly to
increased CaCO3 saturation (Green et al., 2009). In a similar and more
recent study, Green et al. (2013) tested their experiment again to examine if
clam larvae respond positively to increased CaCO3 saturation in both a lab
observation and field manipulation study. They found that aragonite
saturation state rose from Ω = 0.68 to Ω = 1.30 in sediments that were
buffered with shell material. Further, M. mercenaria increased their
burrowing recruitment in the buffered sediments, suggesting that shell
recycling could indeed provide alkalinity regeneration to sediments and
positively influence shellfish that depend on an elevated saturation state.
(Waldbusser et al., 2011) examined different types of intact oyster
(Crassostrea virginica) shell at different levels of pH to determine if fresh,
weathered, or dredged shells would have differing dissolution rates in a lab
setting. Fresh shells were collected from a local oyster house and had their
tissue removed 24 h before the experiment; weathered shells had been placed
in a land-based area for 2 y before the experiment; and dredged shells were
collected from a marina, where they had been on the seafloor for up to several
hundred years (Waldbusser et al., 2011). Fresh and weathered shells had the
highest in shell mass losses, followed by the fossilized, dredged shell. Fresh
shells dissolved slightly faster than weathered shells, though not
significantly. This could be because fresh shells can lose up to 10-25% of its
21
weight almost immediately after death (Waldbusser et al., 2011). One
possible explanation for the higher dissolution found in fresh shells may be
attributed to remineralization of organic matter by microbes on the fresh
shell surface, and the consequent event of metabolic CO2 production
(Waldbusser et al., 2011). Remineralization of the shells’ organic material
would contribute to a mass loss without an increase (and potentially a
decrease) in alkalinity (Waldbusser et al., 2011). Further research is needed
to address if shell mass losses can be attributed to CaCO3 dissolution or
organic decomposition.
Shell size as a factor for dissolution has not been formally studied.
From a first-order perspective, more surface area accelerates thermodynamic
shell degradation (Waldbusser et al., 2011), promoting higher alkalinity
levels. This thesis examined how different shell sizes could impact
dissolution.
Shell Budget
Altering the shell balance via shellfish harvest or shell recycling in an
estuary may have significant geochemical (Waldbusser et al., 2011) and
biological implications (Kidwell and Jablonski, 1983). Increasing inputs into
the shell budget encourages settlement of calcifying organisms due to the
higher carbonate availability and suitable substrate. According to the
taphonomic hypothesis (Kidwell and Jablonski, 1983), increasing shell
22
content may provide a positive feedback process that provides cultch, or hard
substrate for larval calcifying organisms to grow on. These calcifying
organisms will eventually die and add a buffer to the waters as their shells
dissolve. As they grow, they are forming their shells out of CaCO3 by using
the surrounding available CO32-. When they eventually die, their shells
dissolve, adding Ca2+ and CO32- back into the water while providing further
substrate for future calcifying organisms, repeating the process. The
taphonomic hypothesis can work with a caveat. Organisms that thrive from
shell additions must remain in the water indefinitely to ensure the positive
feedback process between shell formation and dissolution continues. This
taphonomic feedback hypothesis may have powerful implications for shellfish
aquaculture that disrupts the CaCO3 budget. If shells are taken out of the
system or cannot be achieved by natural populations of molluscs, waters may
need assistance via shell additions in areas with heavy shellfish aquaculture
harvests (Brumbaugh and Coen, 2009; Waldbusser et al., 2011).
In a study on the decline of the Eastern oyster (Crassostrea virginica),
the population decline is often associated with a decline in the shell bed, and
ultimately a decline in the shell resource, which lowers the available CO32- in
the seawater (Powell and Klinck, 2007). Oyster reefs need a balance among
oyster settlement, growth, and mortality to sustain themselves (Waldbusser
et al., 2011). When the shell budget decreases, a negative feedback occurs
where the current shellfish populations utilize the sparse amounts of CO32-
23
from the seawater to form CaCO3, which is not being supplemented by shell
dissolution. When there is less carbonate in the ocean, there is less
opportunity for carbonate to buffer acids, such as H2CO3. Further, any shell
formation by living shellfish will contribute to a decrease in alkalinity
production and an increase in [H+] (Waldbusser et al., 2013). Because oyster
larvae have lower survival rates in low-alkaline, high-acidity environments
(Barros et al., 2013; Barton et al., 2012), oyster populations tend to decline in
areas where there is not a cycle of CaCO3 replacement by shellfish mortality
or shell additions.
By pulling carbonate from marine waters, calcifying organisms may
play a role in sequestering carbon in the CaCO3 of shells, reducing
concentrations of carbonate that may have once been CO2 as a greenhouse
gas and dissolved into seawater as H2CO3 and reacted into HCO3- and
consequently CO32-. It has been suggested that one oyster can sequester 8.36
grams of carbon every 2 years (Hickey, 2008). However, CaCO3 formation
alone will not be strong enough to curb for the increasing amounts of CO2
absorbed into the ocean via fossil fuel combustion, based a study that
modeled the CaCO3 saturation state for the 21st century, especially
considering dissolution occurs as a result of increasing CO2 absorption
(Andersson et al., 2003). Further, the formation of CaCO3 decreases both
DIC and TA. As a result, the system shifts to higher H2CO3 levels and lower
24
pH levels (Zeebe, 2001). In other words, the current rate of acidification is
outpacing the ocean’s capacity to restore oceanic pH and [CO32-].
The feedback system between shellfish and shells has powerful
implications for Washington State, as it is one of the largest shellfish
aquaculture industries in the nation. Little to no research has been done on
the current fluxes of shell material to the shellfish consumers and back to the
estuary. In commercial shellfish aquaculture, shells return to the waters if
shellfish growers harvest only the meat and return the shells to their
respective bays, usually as a planting material for juvenile oysters. Further
research is needed to establish if shell material can be a renewable, infinite
resource (Powell and Klinck, 2007).
Shell-Recycling Programs
No shell-recycling programs currently exist in Washington State.
Shell enhancement programs do exist for the purposes of native oyster
restoration. As of 2013, the species of shell used for shell additions is locally
grown Pacific oyster (Crassostrea gigas), as it is the most farmed oyster and
shell is readily available from known sources. Shells must sit out for two
years in piles before they can be placed in the water to prevent the spread of
the oyster drill (Cohen and Zabin, 2009; Washington Department of Fish and
Wildlife, 2013). The main reason for the lack of a shell-recycling program in
Washington State can be attributed to the precautionary concern for disease
25
introductions via shell transfers. The transfer of shell material currently
requires a permitting process through Washington Department of Fish and
Wildlife, which also manages invasive species. The Shellfish Transfer Permit
was established to reduce the risk of transferring marine invasive species,
such as oyster drills (Ocicebrellus ornatus), from one water body to another
(WDFW, 2013). Various approaches have been used or recommended to
reduce the risk of transporting oyster drills and harmful shellfish pathogens
from local and non-local sources (Cohen and Zabin, 2009).
On the East Coast and Gulf, shell-recycling programs encourage local
oyster consumers to recycle their oyster shells rather than to send them to
the landfill as trash or employing them for landscaping purposes (Bushek et
al., 2004). With the appropriate protocol, a shell collection and deposition
program could be implemented to help protect local waters against OA, while
also supplementing current native oyster restoration projects and engaging
citizens and businesses with the local impacts of acidification.
One interesting shell-recycling project occurred from 1960 until 2006
in the state of Maryland to restore previously harvested oyster reefs.
Approximately 196 million bushels of dredged oyster shell were replaced in
Chesapeake Bay during a 46-year period from the program’s inception to
termination (Waldbusser et al., 2011). This is likely the largest coordinated
shell-recycling/reef-restoration effort to-date, and also perhaps the largest
alkalinity buffering experiment conducted. Unfortunately, the shell-recycling
26
program has been discontinued because of the lack of accessible shell
material (Southworth et al., 2010).
