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The Role of Mercenaria mercenaria and Mytilus edulis in Coupling Benthic and Pelagic Nutrient Cycles

Kelsey A. Ream1 Mentor: Anne Giblin2

1Departments of Biology and Environmental Science, Allegheny College, Meadville, PA 16335 2 The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543

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Abstract

Bivalves have well-documented bioremediation capabilities, however, these capabilities

differ depending on the preferred lifestyle of a given bivalve species. The Blue Mussel, Mytilus

edulis, prefers to live in aggregate colonies, attached to a hard substrate, suspended slightly

above the benthos. The Hard Shelled Clam, Mercenaria mercenaria, buries itself in the

sediments at the bottom of coastal waters. I chose to study these two species in a microcosm

experiment including micrcosms containing Mytilus alone, Mytilus and sediment, Mercenaria

alone, Mercenaria and sediment, and sediment alone over the course of approximately 3 weeks. I

took water samples in order to measure clearance rates of phytoplankton after a feeding, and to

measure changes in nutrient concentrations over time. Mytilus feed more rapidly, respire more,

and have higher daily fluxes of nitrate, ammonium, and phosphate than do Mercenaria. Mytilus

appear to recycle nutrients much faster than Mercenaria, while simultaneously promoting

denitrification. While claims are often made regarding the abilities of bivalves to remove

phytoplankton growth from excess nutrients and prevent eutrophication events, it appears that

these abilities are highly dependent on living habits and other differences between species, and

that further investigation is warranted into the specifics of these differences.

K ey Words: bivalve, Mercenaria mercenaria, Mytilus edulis, nutrient cycling, phytoplankton

clearance, respiration

Introduction

Knowledge of species with bioremediation capabilities is becoming increasingly

necessary in estuarine coastal ecosystems. These systems tend to be overloaded with nutrients

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from waste water inputs, septic plumes, precipitation, fertilizer usage, and other anthropogenic

sources (Valiela et al., 1997). Understanding the capabilities of native species to affect nutrient

levels in the water column can be important for ensuring proper protection and valuation of such

species.

One group species with well-studied bioremediation capabilities is bivalves. Bivalve

species in the North Atlantic are highly varied in their habitat preferences and living habits.

Some species, such as the Hard Shell Clam (Mercenaria mercenaria), prefer to bury themselves

in the sediments of coastal and estuarine environments (Lorio & Malone, 1995). Others like the

Blue Mussel (Mytilus edulis) prefer to form aggregates on a solid substrate, suspending

themselves above the sediment (Bayne, 1964). Both species are filter-feeders, removing

phytoplankton and suspended organic matter from the water column as a food source. Both

species excrete digested matter onto the sediment in the form of inorganic nutrient-rich

biodeposits.

By removing nutrient-rich phytoplankton and other suspended organic material from the

water column while depositing fecal matter in the sediment and stimulating microbial activity

there, bivalves serve as an important link in the nutrient cycles of both of these locations within

an ecosystem. The ability of these animals to stimulate denitrification through their additions of

inorganic nitrogen to the sediments is relatively well known (Cornwell, Kemp, & Kana, 1999).

Bivalves are particularly good at stabilizing ecosystem nutrient fluxes because of the fact that

they turn over very slowly (compared to phytoplankton) (Dame, 1996). Their slow turnover rate

means that a large bivalve population represents a substantial nutrient-sink in coastal ecosystems.

Bivalve growth is limited by food availability, and bivalves can continue to filter feed

increasingly rapidly as their food sources become increasingly available. Their ability to exert

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top-down control on phytoplankton populations, thereby helping to control algal blooms, has

often been noted. M. mercenaria, in particular, has a well known ability to exert such control

(Officer, Smayda, & Mann, 1982); (Dame, 1996) during eutrophication events that can

sometimes result in hypoxia (Altieri, 2008). M. edulis appears to be less tolerant in that it

decreases in survivorship under hypoxic conditions (Altieri, 2008). While both of these

organisms play a role in how coastal ecosystems respond to nutrient loading, the unique

lifestyles of M. mercenaria and M. edulis cause important differences in the way they influence

nutrient cycling.

