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Title (dcterms:title)
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Eng
Understanding the Impact of Sea Lettuce (Ulva spp. ) Density on Pacific Oyster(Crassostrea gigas) Growth in Puget Sound, Washington
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Date (dcterms:date)
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2015
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Creator (dcterms:creator)
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Eng
Lamb, Audrey P
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Subject (dcterms:subject)
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Eng
Environmental Studies
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extracted text (extracttext:extracted_text)
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UNDERSTANDING THE IMPACT OF SEA LETTUCE (Ulva spp.) DENSITY ON
PACIFIC OYSTER (Crassostrea gigas) GROWTH
IN PUGET SOUND, WASHINGTON
by
Audrey Lamb
A Thesis
Submitted in partial fulfillment
of the requirements for the degree
Master of Environmental Studies
The Evergreen State College
December 2015
©2015 by Audrey Lamb. All rights reserved.
This Thesis for the Master of Environmental Studies Degree
by
Audrey Lamb
has been approved for
The Evergreen State College
by
________________________
Erin Martin, Ph. D.
Member of the Faculty
________________________
Date
ABSTRACT
UNDERSTANDING THE IMPACT OF SEA LETTUCE (Ulva spp.) DENSITY ON
PACIFIC OYSTER (Crassostrea gigas) GROWTH IN PUGET SOUND,
WASHINGTON
Audrey Lamb
Biofouling, or the growth of other species besides the cultured species on aquaculture
gear is a frequent challenge for shellfish farmers. In areas with dense macroalgae growth,
shellfish farmers can frequently spend considerable time managing macroalgae through
manual removal from aquaculture gear, particularly during sensitive life stages such as
seed cultivation. However, there is little research examining the effects of macroalgae
density on oyster growth and survival, and as such, it is unclear if these manual removal
techniques are actually needed. This study investigated the relationship between sea
lettuce (Ulva spp.) density and Pacific oyster (Crassostrea gigas) growth and survival on
two commercial oyster farms in Puget Sound in hopes of improving the understanding of
the necessity for macroalgae removal. Juvenile C. gigas were grown in the North Hood
Canal and South Puget Sound from April through October 2015 in grow bags with
different added wet weights of Ulva spp. (3 kg, 1.5 kg, 0 kg). Presence of Ulva spp.
resulted in significantly smaller shell heights of C. gigas at both sites, but did not have a
significant effect on mortality. The presence of Ulva spp., rather than the amount of Ulva
spp. in the treatment, had significant negative impacts on oyster shell height. The wet
weight of Ulva spp. in the treatments decreased in the late summer and fall due to
diminished on-site macroalgae presence. However, the shell height of oysters that were
exposed to Ulva spp. months earlier remained significantly smaller on average. In other
words, it appeared that early exposure to Ulva spp. during growth negatively slows C.
gigas growth, and it has a lasting effect such that growth is still stunted even when the
oysters are no longer exposed to Ulva spp. months later. These findings have significant
implications for aquaculture practices in that managing Ulva spp. on aquaculture gear
early in the growing season can lead to more rapid crop turnover and increased
production in a growing area over time.
Table of Contents
LIST OF FIGURES…………………………………………………………………………….….v
LIST OF TABLES………………………………………………………………………………...vi
ACKNOWLEDGEMENTS………………………………………………………………………vii
INTRODUCTION…………………………………………………………………………………1
LITERATURE REVIEW………………………………………………………………………….4
Pacific Oyster Biology…………………………………………………………………….6
Triploid Oysters…………………………………………………………………..……….7
Ulva spp. growth in Puget Sound…………………………………………………………8
Historical Blooms………………………………………………………………………....8
Anthropogenic factors impacting blooms…………………………………………………8
Impacts of blooms on the nearshore environment………………….……………………10
Ecosystem role of macroalgae………………………………………………………..….11
Ecosystem factors impacting blooms……………………………………………………12
Physical factors impacting ulvoid biomass growth……………………………………...14
Hydrodynamics…………………………………………………………………………..16
Macroalgae management practices on oyster farms…………………………………..…17
Impact of shellfish aquaculture activities on ulvoid presence…………………………...18
Future scenarios of Ulva biomass levels in Puget Sound……………………………..…22
METHODS……………………………………………………………………………………….25
Study design……………………………………………………………………………...25
Study site………………………………………………………………………………....25
Aquaculture methodology………………………………………………………………..27
Oyster methodology…………………………………………………………………..….30
Ulva spp. methodology…………………………………………………………………..29
Data analysis……………………………………………………………………………..30
RESULTS…………………………………………..…………………………………………….32
Ulva spp weights…………………………...…………………………………………….32
Oyster growth rate……………………………………….……………………………….33
Oyster survival…………………………………………..……………………………….36
DISCUSSION…………………………………………………………………………………….43
Ulva spp. dynamics…………………………………………………………………...….43
Oyster response…………………………………………………………………………..45
Effect of Ulva spp. presence on shell height………………………………………….…46
Physical interactions……………………………………………………………………..47
Chemical interactions……………………………………………………………………49
Biological interactions…………………………………………………………………...51
Additional observations………………………………………………………………….51
Recommendations………………………………………………………………………..53
Design considerations…………………………………………………………………....55
INTERDISCIPLINARY CHAPTER……………………………………………………………..57
Phytoremediation…………………………………………………………….…………..58
Food……………………………………………………………………………………...59
Biofuel…………………………………………………………………………………....59
Fertilizer………………………………………………………………………………….60
Other uses………………………………………………………………………………...60
CONCLUSION………………………………………………………………………………..….62
LITERATURE CITED…………………………………………………………………………...64
APPENDICES…………………………………………………………………………………....71
iv
List of Figures
Figure 1 – Map of field sites in Puget Sound………………………………………...….31
Figure 2 – Mean C. gigas shell height (mm) at Peale Passage…………………………..40
Figure 3 – Mean C. gigas shell height (mm) at Thorndyke Bay……………….………. 41
Figure 4 – Mean C. gigas shell height (mm) at both field sites………………………….41
v
List of Tables
Table 1 –Mean Ulva spp. weights of treatments at Peale Passage, Washington…...……37
Table 2 – Mean Ulva spp. weights of treatments at Thorndyke Bay, Washington…...…37
Table 3 – Mean C. gigas shell height (mm) at Thorndyke Passage, Washington….……38
Table 4 – Mean C. gigas shell height (mm) at Peale Passage, Washington…...……...…38
Table 5 – Mean C. gigas survival at Thorndyke Bay, Washington..…………….………39
Table 6 – Mean C. gigas survival at Peale Passage, Washington………………..………39
vi
Acknowledgements
I am deeply indebted to many people that supported me through this project. Thank you
to my advisor Dr. Erin Martin for your support, expertise, and guidance throughout this
project. Thank you to Dr. Joth Davis for providing advisement as well as granting me
access to Baywater Shellfish Farm. Thank you to the folks at Puget Sound Restoration
Foundation including Betsy Peabody and Brian Allen. Thank you to Matt Moretti from
Bangs Island Mussel Company. This project wouldn’t have been possible without support
from Taylor Shellfish Farms including Bill Taylor, Aisha Prohim, Saleh Prohim, Stuart
Thomas, Steve Jones, and Brittany Taylor. Thank you to the many people that assisted
with data collection including Christina Stalnaker, Natalie Sahli, Sydni Baumgart, Kyle
Grossser, and Meg Munkacsy. Thank you to the Evergreen Sustainability Fellowship for
providing funding support for this project.
vii
I.
INTRODUCTION
Washington is the largest producer of farmed oysters in the U.S. A large portion
of commercial shellfish production occurs in Puget Sound, an estuarine body of water
that extends from British Columbia to western Washington. Along with oysters, other
organisms grow in the intertidal zone including macroalgae. Sea lettuce (Ulva spp) is a
species of macroalgae that is commonly found on oyster farms. Commercial shellfish
farms, particularly clam, oyster, and geoduck farms, occur in the intertidal and sub tidal
zones, where much of the macro algae is found. As macroalgae accumulates throughout
the summer, it drifts over aquaculture gear. When visiting a shellfish farm, it’s common
to see biofouling algae covering the gear. Algal blooms have been linked to shellfish
mortality, although there is not a lot of information about this in the literature (Fisher &
Mueller, 2007). There is a perception that macroalgae including sea lettuce could be
negatively affecting oyster growth. There are many anecdotal accounts of growers
observing increased shellfish mortality in areas covered with large mats of algae (Dewey,
2014). Because of this potential impact to production, some shellfish growers remove
macroalgae from aquaculture gear throughout the summer. However, the perceived
negative effect of macroalgae on commercial shellfish production hasn’t been addressed
in the literature.
The modern shellfish industry is a huge economic driver in Washington State,
adding hundreds of millions of dollars to the State’s economy each year and providing
long-term employment in rural areas. Commercial shellfish production is a lucrative
industry that contributes approximately $270 million annually to the Pacific Northwest
1
economy (Adelsman & Binder, 2012). Many of the jobs at commercial shellfish farms
are living-wage jobs in rural shoreline communities with few opportunities for viable
long-term employment. For example, the commercial shellfish industry is the second
largest employer in Mason County (“Commercial Shellfish Growers Settlement,” 2014).
For the shellfish industry to continue to be successful, growers need to ensure that they
can produce oysters successfully in Puget Sound.
Puget Sound Basin currently supports a population of 4.4 million people. This
population has increased almost 250% since 1960. Approximately 67% of Washington’s
residents live in the Puget Sound basin (“Threats to Puget Sound,” n.d.). With many
major roadways right along Puget Sound’s shorelines as well as homes, agricultural land,
and industrial land, our local pollution drains into the Sound. Nutrients such as nitrogen
and phosphorus from fertilizer, leaky septic systems, and runoff enter the water and
create an overabundance of nutrients in the water, which is taken up by primary
producers such as macro algae. Nutrient pollution is problematic because it causes
macroalgae, which occurs naturally in Puget Sound, to bloom and then die, creating areas
of hypoxia in Puget Sound.
Puget Sound is not only being impacted by local anthropogenic inputs, it is also
experiencing impacts from global anthropogenic activity in the form of ocean
acidification. Over the past decade, shellfish growers have seen significant impacts due in
part to ocean acidification, including multiple years of oyster hatchery failures along the
Pacific Coast (Barton, Hales, Waldbusser, Langdon, & Feely, 2012). Scientists have
already documented impacts of ocean acidification in low-flushing areas of the Puget
Sound (Feely et al., 2010). In the near future, scientists anticipate that the waters of the
2
Puget Sound will continue to become more acidic, impacting the ability of shell-building
organisms to construct their shells and survive through vulnerable development phases.
The combined environmental impacts from both local sources along the water and rising
atmospheric carbon dioxide levels driving ocean acidification create water quality issues
that impact the entire marine system. With these challenges in mind, shellfish growers
have to utilize all available tools to adapt to changing water chemistry.
There is an emerging interest in utilizing the ecosystem services that macroalgae
provide to improve water quality in Puget Sound. Phytoremediation in the marine
environment, or using plants to improve environmental conditions is a new field of
interest to potentially improve water quality locally. Scientists and policy makers are
starting to examine the viability of growing and harvesting macroalgae to improve water
quality on a local scale. In particular, removing seaweed during its growth period (before
decomposition) is a new area of interest for its potential ability to removing CO2 and
nitrogen from the marine system. Altering local water conditions by removing seaweed
before it decomposes may potentially have localized benefits including altering pH and
aragonite saturation state (Adelsman & Binder, 2012). However, introducing more
macroalgae to a shellfish farm may be incompatible with current land use. There is a lack
of research that quantifies the local effects of seaweed growth on shellfish, and vice
versa. Having a better understanding of the chemical and biological dynamics between
seaweed and shellfish growth is critical to shaping more informed management practices
to adapt to ocean acidification.