A large percentage of the oysters used for East Coast shell-recycling
efforts come from Gulf of Mexico populations (Brumbaugh and Coen, 2009).
A conscious effort is being made to quarantine the non-local shell, based on
concerns that the shells may harbor nonnative pathogens or exotic
hitchhikers. Using the results from Bushek et al. (2004), recycled shells are
quarantined in a designated land-based area for a minimum of one to three
months before being used for shell recycling. East Coast and Gulf shellrecycling programs values differ, that is, they are willing to risk the spread of
invasive species in exchange for the benefits of recycling shell material. In
Washington, however, precautionary values prevent shell-recycling programs
from existing on the basis that land-based weathering or quarantine may be
inadequate to neutralize all pathogens for disease transfers (Blake, 2013).
Very little research has been done regarding shell material as a vector for
diseases and invasive species. In a review of shell storage, (Cohen and Zabin,
2009) identified concerns regarding invasive species, but did not address
controlling pathogens. More research is needed to address the concerns of
invasive species management in shell-recycling programs.
27
CHAPTER TWO: MANUSCRIPT
Formatted and prepared for: Journal of Shellfish Research
This manuscript is a preliminary draft submitted to fulfill graduation requirements for
The Evergreen State College Master of Environmental Studies program. The following
document has not been edited, reviewed, or otherwise endorsed by any of the listed coauthors and serves only to exemplify the potential final journal submission.
28
EXAMINATION OF BIVALVE SHELL DEGRADATION FOR
ALKALINITY REGENERATION PURPOSES IN
HOOD CANAL, WASHINGTON
LISA ABDULGHANI1 AND ERIN MARTIN1
1Department
of Environmental Studies, Graduate Program on the
Environment, The Evergreen State College, 2700 Evergreen Pkwy, Olympia,
Washington 98505
ABSTRACT: Spreading shell material can buffer corrosive conditions by
providing alkalinity regeneration by dissolution of calcium carbonate
(CaCO3). This research explored how enhancing the seafloor with a
particular size or species of bivalve shell may influence different rates of
shell degradation in Hood Canal, Washington. The differences in
degradation, measured by changes in mass, was examined over an incubation
period of eight weeks among whole and crushed size types for three different
species: Crassostrea gigas, Ostrea lurida, and Mytilus galloprovincialis. All
shell treatments lost mass, while M. galloprovincialis shells degraded the
most mass, losing up to 2.78% ± 0.08% of its shell matter. Each species had a
significantly different rate of mass loss relative to the other species, whether
the shell was crushed (F2,87 = 37.39, p<0.0001) or whole (F2,70 = 18.74,
p<0.0001). For all species, whole shells displayed higher rates of SML than
crushed shells for each of the species examined: M. galloprovincialis (p=0.02),
O. lurida (p=0.003), and C. gigas (p=0.01). Through CaCO3 dissolution,
whole M. galloprovincialis and C. gigas shells may contribute the most g
CO32- every year to the seawater (133.3 ± 36.8 and 135.6 ± 18.6 respectively).
Both whole and crushed shells of M. galloprovincialis contribute the greatest
amount of organic matter among all the species through decomposition (11.5
± 3.5 and 9.5 ± 3.4 respectively). Conversely, whole and crushed shells of O.
lurida contributed the least amount of CO32- and organic matter among all
the species (52.7 ± 11.5 and 42.5 ± 25.6; 1.5 ± 0.4 and 1.4 ± 0.4 respectively).
Nonetheless, all shell treatments contributed a substantial amount of CO32-,
relative to organic matter, and are recommended for alkalinity regeneration
purposes.
KEY WORDS: ocean acidification, alkalinity regeneration, calcium
carbonate, pH, shell, dissolution, Hood Canal
SHORT RUNNING TITLE: Shell Degradation for Alkalinity Regeneration
ACKNOWLEDGEMENTS: Thank you to the College of Earth, Ocean, and
Atmospheric Sciences at Oregon State University, Carri LeRoy, Jenna
29
Nelson, Ladd Rutherford, Kaile Adney, Brady Blake, Brian Allen, Andy
Suhrbier, Rolin Christopherson, Dave DeAndre, Saleh Prohim, Charlie Korb,
Wendi Ruef, Sylvia Musielewicz, Gretchen Thuesen, and Mitch Redfern.
This research was partially funded by the Evergreen Foundation Grant and
was completed under the Right of Entry Permit No. 23-090919 and the
Shellfish Transfer Permit No. 14-0144.
30
Introduction
Ocean acidification (OA) is the prolonged reduction of seawater pH
that can cause significant shifts in ocean carbonate chemistry by altering
carbonate speciation. These drastic changes are altering a wide range of
marine organisms and the food webs that depend on them (Buck & Folger,
2009; Cooley et al., 2009; Doney et al., 2009; Fabry et al., 2008; Orr et al.,
2005). Changes in seawater chemistry are reducing ocean carbonate (CO32-)
concentrations, and thus calcium carbonate (CaCO3) saturation. Because
Ca2+ is such an abundant ion in seawater, the CaCO3 saturation state (Ω) in
seawater is controlled by the amount of CO32- available. One well-known and
profound effect of OA is the lowering of CaCO3 saturation states (Zeebe,
2001), which can impact calcifying organisms negatively (Feely et al., 2010;
Green et al., 2013; Green et al., 2009). Calcifying organisms are most
vulnerable to OA, as they have difficulty forming and maintaining their
calcareous exoskeletons in conditions with low CaCO3 saturation (Orr et al.,
2005). This is evident, as shellfish hatcheries have experienced losses in
oyster larvae since 2008 (Barton et al., 2012). Natural recruitment of other
bivalves has also decreased (Place et al., 2008), with ocean acidification as the
main culprit.
Probably the largest concern with OA is the magnitude and rapid pace
of its effects due to increasing anthropogenic CO2 in the seawater. Rates of
atmospheric CO2 emissions have increased exponentially since the Industrial
31
Revolution circa 1751 (Orr et al., 2005). From these cumulative
anthropogenic CO2 emissions, 240 Gt C (~44%) have been stored in the
atmosphere, 150 Gt C (~27%) have been accumulated in natural terrestrial
ecosystems, and 155 Gt C (~28%) have been absorbed by the ocean (IPCC,
2013). It is estimated that surface ocean pH has dropped slightly more than
0.1 pH units from 8.25 to 8.14 since the beginning of the Industrial
Revolution (IPCC, 2013). It is forecasted to decrease another 0.29 pH units
(near 7.85) by 2100 (Jacobson, 2005) as CO2 levels increase in the atmosphere
and ocean.
Washington State is a region that is especially vulnerable to the
synergistic causes of OA. The main mechanism for the invasion of CO2 into
coastal waters is through upwelling events, which bring seawater rich with
CO2 from the deep ocean to the coast. Water that upwells onto Washington’s
coast contains CO2 that has accumulated from past anthropogenic additions,
biological respiration, and physical-chemical processes due to the ocean’s
thermohaline circulation patterns (Chapin et al., 2011; Orr et al., 2005).
In Puget Sound, upwelling events can account for 24-49% of the pH
decrease relative to pre-Industrial Revolution levels (Feely et al., 2010).
Recent studies demonstrate that local sources can also contribute to ocean
acidification in marine waters of OA, such as, nitrogen and sulfur oxide
gases, nutrients and organic carbon from wastewater discharges, and
polluted runoff from land-based activities (Abril et al., 2003; Borges and
32
Gypens, 2010; Feely et al., 2010; Kelly et al., 2011; Washington State Blue
Ribbon Panel on Ocean Acidification, 2012). These local sources of OA can
disproportionally affect coastal ecosystems, economies, and cultures that rely
on vulnerable calcifying organisms (Cooley and Doney, 2009; Cooley et al.,
2009).