The purpose of this experiment was to investigate how bivalve lifestyle habits (burrowing

vs. non-burrowing) affect cycling of ammonium, nitrate, and phosphate in the water column and

sediment of experimental microcosms over the course of approximately 3 weeks.

Methods

Microcosm Set-up

My experiment included five treatments, Mytilus alone, Mytilus and sediment,

Mercenaria alone, Mercenaria and sediment, and sediment alone, with three replicates of each

treatment. I set up fifteen cylindrical plastic cores to contain the various treatments, each of

which was kept in an incubation tank filled with water at approximately 20°C to maintain a

constant temperature within the microcosms themselves. Each microcosm was sealed to prevent

movement of water, and outfitted with a magnetic stir bar to maintain water circulation.

Sediment for use in the microcosms was collected at the Sippewissett Marsh in Falmouth MA.

Microcosms containing sediment were filled with sediment to a height of approximately 10 cm.

All microcosms were filled with seawater.

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Using a living biomass: living biomass+shell ratio, I determined that one Mercenaria

weighing approximately 64 g had a similar biomass to 13 small mussels weighing approximately

42 g. Therefore, in order to achieve approximately equal living biomasses (not including the

shell) in all microcosms containing animals, microcosms containing Mercenaria contained 1

clam, while microcosms containing Mytilus contained 13 mussels. I recorded exact weights of

the bivalve biomass in each microcosm both before the start of the experiment, and after the end

of the experiment. Mytilus were added to the microcosms in mesh bags with large holes in order

to provide them with some substrate to aggregate on while still allowing them to move around

the microcosm. Mercenaria were added to the microcosms without any confinement in order to

allow them to bury themselves in the sediment, where possible. In between periods of water

sampling, the microcosms were bubbled with air-stones continuously in order to ensure proper

oxygenation.

Phytoplankton Clearance

I added a phytoplankton paste, Reed Mariculture Algae Paste Shellfish Diet 1800, to each

microcosm 3 times throughout the course of the experiment in order to provide nutrition for the

clams and measured clearance rates of phytoplankton from the water column. Enough paste was

added to achieve an approximate concentration of 400 cells/ µL in the water column of each

microcosm, and the water in each microcosm was stirred continuously using magnetic stirrers

throughout the duration of water sampling. During the first two feeding trials, water was sampled

0, 15, 30, and 60 minutes after the paste was added to the microcosms. During the third feeding

trial, water was sampled 0, 125, 240, 380, 450, and 480 minutes after the paste was added. Water

samples were analyzed for in-vivo fluorescence using a Turner 450 Fluorometer.

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Nutrient Analysis and Respiration Measurements

In order to measure differences in respiration between Mercenaria, Mytilus, and the

sediment, I sealed each of the microcosms to prevent oxygen exchange with the atmosphere and

took approximately five water samples over the course of the dissolved oxygen (DO)

concentration in the microcosms dropping 2 ppm. DO was measured a WTW oxygen electrode

system. Water samples were then analyzed for nitrate using an automated Lachat Flow Injection

Analyzer and a variation of the method by (Wood, Armstrong, & Richards, 1967), phosphate

using a Shimadzu Spectrophotometer and an adaptation of(Murphy & Riley, 1962), and

ammonium using a Shimadzu spectrophotometer and an adaptation of (Solorzano, 1969).

Sediment Analysis

After the completion of the experiment, sediment samples were taken from the top 2 cm

of the sediment layer in each microcosm containing sediment. Sediments were then dried,

acidified to remove carbonates, and analyzed for carbon and nitrogen using a Perkin-Elmer

Series 2 CHNS/O Analyzer 2400.

Results

I weighed the bivalves in each microcosm to determine whether they gained or lost

weight over the course of the experiment. Microcosms containing mussels lost, on average,

approximately 1 g of living biomass per microcosm (8.4 % of the total microcosm biomass),

while microcosms containing clams lost an average of 0.033 g per microcosm (0.35% of the total

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microcosm biomass) (Figure 1). There were significant differences between the amount of

weight lost by each type of bivalve in sediment and alone. Mercenaria alone lost more weight

than Mercenaria in sediment, while Mytilus in sediment lost more weight than Mytilus alone

(Table 1). No clams died over the course of the experiment, and while 5 mussels total died, no

more than two died in anyone microcosm.

