Investigating Daphnia population dynamics
By: Elizabeth E. Tanner, Advisor—Dr. Rebecca A. Doyle-Morin
Summary
Daphnia are important components of an aquatic ecosystem. They serve both as grazers
on phytoplankton and as prey for fish and other zooplankton, such as Leptodora kindii. The goal
of this study was to document the changes in the Daphnia species populations in a freshwater
lake and determine the factors driving these changes. The study site was Yellowstone Lake near
Blanchardville in southwest Wisconsin. Field samples were taken bi-weekly from June 2012 to
August 2012. Results suggest that the largest factor influencing the Daphnia population
dynamics in Yellowstone Lake is fish predation. Other factors that may also affect the Daphnia
population are inedible algae and Leptodora predation.
Introduction
Daphnia, commonly known as water fleas, are microscopic zooplankton found in variety
of aquatic ecosystems. Daphnia spp. play an integral role in these freshwater habitats. They are
important drivers of the phytoplankton (algae) dynamics of a lake system, but are also very
vulnerable to predation, serving as an important food source for organisms that are higher on the
food chain. Understanding the major drivers behind Daphnia population dynamics may be useful
in maintaining the health of water bodies, particularly recreational lakes. For my study I focused
on the populations of Daphnia in Yellowstone Lake, a recreational freshwater lake in southwest
Wisconsin.
Daphnia are key phytoplankton grazers in aquatic ecosystems and may help regulate their
abundance (Lampert et al., 1986). A study done by Scheffer et. al. studied the effects of Daphnia
on phytoplankton and found that Daphnia helps control phytoplankton populations from the topdown (2001). Low abundance or absence of Daphnia could release phytoplankton from grazing
pressure, potentially leading to an algal bloom. Algal blooms can be unsightly and, if they
produce toxins, may become a health hazard.
While Daphnia may drive algal populations from the top-down, algae abundance may
also drive Daphnia populations from the bottom-up. Only certain types of phytoplankton can be
consumed by Daphnia Excessive nutrient loads can drive the growth of poor food quality
species of algae which can kill Daphnia. For example, Daphnia have difficulty consuming
filamentous algae. These algal types are typically too large for Daphnia to successfully consume
and may even clog their filtering apparatus or poison the Daphnia with neurotoxins if consumed
(Dawidowicz 1990).
Other organisms may affect Daphnia populations via the top-down method. Such
organisms depend on Daphnia as an important food source. Fish regularly depend on Daphnia
as a key prey source, particularly larval fish (Balcer et al., 1984). Predatory invertebrates like
Leptodora kindii also depend on Daphnia as a major food source (Browman et. al. 1989).
Abundance of planktivorous fish and Leptodora kindii may have a significant negative effect on
Daphnia populations.
Some species of Daphnia have an inducible defense against predation. They can increase
their helmet size, effectively making them longer (Brooks, 1967). This is an important adaptation
because fish are limited by gape size (Nilsson et al., 2003). If a Daphnia is too large to fit into its
mouth, the fish cannot prey upon the Daphnia. Leptodora are limited by their feeding basket
size. If the Daphnia are too large to fit into the Leptodora’s ‘baskets,’ the Leptodora cannot feed
on them cite. Presence of helmets is an indicator of predation pressure (Nilsson et al., 2003).
Methods
Study Site
Yellowstone Lake is a 453 acre lake located near Blanchardville, WI. It averages 2.5m in
depth with its deepest point at 4.25m (Department of Natural Resources,2010). Samples were
collected from a central location in the lake with an average depth of 2m.
Picture 1: Aerial view of Yellowstone Lake. Star denotes approximate sampling location
(Wisconsin Department of Natural Resources, 2010).
Yellowstone Lake is situated in an agricultural region and receives abundant agricultural
runoff. Because of this, Yellowstone Lake is hypereutrophic and frequently experiences algal
blooms. The lake is located in Yellowstone State Park and is utilized for many recreational uses
including swimming, boating and fishing. Maintaining the health of the lake is essential for these
recreational activities (Department of Natural Resources, 2010).
Multiple predators of Daphnia are found in Yellowstone Lake. Preliminary research
conducted in summer 2011 detected an active Leptodora population (Doyle-Morin and Tanner,
unpublished). The Wisconsin Department of Natural Resources (DNR) stocks the lake annually
with 7,700 walleye fingerlings measuring from 4.3 to 6.4cm. In 2012, walleye fingerlings were
stocked on June 1st (Sims, pers. comm.). Walleye fingerlings are planktivorous, particularly early
in their development, and may feed upon Daphnia (Jackson et al. 1992). Yellowstone Lake also
contains natural populations of other potentially planktivorous fish including largemouth bass,
sunfish, crappie, catfish, bullheads and big mouth buffalo.