In a collaborative effort to strategically respond to the effects of OA in
Washington State, Governor Christine Gregoire created the Washington
State Blue Ribbon Panel to address the causes and consequences of
acidification. One possible short-term strategy to combat locally intensified
acidification is to return shell material to coastal habitats where shellfish are
present. Spreading shell material (CaCO3) can buffer corrosive conditions by
increasing seawater alkalinity (Washington State Blue Ribbon Panel on
Ocean Acidification, 2012). The dissolution of CaCO3 provides alkalinity
regeneration by buffering weak acids, such as H2CO3, the byproduct of CO2
(Abril et al., 2003; Morse et al., 2007; Waldbusser et al., 2013).
Although few studies have examined shell dissolution for alkalinity
regeneration purposes, those that exist have demonstrated significant results
that increased the CaCO3 saturation in seawater. Green et al. (2009) added
crushed shells of the hard shell clam (Mercenaria mercenaria) to a mudflat in
Southern Maine before seeding it with M. mercenaria juveniles. The addition
of shell material caused the CaCO3 saturation state (Ω) to increase from Ω =
0.25 to Ω = 0.53 (Green et al., 2009), which is a small, yet significant, change
33
(Green et al., 2009). In a similar and more recent study, Green et al. (2013)
tested their experiment again to examine if clam larvae respond positively to
increased CaCO3 saturation in both a lab observation and field manipulation
study. Aragonite saturation state rose from Ω = 0.68 to Ω = 1.30 in sediments
that were buffered with shell material. Further, M. mercenaria increased
their burrowing recruitment in the buffered sediments, suggesting that
CaCO3 additions could indeed provide alkalinity regeneration to sediments
and positively influence shellfish that depend on an elevated CaCO3
saturation state.
Different species of bivalve shells may contain different proportions of
inorganic and organic matter, and thus degrade at different rates. Mineral
dissolution can be principally attributed to dissolution of CaCO3, as 96% of
the inorganic matter in bivalve shell is CaCO3 (Yoon et al., 2003). The
remaining mass was composed of seven other minerals, including silica,
magnesia, and sodium oxide in trivial amounts (Yoon et al., 2003).
Conversely, the organic matter in shell can vary from 0.1 to 10% in bivalve,
based on different species of bivalves (Almeida et al., 1998). The proportions
of inorganic and organic matter in bivalve shells may have implications for
shell degradation, though it is poorly understood.
This study examined how bivalve shell degradation occurs during
winter and early spring conditions in Hood Canal, Washington to determine
if bivalve shells contribute to alkalinity regeneration through CaCO3
34
dissolution. This study also aimed to determine if the degradation rates of
bivalve shells differ among different species of bivalve shells. The species of
shells examined included the Pacific oyster (Crassostrea gigas), Olympia
oyster (Ostrea lurida), and Mediterranean mussel (Mytilus galloprovicialis).
These species were chosen because of their existing influence in shellrecycling projects (C. gigas), their potential to understand native oyster bed
mechanics (O. lurida), and the surplus of shell material from commercial
processing (M. galloprovicialis), as well as a need for general understanding
of shell characteristics among different species.
This study also examined if shell degradation may differ between a
whole and crushed size. From a first-order perspective, crushed shell has
more potential to dissolve than whole shells, as there is more surface area,
and surface area is a first-order control on CaCO3 dissolution (Green et al.,
2013, 2009; Morse et al., 2007), however, shell size as a factor for degradation
has not been formally studied.
Materials and Methods
Study Site
In order to examine shell degradation in a location where it would
naturally occur, research took place in Southern Hood Canal, which forms
one of the major basins of Puget Sound in Washington State. Research was
conducted in February and March of 2014 due to Hood Canal’s low CaCO3
35
saturation state and pH values during winter conditions (Feely et al., 2010).
The lack of flushing in Southern Hood Canal during winter causes
confinement of acidic waters, which makes it an ideal site for studying the
effect of ocean acidification on shell dissolution on the seafloor. The sample
site was ~200 m from the shore in the sublittoral zone of Southern Hood
Canal (47°22'49"N, 122°58'09"W) (Fig. 3). An Oceanic Remote Chemical
Analysis (ORCA) buoy was located ~3 km from the sample site (47°22'30"N,
123° 0'30"W) (Fig. 3).
47°45'N
na
l
47°40'N
Esri, HERE, DeLorme,
MapmyIndia, ©
Legend contributors,
OpenStreetMap
od
Ca
47°35'N
ORCA buoy
Ho
47°30'N
Sample Site
47°25'N
Esri,
©
0 HERE,
3 6 DeLorme,
12 18MapmyIndia,
24Kilometers
OpenStreetMap contributors, and the GIS user
community
47°20'N
123°10'W
123°W
122°50'W
122°40'W
122°30'W
Figure 3. Map of the sites of interest in Southern Hood Canal include the
ORCA buoy and the sample site. The ORCA buoy took pCO2 measurements
every three hours for the duration of this project. The sample site is the
location of shell incubations and bi-weekly water sample collections.
Shell Collection and Preparation
Adult shellfish were collected in December of 2013 from Totten Inlet,
Washington by Taylor Shellfish Farms. The animals had all their meat
removed prior to receipt and were eliminated of all bio-foul by rinsing with
36
deionized water and lightly brushing them. Baking the shells for one hour at
100°C in a drying oven (Yamato Scientific Co., Ltd., Tokyo, Japan) provided
consistent dry mass measurements, but was also required for the shell
transfer permit through Washington Department of Fish and Wildlife
(WDFW). Heat treatment, such as baking, ensures that invasive biota, such
as oyster drills and their eggs, would be killed before returning the shells
back to the water ((Washington Department of Fish and Wildlife, 2013)
Two size types, whole and crushed, were examined for the purposes of
this study. Shells were crushed with a hammer after the baking treatment,
and then processed through a series of sieves to achieve the desired particle
size range (0.3 cm to 1.0 cm diameter) for each species. Whole shells ranged
in size among species, but were consistent within each species.
Shells in 0.028-mm-mesh bags were deployed on long lines anchored to
the seafloor in the sublittoral zone, where water depth varied (range = 6-14
m) depending on tidal cycles. The mesh bags were used to permit water to
flow through without allowing the crushed fragments of shell to escape. A
total of 192 bags were deployed, which included a crushed or whole shell
treatment for each species (Fig. 4). Shells were deployed for a maximum of
eight weeks, with four retrieval dates throughout.
Every other week, shells were collected to measure mass changes over
time. Upon collection, shells from the mesh bags were given the same
treatment as before they were deployed: rinsed and lightly brushed to remove
37
any sediment and bio-foul, then dried for 1 hour at 100°C before taking final
shell mass measurements. A control group that was not deployed was also
rinsed, dried, and measured in two-week increments to assess whether the
experimental treatment caused significant changes in mass.
Figure 4. The shell incubation design. The collection of bags included three
species, two size types, four collections, with eight replicate bags for each
treatment type.
Seawater Collection and Analysis
Seawater carbonate chemistry was analyzed to characterize chemical
changes in Southern Hood Canal for the duration of this study. DIC, pCO2,
alkalinity, and pH constitute four measureable factors in the carbonate
system that can be determined analytically (Wolf-Gladrow et al., 2007). The
knowledge of any two of them allows us to calculate the carbonate chemistry
of a seawater sample. According to Bandstra (2006), alkalinity and pH
measurements suffer when high accuracy is required, so this study focused
on measuring pCO2 and DIC instead.
