The impact of oysters on the fate of nitrogen inputs to ...Oysters Oysters were obtained with the...
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The impact of oysters on the fate of nitrogen inputs to estuarine sediments
Ruby An
The University of Chicago
Chicago, IL 60637
Advisor: Anne Giblin
The Ecosystems Center
Marine Biological Laboratory
Woods Hole, MA 02543
Semester in Environmental Science
Class of 2015
ABSTRACT
Nitrogen pollution from anthropogenic sources is a major threat facing estuaries in the Falmouth
area with severe ecological and economic consequences. Oyster aquaculture is a novel water
quality management strategy that could potentially be implemented in Falmouth estuaries.
Oysters can play an important role in estuarine nitrogen cycle processes by consuming
phytoplankton blooms stimulated by anthropogenic nitrogen inputs. Some studies suggest oysters
may also increase the denitrification potential of estuarine sediments; however, this is
complicated by the ecosystem-level affects of eutrophication on oxygen and nitrate
concentrations. To investigate how oysters alter estuarine nitrogen cycling in the context of
historical eutrophication, I collected sediment cores from two estuaries in the Falmouth area,
Little Pond (highly eutrophic) and Waquoit Bay (less eutrophic). I cultured N15 labeled
phytoplankton and added it to sediment cores with and without oysters from each site. I measured
oxygen, ammonium, and nitrate concentrations, calculated fluxes over a 15-day incubation
period, and used stable isotope analysis to track nitrogen fate. Results indicate that oysters
incorporate about 40% of nitrogen inputs into their tissues and decrease the relative amount of
nitrogen that ends up in sediments by over 50%. Within the timeframe of this project, oysters did
not increase denitrification potential or significantly alter final nitrogen concentrations in the
water column. A discrepancy in initial oxygen exposure likely neutralized short-term differences
in site response; however, overall data suggests that biogeochemical context determined by site
history, in particular oxygen availability, plays a critical role in determining the impact of oyster
aquaculture on estuarine nitrogen cycling.
Key words: oysters, Falmouth estuaries, sediments, nitrogen cycle, stable isotope analysis
INTRODUCTION
Nutrient pollution from anthropogenic sources is a major threat facing estuaries in the
Falmouth area and worldwide. Excess anthropogenic nitrogen entering coastal watersheds from
sources such as septic wastewater and lawn fertilizers stimulates large algal blooms in historically
N-limited estuarine ecosystems. Algal blooms shade out native eelgrass beds that serve as
important habitats for many ecologically and economically valuable species; furthermore, as algal
blooms decompose, they can cause severe hypoxic events that suffocate benthic organisms and
lead to massive fish kills. The widespread degradation of ecosystem services that results from
eutrophic conditions has negative consequences on an ecosystem-level that includes the local
Falmouth community.
Oyster (Crassotrea virginica) aquaculture as a water quality management strategy has
been employed at locations including New York Harbor, Chesepeake Bay, and as of 2013 at
Little Pond, Falmouth, MA (Billion Oyster Project, Oyster Recovery Project, Little Pond
Shellfish Demonstration Project). Oysters are extremely effective filter feeders and can feed on
algae blooms stimulated by excess nitrogen pollution (Team, 2007). While the extent to which
oyster aquaculture can save our estuaries is a controversial subject (Pomeroy et al. 2007), oysters
play an important role in many estuarine nitrogen cycle processes. Oysters incorporate nitrogen
from algae they consume into their own tissues, deposit it to the sediment as feces or
pseudofeces, or excrete it to the water column as ammonia (Fig. 1). Some studies suggest that
oysters not only remove nitrogen through phytoplankton consumption and incorporation into
their own tissues, but may also increase the denitrification potential of estuarine sediments thus
increasing loss of nitrogen from estuaries as N2 gas (Kellog et al. 2013). This possible
relationship is complicated by the fact that microbial denitrification is tightly coupled to
nitrification (Jenkins and Kemp, 1984), a process that requires sufficiently aerobic conditions
uncommon in eutrophic estuaries.
