The effects of parasitism on feeding preferences and litter decomposition rates of amphipods
in the Plum Island Estuary
JoHanna Burton
Ripon College Biology Department
Ripon, WI 54971
Advisors:
David Johnson
Linda Deegan
Abstract
The TIDE project is an LTER project studying the effects of eutrophication on salt marsh
creek ecosystems at the Plum Island Estuary. These salt marshes are detritus-dominated
systems, and are home to a variety of detritivores. Amphipods parasitized by trematodes are
readily visible in these creeks because they turn bright orange and move from the protective
cover of vegetation out onto the exposed creek walls at low tide. I studied the effects of creek
fertilization and parasitism on the shredding rate and feeding preference of these amphipods.
To study the effect on shredding, I incubated amphipods with S. alterniflora litter and measured
mass loss, respiration, and C:N ratio. To examine the effect on feeding preference, I offered
each amphipod three food choices – algae, S. alterniflora, and S. patens – and counted the bite
marks on each substrate. I concluded that parasitism does not affect the shredding rate, while
creek fertilization causes an increase in shredding. From the feeding preference experiment I
concluded that both orange and brown amphipods prefer Spartina to algae. Fertilization did not
affect feeding preference.
Key Words: parasite; amphipod; Plum Island; Spartina; behavior; eutrophication
Introduction
Salt marshes are one of the most productive ecosystems in the world, producing up to
2500 g C m-2 yr-1. Much of the plant biomass produced is not consumed live; instead, it dies
back and forms mats of litter. In detritus-dominated ecosystems such as this, decomposition is
an important process. Detritivores such as amphipods, snails, and isopods and microbial
decomposers are important biological agents of litter processing. Detritivores are often
shredders – meaning they break up litter into smaller pieces with a greater surface area for
microbial communities to colonize. Thus, detritivores may be able to facilitate the
decomposition process.
Some amphipods are parasitized by trematodes. Trematodes are internal parasites that
may alter the behavior of their intermediate hosts. A trematode requires two or more hosts to
complete its life cycle: one or more intermediate hosts (generally benthic invertebrates), and a
definitive host, which is a vertebrate in which the trematode reproduces (Huspeni and Lafferty
2004). Some species of trematode are adapted to specific hosts, while others are general
parasites. Trematodes may also alter the color of their hosts (Johnson 2011; Johnson et al.
2009).
Parasites such as trematodes may alter host behavior for a variety of reasons: to kill the
host, to defend the host, to prolong host survival, or to increase the rate of parasite
transmission from one host to another (Moore and Gotelli 1996). Parasitized hosts may choose
lighter colored substrates, more open habitats, areas with brighter light, or substrates of a
different orientation (vertical vs. horizontal) (Johnson 2011; Johnson et al. 2009; Hechtel et al.
1993). It is often assumed that the parasite alters behavior of the host in order to increase
transmission of the parasite to the next host. Studies have tested hypotheses that suggest the
altered behaviors may have other purposes, such as killing the host or prolonging its survival
(Moore and Gotelli 1996). Hechtel et al. (1993) tested the hypothesis that more active behavior
results from greater feeding activity due to an increased energy demand. The results concluded
that the behavior is not related to a greater energy demand.
Amphipods parasitized by trematodes have been found in the Plum Island Estuary (PIE)
saltmarsh in Massachusetts, where there are ongoing studies on saltmarsh eutrohpication as
part of the TIDE Project (Deegan et al. 2007; Johnson et al. 2009; Johnson 2011). In these
studies, fertilizer is applied to some creeks to simulate anthropogenic eutrophication and its
effects on the marsh. Infected amphipods can be identified by their bright orange color, as
opposed to the normal brown color of un-infected amphipods, and are more often found in
fertilized creeks as opposed to reference creeks (Figure 1, Johnson 2009). The two species of
amphipods that have been found in the saltmarsh are Orchestia grillus and Uhlorchestia
spartinophila. These amphipods are most often found in the high marsh, where Spartina patens
grows, and the low marsh, where Spartina alterniflora grows. Algae grow mainly on the creek
walls, where there is no surface litter. Parasitized amphipods have been found to forage on
these exposed creek walls at low tide, especially in nutrient enriched creeks, exposing
themselves to increased predation (Johnson et al. 2009), but also potentially increasing their
grazing preference on algae.