38
During biweekly shell retrievals, water samples were collected at
multiple slack tides within a daily cycle to examine the within-day variation
of seawater chemistry. Water samples were collected in a Van Dorn bottle
~0.1 m above the seafloor where the shells rested and water depth varied
(range = 6-14 m) depending on tidal cycles. Salinity and temperature were
measured at the same depth of water collections using a YSI Model 85
Handheld meter (YSI, Incorporated, Yellow Springs, Ohio).
Water samples for DIC and pCO2 analysis were carefully transferred
from the Van Dorn bottle into 355 mL amber-glass bottles with polyurethanelined metal crimp-sealed caps, filled to within 1.5 cm of the top, preserved
with 30 µL of a saturated HgCl2 solution to stop biological activity from
altering the carbon distributions in the sample container before analysis
(Dickson, et al., 2007).
DIC and pCO2 samples were analyzed at the College of Earth, Ocean,
and Atmospheric Sciences at Oregon State University. The two analyses are
performed sequentially on the same sample, generally the DIC first followed
by the pCO2. The laboratory system is based on a gas-permeable membrane
contactor, through which a flowing gas stream continuously and
quantitatively strips CO2 from acidified seawater. The CO2 content of this
strip-gas stream, which is proportional to the DIC of the seawater stream via
a simple mass balance, is then analyzed using a non-dispersive infra-red
(NDIR) gas analyzer (Model 840A, LI-COR, Inc., Lincoln, NE) (Bandstra et
39
al., 2006). The pCO2 is also determined using the NDIR. A continuously recirculated air stream equilibrates with the sample and passes through the
NDIR. After inputting the in situ temperature and salinity of the water
samples, a modeling program, created by Dr. Burke Hales, computes the
unmeasured terms in the carbonate chemistry system, including alkalinity,
pH, and calcium carbonate saturation values (Bandstra et al., 2006).
For supplemental pCO2 data, an Oceanic Remote Chemical Analysis
(ORCA) buoy automatically took air and surface seawater pCO2
measurements every three hours. The seawater measurements were taken
at the surface of the seawater, while the air measurements were taken 1-2 m
above the surface. Averaged pCO2 measurements are post-calibrated using a
simple linear regression between original averaged measurements and span
coefficients, a method similar to the post-calibration established by (Feely et
al., 1998). All data from the ORCA buoy are preliminary and have not gone
through the quality control process (Mathis et al., 2014).
Organic Matter in Shells
The proportions of organic matter in shells before and after the
experiment were examined. Fresh shells that were not placed in seawater
were compared to shell treatments that had been incubated in the seawater
for different durations of time. Shells were combusted in a muffle furnace at
475°C for 2 hours, which ignited the organic material, leaving only the
40
inorganic material (Goulletquer and Wolowicz, 1989; Rodhouse, 1990). The
proportions of organic and inorganic matter were determined by measuring
shell mass before and after the combustion process.
Statistical Analyses
All statistical analyses were run in JMP Pro (version 10.0.1.1; SAS
Institute, Cary, North Carolina). Shell degradation values were reported as
% mass remaining, so they were arcsine square root transformed before
statistical analysis. Assumptions of normality and homogeneity of variances
were tested using a Shapiro-Wilke’s test and Levene’s test. Because
variation increased through time, data did not meet the assumptions
initially. However, following natural logarithmic transformations, the data
were normal and homoscedastic. Results were reported as mean with
standard error (S.E.) and an alpha (α) of 0.05.
To examine the relationships between shell degradation for each
species and size of shell, a one-way analysis of variance (ANOVA) and posthoc comparisons (Tukey’s honest significant difference, HSD) were used. To
examine patterns in both treatments and percentage of mass loss (g) through
time, an analysis of covariance (ANCOVA) model was used to determine the
effects of species, size, and collection date (covariate). For each shell
treatment, correlation analysis was used to evaluate whether the shell mass
through time was significant (Pearson’s correlation coefficient, r at α = 0.05).
41
Mass loss rates (% wk-1) were calculated by dividing the percentage of mass
loss value by the number of weeks the shells were incubated.
Results
Seawater Chemistry
The carbonate chemistry values of DIC, pCO2, alkalinity, pH, Ω calcite,
and Ω aragonite from the sample site are shown in Table 1. There were no
correlations or apparent patterns found over time (among or within days) for
any of the carbonate chemistry parameters, so the values were averaged with
a standard error and used to characterize the general seawater chemistry at
the sample site. Water samples collected at the sample site (n = 16) were
consistently undersaturated with respect to aragonite (Ω = 0.59 – 0.91).
Calcite saturation values were sometimes undersaturated, though often
supersaturated (Ω = 0.92 – 1.41). The pH values ranged from 7.60 – 7.85.
Further, pCO2 values were consistently supersaturated (551.11 - 1134.01,
relative to atmospheric CO2 levels (~400 ppm) (Etheridge et al., 1996).
42
Table 1. Carbonate chemistry and calcium carbonate saturation values in
Southern Hood Canal during February-April 2014.
Sample
DIC
-1
(μmol kg )
pCO2
(μatm)
alkalinity
-1
(μmol kg )
pH
Ω calcite
Ω aragonite
2/3/14 8:20
2/3/14 13:45
2/3/14 17:30
2/3/14 23:30
2/18/14 12:30
2/18/14 17:30
2/19/14 7:45
3/4/14 13:00
3/4/14 19:15
3/5/14 8:30
3/18/14 7:30
3/18/14 13:00
3/18/14 18:40
4/3/14 14:30
4/3/14 21:00
4/4/14 7:00
1011
898
982
900
1048
551
900
1066
1134
1004
904
636
696
749
1028
1115
2073
2039
2088
2072
2093
1843
2065
2106
2100
2090
1943
1845
1850
1929
2022
2039
2080
2063
2096
2089
2110
1905
2080
2121
2094
2111
1963
1894
1890
1964
2033
2041
7.64
7.69
7.66
7.69
7.64
7.85
7.70
7.63
7.62
7.65
7.67
7.80
7.76
7.75
7.63
7.60
0.96
1.12
0.98
1.06
1.07
1.41
1.07
1.06
0.92
1.11
1.03
1.32
1.21
1.19
0.99
0.93
0.62
0.73
0.63
0.68
0.70
0.91
0.69
0.68
0.60
0.72
0.67
0.85
0.78
0.77
0.64
0.60
43
Data retrieved from the ORCA buoy were used to characterize air and
surface seawater pCO2 levels in Southern Hood Canal (Fig. 5). Values for air
pCO2 were relatively consistent (range = 400.3 – 441.3 μmol/mol). Values for
surface seawater pCO2 were variable (range = 62.6 – 756.9) with a temporal
trend that decreased as time moved forward from winter to early spring (Fig.
5) (R2=0.75, p<0.0001).
CO2 µmol per mol seawater/air
800
Seawater
700
Air
600
500
400
300
200
100
0
25-Jan
14-Feb
6-Mar
26-Mar
15-Apr
Figure 5. The temporal variation of air and surface seawater pCO2 in
Southern Hood Canal for the duration of this study.
Shell Mass and Degradation Rate
Shell degradation was calculated every week by calculating the
percentage of shell mass loss (SML). All treatments showed decreasing SML
over the eight-week study period (Table 2). Shells of M. galloprovincialis
showed the greatest SML, losing up to 2.61% ± 0.06% as crushed shell and
2.78% ± 0.08% as whole shell after the incubation period of eight weeks. The
next largest change in SML was observed in C. gigas, whose shells lost up to
44
1.23% ± 0.04% as crushed shell and 2.23% ± 0.38% as whole shell. Finally
the smallest change in SML occurred for O. lurida, whose shells lost up to
0.78% ± 0.02% as crushed shell and 0.87% ± 0.05% as whole shell.