Phytoplankton Clearance

In microcosms containing Mytilus only and Mytilus and sediment, phytoplankton was

removed from the water column at approximately the same rate. In all 6 microcosms containing

Mytilus, the concentration of phytoplankton cells decreased by approximately 0.084% of the

original concentration per hour. Microcosms containing Mercenaria and sediment removed

phytoplankton more slowly; the original concentration decreased by 0.06% per hour.

Microcosms containing Mercenaria only removed phytoplankton much more slowly, at a rate of

only 0.024% of the original concentration removed per hour. Phytoplankton concentrations

decreased in the sediment at the slowest rate; 0.0066% of the original concentration settled out of

solution per hour (Figure 2).

Nutrient Analysis and Respiration Measurements

Microcosms containing Mytilus respired nearly twice as quickly as microcosms

containing Mercenaria, with microcosms containing Mytilus and sediment respiring slightly

faster than those containing Mytilus alone. Microcosms containing Mercenaria and sediment

actually respired slightly more slowly than those containing Mercenaria alone, however in both

cases, the differences between microcosms containing bivalves alone and bivalves and sediment

were not significant. The Sediment alone respired at the lowest rate of all the microcosms, and

was significantly less than microcosms containing both Mercenaria and Mytilus (Figure 3).

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When respiration rates were normalized by grams of living biomass per microcosm, the same

trends were apparent (Figure 4). Subtracting the average respiration rate of microcosms

containing Sediment only from respiration rates of microcosms containing sediment and bivalves

shows that Mytilus in microcosms containing sediment respire slightly more quickly than those

in microcosms containing more sediment. In contrast, Mercenaria in microcosms containing

sediment respire slightly more slowly than those in microcosms containing no sediment (Figure

5), however, the difference was not significant.

Ammonia fluxes were higher in microcosms containing Mytilus only than in microcosms

containing Mytilus and sediment, while they were slightly higher in microcosms containing

Mercenaria and sediment than in microcosms containing Mercenaria only. Nitrate fluxes were

similar in all microcosms containing Mytilus, and similar in all microcosms containing

Mercenaria, but were considerably higher in microcosms containing Mytilus than in microcosms

containing Mercenaria. Both ammonium and nitrate fluxes were lowest in microcosms

containing Sediment only. Microcosms containing Mytilus typically had higher fluxes of

dissolved inorganic nitrogen (DIN) than microcosms containing Mercenaria, which in turn had

higher fluxes than microcosms containing sediment alone. Additionally, DIN fluxes were most

variable in microcosms containing Mytilus and sediment (Figure 6). Phosphate fluxes followed a

similar trend to that of the nitrate fluxes in that they were similar in all microcosms containing

Mytilus, similar in all microcosms containing Mercenaria, and significantly higher in

microcosms containing Mytilus than in microcosms containing Mercenaria. Microcosms

containing sediment only had the lowest phosphate fluxes (Figure 7). When nutrient fluxes were

normalized by grams living biomass per microcosm, the same trends were still apparent (Figure

8).

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In order to examine how the presence of the sediment affects nutrient fluxes, I subtracted

the average nutrient fluxes of the sediment from the nutrient fluxes of microcosms containing

sediment and animals. In comparing microcosms containing Mytilus, those containing sediment

had significantly higher ammonium fluxes than those without sediment. While nitrate

concentrations were slightly higher in microcosms containing sediment than microcosms

containing Mytilus only, there were no other significant differences in nutrient fluxes between

Mytilus only and Mytilus and sediment. In microcosms containing Mercenaria, there were no

significant differences between those that contained sediment and those that did not. Ammonia,

however, was slightly higher in microcosms containing Mercenaria and sediment, while

phosphate and nitrate fluxes were lower in microcosms containing Mercenaria and sediment

than in microcosms containing only Mercenaria (Figure 9).