It is unknown whether infected amphipods decompose litter at a different rate than
non-infected amphipods. If the rate of decomposition is greatly altered, the effects on the
ecosystem will need to be examined. Furthermore, since amphipods are not obligate
detritivores, the parasitized ones may be feeding on the benthic algae found on creek walls. It is
unknown whether that is a result of food preference. In this study, I examined the shredding
and grazing function of parasitized and non-parasitized amphipods in both fertilized and
reference creeks. I hypothesized that litter from the fertilized creek would be decomposed at a
faster rate than litter from the fertilized creek, that parasitized amphipods would shred litter at
a slower rate than non-parasitized amphipods, and that parasitized amphipods would prefer
algae over S. patens or S. alterniflora.
Methods
To determine if long-term fertilization (8 years) had changed the abundance of
detritivores, I conducted quadrat (0.25m x 0.25 m) counts at each creek via haphazard sampling
of the high marsh. I did five replicates at each creek. Organisms recorded included orange and
brown amphipods, isopods, snails, and spiders.
Samples were taken from two creeks branching from the Rowley River: Sweeney Creek
and West Creek (Figure 2). Sweeney Creek has been receiving nutrient additions, while West
Creek has been left as a reference creek. At each creek, individuals of the species Orchestia
grillus, an amphipod, were collected. I collected both parasitized and non-parasitized
individuals, which were found under wrack. These amphipods were stored in labeled
Tupperware containers in which I placed damp paper towels along with some detritus for
sustenance. These containers were stored in a refrigerator. I also collected dead Spartina
patens from the high marsh and S. alterniflora from the low marsh at each creek. At low tide, I
was able to collect filamentous algae from the creek walls. These samples were used in my
litter decomposition and feeding preference experiments. Samples from each type of primary
producer were also ground for analysis of carbon and nitrogen isotopes.
Litter decomposition
To determine whether parasitized amphipods shred litter at a slower rate, I randomly
assigned amphipods to one of four treatments: parasitized-fertilized; non-parasitized-fertilized;
parasitized-reference; non-parasitized-reference. Amphipods were only placed with litter from
the creek in which they were found. Each treatment was replicated five times. In each petri
dish, I placed 3 damp glass fiber filters along with approximately 1.85 grams of S. alterniflora
leaves cut to 5.5 cm length. I placed one amphipod in each petri dish and allowed them to
shred the litter for 3 weeks. The incubation was done on a lab bench at room temperature. I
checked each amphipod daily to keep them damp and to replace any dead individuals.
At the end of the three week period, I removed the amphipods from the petri dishes. I
cleaned the litter from each dish by gently wiping away material such as dirt and frass prior to
weighing. The respiration of the litter was measured using a LICOR 6400 with an IRGA
attachment in order to determine the amount of microbial activity on the litter. The litter was
then dried and weighed again. I converted the dry weight of the litter into wet weight using a
wet weight – dry weight ratio calculated earlier. By subtracting the final wet weight from the
initial wet weight I was able to calculate the amount of mass lost due to amphipod feeding.
Two samples of litter from each of the four treatments were analyzed for carbon and
nitrogen content to determine palatability.
Feeding preference
To determine if amphipods prefer algae to Spartina, I offered individual amphipods
three food sources: algae, S. alterniflora, and S. patens. The methods I used in this experiment
are similar to those used by Valiela and Rietsma (1984). Some changes were made to these
methods to better suit my experiment. In order to create a feeding substrate, I made agar using
50 mLs of seawater, 1 gram of agar granules, and 0.25 grams of the desired food type. I poured
the agar into small petri dishes and placed them in a temperature controlled room (10 Celcius)
to set. The six types of agar were: fertilized S. alterniflora, reference S. alterniflora, fertilized S.
patens, reference S. patens, fertilized algae, and reference algae.
Since I had only three substrate choices for each treatment, I placed one small petri dish
of each substrate into a larger petri dish. The four treatments were the same as those in the
litter decomposition experiment with the same number of replicates. I randomized the order of
the treatments. I did not starve the amphipods before placing them in the petri dishes, and the
amphipods were initially placed on an open space and not directly on a substrate in order to
remove bias. The petri plates were placed on a lab bench at room temperature.
Every half hour, I recorded the substrate on which each amphipod was located. I
allowed the amphipods to feed for four hours before removing them, so that the number of
bite marks in the agar would not become uncountable. Using a dissecting microscope, I counted
the number of bite marks on each substrate. Since amphipods feed by trenching, whole
trenches in which individual bite marks were not distinguishable were counted as a single bite
mark.
