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The Use of Benthic Suspension Feeders to Mitigate Eutrophication in Coastal Ecosystems
Lynn Ficarra
Introduction
According to Nixon (1995), eutrophication is “an increase in the rate of supply of organic
matter to an ecosystem.” Nutrient enrichment is a major component of eutrophication in coastal
systems. Nitrogen and phosphorus are especially important nutrients that have anthropogenic
sources including land clearing, fertilizer, human waste, animal production, and fossil fuel
combustion (Nixon 1995). Nutrient enrichment in coastal waters can cause phytoplankton
blooms that block sunlight and reduce benthic primary production (Paerl 1988). Phytoplankton
cells that die sink to the bottom to fuel microbial respiration (Diaz and Rosenberg 2008). The
decreased benthic primary production coupled with increased microbial respiration leads to
declining dissolved oxygen levels and thus, hypoxia or anoxia (Paerl 1988). Reduced benthic
vegetation and hypoxia are two consequences of eutrophication that can negatively impact
community structure in coastal aquatic ecosystems (Rice 2001).
Urban estuaries such as the Neponset River Estuary in Boston, Massachusetts are
susceptible to eutrophication via industrial wastes, sewage outflow, and runoff from adjacent
developed land (Gardner, Chen, and Berry 2005, Tucker et al. 1999). A potential method for
managing nutrient levels in enriched estuaries is to establish populations of suspension feeders to
filter the water. Many suspension feeders are benthic and largely sedentary (Levinton 1972).
They include bivalves, tunicates, cnidarians, sponges, and some polychaete worms. These
organisms strain plankton, dissolved organic matter, organic aggregates, and bacteria either
actively or passively from their flowing water medium (Ruesink et al. 2005, Gili and Coma
1998, Levinton 1972). The amount of food consumed by these organisms is based on their
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filtration rate and efficiency of particle retention (Riisgard 1988). Filtration rate in suspension
feeders varies depending on the species, the size of the individual, the velocity of the water, the
water temperature, and other factors (Comeau et al. 2008, Rice 2001, Eastern Oyster Biological
Review Team 2007).
In this paper, I will evaluate the filtration rate and particle retention efficiency of various
bivalves and assess if there is an optimal species for water quality control. Then, I will outline
important biological and environmental requirements of the superior filter feeding species. Next,
I will discuss studies that support the usefulness of benthic filter feeders to combat
eutrophication. Finally, I will explore possible negative impacts of introducing bivalves to
coastal ecosystems.
Bivalves
Suspension feeding bivalves play an important role in benthic-pelagic coupling by
filtering the water column and transporting materials to the benthos (Doall et al. 2008). These
organisms use gills composed of parallel filaments with specialized ciliary tracts to pump water
and to capture particles (Riisgard 1988). Collected particles are sorted and eventually expelled
either as feces (digested particles) or pseudofeces (rejected particles) which are released to the
sediment surface as mucus-coated aggregates (Newell 2004). Filtration rates in suspension
feeders vary based on many factors including the species, the size of the individual, the velocity
of the water, and the water temperatures (Comeau et al. 2008, Rice 2001, Eastern Oyster
Biological Review Team 2007). This makes it difficult to accurately compare the filtering
efficiencies of different bivalves.
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To determine which bivalves have the highest filtering capacity, Riisgard (1988)
measured the filtration rate and particle retention efficiency of Geukensia demissa (Ribbed
mussel), Spisula solidissima (Atlantic surfclam), Brachiodontes exustus (Scorched mussel),
Mercenaria mercenaria (Northern Quahog), Crassostrea virginica (Eastern oyster), and
Argopecten irradians (Bay scallop). Riisgard (1988) found that C. virginica had the highest
filtration rate, A. irradians and G. demissa had rates that were close to C. virginica, B. exustus
and S. solidissima had intermediate rates, and M. mercenaria had the lowest filtration rate (Table
1). All six organisms were able to retain 100% of the particles that were 4-5 µm or larger in size.
G. demissa, S. solidissima, B. exustus, and M. mercenaria retained about 35-75% of particles that
were approximately 2 µm in size. C. virginica retained about 50% and A. irradians retained only
15%. From these results, the oyster C. virginica and the mussel G. demissa appear to perform
slightly better than the rest of the bivalves in this study.