In this context, the goal of my project was to investigate how the presence of oysters alters
the fate of anthropogenic nitrogen inputs to estuarine sediments. I examined the estuarine
nitrogen cycle by quantifying oxygen, nitrate and ammonium fluxes and tracking the relative fate
of N15 labeled organic matter inputs to sediment cores incubated for 15 days. I used stable
isotope analysis to determine whether nitrogen inputs ended up in the sediment, water column,
incorporated into oyster tissue, or denitrified as N2 gas. I collected sediment cores from two
estuaries in the Falmouth area that historically experience different levels of nitrogen pollution to
determine whether the history of eutrophication altered estuarine sediment responses to oyster
aquaculture.
Site Background
I collected sediment cores from Little Pond and Waquoit Bay. Situated in a highly developed
watershed, Little Pond experiences substantial nitrogen runoff from downtown Falmouth,
Falmouth mall, and surrounding residential homes. As a national research reserve located in a
less-developed watershed, Waquoit Bay experiences less nitrogen runoff and organic matter
loading than Little Pond but has still suffered from the effects of eutrophication (Bowen et al.
2001). In her 2015 SES Project, Em Stone found that Little Pond sediment was 24.2% organic
matter by weight and Waquoit Bay sediment was 15.9% organic matter by weight. Both these
sites are locations where oyster aquaculture occurs. Caged oysters were deployed in Little Pond
beginning in 2013 as part of the Little Pond Shellfish Demonstration project with the goal of
improving water quality. Waquoit Bay is home to Washburn Island Oysters, a commercial
aquaculture business. Little Pond cores were collected on 11/15 from the middle of the pond near
Narragansett Street on (Fig. 2 – left). Waquoit Bay cores were collected three days later on 11/18
(due to 11/15 equipment malfunction) from the center of the head of the bay (Fig. 2 – right).
METHODS
Experimental Design
We collected six sediment cores per site and subjected them to three treatments (two cores
per treatment): control (CTRL), N15 organic matter addition (OM), and N15 organic matter
addition with oysters (OM+OYST). We used sediment core tubes that were 9.5 cm in diameter
and 30 cm in height. During collection, we aimed for a sediment height of 10 cm. Post-collection,
cores were incubated in room-temperature seawater (16-20 °C), bubbled with air to maintain
oxygen levels, and kept well mixed with magnetic stir bars (Fig. S2). Water level was maintained
at about 2 cm below the top of the core tube, except during oxygen flux measurements.
Treatments and sampling consisted of initial flux measurements followed by a 15-day
incubation period. Once all cores were collected, the overlying water in each core was drained
and replaced with 25 mm GF/F filtered site water (collected at the same location as the cores).
This allowed us to measure initial oxygen, nitrate, and ammonium fluxes from the sediment.
After these measurements, oysters were placed in each of the OM+OYST cores. Core treatments
and subsequent sampling occurred over the next 15 days. N15 organic matter was added daily to
both OM and OM+OYST cores on days 1-6. On day 7, oysters were removed and all cores were
closed up for oxygen and N2 flux measurements (Post OM+ flux). Subsequently, all cores were
incubated under the same conditions until day 15 on which a final oxygen and N2 flux was
performed.
N15 Algae Aquaculture
For the N15 organic matter additions, we cultured 22 L of the microalgae Isochyysis
galbana (T.Iso) in F/2 growth media enriched approximately 20% with additional 15N. To do so,
40 mL of 100 mM 15NO3 was added to 18 L of F/2 media and inoculated with about 4L of an
existing T.Iso culture (>3 million cells/mL). This culture was placed under constant light for six
days prior to the start of experimental treatments.