Data analysis
Two-way ANOVAs were used to analyze the % nitrogen and C:N ratios of the primary
producers, as well as the results of the feeding preference experiment. T-tests were used to
analyze all other data.
Results
Quadrat counts
On average, more animals were found in the high marsh of the fertilized creek than the
high marsh of the reference creek (Figure 3). This difference, however, was not significant (Ttest, P > 0.1). At both creeks, no parasitized amphipods were found in the quadrats (Table 1),
while the majority of the animals found were snails. Many of the snails were found clumped
together. Parasitized amphipods were, however, observed on creek walls (personal
observation). No densities were estimated.
CHN and isotope analysis
Algae and both Spartina spp. from each creek were analyzed for nitrogen content and
carbon and nitrogen isotopes as references. The average percent nitrogen content of species
decreased from the low marsh to the high marsh in both creeks (Figure 4). Plant species had a
significant effect on the nitrogen content (ANOVA, P = 1.4 * 10-7). The plants in the fertilized
creek had a significantly higher percent nitrogen content than the reference creek (ANOVA, P =
0.03). There is also a significant interaction between nutrient factor and plant species (ANOVA,
P < 0.05).
The average carbon to nitrogen ratio (C:N) in the species analyzed increased from the
low marsh to the high marsh (Figure 5). As with the nitrogen content, plant species had a
significant effect on the C:N ratio (ANOVA, P = 2.7 * 10-7). Also, the plants in the reference creek
had a significantly higher C:N than the plants in the fertilized creek (ANOVA, P = 0.003). It is
interesting to note that plants with low nitrogen content have higher C:N ratios (Figures 4 & 5).
In both the fertilized and reference creeks (Sweeney and West), the Spartina spp. had
less negative delta 13C values than the algae (Figures 6 & 7). The carbon isotope values, which
are good indicators of what an organism eats, for the brown and orange amphipods are much
closer to algae than to either of the Spartina species. The isotope data for the amphipods was
taken from data collected in 2009.
Litter decomposition
Quantitatively, the litter that was incubated with a non-parasitized (brown) amphipod
had higher respiration; however, there was no significant difference between litter incubated
with parasitized amphipods and litter incubated with non-parasitized amphipods (T-test, P >
0.05; Figure 8). Fertilization treatment did not have a significant effect on respiration of the
litter (T-test, P > 0.05).
In treatments containing litter and amphipods from the reference creek, there was no
measurable mass loss (Figure 9). In this case, mass lost from litter from the fertilized creek was
significantly higher than mass lost from litter from the reference creek (T-test, P = 0.007). There
was no significant difference in mass loss between litter eaten by parasitized amphipods and
litter eaten by non-parasitized amphipods (T-test, P > 0.1).
In all cases, the carbon to nitrogen ratio (C:N) of the litter decreased after the three
weeks of incubation (Figure 10). Parasitism did not have a significant effect on the change in the
C:N (T-test, P > 0.05). There was a significant difference in the change in the C:N between
creeks (T-test, P = 0.0004). Interestingly, litter from the fertilized creek showed a greater
change in the C:N ratio than the reference creek, which could be related to the greater mass
loss from fertilized litter (Figures 9 & 10).
Feeding preference
Overall, there were significantly more bite marks in the substrates containing Spartina
spp. than in the substrate containing algae (T-test, P < 0.05; Figure 11). There was no significant
difference in number of bite marks between the substrates containing S. alterniflora and S.
patens (T-test, P > 0.1).
Eliminating nutrient treatment as a factor, I looked at orange amphipods only. The
difference in bite marks was significant between algae and S. alterniflora ( T-test, P < 0.05,
Figure 12). There was no significant difference in preference between either of the Spartina
species (P > 0.05).
Looking only at the algae substrate, no significant patterns emerge (Figure 13). There
was no significant difference in number of bite marks between parasitized and non-parasitized
amphipods (ANOVA, P > 0.05), nor was there a significant difference in bite marks between the
fertilized creek and the reference creek (ANOVA, P > 0.05).