3
II. LITERATURE REVIEW
Introduction
With 2,800 miles of marine waters and 2,500 miles of shoreline, Puget Sound is
the second largest estuary in the United States (“Saving Puget Sound,” n.d.). The Sound
provides critical habitat connectivity between 10,000 rivers and streams and the Pacific
Ocean, through the Strait of Juan de Fuca. The brackish shorelines of Puget Sound also
provide a productive area to grow shellfish for food production, a tradition occurring in
the region for thousands of years. Shellfish resources including clams, oysters, and
geoduck have been an important component of tribal culture and a critical food resource
for coastal tribes in Washington for thousands of years (“Traditional Use of Shellfish,”
2012). In the 19th century, settlers began commercial harvest of oysters and clams. In the
early 20th century, shellfish growers started commercially farming Puget Sound’s
tidelands by importing Pacific oyster (Crassostrea gigas) seed from Japan (“Where We
Work: Washington,” 2013). Today, the shorelines of Puget Sound continue to provide an
important place for shellfish farming for both Tribal and non-tribal communities.
Shellfish farming typically has minimal negative to net positive impacts on the
marine environment, and negative impacts are typically short-term. Shellfish farming can
often have positive environmental impacts to the marine environment through three
dimensional structure and sediment stabilization (Dealteris, Kilpatrick, & Rheault, 2004)
(Straus, McDonald, Crosson, & Vadopalas, 2013). Unlike terrestrial forms of farming,
shellfish aquaculture requires no inputs (except for the larval grow-out phase in a
hatchery). Shellfish consume the nutrients that are overabundant in Puget Sound,
cleaning the water to create improved water quality for other species. Shellfish are
4
biological filters that consume phytoplankton, removing these nutrients from the water
column. Over the period of an hour, an adult Pacific oyster can filter up 20 gallons of
water (“Oyster Restoration Reaches New Depths,” 2014). When shellfish are harvested,
these nutrients are removed from the system, reducing the nutrient loading in the water.
Ulvoids or sea lettuces (Ulva spp) are a genus of seaweeds that grow abundantly
throughout the world. While Ulva provides important ecosystem functions such as shelter
for crustaceans and nutrient cycling, it can be problematic when it blooms and
decomposes, creating dead zones in the intertidal zone (Scigliano, 2012). In Puget Sound,
ulvoids can form large mats, shading out native sea grasses and creating odor problems.
Some shellfish growers view ulvoids as a management issue due to their ability to take
over aquaculture beds, causing suffocation for cultured bivalves as well as restricted
water flows (Mallet, Carver, & Hardy, 2009).
This literature review will cover the dynamic relationship between Pacific oyster
(Crassostrea gigas) farming and Sea Lettuce (Ulva spp.), a macroalgae that naturally
occurs on shellfish farms. The review will discuss the biology of C. gigas including the
seasonality of growth, growth rates, and triploid oysters and current macroalgae
management practices on shellfish farms. The review will then highlight the habitat
requirements of ulvoids including biological, chemical, and physical requirements. It will
then cover the presence of ulvoids in Puget Sound and factors that affect their growth
such as shoreline development, wastewater treatment plants, and agriculture. Finally, the
review will provide a brief section on how ulvoids affect C. gigas.
5
Pacific Oyster Biology
C. gigas is the most commonly grown oyster in the world. Originally from Japan,
it has been cultivated in Washington since the 1920’s. The current value of Pacific oyster
culture in Washington is approximately $85 million annually (Harris, 2008). It grows on
rocks or soft substrates in the intertidal and subtidal zones. The oyster begins its larval
stage as zooplankton for 20-30 days until it sets on a hard surface. The oyster develops as
a male for the first year, and then matures either as a male or female in its second year. It
is a fast growing oyster, reaching 10-15 cm after 2-4 years depending on the conditions of
the bay and growing up to five times faster than other oyster species. C. gigas can survive
in a larger range of temperature and salinity than other oysters, making it a common
oyster to grow in different areas because of its adaptability. It performs well in
temperatures between 8-22 degrees Celsius and salinities of 24 to 28 parts per trillion
(Harris, 2008).
C. gigas growth rate is highly dependent on the body of water where the oysters
are growing. C. gigas growth is variable and is determined by water temperature, salinity,
turbidity, and food concentration. The growth rates are typically fastest in the spring and
summer (Hyun et al., 2001). Pacific oysters require various levels of dissolved oxygen
depending on their life stage. Dissolved oxygen levels of 4 mg/L or less can be harmful
to larvae, and adults require approximately 100 mg/L (“Crassostrea gigas (Pacific
oyster),” 2015). According to a study by Bourgrier et al, oysters that are not in a postspawning state consume oxygen 80% of the time they are immersed in water (1998).
Hypoxia is an on-going issue in Puget Sound, with the most serious hypoxia occurring in
South Puget Sound and Hood Canal. The Washington Department of Ecology is working
6
to determine how much these conditions are being influenced by anthropogenic nitrogen
inputs and how they affect oysters grown in the region (“Nitrogen in the Puget Sound
Ecosystem: Nitrogen Trends in Puget Sound,” n.d.).
Triploid Oysters
Recent genetic advances in shellfish farming have allowed farmers to increase
their year-long yield. Oysters are typically not as marketable in the summertime because
they are using their energy to reproduce. In 1985, shellfish farmers began commercially
producing a sterile oyster that is marketable year-round called the triploid oyster. Triploid
oysters have three gametes instead of two, and are marketable year-round due to their
good flavor and firm texture.
Triploid oysters were historically bred using cytochalasin B (CB) to block the
release of the second polar body, rendering the oyster sterile. However, CB is very toxic
and most triploids are now bred in a much safer way. The more common way to breed
triploids is to cross a tetraploid male with a diploid oyster, producing a sterile oyster with
three gametes. In 2000, one third of all oysters produced in Washington and Oregon
hatcheries, or 12 billion larvae were triploid oysters grown by breeding a tetraploid with a
diploid oyster. Since then, other countries including Australia and France have increased
triploid production (Nell, 2002). Triploid oysters are popular in Washington. Not only do
they retain their flavor year-round, but they also grow more quickly than diploids at
lower latitudes. A study in Hiroshima, Japan found an increase of 81% in whole weight
of triploid oysters after 8 months (Nell, 2002).
7
Ulva spp. growth in Puget Sound
Ulva spp. is a dynamic macroalgae that responds to both human-caused and
natural impacts, with effects locally for marine organisms. There are many different
factors that determine the presence and biomass of Ulva in a given area including day
length, turbidity, weather, bathymetry, nutrient composition of the water, and many
others. In general, Ulva biomass is highest in the summer and fall and the lowest in the
winter and spring. This section will discuss some of these factors to demonstrate how
Ulva grows in Puget Sound, as well as discuss its role in the marine environment.
Historical Blooms
Ulva spp. is native to Puget Sound and it frequently accumulates in mats over the
growing season. Ulvoid mats have historically been a component of Puget Sound’s
marine ecosystem with the first written historical record of presence in the 1930’s.
However, scientists and shoreline residents believe the ulvoid blooms have increased
since then. Scientists and shoreline residents have observed large biomass of blooms in
areas where they do not typically occur on an annual basis (T. A. Nelson, Nelson, &
Tjoelker, 2003). There are also anecdotal records of variation of the intensity of blooms
in different areas in Puget Sound, particularly in areas that have not typically had ulvoid
blooms present (T. A. Nelson et al., 2003) (Frankenstein, 2000).
Anthropogenic factors impacting blooms
Shoreline development
Ulva can outcompete other species in commercial and public areas such as docks
and marina. Almost one third of Puget Sound shorelines are armored, meaning they have
8
manmade bulkheads that alter sediment movement (Fresh et al., 2011). Beach armoring is
positively correlated with gravelly substrates, which provide structure for ulvoids to
attach to. These areas typically have high nutrient inputs (from lawn fertilizer, for
example) and obstructions such as jetties and docks, which alter hydrodynamics. The
turbidity occurring in these areas from boat traffic also lowers the light intensity in the
water column. The lowered light intensity provides an advantage for ulvoids over other
seaweeds and seagrasses because it doesn’t need as much light intensity as other species
grow, and thus outcompetes seagrasses and macroalgae in these areas (Frankenstein,
2000). As more of the shoreline gets developed, Ulva has increased habitat where it may
not have been able to grow historically.
Wastewater Treatment
The population of Puget Sound basin has increased by almost 250% since 1960
(“Threats to Puget Sound,” n.d.). With development comes an increase in nitrogen inputs
into groundwater, which eventually reaches the Sound (Frankenstein, 2000). These
nitrogen inputs can result in blooms of ulvoids, as nitrogen is a limiting nutrient for Ulva
spp. in Puget Sound (Frankenstein, 2000). Sewage treatment plants, storm water
treatment plants, and storm drains all contribute nutrient additions to the water. Nitrogen
is not typically removed from wastewater treatment effluent before its released into Puget
Sound. In the summertime, wastewater treatment plants are the largest source of nitrogen
into Puget Sound (Mohamedali, Roberts, Sackmann, & Kolossues, 2011). Currently, one
of the only wastewater treatment plants that removes nitrogen from their effluent is the
LOTT Center in Budd Bay, which removes nitrogen from April to October (“Budd Inlet
Treatment Plant,” 2015). Failing septic tanks are another significant source of nutrient
9
inputs. Studies have shown that failing septic tanks are one of the largest sources of
nutrients entering the groundwater. This water will eventually reach the Sound, providing
opportunities for additional ulvoid blooms (Frankenstein, 2000).
Agriculture
Agriculture activities along the shoreline or even further upstream contribute to
nutrient inputs in the Puget Sound in the form of non-point nutrient additions and point
source additions. Over fertilization of crops as well as animal waste contributes to
nutrient run-off that enters the water and increases the amount of available nitrogen and
phosphorus. Additional nitrogen inputs into Puget Sound create optimal conditions for
increased ulvoid blooms (Frankenstein, 2000).
Impacts of blooms on the nearshore environment
Growth period of Ulva spp.
Ulvoids provide shelter and forage habitat early in the growing season for many
marine species. During the growth period, ulvoid blooms are associated with increases in
the number and density of grazers and crustaceans that use mats for food and shelter.
Seasonality is a significant factor in ulvoid growth. One study from San Quentin Bay,
Mexico found a 30 to 40 fold increase in Ulva spp. in May and June compared to winter
(Zertuche-Gonzalez et al., 2009). Ulvoid mats alter the habitat and food resources for
juvenile fish, with resources for seabirds, fish, and mollusks increasing and shelter for
perch, greenling increasing as well. Consequently, resources for species such as cod,
which feed on crustaceans, are more limited because the algal mats shelter crustaceans
(Frankenstein, 2000).
10
Ecosystem role of macroalgae
Seaweed beds provide community structure roles in the marine environment
including substrate stabilization, food and larval deposition, buffering residents from
stresses, and providing protection from predators (Bertness, Leonard, Levine, Schmidt, &
Ingraham, 1999). One study examined the impact of algal canopy in high and low tidal
elevations. The authors found a positive effect on canopy cover effect for understory
organisms at stressful high tidal heights. Recruitment, growth, and survival of understory
organisms were higher at high tidal levels with canopy cover. At lower tidal heights, the
effects were neutral or negative. The authors also noted that consumer pressure was
“severe” at low tidal heights. (Bertness et al., 1999).
Decomposition period of Ulva
Large ulvoid blooms can create problematic physical and chemical conditions in
the marine environment. Ulvoid blooms can physically block larvae from settling and
produce toxic exudates which impact barnacles and herring (Frankenstein, 2000). As
ulvoid mats decompose, macro invertebrate use declines. For example, in False Bay on
San Juan Island, invertebrates including amphipods, oligochaetes, and polychaetes
utilized mats and sediments in the summer. As mats decomposed in the fall, densities of
these species decreased (Frankenstein, 2000).
Shading issues
Blooms of ulva can break up and destroy sea grasses such as eelgrass by shading
grass out, which reduces sea grass’s ability to photosynthesize, and can cause sea grasses
to die back. Additionally, ulvoid mats can cover sea grasses completely by growing in
large mats. This is problematic because sea grasses create a nursery habitat which is
11
critical to supporting a number of juvenile fish species, such as salmon (T. A. Nelson et
al., 2003). Movements from sea grass dominated areas to ulvoid-dominated areas are
associated with increased availability of nitrogen (T. A. Nelson et al., 2003).