Field Methods
Samples were collected every other week from June through August 2012. Zooplankton
and phytoplankton samples were taken at every half meter depth increment using a 2.2L Van
Dorn Bottle. Zooplankton samples were also collected by towing a 65µl Wisconsin net through
the water column, both vertically and horizontally.
Dissolved oxygen and temperature measurements were recorded using a YSI model
handheld dissolved oxygen meter. Secchi readings and depth measurements were taken using a
Secchi disk.
Laboratory Methods
Daphnia and Leptodora population counts were recorded for every half-meter depth
using a dissecting microscope. For each depth, the entire 2.2 liters was analyzed. Depth interval
samples were also qualitatively assessed under a compound microscope to determine what
phytoplankton were present at each depth increment.
Daphnia and Leptodora body lengths were measured from the full water column tow
samples. These measurements were quantified using a compound microscope fitted with an
eyepiece micrometer. These tow samples were also analyzed under a dissecting microscope to
determine the presence of other zooplankton. Using the Leptodora body length samples, I was
able to calculate their approximate basket size using a linear regression model calculated by
Manca and Comoli (1995). The equation is Y=0.152+0.145x, where x is the length of the
Leptodora kindii body and Y is the length of the basket.
Data was analyzed in Microsoft Excel version 10. One-way ANOVA was used to
determine if the size of Daphnia or the size of Leptodora baskets changed throughout the
summer using an alpha value of 0.05. A regression test was used to determine if there was a
correlation between the populations of Leptodora and Daphnia throughout the summer.
Results
Two species of Daphnia were present in Yellowstone Lake in the summer of 2012:
Daphnia galeata mendotae and Daphnia rosea. Helmeted D. galeata mendotae were present in
every sample while the larger non-helmeted D. rosea individuals were only present in my first
two samples. On June 13, D. rosea had a density of 8.05/L in the Van Dorn bottle samples. On
June 27, only 6 individuals were recovered in the Wisconsin net samples (non-quantitative).
None were recovered in the Van Dorn bottle samples.
35
0.3
30
0.25
25
0.2
20
0.15
15
0.1
10
5
0.05
0
6/13
0
6/27
7/11
7/25
Summer 2012
8/8
Leptodora kindii (#/L)
0.35
D. galeata mendotae (#/L)
40
D. Galeata
Mendotae
Leptodora
8/22
Figure 1: 2012 Leptodora kindii and Daphnia galeata mendotae summer population dynamics (n=1).
Measurements include individuals found at each depth sampled.
2012 population dynamics demonstrated a potential negative correlation between D. galeata
mendotae and Leptodora kindii populations (Figure 1). At the beginning of the summer, when
Daphnia populations are high, Leptodora populations are low and the opposite is true for the end
of the summer. To test this, I ran a regression test of the Leptodora and D. galeata mendotae
populations (Figure 2). The result is a significant negative correlation (R²=0.665, p=0.04),
confirming that as Leptodora populations increase, Daphnia decrease.
Daphnia galeata mendotae (#/L)
40
35
30
25
20
y = -76.868x + 23.355
R² = 0.665
15
10
5
0
0
0.05
0.1
0.15
0.2
Leptodora kindii (#/L)
0.25
0.3
0.35
Figure 2: Seasonal regression between Leptodora kindii and Daphnia galeata mendotae densities
(p=0.04).
To determine whether or not the Leptodora I sampled could prey upon the D. galeata
mendotae present at the lake, I measured their sizes. The D. galeata mendotae I sampled were far
larger than the basket size of the Leptodora I sampled (Figure 3). There was a significant
difference in the size of D. galeata mendotae over time. Using a t-test in Microsoft excel, it was
found that the first two sample dates were significantly larger from the last four (Figure 3).
2.5
Average Size (mm)
2.3
2.1
p < 0.01
1.9
1.7
1.5
1.3
6/13
p = 0.4
6/27
7/11
7/25
Summer 2012
8/8
8/22
Daphnia
galeata
mendotae
Leptodora
Figure 3: Daphnia galeata mendotae body length and Leptodora kindii basket size over time
(n=30). Error bars based on standard error.
On June 27, Aphanizomenon, a filamentous algae typically inedible to Daphnia (Noqueira et al
2006), was found in abundance. On July 11, Spirulina, another filamentous algae potentially
inedible to Daphnia (Haney 1987), is one of the dominant algal species in the water column.
Discussion
The Daphnia population in Yellowstone Lake fluctuated throughout the summer.
Abundance was highest at the beginning and end of the summer with low abundance throughout
July. Leptodora populations show the opposite. They are in low abundance in the beginning and
end of summer but are found in high abundance in the midsummer. This shows a significant
negative correlation between these two organisms (Figure 2). Leptodora may be driving Daphnia
population dynamics through predation.
