Download The Food Web Structure in Merrymeeting Bay

The Food Web Structure in Merrymeeting Bay
Mark Burton Scot McFarlane, Bowdoin College Department of Biology
Rusack and Howard Hughes Fellowship
Introduction:
Merrymeeting Bay is unique because of its status as a freshwater tidal ecosystem. As a result, it
provides key habitat for a number of ecologically important species; including several anadromous
fishes (i.e. American Shad, Atlantic Salmon, Short-nosed and Atlantic Sturgeon), snapping turtles, bald
eagles and many others. In order to better understand how to conserve and protect the flora and fauna
that make their home here, it is important to understand how the ecosystem functions and how
anthropocentric influences have and continue to influence the system.
The settlement of the Androscoggin and Kennebec river watersheds and their associated
development in the 1800’s led to the pumping of large quantities of nutrients (nitrogen and
phosphorous) into the system. This change also caused a shift in the food web structure in the bay.
One possibility is that the eutrophication, led to a shift from a benthic to a planktonic-based trophic web
(Koster et al.). A benthic food web is characterized by clear water and high productivity at the upper
trophic levels (i.e., fish), whereas a planktonic food web is characterized by cloudy or turbid water and
usually produces less biomass at the upper trophic levels (i.e., fish). Increased development in the
catchment area of the Bay’s tributaries has resulted in increased inputs of nutrients such as nitrogen
and phosphorous from industrial processes (paper mills) and municipal wastewater treatment plants.
These fertilizers increase the growth of planktonic micro-algal in upper layers of the water column. The
extra biomass, in turn, causes a large increase in the turbidity of the system. This turbidity prevents
light from penetrating through the water column to submerged aquatic vegetation (SAV) causing their
decline. SAVs ability to colonize is largely limited by light availability; the photic zone exists at 1 percent
of the incident light and is the threshold below which plants are unable to survive (Harley and Findlay).
The alterations in turbidity levels influence the size of the photic zone and therefore, the area and
depths at which plants are able to survive. As a result, beds of SAV such as sea grasses and benthic
macro-algae, have largely disappeared from much of the ecosystem. Their productivity was replaced by
growth of free-floating algae. This shift is important because fewer invertebrates, fish, and waterfowl
can inhabit an ecosystem with little SAV. Many species of invertebrates and vertebrates (particularly
juvenile fish) depend on the vegetation for both habitat (nursery) structure and forage and cannot
survive in the long term without them (Wigand et al.).
These shifts in the food web structure of the bay are reinforced by feedback loops. A possible
hypothesis is that the Bay has shifted to an alternative stable state (Scheffer et al.). The principle of
stable states implies that an ecosystem is highly resilient to change unless a major event causes a shift.
It is quite possible that anthropocentric influences have caused exactly that and hence the bay has
shifted to a turbid stable state whose feedback loops are preventing recovery to the original state. An
example of the feedback loops working in the system are the macrophytes found in the benthic
community. In a clear water system, they will reduce current flow and encourage the settlement of
particulate matter, thus clearing the water and allowing further colonization by SAV beds. However, the
absence of SAV’s means that sediments are subject to forces of re-suspension, which further clouds the
water and shades out any attempted growth by benthic flora.
The overall goal of the study this summer was to get a better understanding of how the food
web in Merrymeeting Bay is structured. This process will in turn help to illuminate many aspects
pertaining to the health of the system, particularly why the bay has not reestablished its highly abundant
populations of fish and other fauna that occupy higher trophic levels. This understanding will help us to
understand how to better employ conservation methods in order to help the bay regain its natural
populations and composition of species.
Discussion:
Figure 1. shows quite clearly one of the major values of SAV: that they provide
crucial habitat for fish. The impact that the smaller rivers have on Merrymeeting Bay is striking.
While the smaller rivers account for a minute portion of the water entering the Bay they more than
double the turbidity levels of the Bay compared with the levels in the Kennebec and the
Androscoggin (the two major rivers). So the impact that lowering those levels in the smaller rivers
would have on the turbidity levels in the bay would be significant. Because we now know that
SAVs in the Bay are completely light limited at depths below 1.5 m (Fig. 5) then every unit of
turbidity decreased would result in a slight increase in depths that the SAVs could colonize. The
fact that there were some locations where we found peak Tape grass biomass and other locations
at the same depth which had no Tape grass suggests that either SAVs are currently in the process
of colonizing the Bay or that there is some other factor which can limit SAV growth. Running
large-scale transplant experiments in the future may provide an answer to this question. The
results of the difference in the photic zone at low and high tide are also surprising. While there is a
tidal variation of ~1.5 m; the fact that .6 m of that difference in the smaller rivers is compensated
by the decreased turbidity due to the inflow of clearer water from the large rivers shows just how
much of an impact decreased turbidity levels could have on increasing the depth of the photic
zone.
Figure 6. Merrymeeting Bay
Results:
The catch per unit effort for all fish species was significantly higher at the vegetated than the unvegetated sites (p=0.001).
Turbidity levels were much higher in the smaller tributaries than the two major rivers and an average of the levels in the
major and minor rivers was found exiting the Bay through Chops Point. The total suspended solids results exhibited the
same trends as the turbidity data. Turbidity levels in the Bay and the minor rivers were found to be higher at low tide than
at high tide. With a tidal variation of ~1.5 m the high tide water level still dictated the depth of the photic zone. The light
a
response
curve for Tape grass showed that it was extremely low-light tolerant and had a compensation point of ~0.2% of
c
incident radiation. Depth distribution of Tape grass was dictated by water level and light levels with no plants found below
1.5 m at low tide.