Ulvoids blooms have impacts to birds as well. Blooms create a physical barrier
that blocks access to near shore food resources such as crustaceans for carnivorous birds.
Consequently, blooms are associated with an increase in herbivorous birds such as ducks
and geese (Frankenstein, 2000).
Ecosystem factors impacting blooms
Growth and decomposition
Earlier research found that Ulvaria obscura was a fairly rare species and a spring
ephemeral. However, more recent studies have found that Ulvaria obscura is more
prevalent than previously thought. Nelson and authors (2003) found Ulvaria obscura
dominating the sub tidal zone throughout the spring and early summer. It can also live in
the intertidal zone, but it is not particularly tolerant to desiccation and thus dies off
throughout the summer (T. A. Nelson et al., 2003). Blooms typically grow throughout the
summer due to solar radiation and decline in the fall as light conditions decrease
(Frankenstein, 2000). However, certain parameters can cause bloom declines in summer.
Anoxic conditions can cause a decline in blooms. Additionally, high water temperatures
can cause bloom declines during the summer (T. A. Nelson et al., 2003).
The growth of ulvoid mats throughout the spring and summer are a large sink for
nutrients in the water. Decomposing ulvoid mats input nutrients back into the sediment
and the water column. These nutrients are used by algae to support productivity in the
12
summer (Frankenstein, 2000). As ulvoids decompose, particles sink into the sediment
where they remain. Studies have found that sediments in ulvoid-rich areas contain more
nutrients, particularly nitrogen, than areas without ulvoid accumulations. Eventually,
these nutrients are released from sediments into the water column (Frankenstein, 2000).
As ulvoid mats decompose, they become a source of nutrients such as particulate organic
matter and dissolved organic matter. The absorption and release of organic matter helps
distribute nutrients throughout the water column and sediments and contributes to low
oxygen concentrations as this material breaks down (Frankenstein, 2000). The
decomposition of ulvoids contributes to low oxygen concentrations as this material
breaks down (Frankenstein, 2000).
Ulvoid mats can have a negative impact on human usage of beaches and
shorelines. The presence of large mats prevents recreation such as swimming, fishing and
foraging for clams. The odor of the decomposition mats, particularly the hydrogen sulfide
associated with anaerobic decomposition of mats, can be extremely strong and prevent
people from recreating on shorelines (Frankenstein, 2000). While the strongest odors
typically occur in summer, the odors have been documented in the winter as well.
Nutrient Uptake
The two most important nutrients controlling growth of macro algae are nitrogen
and phosphorus. Of the two, nitrogen is the limiting nutrient and ulvoids are thus
controlled by the availability of nitrogen in the water. Another important factor
controlling ulvoid bloom growth is the supply of nitrogen. If nitrogen availability is low,
but it is in constant supply throughout the spring and summer, this could allow ulvoids to
grow at a similar pace to higher nitrogen available conditions (Frankenstein, 2000).
13
Light, temperature, water motion, and CO2 and O2 levels all impact nutrient
uptake, particularly nitrogen and phosphorus (Gao & McKinley, 1994). Temperature has
been associated with an increase in biomass in some studies, and a decrease in biomass in
other studies. The study by Nelson found that increasing temperature was positively
correlated with an increase in biomass (2003).
Physical factors impacting ulvoid biomass growth
There are many parameters that regulate macro algal biomass growth including
nutrient conditions, light conditions, water cycling movement, and day length. These
relationships are very complex and researchers are not in agreement about the most
significant factors. One study claims that daylength, temperature, and dissolved inorganic
nitrogen are the most important factors controlling biomass growth of seaweed (T. A.
Nelson et al., 2003). Another study claims that temperature, grazing and stand density
were the most important factors (Gao & McKinley, 1994). A third study named shoreline
alteration, nutrient inputs, intertidal/subtidal ulvoid beds and hydrodynamics as the most
important factors impacting ulvoid blooms (Frankenstein, 2000). Recognizing that all of
these parameters may play a different role based on the particular bay, it is important to
establish an understanding of what some of the impacts may be.
Light
Irradiance is an important factor controlling the growth of ulvoid blooms.
Irradiance, the solar power per unit area, is largely impacted by light conditions, which
vary diurnally and seasonally. Along with daylength, irradiance is impacted by local site
factors including shading and depth of seaweed. Puget Sound experiences much longer
days in summer and fall than spring and winter, allowing for long irradiance periods
14
during the mid and late summer. These changes in light conditions have an impact on
photosynthetic rates. In a study of Ulva rotunda, researchers found that increasing light
conditions in nitrogen-sufficient plants from low conditions to high conditions resulted in
a 600% increase in daily surface area growth and a 50% increase in light-saturated
photosynthetic capacity (Gao & McKinley, 1994).
Recent research showed a positive correlation between biomass, daylength, and
water temperature (Nelson et al. 2003). In this particular study, light was thought to be
the primary limiting factor impacting biomass growth. However, light availability for
biomass is not completely represented by the daylength variable, as some biomass is
growing in sub tidal areas in deeper, more shaded conditions. Photosynthetic rates vary
by season and time of day. Furthermore, shading and depth impact the pigmentation of
seaweeds (Gao & McKinley, 1994). Water clarity is often lowest in the summer and fall,
and highest in the winter, impacting the amount of light that is available to sub tidal
macroalgae (T. A. Nelson et al., 2003).
Density
The density of macro algae can impact biomass growth. In areas that have a dense
mats of macro algae, light availability, nutrients, and access to inorganic carbon can be
limited due to competition (Gao & McKinley, 1994). These conditions can impact
aquaculture operations and contribute to growers’ desire to remove ulvoids from their
gear. Nelson et al recorded blooms large enough to create hypoxic conditions at night
under the ulvoid mat. However, anoxia was not observed in the specific case (T. A.
Nelson et al., 2003).
15
Hydrodynamics
The three topographic features in which ulvoid blooms were most common
include enclosed shallow coves with low energy hydrodynamic environments, broad
embayments with sloping intertidal flats, and exposed beaches with high wave energy.
Additionally, areas with a long-shore trough and soft substrate, a feature that is correlated
to beach armoring, accumulate drift ulvoid mats. These areas often have high anaerobic
activity, and high odor problems. Substrate is another factor that impacts ulvoid blooms.
Ulva grows well on exposed gravelly beaches, because it attaches to the gravelly
substrate and grows in the intertidal zone (Frankenstein, 2000).
Turbidity
Turbidity has a large impact on biomass growth. Gao and McKinley found that
when the current speed is increased from still to faster currents, macro algae can
photosynthesize up to four times as fast (Gao & McKinley, 1994). For areas that
experience faster currents, higher photosynthetic rates and thus higher biomass can be
expected. Nelson et al found that study sites in isolated bays such as Hood Canal had low
ulvoid biomass in comparison to sites with stronger currents and larger water masses,
such as Blakely Island (T. A. Nelson et al., 2003).Water motion increases macro algae’s
ability to uptake nutrients, even in areas that are nutrient limited. This is because macro
algae can take in nutrients more easily in flowing water (Gao & McKinley, 1994).
Tidal currents
The primary forces that control the surface currents in Puget Sound are tidal
range, tidal height, and wind. The large majority of sites with ulvoid blooms have eddies
off shore during most of the tide. These eddies capture and concentrate drifting ulvoids.
16
Ulvoids are also impacted by littoral drift due to drift cells carrying ulvoids towards an
obstruction such as jetties, where ulvoids tend to accumulate (Frankenstein, 2000). The
area of Puget Sound that experiences the most eddy activity is central Puget Sound.
Central Puget Sound is also where most of the ulvoid blooms are reported (Frankenstein,
2000). Eddies also impact the concentration of nutrients in the water, with higher nutrient
concentrations in areas that have eddies. Eddies and currents are a determining factor in
the amount of nutrients that remain in near shore areas (Frankenstein, 2000).
Ulva and grazing
Bivalves are opportunistic feeders, utilizing resources in the water column
including ulvoids (Frankenstein, 2000). Invertebrates also graze on Ulva including the
mud snail (Ilyanassa obsoleta) in the Eastern US (Giannotti & McGlathery, 2001).
However, ulvoid mats have been documented to be problematic for bivalves and other
invertebrates. Studies have shown that mats have reduced numbers and densities of
benthic macro invertebrates such as bivalves and some polychaetes (Frankenstein, 2000).
A 1999 study of clam resources in Dungeness Bay found a positive correlation between
clam densities and lack of ulvoid mats (Frankenstein, 2000). In areas where ulvoids were
present, there was a positive correlation between clam size and presence of ulvoids.
There was also an increase in dead clams on beaches where ulvoids were present
compared to beaches where ulvoids were absent. The correlation between dead clams and
ulvoid presence is consistent with the literature’s previous findings (Frankenstein, 2000).
Macroalgae management practices on oyster farms
On oyster farms, ulvoid biofouling is sometimes a constant management issue.
Biofouling, or the growth of other species besides the cultured species on aquaculture
17
gear is a frequent challenge for shellfish farmers. Aquaculture gear provides threedimensional habitat in the intertidal area that can increase habitat for macroalgae.
Macroalgae will either latch on to the three-dimensional habitat or drift macroalgae will
get stuck on the gear. The accumulation of macroalgae on shellfish farms can be an issue
for growers when the macroalgae gets extremely dense. Removing algae from
aquaculture gear by scraping it off grow bags and other equipment is time-consuming and
costly for shellfish farmers (Mallet et al., 2009). Up to 15% of annual operating costs can
be spent on controlling biofouling issues including macroalgae (Adams, Shumway, &
Whitlatch, 2011). Growers often pile removed algae on the beach adjacent to the shellfish
harvest area. These piles, which initially removed the algal biomass from the water,
create a subsequent problem later in the summer. As the seaweed decomposes, the
nutrients from the algae reenter the water. Hypothetically, these nutrient additions alter
water chemistry, potentially increasing nutrient loads of harmful CO2 back in the water
column as the nutrients disperse locally. Shellfish, already severely impacted by the
unavailability of CaCO3 due to anthropogenic CO2 emissions (Feely et al., 2010), may
then experience local impacts from CO2 entering the water column from algal
decomposition.
Impact of shellfish aquaculture activities on ulvoid presence
Multiple studies have noted that shellfish aquaculture practices often increase
algal biomass by providing structure for algae to attach to in the intertidal environment. A
study of clam farming in coastal BC found that Ulva spp. increased as a result of farming
practices. The study found that when predator nets were installed, Ulva sp. increased. As
compared to the reference sites, the farm site had noticeably more Ulva spp. One study
18
found that biofouling increased the weight of a predator net by 200 fold, causing
increased dragging (Swain & Shinjo, 2014). A study by Bendell (2014) found that there
was as significant difference in macroalgae biomass per bottom unit between a sandy
beach and aquaculture mesh (Bendell, 2014). Another study by Van Alstyne et al found
an increase in macroalgae with shellfish farming activities (K.L. Van Alstyne, Flanagan,
& Gifford, 2011). One explanation for the increase in macroalgae is the intertidal pools
left by recreational harvesters that were enriched with nutrients including ammonia that
supported macroalgal growth (Bendell, 2014).
Oyster Recruitment
Thomsen et al. researched the effects of drift algae and sediment on Crassostrea
virginica reefs and associated sessile organisms. They found that accumulation of drift
algae as well as sediments reduce richness and abundance of sessile organisms associated
with oyster reefs. They also found that oyster recruitment was reduced due to drift algae,
which they attributed to reduced light penetration, smothering, and interference with
feeding apparatus (Thomsen & McGlathery, 2006).
Allelopathic properties
Nelson tested the effects of Ulva fenestra and Ulvaria obscura extracts on larval
development for Pacific oyster as well as Ulva, Ulvaria, and Fucus gardneri growth in
Puget Sound. They found that Ulva and Ulvaria slowed oyster larvae development
(2003). The authors found that Ulva had a larger effect than Ulvaria. Based on their
results they concluded that Ulva and Ulvaria likely have allelopathic properties.