The substrates containing S. alterniflora and S. patens display interesting patterns
(Figures 14 & 15). In the fertilized treatment, the orange amphipods had a higher preference for
S. patens, while in the reference treatment brown amphipods had a higher preference for S.
patens. In the S. alterniflora substrate, brown amphipods appear to eat more of this substrate
than the orange amphipods in the fertilized treatments, but the orange amphipods appear to
eat more in the reference treatment. These opposite trends indicate a significant interaction
between fertilization and parasitism effects (ANOVA, P < 0.05). Fertilization and parasitism
effects alone are not significant for either substrate (ANOVA, P > 0.05). Interestingly, the
fertilized treatment of the S. alterniflora substrate has a similar trend to the fertilized treatment
in the litter decomposition experiment (Figures 9 & 14).
Based on each amphipod’s location at each half hour, there is a positive correlation
between the amount of time spent on a substrate and the proportion of the total number of
bites taken on that same substrate (Figure 16). Although the strength of the correlations ranges
from 0.37 to 0.58, the amount of time spent on a substrate is still a fairly good predictor of the
proportion of total bites taken.
Discussion
Quadrat counts
Because I conducted the quadrat counts in the autumn, it is likely that I found fewer
animals than if I had done the counts in summer. At this time of year, many of the animals were
either dying or burying themselves in the mud to protect against the cold. The snails were also
clumped together in high density stem areas to ward off the cold. Johnson (2011) found higher
densities of detritivores in the fertilized creek. Though my results also show a higher number of
organisms found in the fertilized creek, the difference is not significant. My results may differ
from Johnson’s because his sampling occurred in the summer.
CHN and isotope analysis
Since the C:N ratio is the ratio of the amount of carbon to the amount of nitrogen in an
organism, it is logical that the C:N ratio will decrease as the amount of nitrogen present
increases.
Algae had a greater percent nitrogen content and a lower C:N ratio than the Spartina
spp. because it has less lignified structure. The algae grow on the creek walls, which are often
inundated by the tides. Stiff, lignified structures would cause the plant to break under the
pressure of the current. In the fertilized creek, the nutrients are added to the water column at
high tide. Thus, the algae has the greatest exposure to the nutrient enrichment which increases
its nitrogen content and decreases its C:N ratio.
S. alterniflora grows in the low marsh, which is still inundated by the tides, but for a
shorter period of time than the creek walls. S. alterniflora is a grass, which means it has more
lignified structures than algae because it needs to grow taller. This grass had a higher percent
nitrogen content in the fertilized creek because it is exposed to the nutrient additions when
inundated by the tides. These results are consistent with Galvan (2008), who also found that
macrophytes had higher percent nitrogen in fertilized creeks.
S. patens grows in the high marsh and had the lowest percent nitrogen and the highest
C:N ratio of the three plant species sampled. The high marsh is the least inundated by the tides
and the grass is less exposed to the nutrient enrichment in the fertilized creeks. This species of
grass probably also has more rigid structures such as lignin and cellulose, increasing the carbon
content of the plant relative to the nitrogen content.
The isotope analysis showed that the amphipods had a carbon isotope fractionation
similar to that of algae, yet their nitrogen fractionation values were lower than that of algae.
Carbon isotopes indicate what sources an organism’s diet is coming from; these results suggest
that the amphipods are eating primarily algae. Although algae has been found to be an
important food source in salt marshes, the amphipods should have a higher nitrogen
fractionation than the algae if that is indeed their diet source (Galvan 2008). This strange
discrepancy in the data is likely caused by the old data I received on amphipod isotope values.
In the two years between when that data was recorded and now, it is possible that the isotope
fractionation of the creeks has changed. Fertilization could cause a change in the fertilized
creek. The shift in fractionation of the reference creek is probably due to natural causes. In
order to obtain accurate data, isotopic fractionation values from the same year should be
compared.
Litter decomposition
Respiration was a measure of the microbial activity on the litter. Greater microbial
activity should have been an indication of a greater amount of shredding, since that would
create a greater surface for microbes to colonize. No significant difference in the respiration
rates is an indicator that there was little difference in the shredding that occurred in each
treatment. A trend does seem to be appearing, however. Perhaps a longer experiment would
accentuate the differences between the shredding of parasitized and non-parasitized
amphipods.
The lack of measurable mass loss in the reference creek suggests that the amphipods in
those treatments did not eat anything at all; however, in order to survive for the three weeks,
the amphipods must have been eating something. The other alternative is that the mass loss
was too small to measure accurately. A longer incubation or more amphipods per dish would
have rectified this problem. In the fertilized creek, at least, I did see mass loss. Both orange and
brown amphipods ate approximately fifteen percent of the total mass in the dish. This is a small
amount of mass loss, with a large variation. If I had been able to incubate these dishes for a
longer amount of time, or add more amphipods per petri dish, I might have seen a more
distinct, less variable mass loss.