Dissolved inorganic nitrogen: phosphorus (N:P) flux ratios were not significantly

different from eachother in any of the 5 treatments, however, were all relatively close to a ratio

of 16:1, with microcosms containing Mytilus and sediment alone having the lowest N:P ratio,

and microcosms containing Mercenaria and sediment having the highest (Figure 10). Oxygen

(O2): nitrogen ratios (O:N) were not significantly different in microcosms containing Mytilus and

sediment, Mercenaria only, Mercenaria and sediment, and Sediment only. Microcosms

containing Mytilus only had significantly lower O:N ratios than any other treatments (Figure 11).

Sediment Analysis

CHN analysis of sediment from each type of microcosm did not yield any significantly

different results. Sediment from microcosms containing Mytilus and Mercenaria did, however,

have a slightly a slightly higher percentage of nitrogen than sediment from microcosms

containing sediment only (Figure 12). Percentage of carbon also did not differ significantly

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between the 3 types of microcosms, however, it was slightly higher in microcosms containing

Mercenaria than in microcosms containing either Mytilus or sediment alone (Figure 13).

Discussion

Over the course of my experiment, the weight of the total living biomass in each

microcosm decreased, with significant differences between microcosms containing bivalves

alone and bivalves in sediment (Figures 1, Table 1). Microcosms containing Mytilus lost, on

average, much more weight than microcosms containing Mercenaria (Figure 2) due to the fact

that several Mytilus died over the course of the experiment, while no Mercenaria died. Mytilus

are known to be sensitive to hypoxic conditions (Altieri & Witman, 2006), so this result is to be

expected. The more extensive weight loss in microcosms containing Mytilus and sediment may

Tbe related to stress from higher nutrient concentrations, such as ammonium, in microcosms

with this treatment type. It is likely that Mercenaria may have lost weight for other reasons;

because they appeared to be respiring more and feeding less, it is likely that they were somewhat

more stressed than the Mytilus. Mercenaria in microcosms without sediment were particularly

stressed, undoubtedly because living in an exposed environment without being buried in

sediment is unnatural, and something that would not occur in the wild. Additionally, it is possible

that I did not feed the bivalves enough over the course of the experiment; because the Mytilus

used for the experiment were younger than the Mercenaria, a lack of proper nutrition may

explain why they were more sensitive, lost more weight, and had more deaths.

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Phytoplankton clearance

Mytilus appeared to filter phytoplankton from the water column much more rapidly than

Mercenaria did (Figure 2). This could have occurred for several reasons, one of which is that the

mussels are situated directly in the water column itself, and therefore were much closer to the

location at which phytoplankton additions were made during the feeding trials (the top of the

microcosm). Because of this difference, mussels would have been able to remove phytoplankton

from the water as soon as it was added, while Mercenaria would have had to wait until it settled

towards the bottom of the water column, potentially creating a delay in the Mercenaria

measured feeding rates. In addition, it is also possible that the Mytilus were simply more

metabolically active within the microcosms, causing them to feed more. Similarly, Mercenaria

may have fed less because they were less metabolically active. In particular, it is also possible

that Mercenaria in microcosms without sediment were so stressed to be exposed in open water

that they did not wish to expend energy in order to feed. These observations are particularly

interesting in that mussels are not as well documented as clams in their ability to control

eutrophication events, despite the fact that it appears that mussel populations may be more

effective at controlling algal blooms under certain conditions. It is important, however, to

consider this in conjunction with their limited ability to tolerate hypoxic conditions(Altieri &

Witman, 2006). Phytoplankton concentrations decreased at the slowest rate in microcosms

containing sediment only. This rate of decrease represented the rate at which phytoplankton were

falling out of solution and settling to the bottom of the microcosm.

Respiration Measurements

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Mytilus respired significantly more than Mercenaria, regardless of whether the

microcosm housing the bivalves had sediment or not (Figures 3 and 4). This, in addition to the

phytoplankton clearance rates, alludes to the idea that Mytilus were much more metabolically

active than Mercenaria. Mytilus in sediment did tend to respire slightly more than Mytilus alone

(Figure 5) and although this trend was not significant, it is logical that stressed mussels that lost a

large amount of weight over the course of the experiment would have been respiring more than

the less stressed Mytilus in microcosms without sediment. A similar trend exists in Mercenaria;

microcosms containing Mercenaria alone respired slightly more than microcosms with

Mercenaria and sediment (Figure 5). While the trend is not significant, it correlates to the trend

of higher weight loss in microcosms containing bivalves that were stressed.