Gili and Coma (1998) compiled particle capture rate data (expressed in organic carbon
units) for organisms within several phyla (Table 2). Among the bivalves in this study, the mussel
Aulacomya ater, the Iceland scallop Chlamys islandica and the eastern oyster Crassostrea
virginica have the highest particle capture rates. From the two bivalve studies discussed here,
oysters and mussels appear to be good candidates for filtering coastal estuaries. Because they are
both native to the Neponset Estuary region, I will use the oyster Crassostrea virginica and the
mussel Mytilus edulis to continue this discussion (Ruesink 2005, Zagata et al. 2008).
Eastern Oyster
The eastern oyster Crassostrea virginica is a sedentary, reef-building, epifaunal, bivalve
mollusk that is naturally distributed along the Atlantic coast of the United States (Ruesink 2005).
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In the coastal waters of the Mid-Atlantic States, C. virginica generally inhabits depths between
0.6 and 5 m (MacKenzie 1996). The optimal water temperature for the species ranges 20-30°C
(Stanley and Sellars 1986). They can survive freezing and warm (>45°C) temperatures, but their
feeding rate will be affected (Galtsoff 1964, Shumway 1996). Larvae must experience a
minimum water temperature of 17.5°C to survive and develop, but 20-32.5°C is optimal
(Hofstetter 1977, Calabrese and Davis 1970). Adults can survive at salinities from 5-40 ppt and
larvae can survive at 10-27.5 ppt (Eastern Oyster Biological Review Team 2007, Shumway
1996). The eastern oyster required a dissolved oxygen level from 20-100% saturation and larvae
need a pH of 6.75-8.75 (Eastern Oyster Biological Review Team 2007, Calabrese and Davis
1966). Larvae tend to need hard substrate on which to settle while adults can survive in muddy
environments (Eastern Oyster Biological Review Team 2007).
Estimates of the filtration rate and particle retention efficiency of Crassostrea virginica
vary widely. According to Nelson (1938), a C. virginica individual can filter up to 26 L h-1 under
ideal conditions. The Eastern Oyster Biological Review Team (2007) reported that rates vary
from 1.5-10 L h-1g-1 dry tissue and they can capture particles ranging in size from 1-30 µm.
According to Brusca and Brusca (2003), C. virginica can process up to 37 L h-1 at 24°C and can
capture particles as small as 1 µm in size. At 24°C, Pechenik (2005) reported that an individual
moves 30-40 L of water past its gills each hour. In an experiment involving 200 oysters (about
35 mm valve height) within mesocosm tanks, Pietros and Rice (2003) estimated a filtration rate
of 55 L/individual/day. The authors reported their estimate to be high relative to literature values
and adjusted their estimate to 15 L/individual/day based on Powell et al. (1992). These rate
estimates demonstrate the difficulty of establishing standard rates of filtration for suspension
feeders.
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Blue Mussel
The blue mussel Mytilus edulis is a sedentary, epifaunal bivalve that is widely distributed
and can be found from southern Canada to North Carolina (Zagata et al. 2008). This species is
well acclimated to temperatures between 5 and 20°C, but can survive for extended periods in
freezing water and up to 29°C (Goulletquer 2012). They can also survive low salinities from 418 ppt, but growth rates decrease below 18 ppt (Goulletquer 2012). M. edulis exists in depth
ranges of 1-10 m and typically requires hard substrate such as the rocky intertidal, though they
can form mussel beds by growing on top of each other in loose substrate (Zagata et al. 2008).
Bayne and Widdows (1978) determined the filtration rate of M. edulis to be 1.34-2.59 L h-1.
Winter (1973) found that filtration rate in mussels increases with body size. On average, mussels
with shell lengths of 8.5 mm filtered algal cells out of water at a rate of approximately 20 ml h-1
whereas mussels with shell lengths of 56.5 mm filtered at a rate of about 1300 ml h-1. Winter
(1973) also found that halving the algal concentration approximately doubled the filtration rates.
These data show how filtration rates can be influenced by many factors.
Oyster versus Mussel
Comeau et al. (2008) compared the filtration rates of the blue mussel Mytilus edulis and
the eastern oyster Crassostrea virginica at low temperatures. At 9°C, M. edulis obtained
significantly higher filtration rates than C. virginica. The average rate for M. edulis at 9°C was
1.82-2.90 L h-1 while a rate of 0.05-1.21 L h-1 was measured in C. virginica. There were similar
results at 4°C, but sample sizes were too small for statistical comparisons. In optimal conditions,
C. virginica filtration rates are generally faster than M. edulis. In cold temperatures, howerver, it
may be more valuable to have populations of M. edulis in estuaries where eutrophication is a
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concern. These results in conjunction with the discussions above suggest that it would be
beneficial to use both species in estuary restoration projects.