The trend that is appearing suggests that litter from the fertilized creek was shred at a
faster rate than litter from the reference creek. This supports my hypothesis that fertilized
creek litter would be shred faster. It is possible that the higher nutrient quality of the litter from
the fertilized creek is responsible for this difference; however, there may be other factors
affecting this, as well, such as lignin content, which I did not measure.
Although the statistics show that on an individual level there is no significant difference
in shredding between parasitized and non-parasitized amphipods, at an ecological level,
amphipods are still moving from the source population in the high and low marsh out to the
creek walls. This is removing them from the litter, thus changing their ecological function from a
shredder to a grazer.
The C:N ratio of the litter decreased over the three week incubation. This must be a
result of the amphipods eating carbon compounds such as cellulose and lignin. If only microbes
were decomposing the litter, I would have expected the C:N ratio to increase as the microbes
consumed nitrogen and left behind tougher compounds such as tannins and lignins. The
decrease in the C:N ratio is not easily explainable, but must be due to the fact that animals, in
this case amphipods, are eating the litter.
Feeding Preference
Within each type of primary producer, there were no differences in preference between
nutrient treatments and parasitism treatments. My hypothesis, however, focused on the
preference of orange parasitized amphipods. When I eliminated nutrient treatment as a factor
and looked at the feeding behaviors of all orange amphipods, it became clear that there was
not a greater preference for algae over Spartina. In fact, the results were contrary to what I
expected: preference for algae was significantly lower than the preference for Spartina. I also
eliminated parasitism as a factor, and looked at the total number of bites on each substrate.
The pattern was similar to that of the orange amphipods – the algae substrate was preferred
significantly less than either of the Spartina species.
Interestingly, my results conflict with those of Galvan (2008), who found that S.
alterniflora was not an important diet source of O. grillus in the fertilized creek. The results of
my feeding preference experiment indicate that S. alterniflora makes up, on average, over 30%
of the amphipods diet (disregarding parasitism). S. patens accounts for approximately 50% of
their diet, and the remaining portion of their diet comes from algae. In my experiment,
however, all three food sources were under equal conditions. In the field, the Spartina spp.
provide cover, while the algae is located in the open, possibly affecting their actual diet.
According to the results I obtained from my CHN analysis, algae had the lowest C:N ratio
of the three plant types analyzed. This should make the algae more palatable, and I would
expect the amphipods to prefer a more palatable food source. Although they do eat algae,
amphipods do prefer S. patens and S. alterniflora. From my data, I cannot determine why this
preference might be.
My findings do not support my hypothesis that parasitized amphipods were moving to
creek walls because they preferred to eat algae. In fact, the data suggests that the amphipods
would rather eat Spartina than algae. This implies that the amphipods are not moving out to
the creek wall in order to eat algae, though they may eat algae once they are out there.
Assuming that the data I recorded every half hour on the location of each amphipod is
representative of the total time spent on each substrate, I can conclude that the longer an
amphipod spends on a substrate, the more bites it will take. I may have seen a stronger
correlation if I had recorded the location more frequently (i.e. every fifteen minutes instead of
every half hour).
Conclusion
My data appears to support my hypothesis that litter from the fertilized creek is
decomposed at a faster rate than litter from the reference creek. Since I was unable to find
measureable mass loss from my reference creek samples, I would suggest further studies with
longer incubations or more amphipods per dish in order to get conclusive results.
From the outcome of my litter decomposition experiment, I can conclude that
parasitized amphipods do not shred litter at a significantly slower rate when offered only S.
alterniflora. This experiment was only at an individual level, however, and not at an ecological
scale. In the salt marsh, amphipods moving out onto the creek walls may still affect the rate at
which litter is decomposed because there are fewer amphipods to shred the litter.
My third hypothesis, that parasitized amphipods prefer to eat algae, was not supported.
In fact, the data shows that amphipods prefer algae less than the marsh grasses. Thus, the
reason for going out onto the creek walls is still unexplained. One possibility for this behavior is
that the parasite moves the host out into the open in order to increase the chances of the
amphipod being eaten and the parasite being passed to the next host. The parasite could
accomplish this by negating or reversing phototaxis, which causes the animals to avoid open,
light areas. Further studies should examine the idea of the effect of the parasite on the
phototaxis of amphipods.