Tape Grass
Future Work:
As mentioned
in the discussion it would be beneficial over the next
a
couple years to gain a better understanding of how introduced fish are
influencing the system. This effect could be particularly pronounced due
to the invasion of carp. Carp are bottom feeders
that stir up large
b
amounts of particulate matter and uproot submerged plants. Thus, their
feeding activities significantly reduce the area habitable to SAVs. This
drastic reduction in SAV populations would have a large and sustained
impact on the food web structure in the bay. A large scale exclosure
program on the one of the smaller tributaries of the Bay, could be
constructed to prevent carp from becoming established upstream. The
system could then be studied for a number of factors including water
quality analysis (turbidity) and SAV colonization. Implementing a
system of transplant experiments would help us to understand if the
SAVs were indeed limited by the light penetration in the water column or
if some other factor was preventing colonization.
0.03
Methods:
0.02
0.015
0.01
b
-1
10
0
250
500
750
1000
Light Intensity
a
-10
Low tide water
level
0.005
Photic zone
~1.7 m
0
Vegetated
b
Aphotic zone
Figure 1. The catch per unit effort (CPUE) for fish was significantly higher for sets in
beds of submerged aquatic vegetation (SAV) as opposed to unvegetated sites.
p=0.001. Error bars represent one standard error.
High tide water level
Low tide water
level
~0.9 m
High Tide
1% of incident radiation
at high tide
Low Tide
Aphotic zone
Figure 2. The photic zone at high tide appears to define the depth of
SAV bed growth in Merrymeeting Bay.
ND
ec
eb
nn
Ke
gg
in
0
sc
o
Turbidity (NTU)
1% of incident radiation
at low tide
10
ud
M
dy
t
n e
t
e r c se
in
st han as
Po
Ea Cat gad
s
ba
op
b
A
Ch
c
d
Depth at low tide (m)
0
~1.5 m
Photic zone
~2.4 m
20
(µmol s-1 m-2)
Figure 4. The photosynthetic light response curve
for Tape Grass shows a strong low light
compensation point at ~0.2% of incident radiation.
1% of incident radiation
Unvegetated
Habitat Type
An
dr
o
a
-1
C PU E (C aptured Fish / trap*hour)
0.025
(nmol O2 gFw s )
Net Photosynthesis
20
Fish sampling was done at vegetated and
unvegetated sites using standard minnow traps. Catch per
unit effort was calculated by number of fish caught by
number of hours the traps were set.
Turbidity measurements were made using a YSI
Sonde 6600 and all measurements were taken within two
hours of each other during high tide or low tide. Total
suspended solids, fixed, and volatile solids were measured
using 500 ml of sample water through a 2.0µm glass fiber
filter. TSS was weighed and measured after one hour in a
drying oven at 105°C and fixed solids were measured after
another forty minutes at 550°C in the muffle furnace.
Light extinction measurements were taken around
midday on a clear day using two light meters (Li-Cor), one
of which recorded ambient light and the other recorded
light every half meter through the water column.
Light response curves of Tape grass were taken from
samples within our SAV sampling site at Center’s Point
(43.98675, -69.85184). A water based O2 electrode
(Hansatech Instruments Ltd.) was used with a circulating
water bath (25°C) and CO2 enriched and filtered bay water.
SAV biomass was measured by harvesting random and
100 m long transect-line plots of 0.25 m2. Plants were
separated by species. They were washed and dried at
56°C. Depth and time of harvest were recorded, and then
depth was corrected using the depth recordings of a sonde
that was in a fixed point throughout the tidal cycle.
c
Carp
References:
0.9 m
-1
Carter, V., N. Rybicki, and R. Hammerschlag. (1991) Effects of submersed macrophytes on dissolved oxygen, pH, and
temperature under different conditions of wind, tide, and bed structure. Vol. 6, No.2: 121-133.
Duartes, C., (1995) Submerged aquatic vegetation in relation to different nutrient regimes. Ophelia. 41:87-112.
1.7 m
Harley, M., and S. Findlay. (1994) Photosynthesis-irradiance relationships for three species of submersed macrophytes in the
tidal freshwater Hudson River. Estuaries. Vol. 17, No. 1B: 200-205.
Köster, D., J. Lichter, P. D. Lea, and A. Nurse. Historical eutrophication in a river-estuary complex in mid-coast Maine.
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-2
0
25
50
75
100
Tape grass biomass (gDW m-2)
Lichter, J., H. Caron, T. S. Pasakarnis, S. Rodgers, T. S. Squiers, Jr., and C. S. Todd. 2006. The ecological collapse and
partial recovery of a freshwater tidal ecosystem in mid-coast Maine. Northeastern Naturalist, in press.
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Ecology and Evolution. Vol. 8, No. 8: 275-279.
Figure 3. The turbidity levels within Merrymeeting Bay
and its six tributaries.
c
d
Figure 5. Depth distribution of Tape grass biomass off
Center’s Point in Merrymeeting Bay. The 0.9 m line
represents what we anticipated as the high tide photic zone.
Wigand, C., J. Wehr, K. Limburg, B. Gorham, S. Longergan, and S. Findlay. (2000) Effect of Vallisneria Americana on
community structure and ecosystem function in lake mesocosms. Hyrdobiologia. 418: 137-146.
Woodley, C., and M. Peterson (2003) Measuring responses to stimulated predation threat using behavioral and physiological
metrics: the role of aquatic vegetation. Oecologia. 136: 155-160.