Effects of the Introduction of an Invasive Plant Species (Myriophyllum spicatum) on the
Fish Community Structure of Pine Lake
Anthony Prisciandaro
Dr. Mark Kuhlmann, Thesis Advisor
ABSTRACT
Since the introduction of Eurasian watermilfoil (Myriophyllum spicatum) in the mid
1980s Pine Lake (Delaware County, NY) has seen an increase in vegetative cover but a
decrease in plant diversity. Milfoil is known to affect aquatic plant communities by outcompeting native species, but it should also have effects on fish by altering food and habitat
types. To examine the impact of the invasion I compared the current plant and fish
communities to earlier surveys of the lake. I sampled the plant community using 1m x 25m
belt transects. Fish population and community structure was measured from a single
electroshocking sample and repeated trap netting. Milfoil has increased from being absent
in 1976 to being present in every transect in 2003, and overall plant diversity in the lake has
decreased. The overall diversity of the fish community has decreased since 1985. Bluegills
have increased from only 33% of the community in 1985 to over 57% in 2003. Chain
pickerel and largemouth bass have also increased in abundance perhaps because of the
increase in food, juvenile bluegills. All other fish species found in 1985 have decreased in
abundance.
INTRODUCTION
Many invasive species are now causing havoc around the world. The increase in
human travel in the past 500 years has come at a cost. We are introducing many species
each day to new environments. Most of them do not survive in their new environment once
they get there, but a few do, and even fewer proliferate to become invasive.
Organisms can proliferate in a new habitat for many reasons. The new habitat may
be more hospitable, there may be few predators, and there might even be an empty niche.
For whatever reason an organism becomes invasive when it starts negatively affecting native
organisms.
One of the most significant invasive aquatic plants in the United States is Eurasian
milfoil, Myriophyllum spicatum. Estimates of the date for the first invasion of Eurasian
milfoil to the United States range greatly from as early as the 1880s (Reed 1977) to as late as
the 1940s (Couch and Nelson 1985). This method of dispersal has allowed milfoil to spread
very rapidly; from an initial infestation in a North Carolina lake of 40ha Eurasian milfoil
spread to 3,200ha in just one year and 32,000ha in just 9 years (Davis and Brinson 1983).
Milfoil forms dense beds, which can block the sunlight from reaching other plants, in
turn killing the other plants (Madsen et al. 1991). The loss of native plant species can cause
a decrease in diversity and abundance of aquatic invertebrates, the food of most juvenile and
many adult fish (Madsen et al. 1991). Fish community structure can change drastically due
to change in habitat and food (Pierce et al. 2000). Milfoil also caused decreased dissolved
oxygen and changes in water temperature (Madsen et al. 1991).
Having seen myself some of the negative effects of other invasives, such as water
hyacinth and zebra mussels, I wanted to look at the impact of Eurasian milfoil. My study
addresses the effects of M. spicatum on both the plant and fish communities of Pine Lake, a
small lake on Hartwick College’s Environmental Campus.
1
Sometime in the past 20 years, Eurasian milfoil was introduced to Pine Lake, most
likely in the late 1980s. The spread of this invasive species has caused many changes in
available habitat types. There were only a few patches of milfoil found in 1988 (Keller
1988) and the rapid spreading of this plant leads me to believe that it was shortly before
1988 when the invasive plant was introduced. Eurasian milfoil spreads by fragments so
recreational activities on the lake may have also increased dispersal throughout the lake
(Smith and Barko 1990).
Many studies have been done on Pine Lake in the past that allow me to track changes
in fish community structure and plant life. Previous studies have been done on the fish of
Pine Lake in 1985 (Major 1985) and in 1994 (Ahl 1994). Previous studies on the aquatic
plants of Pine Lake have been done in 1976 (Smith and Rabler 1976), 1988 (Keller 1988),
and 1993 (Herring 1993).
Seeing that milfoil has been shown to push out native species of plants, I
hypothesized that the abundance and diversity of plant life in the lake would go down. I
also hypothesized that because of the loss of native plant life the fish diversity would also go
down. The top predators in the lake, largemouth bass and chain pickerel, are visual feeders,
so I expected their populations and conditions would specifically suffer because of the dense
milfoil making it easier for prey to hide.