For each species and size treatment, SML increased with time (Fig. 6),
except for whole C. gigas shells (Fig. 6a), which had no significant
relationships with time (Table 3) due to the extraordinary variation within
each week’s measurements. Nonetheless, C. gigas shells lost mass and
showed an insignificant pattern over time.
Table 2. The percentages of shell mass losses over time. These values are
shown in this table as averages with standard errors.
Shell Treatment
Week 2
Whole C. gigas
Week 4
Week 6
Week 8
1.40 ± 0.28
1.69 ± 0.10
2.23 ± 0.38
Crushed C. gigas
0.20 ± 0.05
0.78 ± 0.02
1.10 ± 0.06
1.23 ± 0.04
Whole M. galloprovincialis
0.57 ± 0.03
1.44 ± 0.04
2.40 ± 0.06
2.78 ± 0.08
Crushed M. galloprovincialis
0.26 ± 0.03
1.49 ± 0.10
2.10 ± 0.04
2.61 ± 0.06
Whole O. lurida
0.24 ± 0.02
0.78 ± 0.10
0.86 ± 0.04
0.87 ± 0.05
Crushed O. lurida
0.11 ± 0.01
0.55 ± 0.08
0.79 ± 0.05
0.78 ± 0.02
SML occurred very linearly over time for M. galloprovincialis shells,
decreasing between each week (p<0.003) and explained powerfully by time
(R2=0.92, F1,29 = 335.49, p<0.0001). This relationship is the strongest among
all sizes and species of shells (Table 3) (Fig. 6c). Comparatively, crushed C.
gigas shells had consistent SML that decreased significantly between weeks
45
2 and 4 (p<0.0001) and week 4 and 6 (p=0.017), but not between week 6 and
week 8 (p=0.58), demonstrating a steady SML for the first 6 weeks that slows
for the last 2 weeks (Fig. 6a). Changes in SML of crushed O. lurida shells
resemble the changes in crushed C. gigas shells, where SML slows with time
(Fig. 6a and 6b) between weeks 6 and 8 (p=0.99). For the whole O. lurida
shells, the SML slowed more promptly, that is, shells were different between
weeks 2 and 4 (p<0.05), but not significantly different between weeks 4 and 6,
nor weeks 6 and 8 (p=0.67, p=0.99).
Table 3. The relationships between time (covariate) and the percentage of
shell mass loss for each shell treatment using Pearson’s correlation
coefficient, r and the R2 and F-value results from the ANCOVA test.
Asterisks denote p-values that are statistically insignificant (α >0.05).
2
Shell Treatment
r
R
F-value
Whole C. gigas
0.51*
0.25*
F1,9 = 3.06*
Crushed C. gigas
0.89
0.79
F1,28 = 105.86
Whole M. galloprovincialis
0.94
0.92
F1,29 = 335.49
Crushed M. galloprovincialis
0.92
0.84
F1,29 = 157.26
Whole O. lurida
0.72
0.58
F1,29 = 39.99
Crushed O. lurida
0.86
0.73
F1,27 = 72.60
The rates of shell degradation were calculated by dividing SML by the
amount of time (weeks) the shells had been incubated to yield values in units
of percentage of mass change per week. M. galloprovincialis shells showed
46
significantly higher rates of shell mass loss compared to C. gigas and O.
lurida shells for both size treatments (p=0.006, p<0.001) (Fig. 7). Shells from
C. gigas and O. lurida had similar rates of loss (Fig. 7). For all species, whole
shells displayed higher rates of SML than crushed shells for each of the
species examined (Fig. 8): M. galloprovincialis (p=0.02), O. lurida (p=0.003),
and C. gigas (p=0.01) (Fig. 8).
47
C. gigas
Shell Mass Remaining (%)
100%
Whole
Crushed
99%
98%
97%
0
2
4
6
8
O. lurida
Shell Mass Remaining (%)
100%
10
B
99%
98%
97%
0
2
4
6
8
100%
M. galloprovincialis
Shell Mass Remaining (%)
A
10
C
99%
98%
97%
0
2
4
6
8
10
Time (weeks)
Figure 6. The shell mass loss throughout the eight-week study for the
crushed and whole size varieties of C. gigas (A), O. lurida (B), and M.
galloprovincialis (C) species.
48
Mass Loss Rate (% wk-1)
0.5%
B
0.4%
A
A
0.3%
A
0.2%
0.1%
0.0%
C. gigas
O. lurida
M. provincialis
Mass Loss Rate (% wk-1)
WHOLE
B
0.5%
0.4%
B
0.3%
A
0.2%
A
0.1%
0.0%
C. gigas
O. lurida
M. provincialis
CRUSHED
Figure 7. The differences of mass loss rates (% wk-1) among the species for
each whole (A) and crushed (B) shell treatment. Letters above each bar
signify statistical differences between species from Tukey’s HSD test, so that
letters connected by the same letter are not significantly different.
49
Mass Loss Rate (% wk-1)
A
0.5%
0.4%
A
0.3%
B
0.2%
0.1%
0.0%
Whole
Crushed
Mass Loss Rate (% wk-1)
C. gigas
B
0.5%
0.4%
0.3%
A
0.2%
B
0.1%
0.0%
Whole
Crushed
Mass Loss Rate (% wk-1)
O. lurida
C
0.5%
A
0.4%
B
0.3%
0.2%
0.1%
0.0%
Whole
Crushed
M. provincialis
Figure 8. The differences of shell mass loss rates (% wk-1) between the size
treatments of C. gigas (A), O. lurida (B), and M. galloprovincialis (C) shells.
50
Organic Matter in Shells
Based on organic matter analysis before and after the experiment, all
shell treatments experienced organic matter decomposition during incubation
in seawater (Table 4) (Fig. 9). Whole M. galloprovincialis shells had the
highest amount of organic matter among all shell treatments, before and
after the experiment (5.47% ± 0.11%, 4.77% ± 0.14% respectively) and also
exhibited the highest loss of organic matter during incubation (0.70% ±
0.12%). Shells of C. gigas and O. lurida had relatively similar proportions of
organic matter in their shells. For all species, whole shells contained a
higher proportion of organic matter than their respective crushed shell types.
Crushed C. gigas and whole M. galloprovincialis shells experienced
significant correlations over time, demonstrating that organic matter
decomposition slowed with time as CaCO3 dissolution quickened (Fig. 10)
(p=0.0246, p<0.01). All other shell treatments show generally consistent
pattern, suggesting that organic decomposition occurred very linearly
alongside CaCO3 dissolution.
51
Table 4. The differences in proportions of organic matter (%) for each shell
treatment before and after placing shells in the seawater.
Before
After
Difference
Whole C. gigas
2.35 ± 0.08
1.95 ± 0.17
0.41 ± 0.12
Crushed C. gigas
1.79 ± 0.14
1.44 ± 0.02
0.35 ± 0.08
Whole O. lurida
2.08 ± 0.18
1.44 ± 0.08
0.64 ± 0.13
Crushed O. lurida
1.99 ± 0.03
1.98 ± 0.10
0.01 ± 0.07
Whole M. galloprovincialis
5.47 ± 0.11
4.77 ± 0.14
0.70 ± 0.12
Crushed M. galloprovincialis
4.67 ± 0.41
4.51 ± 0.04
0.15 ± 0.22
Percentage of Organic Matter in Shell
Shell Treatment
6%
Before Experiment
5%
After Experiment
4%
3%
2%
1%
0%
Whole
Crushed
C. gigas
Whole
Crushed
O. lurida
Whole
Crushed
M. galloprovincialis
Figure 9. The percentage of organic matter in each shell treatment before
and after the experiment.
