Post on 07-Aug-2020
The effects of salinity and nutrient limitation on microbial
processes in coastal sediments
Emily Waters
Hampshire College
December 2011
Mentor: Anne Giblin
Abstract
In order to better understand the relationship between salinity, nutrient limitation, and
microbial activity, I collected high and low salinity sediments and overlying water
samples from Little Sippewissett Marsh (Falmoth, MA). I analyzed the overlying water
for NO3-, NH4
+ and PO4
3- concentrations and the sediments for percent carbon and total
phosphate. I gave the homogenized sediments one of four treatments (control, nitrogen,
nitrogen and phosphorus, and phosphorus) and incubated them for three weeks. At each
of four time points, I measured respiration and conducted endopeptidase and phosphatase
assays. Results showed higher phosphate concentrations in the low salinity water and
sediment samples, however significantly higher phosphatase activity than endopeptidase
activity (p<0.0002 for high and low salinity). This suggests that the microbial community
in both high and low salinities is phosphorus limited. Low salinity sediments had an
average percent carbon of 9.48%, while the high salinity sediments had an average
percent carbon of 1.97%, potentially accounting for the significantly higher respiration in
the low salinity samples compared to the high salinity samples (p< 3.5 x10-5
). Both
phosphatase and respiration did not appear to be much affected by the nutrient treatments,
however trends in the endopeptidase activity suggest that endopeptidase activity was
lower in treatments where nitrogen was added. I recommend a further study with a
longer incubation time that accounts for the loss in available carbon over the incubation
period.
Key Phrases: nutrient limitation, extracellular enzymes, coastal sediments, microbial
activity
Key Words: salinity, enzymes, respiration, sediments, phosphatase, endopeptidase,
nitrogen, phosphorus
Introduction
Not only are coastal wetlands extremely productive systems, but they provide many
ecosystem services, contributing to storm protection for coastal communities, buffering
nutrient loads from land to the ocean, and supplying habitat and nursing ground for fish
and birds (Kostka et al., 2002). The key to these environmental and economic services is
the wetlands’ ability to accumulate organic matter and peat. It is likely that this service
will only become more important, because as sea levels rise these marshes will
accumulate sediment, counteracting the potential harm from sea level elevation (Morris
and Bradley, 1999). Peat accumulation in marshes is the result of saturated, and therefore
anoxic sediment. This anoxia limits organic matter oxidation, leading to carbon
accumulating more rapidly than it is decomposed (Amador and Jones, 1993).
As a result of this carbon accumulation, wetlands represent a large sink of global
carbon. (Morris and Bradley, 1999). Therefore, if wetland carbon mineralization were to
accelerate, the consequence of this potentially large CO2 flux could further contribute to
changes in the global climate. One factor that has the potential to increase wetland
carbon mineralization is anthropogenic nutrient loading, which has been shown to alter
the soil carbon storage due to increased decomposition, in some settings (Morris and
Bradley, 1999). Coastal wetlands are especially threatened by nutrient loading because
they receive inputs from surface water runoff and groundwater discharge (Morris and
Bradley, 1999). Besides increased decomposition, a potential consequence of
eutrophication includes increased microbial activity and biomass (Penton and Newman,
2007).
These effects of nutrient loading may result from either nitrogen or phosphorus
inputs, however the responses of different aquatic systems to either nutrient may vary.
The degree of a system’s response may be determined by what nutrient is limiting
production, as well as the vegetation, microbial community, and sediment chemical
quality (Paludan et al., 1999). The typical trends in nutrient limitations of aquatic systems
suggest that freshwater is more often phosphorus limited, while salt water is more often
nitrogen limited. If so, we can presume that over a salinity gradient of an estuary, the
corresponding gradient of limited nutrients would follow. Freshwater may be phosphorus
limited because phosphorus becomes bound in the sediments and is not biologically
available. In salt water, iron is trapped in the sediments in place of phosphorus, making
phosphorus more available. While the scientific community generally accepts these
limitation patterns, there are many examples of systems where this salinity-nutrient trend
does not hold. Additionally, while these patterns may be typical of pristine systems, most
aquatic systems now receive high nutrient loading, which has the potential to shift
nutrient limitation.
Arguably the most important step in the marsh’s food chain is the microbial loop,
which cycles through most of the nutrients and energy in the system (Hill et al., 2006).