Other Suspension Feeders
Mytilus edulis and Crassostrea virginica are both native to the Boston Harbor and are
capable (though it is not ideal) of settling and establishing populations in the soft-bottom benthos
of an estuary. Placing hard structures throughout the estuary would help provide a more optimal
settling substratum. Oyster reefs and mussel beds both provide solid biogenic structures
necessary for many organisms to settle. As a result, these bivalve assemblages promote high
biodiversity (Ruesink 2005). Oyster reefs provide especially large and complex structures to
which other suspension feeders (including mussels) recruit. Organisms that recruit to oyster reefs
include sponges, hydroids, corals, anemones, tunicates, polychaetes, etc. (Ruesink 2005).
According to Gili and Coma (1998), sponges, tunicates, and cnidarians are not insignificant in
their filtering capacities (Table 2). In fact, sponges retain up to 80% of suspended particles
(Milanese et al. 2003). Despite a relatively low pumping rate, sponges are also capable of
capturing miniscule particles, such as bacteria, that other filter feeders may miss (Milanese et al.
2003). According to Pomeroy, D’Elia, and Schaffner (2006) the mean filtering rate of suspension
feeders per gram of dry weight at optimal conditions for 44 species (from sponges to ascidians)
is 7.8 L g-1h-1.
Case Studies
There have been several studies to determine the effectiveness of using filter feeders to
mitigate eutrophication, but there have been few actual experiments involving real introductions
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and ecosystem responses. Dame and Dankers (1991) measured the uptake and release of
materials by mussel beds in the Western Wadden Sea in Holland and the Eastern Scheldt estuary
in the Netherlands. They found that chlorophyll a, seston (suspended particulates), and
particulate organic carbon were taken up by the mussel bed and ammonium, orthophosphate, and
silicate were released from the mussel bed. This study confirms that mussel beds remove
materials from the water and release materials as feces or pseudofeces to the benthos.
Gren, Lindahl, and Lindqvist (2009) measured the nutrient contents of farmed M. edulis
in the Baltic Sea in Scandinavia and found that 1 kg of live mussels contains about 8.5-12 g of
nitrogen, 0.6-0.8 g of phosphorus and 40-50 g of carbon. This study demonstrates the uptake by
mussels of nitrogen, carbon, and small amounts of phosphorus from the water. In Liverpool,
mussels were introduced to a series of docks in permanently eutrophic waters that occasionally
experienced harmful algal blooms. Allen and Hawkins (1993) found that two years after the
mussels were introduced, the water quality and oxygen content of the water column and
sediment improved.
Dame and Prins (1998) calculated the clearance time (time it takes for all bivalves in a
system to filter particles from a volume of water equal to that of the system) of bivalves and the
turnover rates of water and phytoplankton primary production in 11 coastal and estuarine
ecosystems (Table 3). They found that Carlingford Lough, Chesapeake Bay, and Delaware Bay
have very long bivalve clearance times (480.2, 325, and 1278 days respectively) due to small
filter feeding bivalve populations. According to Newell (1988), the pre-1870 oyster population in
the Chesapeake Bay could filter the entire water column during the summer in 3-6 days. Since
1870, oysters have been heavily exploited and populations have declined. Around 1988 when
Newell wrote this paper, he estimated that the present oyster stocks could filter the bay in 325
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days. These studies show how drastically oyster populations can affect their ecosystem.
Transplant studies should be done in order to measure actual effects of bivalves on eutrophic
systems.
Negative Impacts
In addition to the positive effects that increased water filtration can have on coastal
ecosystems, factors that may have negative impacts must be evaluated. Oysters (and other filter
feeders) change the physical and chemical environment of the ecosystem that can lead to altered
food webs, species interactions, faunal assemblages, sedimentation patterns, and water flow
dynamics (Ruesink 2005). While oyster reefs benefit many organisms, they may also exclude
others through competition (Ruesink 2005). Suspension feeding organisms introduced from other
areas may also be important vectors for invasive species and disease pathogens (Ruesink 2005).
There are many factors to take into account when planning a management strategy to counter
coastal eutrophication.