Nutrient Analysis

The lower dissolved inorganic nitrogen (DIN) fluxes in microcosms containing Mytilus

and sediment as compared with microcosms containing Mytilus (Figure 6) only indicate that

some coupled nitrification/ denitrification of the DIN in Mytilus waste may be occurring. It is

likely that some ammonium is being converted to nitrate via nitrification, however, some nitrate

must be lost via denitrification in anaerobic areas of the sediment in order to account for the

lower DIN flux in microcosms containing Mytilus and sediment. The tendency of mussels to

attach themselves to a hard substrate above the sediment, rather than burrowing into it, is likely

conducive to the anaerobic sediment conditions that facilitate denitrification. The burrowing

habits of Mercenaria appear to be less conducive to denitrification. As Mercenaria dig their way

into the sediment, they may allow oxygen to enter the sediment where it previously would have

been excluded, effectively slowing denitrification. While some denitrification may have occurred

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in microcosms containing Mercenaria and sediment, it probably played a less significant role

than in microcosms with Mytilus and sediment. It is likely that nitrification and ammonification

were the more prominent processes in microcosms containing Mercenaria and sediment. This is

particularly likely given the lower rates of phytoplankton consumption and respiration rates in

microcosms containing Mercenaria. Decomposing phytoplankton that were not consumed by the

less metabolically active Mercenaria may have provided a large base of organic N for

ammonification. Finally, DIN fluxes were lowest in microcosms containing sediment only. This

fits with expectation as sediment alone should not produce nearly as much ammonium and

nitrate as metabolically active animals. Additionally, it should be noted that nutrient fluxes in

microcosms containing Mytilus were usually the most variable and therefore had the largest error

bars. This is likely due to the fact that there were a larger number of individual animals in

microcosms containing Mytilus introducing a larger amount of variability.

Phosphate fluxes were significantly higher in microcosms containing Mytilus than in

microcosms containing Mercenaria, while sediment alone had slightly lower phosphate fluxes

than microcosms containing Mercenaria, but not significantly so (Figure 7). Mytilus phosphate

production is likely due to the increased metabolic activity and corresponding waste production

of these bivalves.

N:P ratios all fluctuated around 16:1 (Figure 10), which is the expected Redfield ratio of

marine aquatic environments(Schlesinger, 1997). The ratio is lowest in microcosms with Mytilus

and sediment because of their high DIN fluxes, and lowest in microcosms with Mercenaria and

sediment because of the low DIN fluxes in those microcosms. The closeness to the expected ratio

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serves to validate (to a degree) the phosphate and DIN fluxes I observed. O:N ratios indicated

that were excreting the most inorganic nitrogen per unit respiration and thereby facilitating

denitrification in the presence of sediment.

Sediment Analysis

There were no significant differences in percent nitrogen in sediment between any

treatments, however, the percentage of nitrogen was slightly higher in microcosms containing

bivalves than in microcosms containing sediment (Figure 12). This is because animals inevitably

produce much more waste than bacteria respiring in microcosms containing sediment only. The

percent carbon in sediments was also not significantly different between microcosms containing

bivalves, however, the percentage of sediment C was higher in microcosms containing

Mercenaria than in microcosms containing Mytilus (Figure 13). This is likely due to the fact that

Mercenaria were buried in sediment and depositing any organic material in waste more directly

into the sediment than Mytilus in the water column. More significant differences in % soil C may

have been observable in sediment collected from depths greater than 2 cm as clams have more of

an influence at greater depths than do mussels.

Conclusions

Mercenaria appears to affect benthic nutrient cycling more than pelagic. Hard-shelled

clams seem to become stressed in the absence of sediment as this is a solely experimental

condition that are unlikely to occur in natural settings. Clams appear to add some organic matter

to the sediment, although this requires further exploration of carbon contents of sediments at

various depths to determine to what depth and extent Mercenaria supplement the sediment. In

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my experiment, Mercenaria did not promote much denitrification in the sediment and appeared

to be slower at recycling nutrients than Mytilus. The Blue Mussel seems to affect pelagic

nutrient cycling much more than it does benthic. The presence or absence of sediment seems to

have a small effect on the feeding rates of Mytilus, as compared to its great importance for

Mercenaria, and, in general, Mytilus seemed to be more metabolically active per unit biomass

than Mercenaria. Additionally, Mytilus appeared to promote denitrification in the presence of

sediment.