The OM addition process required a concentration of algal cells to avoid adding large
amounts of N15 from the growth media, which would obscure the N15 signal from the organic
matter. Since T.Iso cells were too small to be filtered (4-6 µm), 50 mL Falcon tubes were used to
centrifuge down 45 mL of algal mixture at a time and excess liquid poured off, resulting in a
pellet of organic matter. This process was repeated three times, such that each tube contained the
concentrated algae from 4 x 45 mL of algal mixture. To add to the cores, this algal pellet was
frozen for 30 minutes to kill algal cells, thawed, resuspended and rinsed out with about 10 mL of
site water. Two tubes, the equivalent of 360 mL of algal mixture, were added per core per day.
This was roughly calculated as the amount of organic matter equivalent to 3% the dry weight of
the oysters. Since algal cell concentration likely varied within the culture over the six day period,
an additional two tubes each containing 45mL of algal mixture were centrifuged and frozen per
day to track the total amount of N15 added.
Oysters
Oysters were obtained with the help of Falmouth resident Sia Karplus from an oyster reef
in West Falmouth Harbor near Chappaquoit bridge. Sia Karplus started the reef from oyster spat
on shells as part of ongoing efforts by the town of Falmouth to improve water quality. These
oysters were collected 10 days prior to the start of experimental treatments. In the interim, oysters
were kept in running seawater and fed a mixture of diatoms and flagellates. Three oysters of
equal size distribution were added to each OM+OYSTR core in hanging baskets constructed from
plastic netting to simulate caged oyster aquaculture (Fig. S1). After oysters were taken out of the
cores on day 7, they were fed non-enriched phytoplankton several times to clear their guts before
they were frozen on day 9.
Sampling
To measure oxygen fluxes, tubing and air stones for core aeration were removed and the
cores closed with specialized tops to prevent air from entering the core water. Oxygen
concentrations were measured using WTW probes and recorded in each core every couple hours
until the concentration dropped below 5 mg O2/L. Time intervals varied, as some cores took
overnight to reach desired O2 concentrations. After initial oxygen flux measurements, core tops
were removed and aeration resumed. After the day 7 oxygen flux (Post OM+), core tops were left
on with some headspace and aeration resumed.
To measure nitrate and ammonium fluxes, water samples were collected and filtered using
25 mm GF/F swinnex filters and frozen for later analysis. The total liquid volume removed from
each core (~33 mL per sample due to rinsing) was replaced with 25 mm GF/F filtered site water.
Water samples for initial NH4 and NO3 fluxes were taken at four time points over the same time
period as the initial oxygen flux measurements. Water samples for NH4 and NO3 fluxes over the
15-day treatment incubation were taken daily (with the exception of days 8-10 over Thanksgiving
break).
Following the 15-day incubation period, we analyzed the oysters, sediment, core water
samples, and N2 water samples for N15 enrichment. Oysters were unfrozen and shucked. Oyster
soft tissue was combined per core, cut up, oven dried overnight, and ground to a powder. The top
4 cm of sediment from each core was sectioned off into metal tins, mixed, air-dried overnight and
by oven, and homogenize by hammering in Ziploc bags. Wet and dry weights were recorded for
both oysters and sediments. 100 mL GFF filtered water samples were taken on day 7 and day 15
to determine N15 enrichment of dissolved inorganic nitrogen. These water samples were
combined per core. Devarda’s alloy was used to perform ammonium diffusions. We took water
samples for N2 concentration and N15 enrichment at the end of the day 7 and day 15 oxygen flux
measurements. These samples were stored underwater in the same incubating bath as the cores to
maintain temperature. To measure N2 enrichment, we used a mass inlet membrane spectrometry
(MIMS). OM addition aliquots (12 tubes) used to measure N15 additions were combined into two
replicates of total addition over days 1-6 and analyzed for N15 content.
Nutrient and Data Analysis
Nitrate concentrations were determined using the Lachat method adapted from Wood et
al. 1967. Ammonium concentrations were determined using a modification of the method from
Strickland and Parsons (1972) based on the phenol-hypochloric method from Solarazano (1969).