Shellfish populations in eutrophic waters may be toxic if consumed by humans. When
harmful algal blooms, heavy metals, and other toxins exist in the water column, bivalves will
filter them out and incorporate them into their tissue (MacKenzie et al. 2004). Marine algal
toxins incorporated into shellfish account for at least four toxic syndromes in humans: paralytic
shellfish poisoning, neurotoxic shellfish poisoning, diarrhetic shellfish poisoning, and amnesic
shellfish poisoning (Dolah 2000). In coastal ecosystems, poachers may remove and consume
contaminated shellfish. To reduce the likelihood of this, ecosystems known to contain toxins
should be posted with warnings not to consume shellfish.
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Another concern about introducing non-native shellfish species is that they may carry
disease pathogens that are foreign to native organisms. An Asian oyster Crassostrea ariakensis
has been considered for introduction to Virginia and Maryland for aquaculture purposes (Moss et
al. 2007). Moss et al. (2007) conducted a pathogen survey on C. ariakensis using DNA analyses
and histopathology tests. They found several protistan parasites, two of which are not found in
U.S. waters, several viruses, and cestodes. This study exposes the risks associated with nonnative introductions.
Non-native bivalves introduced to a new area may result in species invasion. The bivalve
itself may become invasive, or species hitchhiking on the bivalves may be released into the new
region and become invasive (Ruesink 2005). The two species that I suggest for oyster restoration
are both native to the east coast. The use of natives prevents any issues that could arise from
invasives. Unfortunately, non-native oysters are being considered for the Chesapeake Bay
because Crassostrea virginica populations have been dwindling since the late 1800s (Moss et al.
2007., Newell 1988) To avoid problems with introducing invasive species, C. virginica
populations should be revived, or other effective native filter feeders such as M. edulis should be
seeded into the bay.
Finally, Dame and Prins (1998) suggest that restoration of oysters to the Chesapeake Bay
may not improve water quality because oysters have been functionally replaced by other bivalves
and deep portions of the bay are not available to the bivalves due to anaerobic conditions. If this
is true and oyster restoration will not improve water quality, then attempts to establish more
bivalve populations will be a waste of money and management effort.
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Conclusions
Bivalves are filter feeders that remove particles and nutrients from the water, which can
help combat eutrophication and promote water clarity. Mytilus edulis and Crassostrea virginica
are especially effective filter feeders as determined by their high filtration rates and particle
retention efficiencies. Both bivalves produce biogenic structures that promote recruitment and
high biodiversity. Other filter feeders such as sponges, tunicates, and cnidarians recruit to
biogenic structures provided by the bivalves and contribute to the filtration of the water.
Dame and Dankers (1991) and Gren, Lindahl, and Lindqvist (2009) showed that mussels
uptake nutrients and carbon from the water and release materials to the benthos. Hawkins (1993)
suggested that introduced mussels cleared eutrophic water near docks in Liverpool. Dame and
Prins (1998) studied the time it takes for bivalves to filter all the water in 11 different coastal
systems. Newell’s calculation of past and present filtering capabilities of Chesapeake Bay
oysters emphasizes the importance of benthic filter feeding on water quality. Possible negative
consequences of combating eutrophication with bivalves include human illness due to
consuming toxic shellfish, introduction of invasive species, vectoring pathogens, and wasting
time and money. There is much more to be learned about bivalves as filter feeders, but there is
promising evidence for an important role in clearing up nutrient enriched waters.
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Filtration Rate
(Rank)
Retention of
Particles >4-5µm
Retention of 2µm
Particles
Crassostrea virginica
(Eastern oyster)
1
100%
50%
Geukensia demissa
(Ribbed mussel)
2
100%
35-75%
Argopecten irradians
(Bay scallop)
2
100%
15%
Brachiodontes exustus
(Scorched mussel)
3
100%
35-75%
Spisula solidissima
(Atlantic surfclam)
3
100%
35-75%
Mercenaria mercenaria
(Northern Quahog)
4
100%
35-75%
Table 1. Data from Riisgard 1988. Relative filtration rate and particle retention efficiency of
six bivalves.
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Table 2. From Gili and Coma 1998. Diets and particle capture rates in carbon units of various
suspension feeders.
Table 3. From Dame and Prins 1998.Water residence time, phytoplankton primary production
turnover, and bivalve clearance time for 11 coastal ecosystems.
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