Researchers estimated that the maximum concentration of algae that Pacific oysters
experience in the field is a layer of seaweed 5 cm deep, or “six layers” of algae thick
19
(Timothy A. Nelson, Lee, & Smith, 2003). However, Ulvaria is the only macroalgae with
documented allelopathic properties (Kathryn L. Van Alstyne, Harvey, & Cataldo, 2014).
Ulva and Ulvaria extracts significantly affect the growth of Ulva and Ulvaria, meaning
that Ulva and Ulvaria don’t grow as well in areas where dense ulvoids are present.
Not only did Ulva and Ulvaria affect the growth of macroalgae, but they also
affected oyster growth and development. Oysters did not develop normally under tested
concentrations. In fact, no oyster larvae developed normally to the D-hinge stage while
92.8% of control larvae developed successfully. One researcher found that Ulva lactuca
extracts inhibited development of crab larvae and crab larvae had zero larvae success.
The combination of Nelson’s research and Johnson’s research point to a general toxicity
of ulvoids for multiple species. The extract was more toxic for Fucus zygotes than oyster
larvae, suggesting that there is a difference in sensitivities to the toxicity of the extract
between animals and plants. For Ulvaria, the plant did not need to die for the toxic
compounds to be released. This phenomenon is very rare among plants: there are few
cases of macrophytes producing toxics that slow growth of other macrophytes (Timothy
A. Nelson et al., 2003).
The properties of the extracts are still not completely understood. One explanation
for Ulvaria is a polymerization of oxidized dopamine that blocks membranes and direct
effects of dopamine. Another explanation is an antiherbivore activated defense system. A
third explanation is bacteria creating toxins or anaerobic conditions in the extracts
(Timothy A. Nelson et al., 2003).
Impact on primary productivity (food sources)
20
Large algal blooms and subsequent desiccation could result in decreased primary
productivity due to hypoxic conditions and resulting reduced colonization success by
other organisms. There is a potential that large blooms would prevent food from reaching
oysters, thus slowing their growth (Timothy A. Nelson et al., 2003, p. 200)
Water flows with seaweed
Multiple studies have discussed how fouled aquaculture equipment slows water
flows. Mallet et al summarizes three studies, which have hypothesized that reduced water
flows from fouled gear slows rate of food supply as well as waste removal (2007).
However, other studies have addressed benefits of fouled gear to culture species. Sea
scallops and pearl oysters have experienced better performance due to fouling (Mallet et
al., 2009). Research into the impact of algal canopies on the subtidal zone has found that
canopies “can increase the re-suspension and deposition of sediment, enhance larval
settlement and increase food supply to filter feeders” (Bertness et al., 1999). However,
more studies lean towards the idea that algal mats slow water flows, impacting bivalves
negatively. One study found that water circulation in aquaculture gear is reduced (Swain
& Shinjo, 2014).
Although there was no direct link to water flows, one study found that sea
scallops increased mass by 68% with regular changing of nets that removed fouling
organisms (Mallet et al., 2009). The same study found that regular bag turning can
provide effective control of fouling for Eastern oysters in bag on bottom culture.
21
Future scenarios of Ulva biomass levels in Puget Sound
Changes in the water conditions in Puget Sound have already been attributed to
ocean acidification, and the conditions will continue to change in the future (Feely et al.,
2010). For example, Dr. Feely’s research attributes 24-49% of the pH decrease in Hood
Canal’s deep waters since pre-industrial levels to ocean acidification (Feely et al., 2010).
Scientists are currently conducting extensive research to better predict how the marine
ecosystem will be impacted by these drastic changes in water quality, and a lot is still
unknown. Scientists are predicting that 49-82% of the pH decrease in Hood Canal will be
attributed to ocean acidification in the future (Feely et al., 2010). These acidic conditions
will make it increasingly difficult for vulnerable calcifiers to build their shells.
Ulva is not CO2 limited and the saturation state is currently much higher than
current atmospheric levels of CO2. In order to create saturation conditions for macro algal
photosynthesis based on CO2, conditions of at least five times ambient air concentration
of CO2 were needed (Gao & McKinley, 1994). Although Ulva is not carbon limited, the
increase in carbon dioxide may result in an increase of algal biomass. The increase in
atmospheric CO2 is predicted to increase primary productivity by phytoplankton,
resulting in altered conditions for macro algae as well (Gao & McKinley, 1994). The
effect that this added biomass will have on shellfish is not yet known. However,
experiments have found that Ulva lactuca utilizes bicarbonate and possibility utilizes
CO2 for cell processes (Gao & McKinley, 2000). This study also found that macroalgal
use of CO2 most often results in elevated pH levels. They found this to be true even with
elevated concentrations of CO2. It is questionable whether the amount of increased
photosynthesis will result in a net gain or a net loss of pH conditions. This would
22
probably vary on a bay-to-bay basis, depending on the algal biomass, rates of
decomposition and photosynthesis, and flushing of nutrients in that particular area.
Conclusion
Ulvoids, ubiquitous in Puget Sound, have impacts on the nearshore environment
through photosynthesis, respiration, and decomposition. Ulvoids interacts with C. gigas
in multiple ways including physically, biologically, and chemically. Ulvoids uptake large
amounts of nutrients including carbon, nitrogen, and phosphorus from the water column,
and transport these nutrients through tidal movement. As nutrient inputs into Puget Sound
continue to grow, ulvoid blooms become a more frequent occurrence with potentially
widespread effects on the marine ecosystem. Biofouling of shellfish aquaculture gear
with ulvoids is well documented by shellfish growers (Adams et al., 2011). However, this
interaction has not been studied closely in Puget Sound. Understanding the interaction
between ulvoids and oysters can help shape management practices for commercial oyster
farms in Puget Sound.
My thesis research explored the impact of a particular genus of seaweed, Ulva
spp. on Pacific oyster (Crassotrea gigas) production. I examined different levels of Ulva
spp. densities in close proximity to planted oysters in grow bags in order to get a better
understanding of the dynamics occurring between Ulva spp. densities, growth of the
seaweed, and growth and survival of Pacific oysters in different density scenarios of Ulva
spp. The goal of my research was to inform commercial shellfish aquaculture
management practices surrounding seaweed issues in aquaculture in Puget Sound.
Getting a better understanding of ulvoid growth throughout the growing season and the
impact of different ulvoid biomass levels on the growth of shellfish is important to
23
establish better management practices surrounding ulvoid management in the shellfish
aquaculture industry.
24
III. METHODS
Study design
This field experiment took place on two commercial oyster farms in Puget Sound
(Washington) to test the effect of sea lettuce (Ulva spp.) density on Pacific oyster
(Crassostrea gigas) growth and survival in aquaculture settings in order to understand
how to better manage Ulva spp. biofouling on commercial oyster operations. The
experiment was designed to determine if Ulva spp. management is necessary on
commercial oyster farms.
Study site
The study was conducted at two different sites in Puget Sound that varied in their
topography, tidal patterns, and flow rates. These sites were selected to provide a basis for
comparison between two areas of the Puget Sound with different site-specific dynamics
in order to better control potential confounding variables. One of the sites was Baywater
Shellfish Farm on Thorndyke Bay in North Hood Canal and the other site was Gosser
Farm in Peale Passage in South Puget Sound (see Figure 1). These sites differed in their
onsite dynamics because the residence time of water in North Hood Canal is typically
shorter than South Puget Sound (Harrington, 2005) (Babson, Kawase, & MacCready,
2006). Hood Canal is a deep fjord with a sill at the entrance that restricts water
circulation from Admiralty Inlet (Roberts, Newton, & Hannafious, 2005). Residence time
of water in Hood Canal can be a year or more. However, Thorndyke Bay is close in
proximity to Admiralty Inlet and has a shorter residence time of water than other areas of
Hood Canal. On average, the residence time of water in Northern Hood Canal is
approximately 23 days, half as long as the residence time of water in South Puget Sound
25
(Babson et al., 2006). Other major differences include the steepness of the beach;
Thorndyke Bay is a wide, shallow bay while Peale Passage has a steeper gradient.
Thorndyke Bay (latitude, 47.8042; longitude, –122.7344), is a shallow bay
(approximately 750 m long) along the North Hood Canal in Jefferson County. Thorndyke
Bay is bordered to the north by North Thorndyke Reach and to the south by the southeast
side of the Toandos Peninsula. Thorndyke Creek runs through timberland into the
estuary. The drift cell runs left to right, so water circulation patterns presumably transport
freshwater from the creek toward the field site (“Coastal Atlas,” 2015). This experiment
took place just east of the delta of Thorndyke Creek (approximately 500 meters to the E
how far N). Thorndyke Bay is an area of ecological significance as one of the best
examples of an intact creek estuary in Jefferson County. The bay is an important regional
site for overwintering waterfowl.
Taylor Shellfish’s Gosser Farm is located in Peale Passage on the southwest side
of Harstine Island directly east of Squaxin Island. The Gosse farm is approximately 450
meters wide and is a commercial farm for oysters, clams, and geoduck. Peale Passage
(latitude, 47.2645; longitude, –122.9231) is a small channel fed by Pickering Passage to
the north and has a right to left drift cell. Water circulation in Peale Passage is limited by
long residence times and slow flushing rates. On average, the residence time of water in
South Puget Sound is approximately 46 days (Babson et al., 2006). The uplands directly
above the site are a mix of residential and timberland.
26
Aquaculture methodology
The experiment was conducted from April 2015 through October 2015. Hard
plastic mesh oyster grow-out bags (9 mm mesh size formed in a box style) were deployed
at both sites in late April with a set amount of oysters (n=150) and various levels of Ulva
spp. based on treatment. The experiment was conducted using a randomized design
where 5 replicates of 4 treatments, totaling 20 grow bags, were attached to a line along a
tidal elevation of 0. The grow bag configuration and tidal elevation was identical at both
sites (see Appendix D). Commercial oyster production was manipulated at two sites by
controlling the amount of Ulva spp. in each bag. The four treatments included:
“Control” (no Ulva spp. accumulation, 150 oysters)
“1.5 kg” (1.5 kg Ulva spp. added, 150 oysters)
“3 kg” (3 kg Ulva spp. added, 150 oysters)
“Ulva” (1.5 kg Ulva spp added, no oysters)
Oysters were randomly distributed into grow bags filled with equal amounts of
oysters (n=150) of similar size (29.1 ± 1.0 mm at Thorndyke Bay on April 9 and 29.9 ±
1.1 mm at Peale Passage on April 21). This size distribution was chosen because it is
consistent with the industry standard for these bags (Davis, 2015). All oysters used in the
experiment were triploid Pacific oysters from the same family that were produced at the
Taylor Shellfish Hatchery in Quilcene and were grown at the Taylor Shellfish Floating
Upwelling Nursery System (FLUPSY) in Oakland Bay, which is located in Shelton, WA.
They were obtained from the FLUPSY as ½ inch juvenile oysters (see Appendix B). The
experiment was conducted during the summer and fall in order to capture the growth of
27
the oysters during the primary growing season of triploid Pacific oysters (Mvungi,
Lyimo, & Bjork, 2012). During their first year of cultivation, oysters are the most
sensitive to impacts(Pogoda, Buck, & Hagen, 2011). Therefore, juvenile oysters were
selected for this experiment for their potential response to the Ulva spp.
Oyster methodology
(a) Sampling
Oysters were sampled on a bi-weekly basis during accessible tides. All grow bags
were sampled during low tide. Oysters were removed from grow bags to measure growth
and assess mortality. All oysters sampled were immediately returned to grow bags.
(b) Growth
Shell height, defined as “the longest measurement from the umbo to the edge of
the shell” (Cotter et al., 2010) was used as the primary measurement for oyster growth
rate. A sub-sample of 10 oysters per bag were measured by sampling every 10th oyster
counted. Shell growth was measured using a digital caliper (Electronic Digital Caliper 0150mm). Initial shell length ranged from 29.1 ± 1.0 mm to 29.9 ± 1.1 mm.
(c) Mortality
Oyster mortality was measured by counting dead oysters, which were identified
by oysters with shells gaping open and unable to close their shell when handled. Dead
oysters were kept in the bag, consistent with commercial aquaculture practices. All
oysters sampled were immediately returned to grow bags.