Ammonium samples for days 1-15 were diluted 5:1. Core water volume calculated from water
height and core diameter was used to convert concentrations to molar amounts.
I used R to perform all data processing and statistical analysis. Nutrient fluxes were
calculated as a linear regression of collected concentration data and times. Fluxes with R2 values
less than 0.6 were omitted from further analysis. Week 1 nitrate and ammonium fluxes were
calculated using concentrations from days 1-6. Week 2 fluxes were calculated using
concentrations from days 12-15. Factorial design ANOVA was used to determine the statistical
significance of flux differences by treatment, site, and treatment:site interaction. N15 enrichment
above natural levels for sediment and water samples was calculated using the controls as
reference. Background oyster N15 levels were taken from previous SES data (Waquoit Bay
oyster).
Methodological Exceptions
On day 6, an extra 180 mL of algal mixture was added by accident to both Little Pond
OM+OYST cores. The magnetic stir bar in one of the Waquoit Bay OM cores (WB4) consistently
became stuck, resulting in reduced mixing and elevated oxygen concentration measurements
throughout the experiment. During oxygen flux measurements, the top of one of the Little Pond
control cores consistently leaked in air, likely elevating O2 concentration measurements.
Ammonium data for a Waquoit Bay OM+OYST cores (WB1) time point 4 was removed from
flux calculations as air entered the sample stream during analysis, artificially elevating the
reading.
RESULTS
Initial oxygen flux measurements showed some variation in cores; however differences by
treatment assignment and by site were not significant (Table 1.1). The average initial O2 flux was
-10.13 mgO2/d for Little Pond cores and -9.31 mgO2/d for Waquoit Bay cores. A week of OM
additions increased oxygen uptake: during the day 7 O2 flux, both OM and OM+OYST cores had
much greater O2 fluxes whereas CTRL fluxes were fairly similar to initial measurements (Fig. 5).
In general, OM cores had a slightly higher O2 uptake than OM+OYST cores. Final oxygen flux
measurements on day 15 were lower than on day 7, but still higher in OM and OM+OYST than
CTRL cores (Fig. 4).
Initial fluxes for both ammonium (NH4) and nitrate (NO3) varied with no significant
difference by treatment assignment or site (Table 1.2-3). Initial NH4 fluxes when significant (R2
> 0.6) were positive and averaged 15.1 µmol/d for Little Pond cores and 11.7 µmol/d for Waquoit
Bay cores. During Week 1, NH4 concentrations increased in all cores, approaching 300 µM in
OM+OYST cores (Fig. S2). Week 1 NH4 fluxes were significantly different by treatment
(p<0.0001) but not site (Table 1.2). OM core fluxes were higher than CTRL cores; OM+OYST
core fluxes were on average 35 µmol/d higher than OM core fluxes (Fig. 5). Week 2 NH4
concentrations exhibited much more variation: notably, OM+OYST and Waquoit Bay OM cores
showed significant negative NH4 fluxes (Fig. 5).
Initial NO3 fluxes were negative in all cores and averaged -7.74 for Little Pond cores and
-4.67 for Waquoit Bay cores. NO3 concentrations were quite low during Week 1, on average only
3.34 µM, but began increasing over the last three days of the experiment (Fig. S3). For most
cores linear regression did not describe the majority of variation (R2<0.6) and for those it did,
fluxes were small and negative. In the last three days of Week 2 (days 12-15), all cores exhibited
significant positive nitrate fluxes (Fig. 6). CTRL cores had higher NO3 fluxes than OM and
OM+OYST cores. In Waquoit Bay cores, fluxes exhibited more variation but were on average
higher than Little Pond cores (Fig. 6).