Acknowledgements
I would like to thank my mentors, David Johnson and Linda Deegan, for working with me
every step of the way, from designing the project to giving me guidance and advice when I
needed it. I truly could not have done this without you. I would also like to thank Jimmy Nelson
for guiding me through the salt marsh and helping me collect my samples. Anika Aarons also
deserves thanks for taking a day to come into the field with me. Not only did she help me
collect amphipods, she also saved me from getting lost on the way to the field site. Thanks also
to the TAs who were patient enough to help me and keep the lab running smoothly. I also, of
course, need to thank Ken Foreman for all of the effort he puts into making SES run smoothly
and for giving me this wonderful opportunity.
Literature Cited
Deegan, L. A., J. L. Bowen, D. Drake, J. W. Fleeger, C. T. Friedrichs, K. A. Galván, J. E.
Hobbie, C. Hopkinson, J. M. Johnson, D. S. Johnson, L. E. Lemay, E. Miller, B. J. Peterson, C.
Picard, S. Sheldon, J. Vallino, and R. S. Warren. 2007. Susceptibility of salt marshes to nutrient
enrichment and predator removal. Ecological Applications. 17(5):S42-S63.
Galvan, K. 2008. The diet of salt marsh consumers. Ph.D. Dissertation. Louisiana State
University. Baton Rouge, LA., USA.
Hechtel, L. A., C. L. Johnson, and S. A. Juliano. 1993. Modification of Antipredator
Behavior of Caecidotea Intermedius by Its Parasite Acanthocephalus Dirus. Ecology. 74(3):710713.
Huspeni, T. C., and K. D. Lafferty. 2004. Using Larval Trematodes That Parasitize Snails to
Evaluate a Saltmarsh Restoration Project. Ecological Applications. 14(3):795-804.
Johnson, D.S. 2011. High marsh invertebrates are susceptible to eutrophication. Marine
Ecology Progress Series 438:142-152.
Johnson, D. S., J. W. Fleeger and L. A. Deegan. 2009. Large-scale manipulations reveal
top-down and bottom-up controls interact to alter habitat utilization by saltmarsh fauna.
Marine Ecology Progress Series: 377:33-41.
Moore, J, and N. J. Gotelli. 1996. Evolutionary Patterns of Altered Behavior and
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salt-marsh detritivores. Oecologia. 63(3):350-356.
Figures and Tables
Figure 1. Amphipods of the species Orchestia grillus. A non-parasitized individual is shown on
the left, and a parasitized individual is shown on the right.
Figure 2. A map of Rowley River, located at the PIE.
Figure 3. The number of animals found in quadrat counts at each creek.
Table 1. The number of animals of each species found in each quadrat at Plum Island.
Figure 4. The nitrogen content of each primary producer from each creek.
Figure 5. The carbon to nitrogen ratio of each primary producer from each creek.
Figure 6. Carbon and nitrogen isotopes of the organisms collected from Sweeney Creek.
Figure 7. Carbon and nitrogen isotopes of the organisms collected at West Creek.
Figure 8. Respiration of litter after three weeks of incubation.
Figure 9. Mass lost from litter after three weeks of incubation.
Figure 10. The change in the C:N ratio of litter after three weeks of incubation.
Figure 11. The total proportions of total bite marks counted on each substrate (disregarding
parasitism and nutrient treatment).
Figure 12. The percent of total bites taken by orange amphipods on each agar type
(disregarding nutrient treatment).
Figure 13. The proportion of total bites taken on algae substrates.
Figure 14. The proportion of total bites taken on S. allterniflora substrates.
Figure 15. The proportion of total bites taken on S. patens substrates.
Figure 16. The correlation of time spent on a substrate and the number of bites taken on that
substrate.
Figure 1. Amphipods of the species Orchestia grillus. A non-parasitized individual is shown on
the left, and a parasitized individual is shown on the right.
Figure 2. A map of Rowley River, located at the PIE.
Total Animals in Quadrat
120
Number of Animals
100
80
60
40
20
0
Fertilized
Reference
Nutrient Treatment
Figure 3. The number of animals found in quadrat counts at each creek.
Table 1. The number of animals of each species found in each quadrat at Plum Island.