Microbial activity is often controlled by their access to nutrients, acquired through the
synthesis and activity of extracellular enzymes (Penton and Newman, 2007). If nutrient
loading causes increased organic carbon cycling and nutrient limitation shifts in wetlands,
the microbial activity controlling these processes would be further driven by the
microbes’ enzyme production and activity (Penton and Newman, 2007).
Microbes produce enzymes in order to access large polymeric substrates in
organic matter that they would otherwise not be able to use (Karner and Rassoukadegan,
1995). In this way, enzymes can speed up decomposition and mineralization (Allison
and Vitousek, 2005). However enzyme synthesis is energetically and nitrogen-
expensive, so microbes will usually only synthesize enzymes when there is a limiting
nutrient (Allison and Vitousek, 2005). In a study by Penton and Newman (2007), N and P
additions resulted in higher carbon mineralization while enzyme activity was lowered.
Allison and Vitousek (2005) found that carbon and nitrogen availability, which is needed
for enzyme synthesis, may also control enzyme production. Temperature, pH, oxygen
conditions, hydrogen sulfide, humic substances, and heavy metals may also regulate
enzyme activities (Nausch and Nausch, 2000).
The measurement of phosphatase and endopeptidase activity is useful in
determining the nutrient limitations of microbial communities. Phosphatases hydrolyze
the esters and anhydrides of organic phosphorus to make inorganic phosphorus available
(Huang and Morris, 2003). Phosphatases are produced by plant roots, fungi, and bacteria
(Olander et al., 2000) and have been correlated to phosphorus stress and plant growth
(Makoi, 2008). Huang and Morris (2003) found phosphatase activity was positively
correlated with with plant biomass and negatively correlated with phosphorous in
porewater, sediments and total phosphorus. There is a well-documented negative
feedback, where phosphatase activity will increase the bioavailability of P, repressing
phosphatase activity and synthesis, decreasing P mineralization, and making P limited
again, back to where there more phosphatase is needed. In the same vein, it has been
suggested that high phosphatase activity may occur when an ecosystem is in a P-
accumulation phase, while low phosphatase activity signals that the ecosystem is P-
neutral (Huang and Morris, 2003).
Endopeptidase is a type of protease, which catalyzes the degradation of proteins,
supplying primarily nitrogen, but also carbon in low molecular weight compounds that
bacteria are able to assimilate (Nausch and Nausch, 2000). They are limited by the
presence of proteins and peptide substrates (Jankiewicz, 2007), and inorganic nitrogen
has been shown to suppress their activity (Nausch, 2000).
In order to better understand the relationships between salinity, nutrient limitation,
and microbial activity, I sampled sediments in high salinity and low salinity areas of a
coastal marsh in Cape Cod, MA. I incubated the sediments with nitrogen and phosphorus
treatments and measured phosphatase and endopeptidase activity at four time points
during the incubation. At each time point, I also measured sediment respiration, as an
additional measurement of microbial activity and carbon mineralization. The relationship
between salinity and nutrient limitation in aquatic systems has been studied extensively,
but the focus has been on limitation in the water column. This study will emphasize the
sediments. I hypothesize that the low salinity site will be more phosphorus limited and
the high salinity site will be more nitrogen limited. Further, I expect higher phosphatase
activity where the sediments are phosphorus limited, and higher endopeptidase activity in
nitrogen limited sediments. I expect these patterns to become more extreme over time, as
the nutrients are being consumed, and to additionally increase as sediment respiration
increases. Cape Cod coasts receive a substantial anthropogenic nutrient load, which
poses a threat to these marshes. For this reason, understanding how nutrient additions
influence the microbial communities is essential to the management of these wetlands.