52
Percentage of Organic Content
6%
Whole C. gigas
5%
Crushed C. gigas
4%
3%
2%
1%
0%
0
2
4
6
8
Percentage of Organic Content
6%
Whole O. lurida
5%
Crushed O. lurida
4%
3%
2%
1%
0%
0
2
4
6
8
Percentage of Organic Content
6%
5%
4%
3%
2%
Whole M. galloprovincialis
1%
Crushed M. galloprovincialis
0%
0
2
4
6
8
Time (weeks)
Figure 10. The percentage of organic matter in each shell treatment over
time.
53
Contribution of Carbonate and Organic Matter to the Seawater
All shell treatments have the ability to contribute a substantial
amount of CO32-, relative to organic matter, and could be used for alkalinity
regeneration purposes (Table 5). These values were calculated using the
SML and organic matter values, so they are highly variable given the
extrapolation of these data. This calculation assumes that all mineral
dissolution is CaCO3 dissolution. Through CaCO3 dissolution, whole M.
galloprovincialis and C. gigas shells may contribute the most g CO32- every
year to the seawater (133.3 ± 36.8 and 135.6 ± 18.6 respectively). Both whole
and crushed shells of M. galloprovincialis contribute the greatest amount of
organic matter among all the species through decomposition (11.5 ± 3.5 and
9.5 ± 3.4 respectively). Conversely, whole and crushed shells of O. lurida
contributed the least amount of CO32- and organic matter among all the
species (52.7 ± 11.5 and 42.5 ± 25.6; 1.5 ± 0.4 and 1.4 ± 0.4 respectively).
Table 5. The calculated annual contribution of carbonate and organic matter
(g) per kg of shell material.
2-
Shell Treatment
g CO3 per kg shell yr
Whole C. gigas
Crushed C. gigas
Whole O. lurida
Crushed O. lurida
Whole M. galloprovincialis
Crushed M. galloprovincialis
135.6 ± 18.6
63.3 ± 17.7
52.7 ± 11.5
42.5 ± 25.6
133.3 ± 36.8
119.9 ± 37.6
-1
g organic matter per kg shell yr
-1
4.4 ± 0.7
1.7 ± 0.5
1.5 ± 0.4
1.4 ± 0.4
11.5 ± 3.5
9.5 ± 3.4
54
Discussion
Seawater Chemistry
Values of DIC, pCO2, alkalinity, pH, Ω calcite, and Ω aragonite from the
sample site showed low variation with no temporal trends that correlated
with changes in shell degradation over time. However, carbonate chemistry
values correspond similarly to previous research that took place in Hood
Canal in February 2008 (Feely et al., 2010). The low variability and lack of
trends in the data may be attributed to measurements being taken near the
seafloor, where water is often confined with less physical activity (Warner et
al., 2001).
Values from the ORCA buoy showed high variability compared to the
sample site. Because measurements were taken at the surface of the
seawater near the ORCA buoy, more variability was expected, as the surface
water pCO2 can vary drastically from seawater depths (Feely et al., 2010) due
to changes in biological and physical conditions (Takahashi et al., 2009).
Data from the ORCA buoy demonstrated a decrease in pCO2 over time
(Fig. 5), potentially caused by an increase in phytoplankton photosynthesis in
early spring. The data from the ORCA buoy may be more informative than
the data from the sample site due to the higher sample size and temporal
resolution. If this is the case, the observed slowing of shell dissolution (Fig.
6) is expected due to the decrease in pCO2 over time (Fig. 5). However,
coastal waters are subject to highly variable carbonate chemistry (Green et
55
al., 2009), so more research over different temporal and spatial scales is
needed to investigate how changes in carbonate chemistry may affect coastal
ecosystem functions, such as shell degradation.
Areas with high pCO2 and low CaCO3 saturation states are likely to be
adversely affected by ocean acidification and may result in substantial
ecological, economic, and cultural effects (Green et al., 2009). Since there
have been no high-quality, long-term measurements of all carbonate
chemistry factors in Southern Hood Canal, it is not possible to determine the
extent of how ocean acidification is affecting the region. Nonetheless,
aragonite saturation was consistently undersaturated (Ω = 0.59 – 0.91) and
calcite saturation was sometimes undersaturated (Ω = 0.92 – 1.41),
demonstrating that Southern Hood Canal would be an ideal site to use shell
recycling for alkalinity regeneration purposes because of its consistently low
CaCO3 saturation state values and high pCO2 levels during winter
conditions.
Shell Degradation: CaCO3 Dissolution and Organic Decomposition
Shells from M. galloprovincialis degraded the fastest and most among
all bivalve species. This can be partly explained by differences in shell
mineralogy, as the shells of oyster species, such as C. gigas and O. lurida, are
mainly composed of calcite (Stenzel, 1963 in (Gazeau et al., 2007), while the
shells of mussel species, such as M. galloprovincialis, can contain up to 83%
56
of aragonite (Hubbard et al., 1981(Gazeau et al., 2007). Aragonite and calcite
are both mineral forms of CaCO3, though aragonite is also both denser and
more soluble than calcite (Morse et al., 2007). During the experiment,
seawater was sometimes undersaturated with respect to calcite, but it was
consistently undersaturated with respect to aragonite. These data suggest
that M. galloprovincialis shells dissolve the most CaCO3 in the form of
aragonite, contributing more CO32- to the seawater.
Another explanation for the higher degradation rates in M.
galloprovincialis shells can be explained by the organic matter decomposition
that is taking place during the shell degradation process. Shells of M.
galloprovincialis contained a higher proportion of organic matter and
contributed the most organic matter to seawater (Table 5), which does not
contribute to alkalinity regeneration. However, M. galloprovincialis shells
also released the most CaCO3 from dissolution.
The decomposition of organic matter in shells has been discussed by
(Waldbusser et al., 2011), who showed that fresh Crassostrea virginica shells
degraded slightly faster than weathered shells (stored at an upland location
for ~2 y). Fresh shells, as were used for this study, have more organic matter
to decompose. Similarly, M. galloprovincialis shells contained the highest
proportions of organic matter and exhibited the greatest degradation.
Although organic matter decomposition does not contribute to alkalinity
regeneration, the consequent event of microbes respiring CO2 may create
57
conditions that are more favorable for alkalinity regeneration via mineral
dissolution (Waldbusser et al., 2011).
All shell treatments generally experienced consistent proportions of
organic matter decomposition and CaCO3 dissolution. For shells of the oyster
species, C. gigas and O. lurida, there was a fast rate of SML followed by a
slower rate of loss after 4-6 weeks, demonstrating that these species of shells
experience a slower rate in both decomposition and dissolution with time.
SML for M. galloprovincialis did not slow over time, indicating that this
species of shells experience a constant rate of both decomposition and
dissolution. These short-term measurements only provide a glimpse into
shell dissolution mechanics, though these data are within the range of other
previous measurements of shell degradation (1.05 – 7 %(Hales and Emerson,
1997; Hecht, 1933; Waldbusser et al., 2011). More research is needed to
examine if shell degradation would continue to occur at similar rates over a
longer study period.
For all species, whole shells had a higher rate of shell degradation
than crushed shells. From a first-order perspective, crushed shells have more
potential to dissolve, as there is more surface area, which is a first-order
control on dissolution (Morse et al., 2007), but this was not observed. One
possible explanation for the consistently higher degradation rates in whole
shells may be attributed to the method of deployment, in which the crushed
shells may have repositioned in heaps during incubation so that diffusion
58
between the shells and seawater decreased. Additionally, too much crushed
shell can result in sulfide generation due to decreased diffusion between
sediment in water paired with hypoxic conditions (Green et al., 2013, 2009).
Another possible reason for the higher degradation in whole shells could be
attributed to the higher proportion of organic matter in whole shells. The
organic matter in shells is found within them as an organic matrix and as a
protective outer layer as a sheath (Simkiss, 1965; Weiner and Hood, 1975).