Ulva methodology
28
(a) Sampling
Commercial oyster production was manipulated at each site by controlling the
amount of Ulva spp. in each bag. The placement of Ulva spp. inside the bags was to
mimic natural accumulation of Ulva spp. in the grow bags. Ulva spp. weights were
adjusted on a bi-weekly basis to maintain a target weight for each treatment. Ulva spp.
was initially added to the oyster grow bags at Thorndyke Bay on April 20 and Peale
Passage on April 21. From April through October, Ulva spp. was weighed and adjusted
on a bi-weekly basis (every other week) except for September and October due to the
tide. Ulva spp. was gathered on site from nearby sources, drained of water in a salad
spinner, and weighed with a kitchen scale. It was measured into the predetermined
amounts for each treatment (1.5 kg, and 3 kg increments), and placed inside the grow
bags on top of the oysters.
(b) Growth
Ulva spp. growth was assessed two weeks following the placement of a
predetermined treatment amount of Ulva spp. in each bag. While effort was made to
maintain consistency in treatments, this was not always the case due to the dynamic
nature of the marine environment. After two weeks, all Ulva spp. was removed from
grow bags using gloves. Ulva spp. was then spun with a salad spinner. The wet weight of
the Ulva spp. was recorded. Ulva spp. quantities were adjusted to maintain target
treatment weights (3 kg, 1.5 kg, 0 kg). Most often, this required supplementing the
current Ulva spp. in the bag with additional Ulva spp. on site. Ulva spp. was identified to
genus to use for the treatments.
29
Data analysis
Oyster growth data were normally distributed and were analyzed using a repeated
measures MANOVA (JMP Pro 11). Oyster growth was assessed over time as well as
within the treatment variation. Both site and treatment effects were analyzed in the
MANOVA. Oyster mortality data were also analyzed using an MANOVA. The following
variables were not normally distributed: mortality. Non-parametric analysis was used to
analyze mortality.
30
Figure 1: Map of field sites in Puget Sound. Thorndyke Bay is located in North Hood
Canal and Peale Passage is located in South Puget Sound on Harstine Island.
31
IV.
RESULTS
Ulva spp. weights
Fluctuations in Ulva spp. levels were recorded at each site and the amount of Ulva
spp. that remained in the bags at the next sampling event was used to determine the mass
of Ulva spp. that the oysters had been exposed to for the preceding two weeks. Both site
and time had a significant effect on Ulva spp. weight. The amount of Ulva spp. in each
treatment was significantly larger at Thorndyke Bay than Peale Passage (F(5,45)=1.864,
p<0.0001). Time also had a significant effect on Ulva spp. weight. There was a
significant difference in the amount of Ulva spp. in each treatment between months
(F(1.45)=13.260, p<0.0001).
Thorndyke Bay
At Thorndyke Bay, Ulva spp. weights varied within each treatment. For the 3 kg
treatment, values ranged from 1.0 kg ± 0.1 kg to 2.5 ± 0.0 kg with the lowest weight
occurring in May and the peak being two months later in July. The mean value for the 3
kg treatment was 2.1 ± 0 kg. Looking at the graph (see Figure 3), values for the 3 kg
target treatment were always less than 3 kg. For the 1.5 kg treatment, values ranged from
0.4 ± 0.0 kg to 1.6 ± 0.1 kg with the lowest value occurring in May and the highest value
occurring in June. The 1.5 kg treatment without oysters (control) had a similar trend, with
values ranging from a low of 0.9 ± 0.1 kg in June to a high of 1.6 ± 0.1 kg in July. The
control (oysters, no, Ulva spp.) only had Ulva spp. detected in late July with a mean value
of 0.5 ± 0.5 kg. Ulva spp. weights were collected in October for half of the bags but due
to an incoming tide not all Ulva spp. weights were recorded. For the bags that were
recorded, Ulva spp. weights were the lowest of any weights recorded during the
32
experiment with mean values for the 3 kg treatments of 0.4 ± 0, 1.5 kg treatments of 0.2 ±
0.1 kg, and 0 kg Ulva spp. for the control and Ulva spp. only treatments (see Table 2).
Peale Passage
When Ulva spp. was detected at Peale Passage, the mean for the 3 kg treatment
was 1.1 ± 0.1 kg and the means for the 1.5 kg treatments with and without oysters were
both 0.5 ± 0.1 kg. By August, there wasn’t sufficient Ulva spp. on site to meet the
treatment levels for the bags. Interestingly, Ulva spp. weight in the 3 kg treatment peaked
one month earlier than Thorndyke Bay in June. Values for the 3 kg treatment ranged from
2.6 ± 0.1 kg in June to 0.1 ± 0.0 kg in August. Oysters in the 1.5 kg treatment were
exposed to less than half of the targeted Ulva spp. weight in most months (see Figure 2).
The maximum value for the 1.5 kg treatment occurred in June with 0.8 ± 0.0 kg and the
minimum occurred in August with 0.1 ± 0.1 kg. The 1.5 kg treatment without oysters
(control) had maximum values of 0.7 ± 0.1 kg and 0.7 ± 0.0 kg in June and July
respectively. The lowest value for the 1.5 kg treatment without oysters was 0.1 ± 0.0 kg
in August. For much of the experiment, Ulva spp. was not detected in the control
treatment (oysters, no Ulva spp.), but there were slight increases detected in June and
July. There was no Ulva spp. detected in any of the treatments in October (see Table 1).
Oyster growth rate
Ulva spp. weight had a significant effect on oyster growth at each site and across
both sites. Shell height in the control treatment (150 oysters, no Ulva spp.) was
significantly larger than both the 3 kg and 1.5 kg treatments at both sites (see Figure 4).
As discussed below, significant differences in oyster height were not detected between
the medium and high Ulva spp. treatments until October.
33
Thorndyke Bay
The significant difference between shell height of the control (oysters, no Ulva
spp.) as compared to the high and medium treatments was not apparent immediately. The
initial mean shell height for oysters at Thorndyke Bay was 29.1 ± 1.0 mm. The treatments
weren’t put into place until a month after the oysters were put into their bags. Shell height
growth rates always increased for the 3 kg and control treatments from April with a mean
of 29.1 ± 1.06 mm to October with means of 61.0 ± 3.2 mm for the 3 kg treatment and
83.8 ± 3.9 mm for the control. Shell height always increased for the 1.5 kg treatment
from a mean of 29.1 ± 1.06 mm to 59.4 ± 3.7 in August. However, mean shell height for
the 1.5 kg treatment decreased from 59.4 ± 3.7 in August to 51.9 ± 3.4 mm in October.
The oysters did not get smaller, so it is likely that smaller oysters were sampled in
October than August on average. Post ad-hoc tests showed that by July, the mean shell
height for oysters in the control treatment was significantly larger than the 3 kg and 1.5
kg treatments at Thorndyke Bay (F(2,12)=5.099, p<.0001). This trend continued
throughout the rest of the experiment. Even as Ulva spp. weights decreased to almost 0
kg in the treatments in October, there continued to be a significant difference between the
control treatment and the 3 kg and 1.5 kg treatments with a difference of approximately
20 mm on average (see Figure 3). Conversely, there was not a significant difference in
size between the oysters in the 3 kg and 1.5 kg treatments F(2,12)=5.099, p<.0001). At the
final sampling date in October the mean shell height of oysters in the control treatment
was 22.8 ± 0.7 mm larger than the 3 kg treatment (see Table 3).
34
Peale Passage
The oysters at Peale Passage exhibited a similar trend throughout the summer
with slight variations. The initial mean shell height when the oysters were planted in
April was 29.3 ± 1.1 mm. The treatments were put into place two weeks after the oysters
were put in their bags. Shell height growth rates always increased from April with a mean
of 29.3 ± 1.1 mm to October with means of 78.0 ± 5.9 mm for the 3 kg treatment, 81.5 ±
3.7 for the 1.5 kg treatment, and 97.6 ± 3.2 mm for the control (150 oysters, no Ulva
spp.) (see Table 4). Post ad-hoc tests showed that by June, the mean shell height for
oysters in the control treatment was significantly larger than the 3 kg and 1.5 kg
treatments at Peale Passage (F(2,12)=7.309, p<.0001). This trend continued through July.
In August, there was a significant difference between the shell height for oysters in the 3
kg treatment and control treatment. A post ad-hoc test showed that shell heights for the
1.5 kg treatment were not significantly different from the 3 kg or control treatments in
September. Shell height increased for all treatments in October, with shell height in the
control significantly larger than the 3 kg and 1.5 kg treatments (F(2,12)=6.03, p<.0001)
(see Figure 2).
Although the site dynamics were different in both sites, shell height response was
consistent at both sites for the summer (see Appendix B). Through September, site did
not have a significant effect on shell height, although it was approaching significance (see
Figure 4) (F(1,25)=0.18, p<.0439). In October, oysters in all treatments at Peale Passage
grew considerably and at that point site did have a significant effect (F(2, 24)=6.10,
p<.0001).
35
Oyster survival
Oyster survival was noted in each bag and dead oysters were counted on a biweekly basis. The 3 kg, 1.5 kg, and control treatments started with 150 live oysters in
each bag. Survival values ranged from 150 ± 1 to 147 ± 4 at Thorndyke Bay (see Table 5)
and 150 ± 1 to 147 ± 3 at Peale Passage (see Table 6) over the entire experiment. Oyster
mortality declined slightly (between 150-147) depending on the treatment, over the
course of the duration of the study. Survival was non-normally distributed and was
analyzed using non-parametric Monte Carlo simulation. Neither site nor treatment had a
significant effect on oyster survival (DIF=1.4, p=0.297).
36
Tables
Table 1
Mean Ulva spp. weights of treatments at Peale Passage, Washington. “3 kg” signifies treatment
with a targeted weight of 3 kg Ulva spp. and 150 oysters. “1.5 kg” signifies treatment with a
targeted weight of 1.5 kg Ulva spp. and 150 oysters. “0 kg” signifies treatment of 0 kg Ulva spp.,
150 oysters. “1.5 kg, no oysters” represents treatment with 1.5 kg targeted Ulva weight and no
oysters.
Date
3 kg
5/17/15
6/16/15
7/1/15
7/13/15
7/28/15
8/30/15
10/29/15
1.2
2.6
1.0
1.0
0.5
0.1
0.0
0.9
Total Mean
±
±
±
±
±
±
±
±
SE
1.5 kg
0.1
0.1
0.1
0.3
0.0
0.0
0.0
0.1
0.4
0.8
0.7
0.5
0.4
0.1
0.0
0.4
±
±
±
±
±
±
±
±
SE
0 kg
0.1
0.0
0.0
0.2
0.1
0.1
0.0
0.1
0.0
0.3
0.1
0.2
0.0
0.0
0.0
0.1
±
±
±
±
±
±
±
±
SE
1.5 kg, no
oysters
SE
0.0
0.2
0.0
0.2
0.0
0.0
0.0
0.1
N/A
0.7
0.6
0.7
0.4
0.1
0.0
0.4
N/A
0.1
0.1
0.0
0.0
0.0
0.0
0.0
±
±
±
±
±
±
±
±
Table 2
Mean Ulva spp. weights of treatments at Thorndyke Bay, Washington. Abbreviations are as defined in Table 1.
Date
5/4/15
5/20/15
6/4/15
6/15/15
7/3/15
7/14/15
7/31/15
8/28/15
10/27/15
Total Mean
3 kg
1.0
2.5
2.5
2.1
2.2
2.2
2.0
1.9
0.4
1.9
±
±
±
±
±
±
±
±
±
±
SE
0.1
0.4
0.0
0.1
0.1
0.2
0.2
0.1
0.0
0.1
1.5 kg
0.4
1.4
1.6
0.6
1.3
1.3
1.5
1.1
0.2
1.0
±
±
±
±
±
±
±
±
±
±
SE
0.0
0.1
0.1
0.3
0.1
0.1
0.2
0.1
0.1
0.1
0 kg
0.0
0.0
0.0
0.0
0.0
0.0
0.5
0.0
0.0
0.1
±
±
±
±
±
±
±
±
±
±
SE
0.0
0.0
0.0
0.0
0.0
0.0
0.5
0.0
0.0
0.1
1.5 kg, no
oysters
N/A
N/A
1.3
0.9
1.6
1.5
1.5
1.4
0.0
1.2
37
±
±
±
±
±
±
±
±
±
±
SE
N/A
N/A
0.1
0.1
0.1
0.1
0.1
0.1
0.0
0.1
Table 3
Mean shell height (mm) of oysters in treatments at Thorndyke Bay. Abbreviations are as defined in
Table 1. Mean shell height was calculated using a weighted mean.