Methods
Field Sampling and Experimental Design
I collected overlying water samples and three replicate sediment samples from a high
salinity (34 ppt) and low salinity (4 ppt) site at Little Sippewissett Marsh in Falmouth,
MA (Figure 1). Sediment samples were taken from the top 5 cm, however not at the
same distance from water. I filtered the water samples and analyzed them for NO3-, NH4
+
and PO43-
concentrations. I homogenized the sediments and sub-sampled them into 24
samples per replicate (total of 96, 48 high salinity and 48 low salinity). Each subsample
was about 15-20 g wet weight (WW), however after the first time point, an additional 10
g WW was added in order to increase respiration fluxes. For each set of 24 subsamples,
there were 4 control (HSC=high salinity control, LSC= low salinity control), 4 nitrogen
treatments (HSN=high salinity nitrogen, LSN- low salinity nitrogen), 4 phosphorus
treatments (HSP= high salinity phosphorus, LSP= low salinity phosphorus), and 4
nitrogen and phosphorus treatments (HSNP= high salinity nitrogen and phosphorus,
LSNP= low salinity nitrogen and phosphorus). I added nitrogen in the form of 1 mL 200
mM NH4+ and phosphorus as 1 mL of 12.5 mM PO4
3-. I set aside 20 g WW from each
sediment sample for initial respiration and enzyme assays. For each time point during the
22 day incubation, I harvested a set of 24 samples, took measurements, and added an
additional nutrient treatment to the remaining samples. The time points occurred on day
5, 9, 18, and 22. Each set of measurements consisted of respiration measurements and an
endopeptidase and phosphatase assay. I incubated the samples at room temperature and
added DI water each day to bring the samples back to their initial WW to regulate for
evaporation.
Overlying water and Sediment Analysis
I followed the phosphate protocol adapted from Murphy and Riley (1962) and used
the Shimadzu UV-1800 UV spectrophotometer (Shimadzu Coroporation; Kyoto, Japan).
For ammonium analysis, I followed the ammonium protocol modified from Strickland
and Parsons (1969) and used the Shimadzu UV-1601 UV spectrophotometer (Shimadzu
Coroporation; Kyoto, Japan). To determine nitrate concentrations, I followed the Lachat
flow injection analyzer (FIA) for measuring nitrate (adapted from Wood, Armstrong, and
Richards, 1967). I performed CHN and Total Phosphorus (adapted from Asplia et al.,
1976) analysis on the sediment samples using the Perkin Elmer Series II CHNS/O
Analyzer 2400 (Shelton, CT) and the Shimadzu UV-1800 UV spectrophotometer
(Shimadzu Coroporation; Kyoto, Japan) respectively.
Respiration
I measured respiration on both the Li-Cor 6200 and Li-Cor 6400 Portable
Photosynthesis System (Li-Cor Biosciences, Lincoln, Nebraska). I used the Li-Cor 6200
for the initial measurements and the first time point and the Li-Cor 6400 on the remainder
of the measurements. The machine measured CO2 every 15 seconds for five minutes per
sample.
Enzymes
I placed 1 gram of fresh sediment in a 15 mL falcon tube and added 8 mL of acetate
buffer (0.1 M) and 400 uM substrate (1 mM, 4-Methylumbelliferyl phosphate and L-
Leucine-7-amido-4-methylcoumarin hydrochloride). At room temperature, I tumbled the
falcon tubes during the incubation to keep the sediment mixing. At 10 min, 30 min, and
60 min, I centrifuged the tubes, withdrew 2.5 mL of supernatant and combined it with 2.5
mL pH 10 glycine buffer (200 mM), which kills the activity due to its high pH. I
analyzed the supernatant and glycine buffer fluorometrically on 10-AU Fluorometer
(Sunnyvale, California) at 445 nm.
Calculations
I averaged the final 8 respiration measurements of the five minutes recorded for each
sediment sample. To calculate enzyme activity, I plotted MUF concentration versus time
and used a linear regression to get activity in nmole l-1 h-1. All data analysis was
completed on Microsoft Excel 2008.
Results
Nutrient analysis of the overlying waters from the low salinity site showed higher
concentrations of NH4+ and PO4
3- than the high salinity site (Table 1). The average
ammonium concentration was prominently greater, with 5.29 uM NH4+ -in the low
salinity water and 0.47 uM NH4+ in the high salinity water. The average phosphate
concentration in the low salinity overlying waters was 2.11 uM PO43-
, while just 1.03 uM
PO43-
in the high salinity water. NO3- concentrations were fairly similar in high and low
salinity waters (0.73 and 0.61 uM NO3-, respectively). Sediment analysis was consistent
with the phosphate concentrations in the water samples. Total phosphate in the low
salinity sediment samples (6.09 mmol/g) was three times as much as that in high salinity
sediments (2.17 mM/g). Additionally, low salinity sediments had a C:N ratio of 21 with
an average percent carbon of 9.48%, while the high salinity sediments had a C:N ratio of
only 15.38 with an average percent carbon of 1.97%. Initial enzyme assays showed that
the low salinity sediments had higher phosphatase and endopeptidase activities as well as
higher respiration compared to the high salinity sediments. It is worth noting that prior to
respiration measurements the sediments were homogenized. This disturbance likely
altered respiration measurements, as the rates are about an order of magnitude higher than
the respiration rates we measured throughout the incubation.