MATERIALS AND METHODS
Pine Lake is a 5km2 Kettle hole lake that is part of Hartwick College’s
Environmental campus in West Davenport, NY. It has one connection to Charlotte Creek,
which eventually runs into the Susquehanna River. It ranges in depth from a shallow swamp
at the southwest end of the lake to 11 meters deep at the center. During the summer months
there is an anoxic zone that forms at between 3 and 4 meters in depth restricting plants and
fish to the shallower depths.
In order to examine changes in the fish community related to the invasion of Pine
Lake by milfoil, both fish and plant communities were sampled in the fall of 2003 and
compared to past studies. Because of biases inherent in sampling any mobile animal, such
as fish, I used two different methods to sample fish. The NYS Department of
Environmental Conservation (DEC) electroshocked Pine Lake on the evening of September
9 with a 220v, 3,500-watt, AC generator. We did one loop around the lake spending _ hr
shocking at 530v and 4.5 amps. Total length, weight, and species were determined for all
fish captured.
Because bottom feeders and fish in thick vegetative cover are usually not easily
collected during electroshocking, I also used trap nets in the fall of 2003. A 1 _ cm mesh
trap with 15m leader was used. I tried to use the same sites as the study done in 1994 (Ahl
1995), with one site by the sauna, the second in the northeast corner of the lake, and the third
by the fishing dock (Fig. 1). Traps were left in for a total of eight 48hr periods with the
length, weight, and species of all captured fish recorded.
Seining was also attempted but the thick milfoil made this an ineffective sampling
method.
A small survey of the plants in Pine Lake was also done during the beginning of
October of 2003. A 25m rope with marks every meter was used to determine presence of
the different plant species in 1m2 areas along the 25 meter belt transect. Six of these belt
transects were spaced around the perimeter of the lake.
2
The plant survey was turned into a presence-absence model for each square meter
plot for analysis. If a plant was present in half of the plots it was given a value of 50%, but
more than one plant species can be found in a square meter so the totals for all species add
up to more than 100%. Fish condition was calculated by dividing the length by the mass.
Statistical analyses were done to compare the two other plant studies and the two other fish
studies of Pine Lake to my current research. I statistically compared the diversity of the fish
community between years using the Shannon diversity index and a modified T-test (Cox
1996).
Fig. 1 Depth profile of Pine Lake with trap net sites at A, B, and C and plant transects at 1-6 Depth
contours are in meters.
3
RESULTS
In the fall of 2003, I found 11 aquatic plant species, which represents a loss of 12
species since the 1988 study. Milfoil was present in over 69% of the area surveyed, a great
increase from not being present at all in 1976 (Smith and Rabler 1976). The second most
abundant plant species was Ceratophyllum demersum, present in 35% of the area surveyed.
Only 2 other species, Potamogeton amplifolus and Nuphar veriegatum, were found in
abundances over 15%.
80
60
Abundance
40
20
1993
2003
0
s
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.f o
N e r w um
v il m
L i ut i c u
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H virg tris s
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T . a l u can
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T . me t a
a ta
L. os era
. h if
V ulb
b or
C. in us
m ll
L . si
s
pu u s t u
P. r i s p i l l a s
c tic iu
P. e n i f o l
.v h s
D mp dru
a y
P . p i h ris
e a
P . ulg r s u m
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d g
C. a r i e r i
. v eb
N chr t u m
s ca
B. s p i t a t a
. o
M ord
.
N
Species
Fig. 2. Relative abundances, percent of quadrats present, for each
species of aquatic macrophytes in 2003 relative to 1993.
In addition, the abundances of all but two of the remaining species, milfoil and
bigleaf pondweed, have decreased since 1993 (Fig. 2). In 1993, there were 5 species of
aquatic plants that were present in over 40% of the area surveyed; today, milfoil is the only
plant that can be found in more than 40% of the survey area. Nymphae ordorata was
actually present at higher levels than milfoil in 1993, but it is now only found in only 9% of
the area surveyed.
In the fish community, both Simpson’s and Shannon diversity indices have
significantly decreased since 1985, with the P value on all t-tests being <0.05(Table 1).
Table 1. Fish diversity in 1985, 1994 and 2003
Number of species
Simpson's Diversity Index
Shanon Diversity index
4
1985
10
4.99
0.82
1994
12
4.33
0.84
2003
10
2.76
0.64
We have lost two species of fish since 1985, but two new species were found in
1994. One of those is now absent but another new species was found in 2003.
The bottom feeders seem to have been the most adversely affected since 1985. Out
of the three species found in 1985 only 1 bottom feeder remains. Even though the brown
bullhead is still present, it has decreased from about 8% of the community in 1985 to less
than 4% in 2003.