Stable isotope analysis indicated significant enrichment of oysters, sediment, and water
samples but no enrichment of N2 compared to controls and background values. OM additions
were highly enriched (on average ∂15
N=16000). However, estimates for total N15 added as
organic matter per core had a large standard deviation, 20.3 ± 3.3 µmol/core (22.0 ± 0.4 for Little
Pond OM+OYST cores). Total recovered OM N15 per core (sum of sediment, water, oysters
when present) was on average 22.7 µmol, within one standard deviation of average total N15
addition (Fig. 7). Core replicates by treatment and site were nearly identical and sites were very
similar (Fig. S4). In OM cores, about 60% of N15 was found in the sediment and 40% in the
water. In OM+OYST cores, about 25% of N15 was found in the sediment, 35% in the water, and
40% in the oysters (Fig. 8).
DISCUSSION
As expected, nutrient fluxes indicated that adding organic matter to simulate eutrophic
conditions increased sediment core oxygen uptake. Lower oxygen uptake by sediment in
OM+OYST cores suggests oysters decreased the amount of organic matter readily available to be
oxidized either by incorporating it into their tissues or repackaging it as biodeposits. This was
corroborated by visual observations that water in OM+OYST cores was much clearer than in OM
cores. It is important to note that while total oxygen uptake by sediments in OM+OYST cores was
lower, this flux was measured with oysters removed. Based on oyster respiration rates of 0.1-0.5
mgO2/hr/ind, oyster respiration per core (3 oysters) would have been in the range of 7.2-36.0
mgO2/d (Uline 2013). Thus, total oxygen uptake in OM+OYST cores would likely have been
significantly greater than in OM and CTRL cores. These calculations suggest that oyster
aquaculture has the potential to further stress estuaries already experiencing hypoxic conditions.
While one would anticipate that oxygen fluxes would be distinct by site that Little Pond
sediment cores would have greater oxygen uptake rates due to higher organic matter content,
oxygen flux data was surprisingly uniform between sites. This result was likely explained by the
three-day difference in sediment core collection time. Since Little Pond cores were collected
three days before Waquoit Bay cores and incubated with aeration, the additional exposure to
oxygen rich conditions (~8 mgO2/L) likely oxidized much of the readily available organic matter
before treatments began. This highlights the importance of oxygen availability in regulating
subsequent sedimentary nitrogen cycle processes (Fig. 1) and explains the relative lack of site-
specific variation in the nitrogen flux data and stable isotope analysis.
The major difference between sites occurred in nitrate flux measurements at the very end
of the incubation period. Large negative ammonium fluxes coupled with positive nitrate fluxes in
Waquoit bay OM and OM+OYST cores suggesting that nitrification begins to occur during the
last three days of the incubation period (Fig. 5-6, S3). Data from Little Pond cores fits the same
pattern, but is much less conclusive and total nitrate concentrations remain lower. This implies
that over longer time periods, site history of eutrophication and oxygen availability may condition
the future response of the sedimentary nitrogen cycle. Since denitrification is tightly coupled to
nitrification, this suggests that the effect of oysters on long-term denitrification potential may be
dependent on site history; however, data from this 15-day experiment cannot be conclusive.
The lack of any evidence for denitrification of N15 enriched inputs is not surprising in the
context of the high oxygen uptake and low nitrate concentrations, which were the baseline
conditions of this experiment until the last three days. Without sufficient oxygen to oxidize
ammonium to nitrate, nitrate conditions were simply too low for denitrification to occur.
Although oysters did not increase denitrification potential, they did act as effective filter feeders,
incorporating 40% of added nitrogen into their tissues. This decreased the relative amount of
nitrogen that ended up in the sediment from 60% to 25%, but did not significantly alter water
column concentrations, likely due to the fact that oyster actively excreted ammonia while
metabolizing. These results indicate that oysters that care must be taken to consider nitrogen
recycling and oxygen availability in the context of site history when considering oyster
aquaculture as a water quality management strategy.