However, as shown in Figure 3, the Daphnia taken from Yellowstone Lake are too large
to be preyed upon by the Leptodora sampled. Leptodora may still be having an effect on the
Daphnia population by feeding on juveniles. Juveniles that fit into the Leptodora basket size
range were relatively absent from my samples, suggesting that the Leptodora may be preying
upon them. However it is unlikely that Leptodora are the main drivers of Daphnia population
dynamics. If Leptodora was driving Daphnia’s population dynamics, one would expect Daphnia
size to increase over the summer due to the selective removal of smaller Daphnia. However, the
opposite is true. Daphnia sizes decrease over the summer (Figure 3). This suggests that some
other factor, selecting for larger Daphnia, is likely driving Daphnia’s population dynamics.
One such factor may be planktivorous fish. Yellowstone Lake was stocked with walleye
fingerlings on June 1st, 2012. This was one month before D. rosea disappeared and the D.
galeata mendotae population crashed. In a stocked South Dakota reservoir, Jackson et al. (1992)
demonstrated summer YOY walleye growth rates of approximately 50 mm/month. They also
showed prey electives that progressed from copepods to Daphnia to Leptodora as the walleye
grew through a similar size range as we predict occurred in the 2012 population (given stocked
size range).
In the beginning of the summer when walleye were first stocked, they may have fed on
zooplankton smaller than Daphnia, such as copepods if they followed the same prey selection
pattern as other lakes. At this size, the walleye would not be able to consume the large D. rosea
and helmeted D. galeata mendotae. As the walleye increased in size, they may have begun to
feed on Daphnia. Because fish are visual predators, the walleye would have first consumed the
much larger D. rosea. Once the D. rosea population was eliminated, the walleye began to feed
more heavily on D. galeata mendotae. This would explain the decline and disappearance of D.
rosea and the population crash of D. galeata mendotae during midsummer.
As previously stated, walleye are visual predators. One would anticipate walleye to feed
on the largest D. galeata mendotae, potentially causing their decrease in size over the summer.
D. galeata mendotae size did not decrease in the beginning of summer, when it is likely that the
walleye were preying upon other zooplankton. The size decrease of the Daphnia correlated with
the walleye growth rate. About the time the walleye were calculated to be large enough to feed
on the Daphnia, the average Daphnia size decreased.
Other studies of Daphnia populations show a similar Daphnia summer population
pattern. A study done in Lake Erie found similar population dynamics, with minimal Daphnia
abundance around July (Wu et. al 1994). In a German lake, another study that found midsummer
decline of Daphnia abundance (Mehner et. al 1998). Both of these studies found evidence that
planktivorous fish may be driving the Daphnia population dynamics in their systems.
Despite the strong evidence that walleye are the major drivers of Daphnia population
dynamics, there are other potential factors that may also have an effect. Other planktivorous fish
that are present in Yellowstone Lake may also be affecting the Daphnia populations. Inedible
algae may also be having an effect. Aphanizomenon and Spirulina, both inedible algae, may have
an adverse effect on Daphnia populations by outcompeting high quality algae and…. Their
abundance during Daphnia’s population crash may be the cause of the Daphnia decline.
To further explore some of these variables, I suggest a few methods for further research.
One method would be taking quantitative phytoplankton samples to statistically compare to
Daphnia. The same can be done with fish densities since the population dynamics are not
currently known for the naturally occurring fish in Yellowstone Lake. Fish stomach contents
could be analyzed to determine if and when the fish are consuming Daphnia.
Daphnia are important to aquatic ecosystems. They are key grazers on phytoplankton and
serve as a key food source for other organisms like fish. Understanding the population dynamics
gives key insight on how to regulate Daphnia populations. Regulating their population may be
helpful in maintaining the health of a freshwater recreational lake like Yellowstone Lake.
Acknowledgements
I would like to acknowledge UW-Platteville for funding through the Pioneer
Undergraduate Research Fellows (PURF) grant program namely, the Association of Retirees for
Excellence, the UWP Foundation and UW-Platteville. I would also like to acknowledge the
UW-Platteville Biology Department for shared resources, facilities, and support (particularly
Ann Bauer-Leffler, Jeff Huebschman, Gloria Stuckey, and Kris Wright). Also I would like to
acknowledge Brian Hardyman and Erika Donar for assisting us with the field work. I also would
also like to acknowledge the Wisconsin Department of Natural Resources for allowing us to
utilize Yellowstone Lake and providing information upon request. Finally I would like to give a
big thanks to my faculty advisor Rebecca Doyle-Morin.
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