The ability of bivalves to remove nutrients from overloaded estuarine systems is often

touted as a tragically under-utilized way to prevent eutrophication, however, my experiment

suggests that bivalves may accelerate nutrient cycling on short time scales. The comparison of

mussels and clams is especially interesting in that Mytilus increase DIN fluxes more than

mercenaria, but also promote denitrification more than Mercenaria. More research is certainly

warranted in order to compare M. mercenaria and M. edulis to other bivalve species in order to

understand the range of denitrification capabilities of bivalve species in nutrient overloaded

coastal areas such as Cape Cod.

Acknowledgements

This project would not have been possible without the guidance of my mentor Dr. Anne

TAs Rich McHorney, Carrie Harris, Stef Strebel, and Laura Van der Pol, and the clams, mussels,

shellfish paste food, and advice from Emma Green-Beach

Marine Resource Center.

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L iterature C ited  

Altieri, A. H. (2008). Dead zones enhance key fisheries species by providing predation refuge.

Ecology, 89, 2808-2818.

Altieri, A. H., & Witman, J. D. (2006). Local extinction of a foundation species in a hypoxic

estuary: Integrating individuals to ecosystem. Ecology, 87(3), 717-730.

Bayne, B. L. (1964). Primary and secondary settlement in mytilus edulis L.(mollusca). The

Journal of Animal Ecology, 33(3), 513-523.

Cornwell, J. C., Kemp, W. M., & Kana, T. M. (1999). Denitrification in coastal ecosystems:

Methods, environmental controls, and ecosystem level controls, a review. Aquatic Ecology,

33(1), 41-54.

Dame, R. F. (1996). Ecology of marine bivalves: An ecosystem approach. Boca Raton, FL.: CRC

Press Inc.

Lorio, W. J., & Malone, S. (1995). Biology and culture of the northern quahog clam (mercenaria

mercenaria). Southern Regional Aquaculture Center, SRAC Publication No.433

Murphy, J., & Riley, J. (1962). A modified single solution method for the determination of

phosphate in natural waters. Analytica Chimica Acta, 27, 31-36.

Officer, C. B., Smayda, T. J., & Mann, R. (1982). Benthic filter feeding: A natural eutrophication

control. Marine Ecology Progress Series, 9, 203-210.

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Schlesinger, W. H. (1997). Biogeochemistry: An analysis of global change. Geochimica Et

Cosmochimica Acta, 56(2)

Solorzano, L. (1969). Determination of ammonia in natural waters by the phenolhypochlorite

method. Limnology and Oceanography, 14(5), 799-801.

Valiela, I., Collins, G., Kremer, J., Lajtha, K., Geist, M., Seely, B., . . . Sham, C. (1997).

Nitrogen loading from coastal watersheds to receiving estuaries: New method and

application. Ecological Applications, 7(2), 358-380.

Wood, E., Armstrong, F., & Richards, F. (1967). Determination of nitrate in sea water by

cadmium-copper reduction to nitrite. J.Mar.Biol.Assoc.UK , 47(1), 23-31.

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1A 2A 3A 1B 2B 3B 1C 2C 3C 1D 2D 3D Average  -­‐

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Average  -­‐ Clams

Weight  (g)

Pre-­‐ExperimentPost-­‐Experiment

Mussels Clams

Average          AverageMussels            Clams

Figure 1 contained Mytilus Mytilus Mercenaria only, and microcosms

Mercenaria and sediment.

Appendix. Ream 18

10%

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100%

0 100 200 300 400 500 600

%  Orig

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centratio

n

Time  (minutes)Figure 2. Clearance rates of phytoplankton cells from the water column in microcosms containing Mytilus only, Mytilus and sediment, Mercenariaonly, Mercenaria and sediment, and Sediment only. Phytoplankton shown as percent of original concentration (cells/ uL) remaining. The Legend displays the slope of the line of best fit for each microcosm type.