ACKNOWLEDGEMENTS
This project would not have been possible without the help of many. In particular, I would like to
thank my adviser Anne Giblin for her incredible patience, perspective, and support. Special
thanks are due to Dave Bailey and the Lindell lab, Sia Karplus, Tyler Messerschmidt, Sam
Kelsey, Jane Tucker, Sam Kelsey, Marshall Otter, and the fantastic SES TA team of 2015.
LITERATURE CITED
Bowen, Jennifer L., and Ivan Valiela. "The ecological effects of urbanization of coastal
watersheds: historical increases in nitrogen loads and eutrophication of Waquoit Bay
estuaries." Canadian journal of fisheries and aquatic sciences 58.8 (2001): 1489-1500.
Hoellein, Timothy J., and Chester B. Zarnoch. "Effect of eastern oysters (Crassostrea virginica)
on sediment carbon and nitrogen dynamics in an urban estuary." Ecological Applications 24.2
(2014): 271-286.
Jenkins, Mark C., and W. Michael Kemp. "The coupling of nitrification and denitrification in two
estuarine sediments1, 2." Limnology and Oceanography29.3 (1984): 609-619.
Kellogg, M. Lisa, et al. "Denitrification and nutrient assimilation on a restored oyster reef." Mar
Ecol Prog Ser 480 (2013): 1-19.
Mortazavi, Behzad, et al. "Evaluating the impact of oyster (Crassostrea virginica) gardening on
sediment nitrogen cycling in a subtropical estuary."Bulletin of Marine Science 91.3 (2015): 323-
341.
Pomeroy, Lawrence R., Christopher F. D'Elia, and Linda C. Schaffner. "Limits to top-down
control of phytoplankton by oysters in Chesapeake Bay." (2006): 301.
Solarzano L. 1969.” Determination of ammonium in natural waters by phenol hypochloric
method.” Limnological Oceanography 14:799-800.
Strickland JDH and Parsons TR. 1972. A practical handbook of Seawater Analysis. Ottawa,
Fisheries Research Board of Canada. 2nd Ed.
Team, Eastern Oyster Biological. "Status review of the eastern oyster (Crassostrea
virginica)." Report to the National Marine Fisheries Service, Northeast Regional Office (2007).
Wood, Elwyn Devere, F. A. J. Armstrong, and Francis A. Richards. "Determination of nitrate in
sea water by cadmium-copper reduction to nitrite." Journal of the Marine Biological Association
of the United Kingdom47.01 (1967): 23-31.
FIGURES
Figure 1. Oyster N-cycle diagram from Kellog et al. 2013.
Figure 2. Little Pond (left) and Waquoit Bay (right) sediment core collection sites, circled in red.
Figure 3. Aerial view of experimental set-up of sediment core incubations.
Figure 4. Initial, Post OM+, and Final oxygen fluxes for all twelve cores by site and treatment.
Figure 5. Initial, Week 1, and Week 2 ammonium fluxes.
Figure 6. Initial, Week 1, and Week 2 nitrate fluxes.
Figure 7. N15 mass balance stacked bar graph showing the location of N15 recovered.
Figure 8. Pie charts exhibiting relative fate of recoverd N15.
SUPPLEMENTARY FIGURES
Figure S1. Oysters added per core and basket mechanism.
Figure S2. NH4 concentration time series (days 1-15).
Figure S3. NO3 concentration time series (days 1-15).
Figure S4. N15 recovery separated by site and location.
TABLES
Table 1. p-values from Factorial ANOVA analysis of nutrient fluxes .
Table 2. Mass balance of recovered N15 per core (µmol).
Table 3. Relative recovery of N15 per core (%).
Figure 1. Oyster N-cycle diagram from Kellog et al. 2013.
Figure 2. Little Pond (left) and Waquoit Bay (right) sediment core collection sites, circled in red.