Date
9-Apr
20-Apr
4-May
20-May
4-Jun
15-Jun
3-Jul
14-Jul
31-Jul
28-Aug
27-Oct
3.0 kg
29.1 ±
30.2 ±
32.3
40.7
40.4
42.2
50.5
53.6
59.4
60.7
61.0
±
±
±
±
±
±
±
±
±
SE
1.0
0.9
0.7
1.3
1.5
1.4
1.8
2.0
2.6
3.3
3.2
1.5 kg
29.1 ±
30.6 ±
37.4
38.0
37.4
42.9
47.2
49.2
51.1
59.4
51.9
±
±
±
±
±
±
±
±
±
SE
1.0
0.9
0 kg
29.1 ±
33.5 ±
SE
1.0
0.8
1.0
1.3
1.3
1.3
2.2
2.7
2.9
3.7
3.4
37.1
38.1
42.2
44.9
51.7
54.1
61.6
71.9
83.8
1.0
1.4
1.3
1.7
1.7
2.6
2.0
2.7
3.9
±
±
±
±
±
±
±
±
±
Table 4
Mean shell height (mm) of oysters in treatments at Peale Passage, Washington. Abbreviations are as
defined in Table 1. Mean shell height was calculated using a weighted mean.
Date
21-Apr
17-May
16-Jun
1-Jul
13-Jul
28-Jul
30-Aug
29-Oct
3.0 kg
29.3
34.9
41.2
42.4
42.2
46.5
61.1
78.0
±
±
±
±
±
±
±
±
SE
1.1
0.9
1.1
1.7
1.7
1.8
3.7
5.9
1.5 kg
29.3
33.7
40.5
45.8
45.1
48.2
60.6
81.5
±
±
±
±
±
±
±
±
SE
1.1
1.2
1.8
1.7
2.1
2.8
3.0
3.7
0 kg
29.3
35.9
41.9
48.5
54.8
59.7
75.1
97.6
38
±
±
±
±
±
±
±
±
SE
1.1
1.0
1.6
1.7
1.9
1.9
2.9
3.2
Table 5
Oyster survival in treatments at Thorndyke Bay, Washington. Abbreviations are as defined in
Table 1.
April
May
June
July
August
October
3.0 kg
150
149
149
149
148
148
±
±
±
±
±
±
SE
0
1
1
0
1
1
1.5 kg
149
149
149
149
148
148
±
±
±
±
±
±
SE
1
1
1
1
1
1
0 kg
150
148
147
147
147
147
SE
1
3
4
4
4
4
±
±
±
±
±
±
Table 6
Oyster survival in treatments at Peale Passage, Washington. Abbreviations are as defined in
Table 1.
May
June
July
August
October
3.0 kg
150
N/A
147
147
147
±
±
±
±
±
SE
1
N/A
3
3
3
1.5 kg
150
N/A
148
148
148
±
±
±
±
±
SE
1
N/A
2
1
1
0 kg
150
N/A
149
149
149
±
±
±
±
±
SE
0
N/A
1
1
1
39
Figures
100.0
3.0
90.0
Shell Height (mm)
70.0
2.0
60.0
50.0
1.5
40.0
1.0
30.0
20.0
Ulva weight (kg)
2.5
80.0
0.5
10.0
0.0
0.0
April
May
June
July
Aug
Oct
Figure 2: Mean C. gigas shell height (mm) at Peale Passage for length of the experiment
(primary y-axis). Patterned bar signifies treatment with 3 kg target weight Ulva spp. and 150
oysters. Grey bar signifies treatment with 1.5 kg target weight, 150 oysters. Dark grey bar
signifies “control” treatment with 0 kg Ulva spp. target weight, 150 oysters. Secondary y-axis
represents actual mean Ulva spp. weight in bags (kg) after two weeks. Dashed line represents 3
kg Ulva spp. target weight with oysters. Dotted line represents 1.5 kg target weight Ulva spp., no
oysters. Outlined dashed line represents 1.5 kg target weight Ulva spp. with oysters. Black line
represents 0 kg target weight Ulva spp. with oysters.
40
100.0
2.5
80.0
2.0
70.0
60.0
1.5
50.0
40.0
1.0
30.0
20.0
0.5
Ulva weight (kg)
Shell Height (mm)
90.0
10.0
0.0
0.0
April
May
June
July
Aug
Oct
Figure 3: Mean C. gigas shell height (mm) at Thorndyke Bay for length of the experiment
(primary y-axis). Patterned bar signifies treatment with 3 kg target weight Ulva spp. and 150
oysters. Grey bar signifies treatment with 1.5 kg target weight, 150 oysters. Dark grey bar
signifies “control” treatment with 0 kg Ulva spp. target weight, 150 oysters. Secondary y-axis
represents actual mean Ulva spp. weight in bags (kg) after two weeks. Dashed line represents 3
kg Ulva spp. target weight with oysters. Dotted line represents 1.5 kg target weight Ulva spp., no
oysters. Outlined dashed line represents 1.5 kg target weight Ulva spp. with oysters. Black line
represents 0 kg target weight Ulva spp. with oysters.
100.0
90.0
Shell Height (mm)
80.0
70.0
60.0
50.0
40.0
30.0
20.0
April
May
June
July
Aug
Oct
Figure 4: Mean C. gigas shell height (mm) at Thorndyke Bay and Peale Passage for the length of
the experiment. Black lines signify Peale Passage treatments with solid line representing control
41
treatment (oysters, no Ulva spp.), dashed line representing 1.5 kg Ulva spp. treatment, and dotted
line representing 3 kg Ulva spp. treatment. Grey lines signify Thorndyke Bay with solid line
representing control treatment (oysters, no Ulva spp.), dashed line representing 1.5 kg Ulva spp.
treatment, and dotted line representing 3 kg Ulva spp. treatment.
42
V. DISCUSSION
Introduction
These results are one of the first studies to provide evidence for the negative
impact that Ulva spp. can have on juvenile C. gigas growth. The results demonstrated
that when Ulva spp. was present, it significantly decreased the shell height of oysters.
The presence of Ulva spp., rather than the amount of Ulva spp. added, had significant
negative impacts. The amount of Ulva spp. in the treatments decreased in the late summer
and fall due to diminished on-site Ulva spp. presence. However, the shell height of
oysters that were exposed to Ulva spp. months earlier remained significantly smaller on
average. In other words, it appeared that early exposure to Ulva spp. during growth
negatively slows C. gigas growth, and it has a lasting effect such that growth is still
stunted even when the oysters are no longer exposed to Ulva spp. months later. These
findings have significant implications for aquaculture practices in that managing Ulva
spp. on aquaculture gear early in the growing season can lead to more rapid crop turnover
and increased production in a growing area over time.
Ulva spp. dynamics
Ulva spp. accumulation and residence time affects whether it interacts with
aquaculture on-site. Ulva spp. accumulation varies on a site-by-site basis depending on
currents, freshwater inputs, and nitrogen and light availability, among other factors
(Frankenstein, 2000). Ulva spp. production is driven by time periods when nitrogen is
abundant and light is available. The annual cycle for ulva spp. in Puget Sound begins in
springtime when daylength increases, peak growth occurs mid-summer due to increased
43
primary productivity in the water as well as increased irradiance, and then ulva spp.
decomposes in late summer and fall after it blooms. Ulva spp. occurs year round in low
levels and it has the ability to overwinter in some areas. However, it is absent from other
sites (or present at very low levels) starting in late summer through early spring. Ulva
spp. growth is very site-specific, and thus its interactions with aquaculture are site
specific as well.
This study saw a higher amount of ulva spp. on site at Thorndyke Bay relative to
Peale Passage both in quantity as well as residence time. Peak ulva spp. growth occurred
in July in Thorndyke Bay where ulva spp. weight was 2.5 times larger in the 3 kg
treatment as compared to the control. Ulva spp. at Peale Passage peaked in June with 2.2
times more Ulva spp. in the 3 kg treatment as compared to the control. Ulva spp.
resources decreased in August to almost 0 at Peale Passage and less than one third of the
treatment target weights at Thorndyke Bay. Ulva spp. weight continued to decrease at
Thorndyke Bay through the remainder of the experiment.
Differences in on-site topology, water quality dynamics, and biological
communities could potentially explain some of the variability in Ulva spp. presence
between the two sites. Thorndyke Bay is a very shallow bay as compared to Peale
Passage, which has a steeper gradient. The shallow depth at Thorndyke Bay allows
increased light penetration during more of the tidal cycle and larger surface area allows
for increased light penetration deeper in the water column as well as further into the bay,
which could provide favorable conditions for Ulva spp. to grow in relative to Peale
Passage. There is also a freshwater stream, Thorndyke Creek that empties into the bay,
providing a possible source of nitrogen throughout the year. North Hood Canal also has a
44
shorter residence time for water than South Puget Sound, which could increase nutrient
exchange and thus influence availability of nutrients for Ulva spp. (Harrington, 2005).
Ulva spp. is typically present at Thorndyke Bay year-round and can be observed
overwintering in oyster and clam bags. Shellfish farmers at Peale Passage have observed
Ulva spp. seasonally, with some Ulva spp. occurring in the summer and hardly any
growing on-site for the rest of the year (Prohim, 2015). Ulva spp. does not typically
accumulate in bags at Peale Passage during most of the year. These site-specific
dynamics influenced the presence of Ulva spp. at both sites throughout the experiment.
Oyster response
Oyster growth rates varied between the two sites, although not significantly. It is
common to see differences in oyster growth rates between sites. Growth performance can
be affected by a number of variables including food availability (phytoplankton quantity
and quality), turbidity, seston concentration, temperature, salinity, and pollution
(Kochmann & Crowe, 2014) (Cassis, Pearce, & Maldonado, 2011). Specific parameters
for these two sites are currently unknown. It is important to note that Washington
produces some of the fastest-growing oysters in the world and the growth rates in
Washington are higher than most studies from the East Coast of the US as well as
Europe. As a means of comparison, Diedrich et al conducted a study in the Wadden Sea
in North Germany examining growth rates of oysters in different substrates (2006). The
experiment started with juvenile oysters planted in June 2002 that had a mean shell height
of 27 ± 1.0 mm, and oysters grew to a mean length of 45.9 ± 1.0 mm by November 2002
(Diedrich, 2006).
45
Effect of Ulva spp. presence on shell height
Shell height for all three treatments almost doubled between July and October,
which is consistent with seasonality for oyster shell height development (Hyun et al.,
2001) (see Figure 4). Oyster growth at Peale Passage in the late summer and fall was
greater than growth at Thorndyke Bay, suggesting that on-site variability between the two
sites was a potential factor. One potential explanation is the absence of Ulva spp. at Peale
Passage during that time period. At both sites, there was a similar trend between the
treatment weight (3 kg vs. 1.5 kg) and the shell height response, where oysters exposed to
1.5 kg of Ulva spp. had larger shell heights than oysters in the 3 kg bags (except for the
1.5 kg treatment at Thorndyke Bay in October). However, this response was not
significant and the presence of Ulva spp. was more of a determining factor for shell
height than amount of Ulva spp.
The presence of Ulva spp. had a strongly significant effect on shell height at both
sites in July through the rest of the experiment. At the final sampling date in October, C.
gigas mean shell height for the control were approximately 25% larger than the 3 kg
treatment and 20% larger than the 1.5 kg treatment at Peale Passage. This difference was
more pronounced at Thorndyke Bay, with mean shell height of the control almost 40%
larger than the 3 kg treatment and 60% larger than the 1.5 kg treatment. These differences
are not only significantly different statistically, but could also be significant in the
marketplace. Oysters that are consistently smaller by 20 mm or more could mean the
difference between an oyster ready for market in October and an oyster that wouldn’t be
ready for market until the following year. This size difference could result in thousands
of dollar of lost profit as well as additional costs of continued management of the current
46
crop and a delayed outplanting of new crops. For these biological and commercial
reasons, it is important to identify variables that could explain the causes for these
differences in shell height.