Over the course of the incubation, respiration measurements in all nutrient treatments
and in both salinities initially increased (Figure 2). At the second measurement,
respiration slightly decreased and the congruence deteriorates. HSN, HSP, and LSP drop
off, HSC and HS N+P increase, and LSN and LSN+P show no change. Respiration was
significantly higher in the low salinity sediments compared to the high salinity sediments
(p< 3.5 x10-5
) with an average respiration rate across the time points of 0.104 ug CO2/g
dry sediment/min in high salinity sediments and .351 ug CO2/g dry sediment/min in low
salinity sediments.
At the beginning of the incubation, endopeptidase was highest in the phosphorus-
treated samples regardless of salinity. However, by day 5 these activities decreased and
we saw the highest activity in the control samples (Figure 3). Activity tended to decrease
until the last time point, where activity was highest in the phosphorus-treated and control
samples. Activity in the samples treated with nitrogen or both nitrogen and phosphorus
were close to zero throughout the incubation. The patterns of the high and low salinities
were fairly similar, although measurements from the first time point showed higher
endopeptidase activity in the low salinity P-treated sediments compared to the P-treated
high salinity sediments.
Phosphatase activity was significantly higher than endopeptidase activity in both high
salinity and low salinities (p<0.0002 for high and low salinity). There was a positive
correlation between the phosphatase activity in high and low salinity (r2= 0.79), where
phosphatase activity decreased over the course of the incubation, regardless of the
sediment salinity. Within this trend, activity varied among the nutrient treatments within
a given salinity (Figure 4). However, there were no significant differences between
treatments, except that the high salinity control was significantly higher (p<0.02) than the
high salinity nitrogen treatment. At the first time point in the high salinity sediments, the
highest phosphatase activity was in the samples treated with both N and P and lowest in
the sediments treated with N. However, at day 22, the phosphorous treatment and the
control had the highest activity. The phosphorus-treated and control sediments had the
highest phosphatase throughout the incubation in the low salinity sediments. All low
salinity treatments showed decreasing activity, although the control samples had
increased phosphatase activity. Overall, there was much higher and more frequent
phosphatase activity than endopeptidase activity.
Until now, we have only considered the patterns of a single measurement over the
incubation period. However, in some treatments, emergent patterns suggest that these
microbial processes may be working in conjunction with each other. For example, the in
the low salinity control sediments, all three measures increased between day five and day
nine (Figure 5). Between day nine and eighteen, endopeptidase activity stopped, while
phosphatase activity continued to increase and respiration declined slightly. Between the
final time points, respiration and phosphatase decreased dramatically, while
endopeptidase activity revived itself.
Just as in the low salinity control samples, the high salinity controls showed microbial
activity increases in all three processes between the first and second time point (Figure
6). Opposite to the low salinity, for the remainder of the incubation, endopeptidase and
phosphatase activities declined while respiration increased. These considerations may
help determine what nutrients are limiting the microbial communities in these sediments.
Discussion
The results of the initial enzyme assays and respiration measurements showed higher
microbial activity in the low salinity sediments compared to the high salinity sediments.
Initial respiration measurements are higher than those throughout the incubation, likely
due to homogenizing the sample. Kostka et al. (2002) found that homogenizing the
sediments resulted in increased respiration by a factor of two to six, however the
measurements recovered to typical levels after two days. If our sediments followed this
recovery, then the respiration measurements throughout the incubation are likely a good
reflection of typical fluxes and were not hampered by the initial homogenization.
Additionally, there were higher ammonium and phosphate concentrations in the
overlying waters of the low salinity site, and the nitrate concentration was only slightly
lower than the overlying waters of the high salinity site. Sediment total phosphate was
about three times as high in the low salinity site. While we would expect higher enzyme
activity where nutrients are limited, this suggests that the microbial community in the
high salinity sediments was fairly nutrient limited at the start of the experiment, perhaps
limiting its ability to synthesize enzymes. On the other hand, the low salinity sediments
had a higher C:N ratio than the high salinity site (21.00 versus 15.38). With more
available carbon, the microbial community in the low salinity site may be more active,
which is reflected in the higher respiration. Despite the site being more nutrient rich, the
microbes may require more nutrients in order to sustain this level of activity, which is
reflected in the higher enzyme activity. High soil organic matter often suggests high soil
fertility and productivity (Bolton Jr. et al., 1985). Additionally, the high salinity
sediments had a comparatively lower C:N ratio and percent carbon, which suggests that
there is less available carbon for mineralization and may explain the lower respiration.