During the crushing process, if organic matter crushed into smaller pieces
more easily, it is possible that small pieces of the organic matter found in
shells were crushed too small (<0.3 cm) to be used for this study. This
hypothesis is supported by the fact that crushed shells contained less organic
matter than whole shells even before shells were placed in the water for the
experiment (Fig. 9).
Future Considerations of Shell-Recycling Programs
In Washington State, shell is sometimes introduced back into the
seawater as a settling surface to encourage the establishment of juvenile
bivalve populations. The use of whole shells is the traditional technique for
shell enhancement as a substrate to seed native and cultured oyster beds.
Whole shells would be more suitable due to their higher rates of dissolution
and existing influence in shell enhancement projects. When shells are overplanted in soft muds, burial can occur, which slows dissolution enormously
59
and results in CaCO3 preservation (Morse et al., 2007). A thin base layer to
the seafloor is recommended so that burial is minimal and so that there is
space for dissolution and juvenile oyster larvae settlement.
The main reason for the lack of a shell-recycling program in
Washington State can be attributed to the precautionary concern for disease
and invasive species introductions by shell transfers. The Shellfish Transfer
Permit was established to reduce the risk of transferring marine invasive
species, such as oyster drills (Ocicebrellus ornatus) from one water body to
another (WDFW, 2013). Shells must weather for two years in piles before
they can be placed in the water to prevent the spread of the oyster drill
(Cohen and Zabin, 2009; Washington Department of Fish and Wildlife, 2013).
The use of fresh shells may not be appropriate for shell-recycling
purposes because of possible disease transfers (Washington Department of
Fish and Wildlife, 2013) and their high proportions of organic matter content.
The process of land-based weathering is highly recommended for all shells, as
it allows the organic matter to break down before returning shells to the
water. Weathered shells can still contribute to alkalinity (Waldbusser et al.,
2011), so a land-based weathering treatment can help minimize organic
decomposition. Weathering has other positive effects, such as minimizing the
transfer of the invasive oyster drill by storing it in a land-based area for ~3
months (Cohen and Zabin, 2009).
60
The use of M. galloprovincialis and C. gigas shells is highly
recommended for alkalinity regeneration purposes because of the high rates
of CaCO3 dissolution that may occur with these species of shells. However,
all bivalve species and sizes of shells from this study can contribute to
alkalinity regeneration. This research may guide restoration organizations
manage their approach to shell-recycling by understanding the potential to
use different types of available shell material for alkalinity regeneration
purposes.
61
CHAPTER THREE: General Conclusions and Discussion
Introduction
This study presents the first of its kind investigating shell degradation
for alkalinity regeneration purposes in Washington State. Southern Hood
Canal is an ideal site to use shell recycling for alkalinity regeneration
purposes because of its consistently low CaCO3 saturation state values. The
use of M. galloprovincialis shells is highly recommended for alkalinity
regeneration purposes because of the high rates of CaCO3 dissolution that
may occur with these species of shell. However, all bivalve shell species from
this study can contribute to alkalinity regeneration. More research is needed
to examine if shell degradation would continue to occur at similar rates over
a longer study period. This research may guide restoration organizations
manage their approach to shell-recycling by understanding the potential to
use different types of available shell material for alkalinity regeneration
purposes. The current limitations regarding shell recycling include, but are
not limited to: disease and invasive species management, alkalinity
regeneration performance, lack of infrastructure, and ethical considerations.
These potential consequences should be addressed before any operations take
place.
62
Alkalinity Regeneration Performance
This study demonstrated that shells have the potential to offer
alkalinity regeneration to marine waters. Preliminary results also show how
much these amounts are. However, these results may change as seawater
chemistry naturally changes on a seasonal basis. Further, these results may
be perceived as idealistic, in that shells did not have any organic tissue
remaining on them and contributed the minimum amount of anticipated
organic matter to marine waters. Realistically, more organic matter would
be contributed to the seawater, due to processing and handling of shells.
Additionally, eight weeks is a short period of time and this study only
provides a glimpse into shell dissolution mechanics. Some possible
mechanisms that may deter shell dissolution from occurring over time
include shell burial, algae and bio-foul growth on shell substrate, and piling
shells too high. More research is needed to examine if shell degradation
would continue to occur at similar rates over a longer study period.
Disease and Invasive Species Management
The main reason for the lack of a shell-recycling program in
Washington State can be attributed to the precautionary concern for disease
and invasive species introductions by shell transfers, as shells can be a vector
for disease (Cohen and Zabin, 2009). Shells from restaurants, oyster bars,
and processing operations from outside Washington State may have
63
potentially infected locations that may enter the markets without testing for
invasive species or pathogens. Various approaches have been used or
recommended to reduce the risk of transporting and introducing undesirable
organisms with shells (Cohen and Zabin, 2009), but more research is needed
based on the lack of knowledge in this field.
A large percentage of the oysters used for East Coast shell-recycling
efforts come from non-local populations (Brumbaugh and Coen, 2009). A
conscious effort is being made to quarantine the non-local shell, based on
concerns that the shells may harbor pathogens or invasive species. Recycled
shells are weathered in a designated area for a minimum of one to three
months before being used for shell enhancement (Bushek et al. 2004), though
it is unknown if this time frame is sufficient for pathogen removal. East
Coast and Gulf shell-recycling programs values differ from the West Coast by
their willingness to risk the spread of invasive species in exchange for the
benefits of restored oyster beds and improved water quality through bivalve
filtration (Brumbaugh and Coen, 2009; Cohen and Zabin, 2009).
Currently, land-based weathering is the treatment established in
Washington State to reduce the spread of oyster drills (Ocicebrellus ornatus)
from one water body to another (Cohen & Zabin, 2009, WDFW, 2013), but it
is not known to be effective strategy to neutralize all biota and disease. In a
review of land-based weathering, (Cohen and Zabin, 2009) identified concerns
regarding invasive species, but did not address controlling pathogens. The
64
research by (Bushek et al., 2004) investigated if land-based weathering would
be a sufficient treatment for the protozoan oyster disease, Perkinsus marinus.
While a storage period of 90 days eliminated most of P. marinus, cells with
unknown viability was observed from shell material that had been stored for
6 months. One disease of immediate concern is the oyster herpes virus 1
(OsHV-1), which can cause up to 100%, mortalities in young Crassostrea
gigas (Segarra et al., 2010). Currently this disease has been observed as
virulent in C. gigas stocks cultivated in France, Australia, New Zealand and
present in Japan and the United Kingdom (Friedman et al., 2005; Le Deuff et
al., 1996; Renault et al., 1995, 1994). An outbreak of this disease puts at risk
the natural and commercial populations of shellfish in Washington State,
which provide many important ecosystem services and economic provisioning.
No study has examined if land-based weathering would be an effective
approach to prevent pathogenic outbreaks during shell transfer,
demonstrating a need for treatment and monitoring strategies.
The conditions that are needed to permit non-local shell transfers
should include a treatment strategy, either by a heat, weathering, or
chemical treatment, so that no viable viral, bacterial, or protozoan agents
remain viable. The effectiveness of land-based weathering is based on time,
climate, pile location, maintenance, and the tolerations of the organism of
concern. More research is needed to address these factors, especially to
identify the ideal amount of time for land-based weathering to neutralize
65
pathogens and minimize the organic matter in different species of shells.
Heat is another method that could be used to treat shell. The Coast Seafoods
plant in Willapa Bay used a propane blast heater on an external conveyor
system to prevent the spread of an invasive crab (Carcinus maenas), which
had become established in Humboldt Bay a few years earlier (Cohen and
Zabin, 2009). However, the stainless steel conveyor belt paired with the
saltwater environment did not hold up well (T. Morris, pers. comm.) and they
eventually switched to land-based weathering as a form of treatment. A
monitoring program would be necessary to ensure the effectiveness of any
treatment. The shell-recycling program in Washington State must be
prepared to both treat and monitor all shells to ensure no disease outbreak in
marine waters.