Physical interactions
Shading
Several observations of Ulva spp. negatively impacting oysters are consistent with
observations of the same phenomenon in other studies. Multiple studies have noted an
effect of macroalgae on oyster recruitment and oyster larval development. Thomsen and
McGlathery (2006) researched the effects of drift algae and sediment on Crassostrea
virginica reefs and associated sessile organisms. They found that oyster recruitment was
reduced due to the presence of drift algae, which they attributed to smothering,
interference with feeding apparatus, and the indirect effects of light penetration limiting
food productivity (Peckol & Rivers, 1996). Ulva intestinalis has been shown to decrease
light penetration by decreasing surface irradiance by 50% with one layer and over 80%
with three layers (Mvungi et al., 2012).
Water flow
One explanation for this trend is that water circulation in bags with Ulva spp. was
slower than water circulation in the control bags. When the grow bag is submerged, the
macroalgae in the bag slows down the water flow over the oysters (Mallet et al., 2009).
Water flow has a strong effect on passive filter feeders (Sanford, Bermudez, Bertness, &
Gaines, 1994). Oysters access their food, phytoplankton, through the water column and
current spend affects food availability. Thus, with slower water flow rates, the oysters
47
have less opportunity for consumption and growth (Cassis et al., 2011). Water flow rate
was not studied specifically in this experiment, but is a potentially explanatory factor that
should be studied further. This potential finding has repercussions for other types of
aquaculture where flow is inhibited by biofouling of bivalves on other organisms. For
example, multiple studies have seen an effect of algal biofouling on flow rates in
aquaculture. Claereboudt et al studied the effect of biofouling of mollusks on juvenile sea
scallops (Placopecten magellanicus) and found that changing the nets regularly resulted
in a 68% increase in muscle mass and a 4.8% increase in shell height (Claereboudt,
Bureau, Cote, & Himmelman, 1994).
Sedimentation
Macroalgae can alter water currents or act as a physical barrier to oyster
settlement. Macroalgal canopies have physical impacts that have cascading effects,
including concentrating pelagic larvae and decreasing water flow leading to
sedimentation (Kathryn L. Van Alstyne et al., 2014). Kochmann and Crowe examined the
effect of biotic interactions on the establishment of C. gigas including the effect of
macroalgae cover of Fucus serratus on C. gigas juvenile development and survival. The
authors found that macroalgae cover did not have a significant effect on oyster growth or
survival, and there was a non-significant trend toward greater oyster growth in cages with
macroalgae. The authors suggested that these results may be due site-specific dynamics
including communities of other organisms that oysters compete with (Kochmann &
Crowe, 2014).
48
Chemical interactions
Water quality
There are a number of factors that may explain why oysters with Ulva spp.
present in the bag had a decreased growth rate as compared with the control. The
presence of Ulva spp. blooms can influence local water quality dynamics in a bag
through the growing cycle of the macroalgae. For example, a study by Mvungi et al
examined the impact of Ulva intentinalis layers on Zostera marina productivity (2012).
The authors observed increases in pH due to Ulva spp. photosynthesis up to 10.6
(Mvungi et al., 2012). Increased growth rates in the bags with 3 kg of Ulva spp. as
compared to the other two treatments were observed at Peale Passage in May and
Thorndyke Bay in June, although these differences were not significant. This could
potentially be explained by the growth of Ulva spp. during this period and subsequent
water quality improvements, but more research would be needed to confirm this finding.
Lomstein et al found an increase in dissolved oxygen (DO) concentrations when Ulva
lactuca was added to a lab experiment from 11 to 23 mmol m−2 d−1 that remained
through the incubation period (Lomstein, Guldberg, Neubauer, & Finster, 2006).
Although pH may increase locally during growth, it could also decrease during
decomposition (Felix & Pradeepa, 2012). Ulva spp. can decrease DO concentrations
during decomposition (Rinehart, Guidone, Ziegler, Schollmeier, & Thorner, 2014).
Decomposition was observed at Peale Passage on July 13th (see Appendix C) and was
not observed at Thorndyke Bay during sampling. The impact of decomposition could be
another explanatory factor in the significant difference between treatments with Ulva spp.
present and without Ulva spp. Decay was observed at Peale Passage in June and
49
Thorndyke in August. According to Brush, decay occurs when oxygen levels are low and
respiration is reduced (2010). Decay of macroalgae is directly related to temperature and
indirectly related to C:N and C:P ratios of the macroalgae (Brush & Nixon, 2010). More
research is needed to quantify the impacts of the water quality dynamics.
Allelopathy
Chemical interactions could also play a role in the impact of Ulva on oyster
growth. Ulva has been observed to inhibit the growth and development of other plants
and animal species (Timothy A. Nelson et al., 2003), suggesting a potential for general
toxicity, or allelopathic properties. Nelson also noted another study that found Ulva
lactuca extract inhibited development of crab larvae and the larvae had zero success, or
100% mortality (Timothy A. Nelson et al., 2003). Currently, Ulvaria obscura is the only
known algae to produce dopamine. Ulvaria releases dopamine during desiccation and
rehydration process, which occurs frequently during tidal cycles (Kathryn L. Van Alstyne
et al., 2014). Van Alstyne et al noted that Ulvaria released 7-100% of dopamine in their
tissues when desiccated on the beach for 75 minutes and then re-immersed in seawater
(2014). The authors also observed a rapid 0.7 drop in pH in the surrounding seawater.
The authors did not see the same drop in pH when Ulva lactuca was exposed to similar
conditions (Kathryn L. Van Alstyne et al., 2014). However, Nelson’s research suggests
that Ulva exhibits slowed growth when exposed to its own extract and could be a
potential explanatory factor for the stark difference in size between C. gigas exposed to
Ulva and the control.
50
Biological interactions
Ulva spp. concentrations varied between sites and decreased over time in the
course of the experiment. There are a number of factors that influence macroalgal loss
including decay, respiration, grazing, and decay through drift (Brush & Nixon, 2010).
One study hypothesized that Ulva biomass can be lost due to marginal tissue becoming
reproductive and producing spores as it is integrated into the water column (ZertucheGonzalez et al., 2009). There is no data available for Peale Passage or Thorndyke Bay
that addresses the strongest influences at either of those sites. However, the increased
consistent loss of Ulva spp. at Peale Passage as compared to Thorndyke Bay suggests that
the Ulva spp. at Peale Passage may have experienced a more robust grazing regime.
Invertebrates including snails and crabs were found inside the bags at both sites.
Additional observations
Another interesting finding was that actual Ulva spp. weights for the targeted 1.5
kg treatment differed in the bags that had oysters and bags without oysters, which both
had 1.5 kg of Ulva spp. added to it. At Thorndyke Bay, where there was more Ulva spp.
on site, bags with a treatment weight 1.5 kg of Ulva spp. and 150 oysters had less Ulva
spp. remaining in the bags after two weeks than bags with a treatment weight of 1.5 kg of
Ulva spp. and no oysters by 15-50%. At Peale Passage, a similar trend was observed in
July when the bags with a treatment weight 1.5 kg of Ulva spp. and no oysters had
approximately 37% more Ulva spp. than bags with a treatment weight of 1.5 kg and
oysters. This trend was not significant but it did occur throughout multiple sampling
periods and at both sites. This suggests that there may be a feedback of oysters on Ulva.
51
This observation differs from findings of the literature by Ale et al, who found
that Ulva lactuca showed a favorable growth response to ammonium, which oysters
secrete, as a nitrogen source (Ale, Mikkelsen, & Meyer, 2011). In their experiment, Ale
et al observed that Ulva lactuca prefers ammonium over nitrate as a source of nitrogen
(Ale et al., 2011). Their findings suggest that oysters may provide beneficial conditions
for Ulva to grow in and thus may support more Ulva growth on site. Lomstein et al found
that in a controlled lab experiment, ammonium efflux accounted for 83% of the total
nitrogen efflux after Ulva lactuca was added to the flow-through mescosm (2006).
However, this experiment suggests that other negative impacts may dominate this
positive impact.
Shell height was selected as the metric or proxy for oyster growth in this study
due to time constraints of collecting data during a low tide. Shell height is one parameter
to measure growth, but there are other important parameters that help tell a complete
story. Many studies examine meat weight (dry weight) as a parameter of growth. Others
measure shell height as well as width. Shell height provides data on the length of the shell
growth but it does not always reflect characteristics that are important qualities in a
commercial oyster. Commercial oysters are marketed based on the shell size, meat
weight, meat quality and color, and shell appearance. For example, the oysters at Peale
Passage that made up growth in the late summer and fall were what’s known in the
industry as “skinny” (Davis, 2015). Skinny oysters have a long narrow shell and thus the
internal meats are also long and skinny. Skinny oysters can occur when oysters
experience rapid growth or when bags are not turned on a regular basis
52
This type of oyster is less desirable in the marketplace because of its shape and smaller
meat size (Davis, 2015). Oysters under the “control” treatment at Peale Passage were not
observed to be skinny, while oysters under both Ulva treatments were skinny by late
October. This trend was not observed at Thorndyke Bay where growth rates in the late
summer and early fall were slower for the oysters in the two Ulva treatments. While the
shell height metric portrayed a lot of growth in the late summer and fall particularly for
oysters at Peale Passage, this may not have reflected the true size of the oyster.
There were also noticeable differences in shell color at both sites between the
different treatments. The difference in shell color was more apparent at Thorndyke Bay,
where oysters in the Ulva treatments often had light-colored shells and oysters in the
control treatment had external shells with more purple coloring to them. One explanation
for this is that shell color differed based on the level of stress that the oyster was
experiencing. Changes in shell color under stress is a phenomenon that other researchers
have observed with bivalves include conversion of purple internal shell to white in
Corbicula fluminea.
Recommendations
Ulva presence had a persistent effect on diminished oyster height throughout the
experiment. Even though Ulva weights in treatment bags diminished considerably at both
sites by August, a discrepancy in growth rates between treatments exposed to Ulva and
the control remained through the production period. This preliminary research suggests
that to maximize growth, farmers should begin to think about ways to move Ulva from
seed growing areas. For tidelands where Ulva growth accumulates in bags, efforts should
be focused on removal early in the growing season (May-July). Oysters that were
53
exposed to treatment levels during this period did not recover in their growth rates in this
experiment. This is also typically the peak growing season for Ulva. Removing Ulva
early in the season increases the likelihood that oysters will be ready for market earlier
and growers can introduce another crop earlier in the growing season. Other suggestions
include regular bag turning to help control fouling on top of bags (Mallet et al., 2009).
Ulva growth is very site specific and growers need to account for site-specific
differences when determining how to manage their beds. For example, Ulva naturally
accumulates in grow-out bags at Thorndyke Bay but not at Peale Passage. Managing
Ulva in bags is not necessarily a critical issue for growing areas that have more
ephemeral Ulva populations.
This experiment was conducted at two field sites with different site dynamics and
produced consistent results at both sites. Studies examining the interaction between
oysters and macroalgae are rare and more research needs to be done before general
conclusions about the interactions between Ulva spp. and C. gigas in Puget Sound can be
concluded. However, the benefit of having two field sites illustrates that this phenomenon
is not site-specific. Although site-specific dynamics were very different at both sites,
shell height growth rates were strongly significant and had similar trends at both sites in
the 1.5 and 3 kg treatments. This experiment demonstrates that the interaction between
these two species is worth examining further.
Future experiments on this issue should also consider examining water quality
parameters as a key component of the study. Water quality measurements including pH
and DO were included in the study design. However, due to the limited scope of the
54
project, these measurements were not taken on a regular basis. It would be useful to use
equipment that could take frequent measurements in the bag throughout the day (pulse)
instead of obtaining measurements only during field collection in order to get a better
understanding of the water quality dynamics. The toxicity of Ulva to itself as well as
other organisms is another emerging field that needs more research in order to better
understanding the interactions occurring.