%  Original  Concentration  Lost/  Minute

Mytilus Only -­‐0.0014

Mytilus+Sediment -­‐0.0014

Mercenaria Only -­‐0.0004

Mercenaria+Sediment -­‐0.001

Sediment Only -­‐0.00011

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Figure 3. Oxygen fluxes of microcosms containing Mytilus only, Mytilus and sediment, Mercenaria only, Mercenaria and sediment, and Sediment only.

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ass  -­‐

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Figure 4. Oxygen fluxes of microcosms containing Mytilus only, Mytilus and sediment, Mercenaria only, Mercenaria and sediment, and Sediment only normalized by grams of living biomass per microcosm.

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Oxygen  Flux  (

moles  m

-­‐2   day  -­‐1

)Microcosm  Type

Figure 5. Effect of the presence of sediment on oxygen fluxes of Mytilus and Mercenaria. The Mytilus only (-sediment) and Mercenariaonly (-sediment) bars represent the oxygen fluxes of microcosms containing bivalves and sediment with the average oxygen flux of microcosms containing only sediment subtracted.

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moles  m

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y  -­‐1)

Microcosm  Type

Nitrate

Ammonium

Figure 6. Total dissolved inorganic nitrogen fluxes in microcosms containing Mytilus only, Mytilus and sediment, Mercenaria only, Mercenaria and sediment, and Sediment only.

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Figure 7. Total phosphate fluxes in microcosms containing Mytilus only, Mytilus and sediment, Mercenaria only, Mercenaria and sediment, and Sediment only.

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Figure 8. Nutrient fluxes normalized by grams living biomass per microcosm in microcosms containing Mytilus only, Mytilus and sediment, Mercenaria only, and Mercenaria and sediment.

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-­‐2da

y  -­‐1)  

Microcosm  Type

AmmoniumNitratePhosphate

Figure 9. Effect of the presence of sediment on nutrient fluxes of Mytilus and Mercenaria. The Mytilus only (-sediment) and Mercenariaonly (-sediment) bars represent the nutrient fluxes of microcosms containing bivalves and sediment with the average nutrient flux of microcosms containing only sediment subtracted.

Ream 25

0

5

10

15

20

25

Mytilus  Only Mytilus  +  Sediment Mercenaria  Only Mercenaria  +  Sediment

Sediment  Only

N:P  Ratio

Microcosm  Type

Figure 10. Total dissolved inorganic nitrogen: phosphorus flux ratios in microcosms containing Mytilus only, Mytilus and sediment, Mercenaria only, Mercenaria and sediment, and Sediment only.

Ream 26

0

5

10

15

20

25

Mytilus  Only Mytilus  +  Sediment Mercenaria  Only Mercenaria  +  Sediment

Sediment  Only

O:N  Ratio

Microcosm  Type

Figure 11. Oxygen: total dissolved inorganic nitrogen flux ratios of microcosms containing Mytilus only, Mytilus and sediment, Mercenaria only, Mercenaria and sediment, and Sediment only.

Ream 27

0

0.005

0.01

0.015

0.02

0.025

Mytilus   Mercenaria Sediment  Only

%  Nitrogen

Microcosm  Type

Figure 12. Percentage of nitrogen in sediment samples from microcosms containing Mytilus and sediment, Mercenaria and sediment, and Sediment only.

Ream 28

0

0.05

0.1

0.15

0.2

0.25

Mytilus   Mercenaria Sediment  Only

%  Carbo

n

Microcosm  TypeFigure 13. Percentage of carbon in sediment samples from microcosms containing Mytilus and sediment, Mercenaria and sediment, and Sediment only.

Ream 29

Weight Change (g)

Mercenaria Only 0.05 0.0201 (SE)

Mercenaria+Sediment 0.02 0.0039 (SE)

Mytilus Only 0.59 0.5340 (SE)

Mytilus+Sediment 1.59 0.2152 (SE)

Table 1. Change in living biomass over the course of the experiment in all microcosms containing Mercenaria and Mytilus.

Ream 30