The only fish species that seem to be prospering in the lake are the bluegill sunfish,
58%, and its predators the largemouth bass, 12%, and the chain pickerel, 11%. The other
two species of sunfish, the pumpkinseed and the redbreast, have seen dramatic decreases in
abundance since 1985, from 22% down to 2% and 12% to 5% respectively (Figure 3).
60
50
40
30
20
Percent
1985
10
1994
0
2003
Bluegill Sunfish
Redbreast Sunfish
Pumpkinseed Sunfish
Chain Pickerel
Largemouth Bass
SPECIES
Fig. 3. Relative abundances (% of all fish captured) of
5 of the most abundant fish species from Pine Lake
Some changes can also be seen within species. The majority of bluegills in 1994
were adults over 160mm while the majority of bluegills in 2003 were juveniles under 90mm
(Fig. 4). The largemouth bass and chain pickerel have seen changes in condition between
1994 and 2003. In both species the juveniles had higher conditions in 1994 compared to
2003 while in 1994 the adults had lower conditions compared to 2003 (Fig. 5, 6).
5
1994 Bluegill Size Distribution
40
Number of Individuals
30
20
10
0
20.0
60.0
40.0
100.0
80.0
140.0
120.0
180.0
160.0
220.0
200.0
Length (mm)
2003 Bluegill Size Distribution
40
Number of Individuals
30
20
10
0
20.0
40.0
30.0
60.0
50.0
80.0
70.0
100.0
90.0
120.0 140.0 160.0 180.0 200.0
110.0 130.0 150.0
220.0
170.0 190.0 210.0
Length (mm)
Fig. 4. Size distribution for bluegill sunfish, 1994 vs. 2003.
6
6
Condition- Mass/Length
4
2
DATE
2003
0
1994
0
1000
2000
Length mm
Fig. 5. Largemouth bass condition vs. length in 1994 and 2003.
5
4
Condition- Mass/Length
3
2
DATE
1
2003
0
1994
0
100
200
300
400
500
600
700
Length mm
Fig. 6. Chain pickerel condition vs. length for 1994 and 2003
7
DISCUSSION
Since the invasion of Eurasian milfoil, the overall diversity of plant life in Pine Lake
has decreased as hypothesized. The three plant species that did not suffer as much as the
others each have at least one competitive advantage over milfoil. Bigleaf pondweed
(Potamogeton amplifolus) is the only species, besides milfoil, that has increased in
abundance since 1993. This may be because besides having some floating leaves, it is also
able to survive at greater depths than Eurasian milfoil. Coonstail (Ceratophyllum demersum)
is still found at relatively high densities. It is a free-floating plant and is not rooted so it can
be pushed away by the milfoil but it isn’t usually shaded by milfoil. Most of the coonstail in
2003 was found in the shallow areas where milfoil was not as prevalent. The bullhead lily
(Nuphar veriegatum) is also still found at relatively high densities. This lily pad comes up
earlier in the spring than the milfoil (Pers. obs.). It has leaves that float on the surface so the
milfoil can’t block out the light and the petiole is stiffer than other lily pads not allowing the
milfoil to push it around.
As expected, the diversity of the fish community has also decreased since Eurasian
milfoil was first found in Pine Lake. Seeing that this was only a small patch of milfoil found
in 1988 (Keller 1988), and milfoil has been shown to spread very rapidly (Davis and
Brinson 1983), I am assuming that if there was any milfoil present in 1985 it did not have
any major effects on the fish community.
Even though there was an overall decrease in diversity, three new fish species, 2 of
these species being represented only by individual fish, have been found since 1985. This
increase in diversity can be explained by the more in-depth search methods used in the 1994
and 2003 studies compared to the 1985 study, but the loss of species is not as easily
explained. The two bottom feeder species that were lost, the creek chubsucker and white
sucker, had relatively large population sizes before 2003. Their decline may be explained
by the change in available habitat types following the introduction of Eurasian milfoil.
Milfoil has been shown to change the average size of the particulate matter along
with the carbon content of the bottom sediment (Keast 1984). Keast found in his study that
most of the substrate in the milfoil areas consisted of an “organic ooze” consisting of an
average particle size <0.005mm along with an organic carbon content between 40 and 70%.