Design considerations
The study design was effective and allowed researchers to observe an ecosystem
effect. However, in a future follow up study, there are some factors that should be
considered. Utilizing a power analysis in the study design in order to determine the
amount of samples may have helped increase the F-value for shell height and provided
more informative results for this measurement. The amount of replicates (5) was quite
large and could probably be reduced while samples per bag (10) could be increased.
These values would provide more information about the growth dynamics occurring in
each bag. Now that a significant effect has been observed for treatments of 3 kg and 1.5
kg, it would be informative to see if there is still an effect with less Ulva. Treatments of
0.5 and 1 kg should be considered for a follow-up experiment. Measuring the amount of
macroalgae that naturally accumulates in grow bags at different locations could help to
inform this parameter. Additionally, including water quality measurements in the study
design would be informative to identify site-specific factors that may help to explain the
differences in shell height. Water flow, pH, temperature and DO should all be considered
for future follow up studies.
Conclusion
55
There are many factors that potentially affect the growth of C. gigas in
commercial operations. In this experiment, there was a significant size difference
between C. gigas exposed to Ulva spp. in the bag and without Ulva spp. in the bag. This
indicates a relationship between Ulva spp. and oyster growth. With the introduction of
Ulva spp in a bag comes many potential changes in the local environment including
physical, chemical, and biological changes. This study established that there was an
effect of Ulva spp. on C. gigas growth. Additional research is needed to explain the
particular components of Ulva spp. presence that had the strongest effect on C. gigas
growth. Understanding the key components of Ulva spp. presence that affect oyster
growth is very important to developing successful growing practices on oyster farms.
56
VI. INTERDISCIPLINARY CHAPTER
Introduction
Washington State has been proactive on addressing ocean acidification through
the creation of the Governor’s Blue Ribbon Panel (2012) and Marine Resources Advisory
Committee (2013-present), and the Washington Shellfish Initiative (2012) by providing
funding and resources to support research and collaborative efforts between growers,
scientists, and policymakers. Through the Marine Resources Advisory Committee,
policymakers and scientists have set priorities for Washington to focus on in order to
better equip the state to adapt to ocean acidification. One of the key early action items in
the 2012 Blue Ribbon report is to “develop vegetation-based systems of remediation for
use in upland habitats and in shellfish areas” (Adelsman & Binder, 2012). This action
item recognizes that seaweeds are a prolific resource and have the ability to potentially
improve water quality for calcifying organisms.
The abundance of algae in Puget Sound presents an opportunity to both improve
environmental conditions and create a product from seaweed that could be important in
the marketplace. The total of all products made from seaweed worldwide is $5.6 billion
annually, with many value-added products made from seaweed (“A guide to the seaweed
industry,” 2015). Currently, the vast majority of seaweed production occurs in Asia.
Commercial harvesting of seaweed in the U.S. is restricted by permit regulations.
Currently, Maine is the only state that has a commercial seaweed harvesting industry.
Commercial harvesting is not permitted at this time in Washington.
Phytoremediation
57
There has been a lot of interest around harnessing the potential of seaweed here in
Washington. The Puget Sound Restoration Foundation (PSRF) was awarded a $1.5
million Ocean Challenge grant through the Paul Allen Foundation in 2015. The challenge
was targeted toward the scientific community to develop science-based solutions to the
social and environmental effects of ocean acidification (Hickey, 2015). PSRF is currently
developing a seaweed demonstration farm in Hood Head, North Hood Canal to look at
the impact of growing sugar kelp on the marine environment. They hope to have a better
understanding of the impact of seaweed farming on surrounding water chemistry. PSRF
is partnering with other research institutions including University of Washington to look
at the impact of seaweed farming for sensitive calcifiers such as pteropods.
Food
Ulva is an edible seaweed that can be harvested for human consumption. It is one
of many types of seaweed in the Pacific coastal region that are edible. Seaweed has been
harvested as a food product in coastal communities for thousands of years, particularly
Asian countries. It is now being consumed in many other countries as well. Seaweed is
extremely nutritious and contains high levels of iodine and iron. It also contains calcium,
magnesium, potassium, sodium, iron, chromium, copper, and trace minerals which are all
important for bodily functions (“Nutritional Information,” 2015). Ulva in large quantities
is a good source of fiber, protein, plant pigments, and plant sterols (Moll, 2013). Seaweed
is harvested commercially in 35 countries and the top three producing countries for edible
seaweeds are China, Korea, and Japan. There has not historically been as much demand
for edible seaweed in the United States, but this is starting to change. Chefs in high-end
restaurants in New York are featuring seaweed on their menus as a sustainable local food.
58
Developing a regional market for seaweed is critical to establishing seaweed harvesting
on a larger scale.
Biofuel
Seaweed represents an untapped biofuel market. Seaweed is extremely prevalent
worldwide and utilizing this resource could significantly reduce our dependence on fossil
fuel. Gao and McKinley estimate that substituting seaweed for fossil fuels to offset
carbon dioxide emissions could be a net cost of zero or potentially less than zero (Gao &
McKinley, 1994). Compared to first-generation biofuel sources such as bioethanol,
seaweed has much lower growing and processing costs (Dibenedetto, 2010). In addition,
replacing first-generation biofuels such as bioethanol with seaweed would free up
terrestrial land that could be used for growing food crops. Researchers in Norway are
looking at the viability of extracting bio-oil from sugar kelp (Laminaria saccharina) to be
used as biofuel. Their preliminary results suggest that it is possible to get bio-yields of
79% with very high heating mechanisms. Using seaweed for bioenergy (ethanol, etc.) can
also free up terrestrial land for farming food products or other land uses. Of course, there
are technical challenges that prevent seaweed biomass from being utilized for bioenergy
on a global scale on the magnitude of replacing carbon. Understanding the role that algal
blooms play in the ecosystem is of primary importance before any type of
implementation efforts can be explored.
Fertilizer
Seaweed has been used as a fertilizer for thousands of years. Salish tribes in the
San Juan Islands used to fertilize their camas gardens with seaweed (Wagner, 2012).
Seaweed contains concentrated amounts of nutrients such as phosphorus and nitrogen
59
that have high demand in agriculture. For example, phosphorus is a finite material that is
currently mined with significant environmental impacts. Seaweed contains phosphorus
that can be utilized in much less impactful way. Seaweed can be used to make excellent
local fertilizer for the Pacific Northwest. It is processed commercially for liquid seaweed
extracts, a rapidly growing market for organic fertilizer.
Other uses
The seaweed industry extends beyond edible seaweeds for consumption. Seaweed
is also used as a food additive and an anti-bacterial agent. It is harvested for its
hydrocolloids that are used a thickening agent in cosmetics. There is also potential to use
seaweed for wastewater treatment through absorption of heavy metal ions in polluted
water (“A guide to the seaweed industry,” 2015).
Ulva spp. has potentially beneficial chemical effects that could promote human
health and safety. The extract of Ulva lactuca has been found to slow the growth of
harmful algal blooms (HABs) in lab experiments. Authors Tang and Gobler found that
environmentally realistic levels of Ulva lactuca strongly slowed the growth of seven
common HABs due to heat-stable allelochemicals (Tang & Gobler, 2011). This research
demonstrates that there are many other potential uses for seaweed, particularly Ulva that
could be explored through additional research.
Conclusion
The Puget Sound region has the ability to contribute to a profitable worldwide
industry of seaweed harvesting and products. There is a lot of emerging interest in
harvesting seaweed from Puget Sound as well as growing it commercially. The seaweed
60
industry is very profitable; total worldwide value of all industrial products made from
seaweed is $590 million (“Prospects for seaweed production in developing countries,”
2015). There are also implications of harvesting macroalgae from Puget Sound for
commercial use that must be addressed before seaweed is harvested on a large scale.
Removing seaweed, full of micronutrients out of the water will likely have impacts on the
marine ecosystem. It is unknown whether these impacts would be significant or not. This
issue is further complicated by the fact anthropogenic nitrogen plays a role in seaweed
growth in Puget Sound. Before seaweed harvesting is explored on a commercial scale,
it’s important to understand the potential implications of this practice to the marine
ecosystem.
61
VI. CONCLUSION
Washington is one of the largest producers of farmed shellfish in the United
States, producing approximately 85% of the farmed shellfish on the West Coast
(Adelsman & Binder, 2012). A major portion of this production occurs in Puget Sound.
Shellfish farming is an important economic driver in rural coastal towns along Puget
Sound, and is also a source of local, sustainably grown food. Unlike terrestrial farming,
which can often have harmful environmental impacts including problematic waste
streams and point-source pollution, shellfish farming often has a net positive effect on the
local ecosystem (Fisher & Mueller, 2007). Shellfish, as filter feeders, consume nutrients
from the water that are then removed when the shellfish are harvested. Additionally, by
consuming phytoplankton, shellfish improve the clarity of the water and allow sunlight to
penetrate deeper into the water, supporting the growth of native species including
eelgrass (Zostera marina) (Fisher & Mueller, 2007). In addition, shellfish farming
provides an impetus for political action to improve clean water in Washington State. This
political pressure has far-extending impacts for the local population and the environment
far beyond shellfish farming.
The future health of Washington’s marine waters is uncertain. Scientists are
observing the beginning of what could be enormous changes in the water chemistry and
thus species composition of our marine waters. The local shellfish industry, dependent on
highly sensitive shellfish larvae, is the canary in the coalmine for this problem. Shellfish
growers are adapting to this “new normal” proactively by using adaptive techniques
including controlling water chemistry conditions in hatcheries for shellfish larvae and
utilizing hatcheries to grow out shellfish until they are less vulnerable instead of relying
62
on natural sets. However, continuing to produce shellfish in increasingly acidic
conditions will require more adaptation and creativity.
Local impacts on Puget Sound can alter nutrient availability for macroalgae,
impacting the frequency and amount of algal blooms. These impacts contribute to
nutrient pollution, which has detrimental effects for water quality in some areas of Puget
Sound (Mohamedali et al., 2011). Macroalgae exist naturally and are an important
component of marine systems. They provide shelter, transport, and food for many
organisms low on the food chain including shelter for crustaceans from predators
(Frankenstein, 2000) and uptake of anthropogenic CO2 during photosynthesis. They can
also be harmful to water quality, shading out native sea grasses and creating oxygen
deficient environments, killing marine species and potentially leading to local dead
zones. Researchers have linked the increase in nutrient run-off into the Sound to an
increase in algal blooms (Frankenstein, 2000).
Understanding the relationship between seaweed and shellfish growth is key
because macroalgae occurs on many shellfish farms during certain times of the year, and
growers can spend a lot of time managing biofouling on their gear. Quantifying the
impact of Ulva spp. weight on commercially grown C. gigas oysters is critical to making
informed management decisions. When developing plans to introduce macroalgae culture
or harvest to a shellfish farm, the impacts to the growth of the crop must be taken into
account along with macroalgae’s ability to alter local water chemistry. Providing shellfish
growers with the best available tools to adapt to more acidic marine waters is critical to
ensuring the survival of the shellfish industry in Washington State.
63
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70
Appendices
Appendix A. Field Sites in spring and then during peak Ulva spp. bloom at a) Thorndyke
Bay and b) Peale Passage
A (April 2015)
A (July 2015)
B (April 2015)
B (June 2015)
71
Appendix B. C. gigas growth comparison at completion of experiment (October 2015) at
a) Thorndyke Bay and b) Peale Passage. “3 kg” indicates 3 kg Ulva spp. added to
treatment and “control” indicates 0 kg Ulva spp. added.
Initial seed size
A (Control)
A (3 kg)
B (Control)
B (3 kg)
Appendix C. Evidence of Ulva spp. decomposition at Peale Passage on July 13, 2015.
72
Appendix C. Study Design
Graphic representation of study design at each site.
C= Control, 0 kg Ulva spp., 150 oysters
U= Ulva spp., 1.5 kg Ulva spp., 150 oysters
1.5= 1.5 kg Ulva spp., no oysters
3= 3 kg Ulva spp., 150 oysters
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Lamb_AMESthesis2015.pdf