Shell-recycling programs encourage local oyster consumers to recycle
their oyster shells rather than to send them to the landfill as trash or
employing them for landscaping purposes (Bushek et al., 2004). To ensure
that the public does not put shells back in the water themselves, an outreach
component to the shell-recycling program is essential to ensure the success of
disease and invasive species management. With the appropriate protocol, a
shell collection and deposition program could be implemented to help protect
local waters against OA, while also supplementing current native oyster
restoration projects and engaging citizens and businesses with the local
benefits of recycling shell.
66
Ethical Considerations
Further research is needed to address the ethics of adding shells to
marine waters. Shell addition manipulates ocean chemistry, which may have
the potential to damage or alter marine ecosystems in unforeseen ways (Buck
and Folger, 2009). Shell additions can also alter natural habitats, if added in
vast quantities in areas where native oysters would not naturally live and
deposit their shells. These potential consequences should be addressed
before any massive operations take place. Management agencies must ask if
the costs of doing nothing are more harmful than modifying the existing
infrastructure.
Adding shells into the water may be a form of restoration, rather than
a form of new alteration. Historically, reefs and beds formed by oysters such
as the Olympia oyster (Ostrea lurida) were dominant features in many
estuaries throughout their native ranges (Brumbaugh and Coen, 2009).
Recycling shells can restore ecosystem function by providing a positive
feedback process that provides CaCO3 back into the system (Kidwell and
Jablonski, 1983). Shells act as a suitable habitat for shellfish larvae to settle,
grow, and die (Brumbaugh and Coen, 2009). These calcifying organisms will
eventually die and add a buffer to the waters as their shells dissolve. As they
grow, they are forming their shells out of CaCO3 by using the surrounding
available CO32-. When they eventually die, their shells dissolve, adding Ca2+
and CO32- back into the water while providing further substrate for future
67
calcifying organisms, repeating the process. Enhancing an area with shells
may increase shellfish populations as well as restoring the shell resource.
Enhancing an area with shells may increase shellfish populations as well as
restoring the shell resource. If shells are taken out of the system without
being recycled, a negative feedback will occur that make it difficult for
sustainable shellfish population (Brumbaugh and Coen, 2009; Waldbusser et
al., 2011).
Responding to Ocean Acidification in Washington State
The need for action to prepare for ocean acidification (OA) is great in
the inland and coastal waters of Washington State, a region that is especially
vulnerable to synergistic causes and effects of OA (Gazeau et al., 2007). The
absorption of anthropogenic carbon dioxide (CO2) by the ocean and the
localized impacts that exacerbate levels of CO2 in Washington State are
causing pH to decrease and along with it the saturation state of CaCO3 in
seawater, which threatens calcifiers who depend on CaCO3 to survive. Many
of Washington State’s marine species are calcifiers including oysters, clams,
scallops, mussels, abalone, crabs, geoducks, barnacles, sea urchins, sand
dollars, sea stars, sea cucumbers, and some seaweeds (Washington State
Blue Ribbon Panel on Ocean Acidification, 2012). In Washington’s marine
waters, as with the global marine ecosystem, ocean acidification is expected
to significantly impact food web structures and functions, as well as
68
individual species.
This research has strong implications for different fields of study,
including marine biology, ecology, wildlife management, commercial
aquaculture, and environmental policy. Collaboration between these
disciplines is required to tackle ocean acidification at different scales.
Various collaborative organizations among shellfish farmers, commercial
harvesters, the Tribe, state, local, and federal agencies, and other
stakeholders, such as the Blue Ribbon Panel on Ocean Acidification, are
identifying and pursuing research, policy, and education goals that will best
prepare communities in Washington State to adapt to changes in the marine
ecosystem. Continued research and education on ocean acidification will
allow those who depend on the ocean for its ecosystem services to respond to
OA in a sustainable approach.
Economic and Cultural Impacts of Ocean Acidification
Although OA is a global issue, Washington State is particularly
susceptible to its effects, which impact the state’s environment, economy, and
culture. OA can disproportionately affect coastal ecosystems and the
communities that rely on them by negatively affecting shellfish and the
ecosystem services they provide (Borges and Gypens, 2010; Kelly et al., 2011).
Shellfish provide many ecosystem services, including provisioning services
such as food and income; regulating services such as water quality through
69
the control of eutrophication, algal blooms, and hypoxia; supporting services
such as nutrient cycling that maintain ecosystem functions; and cultural
services such as spiritual, recreational, and social benefits (Cooley et al.,
2009; UNEP and Millenium Ecosystem Assessment Board, 2005).
Washington is the top producer of farmed clams, oysters and mussels
in the nation, with an annual value of over $107 million (“Washington State
Shellfish Initiative,” 2011). People have been farming shellfish in
Washington since the mid-1800s (Toba and Nosho, 2004). Today,
Washington State’s shellfish industry directly and indirectly employs over
3200 people and annually contributes an estimated $270 million to the state’s
economy (“Washington State Shellfish Initiative,” 2011). Shellfish farmers
are significant private employers in rural coastal areas of Washington. In
Pacific and Mason counties alone, the industry generates over $27 million
annually in payroll (“Washington State Shellfish Initiative,” 2011).
Washington’s recreational shellfish activities are also economically and
culturally significant. Over 300,000 licenses are purchased annually to
harvest shellfish, providing over $3.3 million of revenue to the state
(“Washington State Shellfish Initiative,” 2011). On average 244,000 digger
trips are made per season for recreational razor clam harvest on
Washington’s coast bringing an estimated $22 million to coastal economies
(“Washington State Shellfish Initiative,” 2011).
70
Shellfish have also played a significant role in the diets and economies
of western Washington Native American tribes for thousands of years
(Northwest Indian Fisheries Commission, 2013). Currently, Washington
tribes engage in subsistence harvest of shellfish (Northwest Indian Fisheries
Commission, 2013), therefore tribes are vulnerable to ocean acidification.
Therefore, tribal leaders are needed as important stakeholders in ocean
acidification policy and research.
Key Policy Changes to Address Ocean Acidification
Because ocean acidification is primarily caused by the increase of
global carbon dioxide emissions, policies that advocate for carbon dioxide
emission reductions are needed to protect marine waters from OA. This is a
comprehensive and challenging task, as it requires collaboration between
regional, national, and international entities. Washington State is leading
the way in OA science and policy. Recognizing the risks of OA to
Washington, former Governor Christine Gregoire created the Washington
State Blue Ribbon Panel on Ocean Acidification (The Panel) to chart a course
for addressing the causes and consequences of acidification (“Washington
State Shellfish Initiative,” 2011). The Panel is comprised of different
stakeholders, including scientists and policymakers, who studied ocean
acidification in depth from different perspectives. In doing so, it recognized
that global CO2 emissions are the leading cause of OA, and that necessary
71
reductions in emissions must be met (Washington State Blue Ribbon Panel
on Ocean Acidification, 2012).
Conclusion
The rapid pace of OA does not give humans, ecosystems, or organisms
much time to adapt. Ultimately, policy and human behaviors need to change
to minimize OA, however, those factors take time. Both short- and long-term
strategies are required to successfully offset the magnitude of OA. Shellrecycling programs can improve water chemistry in Washington State as a
short-term restorative approach to combat OA. Given that shell-recycling
programs encompass several purposes, including mitigation against OA,
providing restoration material for native oysters, and engaging citizens and
businesses in combatting OA, the benefits may very well outweigh the
potential costs.
72
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