In comparison, the areas without milfoil had a particle size between 0.07mm and 0.17mm
and the organic carbon content averaged only 4.8%. Most bottom feeders tend to prefer the
larger particle size and stay away from the “organic ooze.”
The three sunfish species have seen dramatic changes since 1985. The bluegills have
dramatically increased in abundance while the other two sunfish species have seen great
decreased in abundance. This may be because of the feeding styles and habitat choices of
the three species.
Bluegills are generalists and are able to feed on zooplankton and other aquatic
invertebrates both in the vegetated littoral zone and the open water pelagic zone (Ehlinger
1989). They do tend to move to the pelagic zone once they are too big for most bass to eat,
but Ehlinger has found differences in pectoral fin length that allow some individuals to be
better feeders in the pelagic zone (short pectoral fins), or the littoral zone (long pectoral
fins). The longer pectoral fins of some bluegills allow for more mobility in the dense littoral
zone and a higher prey capture rate than individuals with shorter pectoral fins. Being able to
8
feed on a wide variety of invertebrates in the pelagic and littoral zones may have given the
bluegills an advantage over the other sunfish species.
Pumpkinseed sunfish are more specialists. They, like most other species, survive on
zooplankton and small invertebrates in the littoral zone as juveniles. But, unlike bluegills,
pumpkinseeds stay in the littoral zone as adults, feeding on aquatic gastropods that the
bluegill cannot feed on. This allows pumpkinseeds a competitive advantage as adults but
they have to compete with the ballooning bluegill population until they reach around 80mm
TL (Wainwright 1996). Aquatic snails, like many other invertebrates, live on the aquatic
plant life. With the drastic change in plant life in Pine Lake, it makes sense that the snail
population may have also declined, leaving the adult pumpkinseeds with little food if they
are to reach adulthood.
Redbreast sunfish, like the pumpkinseed, also spend most of their life in the littoral
zone. Milfoil may have more of a direct effect on this species because they are significantly
affected by the amount of sunlight they receive. Redbreasts have been shown to be less
active on overcast days or even just when a clouds blocks the sunlight reaching the water
(Burton and Burton 1970). The dense canopies built by the milfoil have been shown to
decrease the amount of light actually getting into the water. This could be a possible
explanation for the decrease in redbreast abundance.
Redbreasts also have been shown to have shorter pectoral fins than both bluegills and
pumpkinseeds. This may be making it difficult for them to maneuver and capture prey in
the dense milfoil.
The abundance of redbreasts in 2003 may also be misleading. This species is known
to form schools in the fall as they prepare to enter the depths of the pelagic zone to over
winter (deBeer 1972). All of the redbreasts found in 2003 were from a single catch in one of
the trap nets, which suggests they may have been schooling to prepare for winter. If this is
true, their actual relative abundance in 2003 should be even lower than it actually is.
Since 1994 there have been some changes within species. The bluegill size class has
changed from being mostly adults in 1994 to being mostly juveniles in 2003. Adult
largemouth bass and chain pickerel condition has increased greatly since 1994, contradicting
my hypothesis that their condition would decrease because the milfoil would make it harder
for them to see their prey. On the other hand, juvenile largemouth bass and chain pickerel
condition has greatly decreased since 1994.
The shift in bluegill size distribution from mostly adults in 1994 to mostly juveniles
in 2003 may actually be the cause for the changes in condition of the largemouth bass and
chain pickerel. Juvenile largemouth bass and chain pickerel compete with the juvenile
bluegill for food. This may be lowering the condition of the juvenile largemouth bass and
chain pickerel in 2003. The adult largemouth bass and chain pickerel can feed on the large
population of juvenile bluegill, which may explain the increase in their condition in 2003.
The invasion and proliferation of milfoil appears to have caused changes in Pine
Lake’s plant and fish community. There are a few ways that milfoil may be dealt with in the
lake if deemed necessary. The native milfoil weevil, Euhrychiopsis lecontei, has been
shown in some cases to dramatically reduce the prevalence of Eurasian milfoil (Mazzei et al.
1999; Solarz and Newman 2001; Newman and Biesboer 2000; Lillie 2000). Other
biological controls have been used such as the larval form of a moth, Acentria ephemerella
(Johnson et al. 1997). Herbicides have also been shown to work (Getsinger et al. 1997).
Mowing the milfoil is also another option that has been tried but is not as good seeing that it
9
may help the spread by creating fragments that may float to other areas (Boylen et al. 1996;
Eiswerth et al. 2000).
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