The microbes in the high salinity sediments also had more nitrogen available to them
compared to the low salinity microbes. Therefore, they may not need to be producing
those enzymes. In considering these initial enzyme activities, it is necessary to recall that
plants as well as microbes may produce enzymes. The sediments were taken within the
root zone of the sediments and it is likely that the sediments contained previously
synthesized enzymes from the surrounding vegetation. While both sites were in
vegetated areas, the vegetation in the low salinity site was denser, although this was not
measured. Huang and Morris (2003) showed that phosphatase activity had a positive
correlation to aboveground biomass.
Respiration increased in all treatments between the first two time points, and then
decreased over the remainder of the incubation, except in HSC, HSN+P, and LSN. This
general decrease over the time may be contributed to the fact that no new carbon was
added to the sediment. While studies have shown an increase in microbial activity and
carbon mineralization with greater availability of inorganic nutrients (Amador and Jones,
1993), we likely didn’t see this pattern, despite the nutrient additions because the
microbial community may have become carbon limited. Further, the initial high
respiration and microbial activity may have reflected the addition of nutrients. At the
second time point, in the high salinity sediments, the two highest measurements are in
sediments with nutrient treatments. In the low salinity sediments, the highest respiration
at the second time point is in the LSP and LSN+P treatments, which may reflect the high
phosphatase activity in the initial measurements. This is consistent with Sundareshwar et
al. (2003), who found increased soil respiration and carbon turnover in plots treated with
N+P. White and Reddy (2000) found a positive correlation between microbial biomass
and high P treatments, suggesting that wetlands are phosphorus limited. Further, they
found that the addition of P made nitrogen more available because the microbial
community was more active and microbes drive the nitrogen cycle. Sundareshwar et al
(2003) additionally found that although the plants in the studied salt marsh were nitrogen
limited, which follows the typical pattern of nitrogen limitation in high salinity pristine
aquatic systems, the microbes were P-limited. Consistent with this study, he found that
the microbial community had a secondary carbon limitation when phosphorus was added.
The enzyme activity of both the high salinity and low salinity sediments supports
the microbial community being phosphorus limited, as phosphatase was significantly
higher than endopeptidase activity. The initial increase in phosphatase activity may be a
result of established active phosphatase that had been synthesized before sampling, as
opposed to new phosphatase synthesis as a result of nutrient treatments (Spier et al.,
1978). However, it has been suggested that the microbial community will respond to
changes in environmental conditions readily, the first response being enzyme synthesis
and activity on the short-term (Karner and Rassoukadegan, 1995).
Nutrient treatments did not follow the expected pattern of inhibiting enzyme activity.
In both high salinity and low salinity sediments, phosphorus additions appeared to
stimulate phosphatase activity, as the highest activities were in the phosphorus addition in
the low salinity sediments and in the nitrogen and phosphorus treatment in the high
salinity sediments. It is possible that the sediments had enough of a phosphorus
limitation that the treatments did not pass a threshold where phosphatase synthesis would
be inhibited. In fact, it is possible that a small enough phosphorus addition may stimulate
phosphatase activity in the microbial community. Penton and Newman (2007) suggested
that until a certain threshold, small shifts in phosphorus concentrations will not change
enzyme-based resource allocation. Another explanation for the lack of responsiveness to
nutrient treatments is that because the sediments were phosphorus limited, the microbes
may be accustomed to producing phosphatase regularly, regardless of the availability of
phosphate. Microbes have been shown to constituently produce enzymes that target the
nutrient that is typically limiting of them, regardless of its availability (Allison and
Vitousek, 2005). While phosphatase activities dropped off by the end of the incubation,
there were not significant differences between the nutrient treatments and the controls,
suggesting that this is probably not a result of the nutrient additions, and possibly a
carbon limitation. The coupling of phosphatase activity and respiration further support
this possibility. In most treatments, there is an initial increase in all activities, followed
by a crash in respiration after the second time point, and a crash in phosphatase activity
after the third time point. If the sediments became carbon limited, this would stop
respiration and carbon mineralization, followed by a shift in the microbial community to
focus its resources on acquiring more recalcitrant carbon. Allison and Vitousek (2005)
suggest that the two most significant regulators of enzyme production are microbial
demand and access to C and N, which are both needed for enzyme synthesis. If carbon
became limiting, which is reflected in the respiration measurements, then the microbial
community would not have carbon to spare for enzyme synthesis.