Figure 3. Aerial view of experimental set-up of sediment core incubations. This picture shows
the six Little Pond cores, with treatments in rows (OM+OYST, OM, CTRL from back to front)
and replicates in columns. The cylindrical apparatus in the center is hooked up to a battery that
causes the central disc to spin, rotating the white magnetic stir bars in each core keeping them
well mixed. Waquoit Bay cores were set up identically in another tub.
Figure 4. Initial, Post OM+, and Final oxygen fluxes for all twelve cores by site and treatment.
Error bars show the replicates. The Post OM+ flux was taken on day 7. The Final flux was taken
on day 15.
Figure 5. Initial, Week 1, and Week 2 ammonium fluxes. Missing values indicate linear
regressions with R2 < 0.6. Week 1 was calculated as the flux from days 1-7. Week 2 was
calculated as the flux from days 12-15.
Figure 6. Initial, Week 1, and Week 2 nitrate fluxes. Missing values indicate linear regressions
with R2 < 0.6. Week 1 was calculated as the flux from days 1-7. Week 2 was calculated as the
flux from days 12-15.
Figure 7. N15 mass balance. The dark green line is the average estimate for total N15 OM added.
The light green lines indicate one standard deviation above and below. The yellow line indicates
the upper standard deviation for the Little Pond OM+OYST cores (see Methods).
Figure 8. Pie charts exhibiting relative fate of recovered N15.
Figure S1. Oysters added to OM+OYST cores. All three oysters per core were placed in
makeshift plastic netting baskets like those shown in row three above.
Oysters
Figure S2. Ammonium concentration timeseries (days 1-15).
Figure S3. Nitrate concentration timeseries (days 1-15).
Figure S4. N15 recovery separated by site and location.
TABLES
Table 1.1.
O2 p-values
Initial Post OM+ Final
Treatment 0.319 0.001 0.015
Site 0.380 0.326 0.414
Treatment:Site 0.115 0.094 0.321
Table 1.2.
NH4 p-values
Initial Week 1 Week 2
Treatment 0.470 5.34*10-5
0.063
Site 0.564 0.073 0.396
Treatment:Site 0.175 0.867 NA
Table 1.3.
NO3 p-values
Initial Week 1 Week 2
Treatment 0.934 0.677 0.245
Site 0.158 0.305 0.144
Treatment:Site 0.111 0.257 0.578
Table 1. p-values from factorial ANOVA analysis of O2, NO3, NH4 fluxes.
MASS BALANCE PER CORE
Core
Oysters
(µmol
N15)
Sediments
(µmol N15)
Water
(µmol
N15)
Recovered
(µmol N15)
OM+
(µmol
N15)
Discrepancy
%
WB1 9.55 6.54 8.03 24.12 20.32 15.76%
WB2 9.80 5.29 7.19 22.29 20.32 8.84%
WB3 -- 12.79 8.29 21.08 20.32 3.62%
WB4 -- 12.25 8.18 20.43 20.32 0.57%
LP1 9.16 5.76 8.03 22.95 22.01 4.10%
LP2 11.44 5.07 8.09 24.59 22.01 10.51%
LP3 -- 14.09 9.13 23.22 20.32 12.50%
LP4 -- 13.21 9.88 23.09 20.32 12.00%
Table 2. Mass balance of recovered N15 per core.
RELATIVE RECOVERY
Site Treatment Oysters Sediment Water
WB1 OM+Oysters 39.6% 27.1% 33.3%
WB2 OM+Oysters 44.0% 23.7% 32.3%
WB3 OM 0.0% 60.7% 39.3%
WB4 OM 0.0% 59.9% 40.1%
LP1 OM+Oysters 39.9% 25.1% 35.0%
LP2 OM+Oysters 46.5% 20.6% 32.9%
LP3 OM 0.0% 60.7% 39.3%
LP4 OM 0.00% 57.20% 42.80%
Table 3. Relative recovery of N15 per core.