Nutrient
Quadrat Parasitized
Level
Non-parasitized
Snails
Isopods
Spiders
Total
Amphipods Amphipods
Animals
Fertilized
1
0
3
68
0
0
71
Fertilized
2
0
8
24
4
3
39
Fertilized
3
0
9
92
17
2
120
Fertilized
4
0
2
52
1
1
56
Fertilized
5
0
4
30
2
1
37
Reference
1
0
9
38
0
1
48
Reference
2
0
1
43
6
0
50
Reference
3
0
0
36
0
0
36
Reference
4
0
1
28
1
1
31
Reference
5
0
13
45
0
0
58
% N content
2.5
Average % N
2
1.5
Fertilized
1
Reference
0.5
0
Algae
alterniflora
patens
Primary Producer
Figure 4. The nitrogen content of each primary producer from each creek.
C:N
120
Average C:N Ratio
100
80
60
Fertilized
40
Reference
20
0
Algae
alterniflora
patens
Primary Producer
Figure 5. The carbon to nitrogen ratio of each primary producer from each creek.
7.0
Isotopes in Sweeney Creek
6.0
del N15
5.0
4.0
Algae
3.0
S. patens
S. alterniflora
2.0
Brown Amphipod
1.0
0.0
-20.0
Orange Amphipod
-15.0
-10.0
-5.0
0.0
del C13
Figure 6. Carbon and nitrogen isotopes of the organisms collected from Sweeney Creek.
7.0
Isotopes in West Creek
6.0
del N15
5.0
Algae
4.0
S. patens
3.0
S. alterniflora
2.0
Brown Amphipod
1.0
0.0
-20.0
Orange Amphipod
-15.0
-10.0
del C13
-5.0
0.0
Figure 7. Carbon and nitrogen isotopes of the organisms collected at West Creek.
Litter Respiration
Respiration (g CO2 per g litter)
6
5
4
3
Brown
2
Orange
1
0
Fertilized
Reference
Nutrient Treatment
Figure 8. Respiration of litter after three weeks of incubation.
Mass Loss
Average Proportion of Mass Lost
0.4
0.35
0.3
0.25
0.2
Brown
0.15
Orange
0.1
0.05
0
-0.05
Fertilized
Reference
Nutrient Treatment
Figure 9. Mass lost from litter after three weeks of incubation.
Change in C:N
Difference (Final - initial)
-35
-30
-25
-20
-15
Brown
-10
Orange
-5
0
Fertilized
Reference
Nutrient Treatment
Figure 10. The change in the C:N ratio of litter after three weeks of incubation.
Feeding Preference Totals
9
Sum of Proportions
8
7
6
5
Sum of % Algae
4
Sum of % alterniflora
3
Sum of % patens
2
1
0
Total
Figure 11. The total proportions of total bite marks counted on each substrate (disregarding
parasitism and nutrient treatment).
Orange Amphipods
80.00
Percent of Total Bites Taken
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
Algae
S. alterniflora
S. patens
Primary Producer
Figure 12. The percent of total bites taken by orange amphipods on each agar type
(disregarding nutrient treatment).
Average Proportion of Total Bites Taken
Feeding Preference on algae
0.5
0.4
0.3
Brown
0.2
Orange
0.1
0
Fertilized
-0.1
Reference
Nutrient Treatment
Figure 13. The proportion of total bites taken on algae substrates.
Feeding Preference on S. alterniflora
Average Proportion of Total Bites Taken
1
0.8
0.6
Brown
0.4
Orange
0.2
0
Fertilized
-0.2
Reference
Nutrient Treatment
Figure 14. The proportion of total bites taken on S. allterniflora substrates.
Average Proporiton of Total Bites Taken
Feeding Preference on S. patens
1.2
1
0.8
0.6
Brown
0.4
Orange
0.2
0
Fertilized
Reference
Nutrient Treatment
Figure 15. The proportion of total bites taken on S. patens substrates.
Relation of Time and Number of Bites
Proportion of Total Bites Taken
1.2
1
R² = 0.5804
0.8
R² = 0.4449
Algae
S. alterniflora
0.6
S. patens
Linear (Algae)
0.4
Linear (S. alterniflora)
R² = 0.3744
0.2
Linear (S. patens)
0
0
0.2
0.4
0.6
0.8
1
1.2
Proportion of Time Spent on Substrate
Figure 16. The correlation of time spent on a substrate and the number of bites taken on that
substrate.