Endopeptidase activity seemed to be more responsive to nitrogen additions than
phosphatase to phosphorus additions. The controls in both low and high salinity
sediments followed the respiration and phosphatase activity pattern, with initial increases
and then dropping off after the second time point. Where phosphorus alone was added,
endopeptidase activity was initially higher than the other treatments, then dropped off
after the first time point, and increased slightly on day 22, the fourth time point. In the
nitrogen and phosphorus as well as nitrogen only treatment, endopeptidase activity was
lowest, especially in the high salinity sediments. These results contrasted with other
studies, which found that endopeptidase activity was less responsive to increased nitrogen
concentrations than phosphatase activity was to phosphorus additions. The explanation
to this trend is that phosphatase directly makes phosphate biologically available, while
endopeptidase indirectly makes nitrogen available from proteins. Moreover, inorganic
nitrogen may be accessed in multiple ways outside of enzyme acquisition, while
phosphatase is a primary pathway for microbes to access phosphate. Accessing nitrogen
from proteins requires a suite of enzymes in addition to endopeptidase, and this process is
often incomplete and does not result in the complete mineralization to ammonium
(Olander et al., 2000). While the treatments did seem to control the endopeptidase
activity, the overall higher activity in phosphatase activity may be contributed to the more
easily-accessed phosphate, as well as the phosphorous limitation, Additionally, the
increase in endopeptidase activity in many treatments towards the end of the incubation
may be contributed to the carbon limitation. Because endopeptidase goes after proteins,
it helps free up carbon, as well as nitrogen. Olander et al. (2000) found that
endopeptidase activity increased when carbon was limiting. Endopeptidase is a type of
protease, which often are excreted in order to cope with starvation (Karner and
Rassoukadegan, 1995).
There are many methodological problems associated with the short term, small scale
nature of this study. The small samples (about 30 g WW) resulted in small CO2 fluxes,
and it is recommended that future studies use larger sediment samples to measure
respiration. While I added water to the sediments daily to bring them back to their
original wet weight, keeping moisture constant, as well as salinity, was difficult. An
additional problem is that these coastal sediments receive new detrital carbon, face daily
tides, and fluctuations between aerobic and anaerobic conditions. These conditions were
not replicated in the lab and the samples were essentially treated like terrestrial samples.
The degree to which this would impact the results is unknown. Finally, in order to better
understand the dynamics between nutrient limitation, salinity, and microbial activity, a
longer incubation would be preferable. Future studies should be done, adjusting for these
problems. Additional studies could assay for enzymes that target carbon, as a way to
determine the carbon limitation as well as measure total N mineralization, in order to
determine the degree to which N acquisition is through enzyme activity.
Acknowledgements
I would like to thank Anne Giblin, my mentor, for her support and guidance through
this project, and inspiring my interest in this topic. I would like to thank Stefanie Strebel
for helping me develop an enzyme assay protocol, as well as Laura van der Polt for being
patient with the Li-Cor, when I couldn’t be. I also need to thank Rich McHorney and
Carrie Harris for their daily help in lab. I couldn’t have done it without you.
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Figures and Tables:
Figure 1: Map of field site, Little Sippewissett Marsh (Falmouth, MA).
Table 1: A comparison of initial and background measurements between a high and low salinity marsh site.
Figure 2: A comparison of sediment respiration in high and low salinity
sediments and across nutrient treatments over the incubation period.
Figure 3: A comparison of endopeptidase activity in high and low salinity
sediments and across nutrient treatments over the incubation period.
0
1000
2000
3000
4000
5000
6000 HIGH SALINITY
control
N+P
N
P
0
1000
2000
3000
4000
5000
6000
5 8 19 22
LOW SALINITY
control
N+P
N
P
Time (days)
Figure 4: A comparison of phosphatase activity in high and low salinity
sediments and across nutrient treatments over the incubation period
Figure 2: How endopeptidase activity, phosphatase activity, and respiration change over time in low salinity control
sediments.
Figure 6: How endopeptidase activity, phosphatase activity, and respiration change over time in high salinity
control sediments.