Drivers of Diel Vertical Migration in Zooplankton and the Role Of

The Great Escape: Drivers of Diel Vertical Migration in Zooplankton and the Role of

Dissolved Organic Carbon

BIOS 35502-01: Practicum in Environmental Field Biology Joseph Nowak Advisor: Amaryllis Adey 2019

Abstract In nearly all aquatic environments, zooplankton are observed to inhabit deeper waters during the day and surface waters at night in a process known as diel vertical migration (DVM). There are several proposed theories for why this phenomenon occurs including the avoidance of UV radiation and visual predators, but not much is known about the role of dissolved organic carbon (DOC). In this study, zooplankton tows were performed on the epilimnion of six lakes, three with high DOC and three with low DOC, during the day and at night. Two zooplankton orders, copepods and cladocerans, along with the zooplankton predator Chaobrous were counted in each sample. Copepods and Chaobrous exhibited significant DVM across the six lakes, while Cladocera and Chaobrous showed greater migration in lakes with high DOC. Though the exact driver of the difference in migration patterns were not identified, theories are provided and recommendations for future studies are given.

Introduction

Diel vertical migration (DVM) is a tactic utilized by the majority of marine and freshwater zooplankton taxa, comprising the world’s largest animal migration with respect to biomass (Hays 2003). During the normal DVM pattern, zooplankton reside deeper in the water column during the day and spend nights at the surface. The transitions between these states occur at dusk (ascent) and dawn (descent). The current literature cites two primary drivers of DMV: ultraviolet (UV) radiation and visual predation.

Sunlight contains short wavelengths of UV radiation that can cause damage to the DNA and membranes of zooplankton that remain in surface waters for extended periods of time

(Rautio and Tartarotti 2010). Consequently, it is hypothesized that zooplankton migrate downwards at dawn in order to avoid potentially lethal UV rays that are present during the day.

Though UV radiation can penetrate well past 10 meters in highly transparent and high-altitude lakes, the depth at which only 1% of 380-nm UV-A penetrates is less than 2 meters in most lakes

(Williamson et al. 1996). As a result, many researchers claim that UV radiation can only be responsible for DVM to depths of a few meters (Lampert 1989; Williamson et al. 2011).

1 Visual predation evasion is therefore most often considered the primary driver of DVM.

The underlying assumption for this theory is that herbivorous zooplankton feed on phytoplankton residing in surface waters (Williamson et al. 2011). However, visual predators such as planktivorous fish and Chaoborus, a type of midge larva, can better locate prey in these brighter waters (Williamson et al. 2011). Zooplankton thus stay in deeper, darker waters during daylight hours, coming up to feed at night when visual predation is considerably more difficult

(Williamson et al. 2011). In other words, the benefit of reduced chance of mortality from predation outweighs the cost of a reduced feeding window, or as one research team aptly puts it,

“better hungry than dead” (Kremer and Kremer 1988). Numerous studies have tested and confirmed the presence of this phenomenon. For example, multiple studies have found that zooplankton DVM increases in the presence of planktivorous fish or fish kairomones, while others have determined that smaller zooplankton migrate less than larger ones since they are less prone to visual predation (Dodson 1988; Van Gool E. and J. Ringelberg 2002; Bollens and Frost

1991; Thys and Hoffmann 2005).

Considering the great diversity of zooplankton, it is important to note that DVM behavior is not consistent among all taxa. For instance, Daphnia and other cladocerans exhibit a stronger propensity to avoid UV radiation, while copepods have greater UV tolerance and may even seek it out (Hessen 1994; Kessler et al. 2008; Williamson et al. 2011). Unfortunately, there is a tendency in the scientific community to focus solely on Daphnia when researching DVM in lakes, especially with respect to changes in dissolved organic carbon (DOC) (Williamson et al.

2011).

In lakes and other freshwater bodies, as DOC concentrations increase there can be significant effects on the structure and processes of ecosystems. DOC most often enters such

2 systems in the form of dissolved organic matter (DOM), which can enter a body of water via two types of pathways, autochthonous and allochthonous (Solomon et al. 2015). Autochthonous sources are internal, derived from primary production within the aquatic system, while allochthonous sources are external, originating from primary production outside the system

(Solomon et al. 2015). As a result of climate change, the latter type of DOC inputs has increased in the majority of regions in a phenomenon known as “global browning” (Solomon et al. 2015).

Consequently, understanding the response of lake ecosystems to changing DOC concentrations is critical.

Due to their position among the lowest level heterotrophs in lake ecosystems, the effects of changes in DOC on zooplankton migration have important implications on higher trophic levels. In addition, DVM causes zooplankton to remove nutrients such as carbon and nitrogen from surface waters at night through feeding and release them in deeper waters during the day via respiration and excretion (Schnetzer & Steinberg 2002). Therefore, changes in DVM may have a profound effect on nutrient cycling that impacts the entire lake environment. While some studies have been conducted, there is still no consensus on how DVM differs in lakes of varying

DOC concentrations.

One potential effect of “global browning” is a reduction in the amount of light penetrating through a given lake and a subsequent decrease in total primary productivity

(Solomon et al. 2015). Though a reduction in phytoplankton would likely decrease zooplankton populations, it is unclear how this would impact DVM. In less transparent lakes, phytoplankton are also more constrained to surface waters, which may increase DVM as long as visual predators are present (Moeller 1994). However, some studies have found that chlorophyll

3 concentrations, which serve as a proxy for herbivore food availability, are higher in deeper waters than near the surface (Kiefer et al. 1972; Fee 1976).

As for predation, more transparent lakes in general have lower fish densities (Downing et al. 1990). In less transparent lakes, on the other hand, turbidity decreases the maximum distance at which fish can identify prey (Sweka and Hartman 2003). Both of these factors are believed to favor an increase in DVM in less transparent (high DOC) lakes due to the increased risk of predation and greater concealment benefit gained from going deeper in the water column.

The effect of DOC on UV radiation is less clear. Some researchers theorize that DOM shields zooplankton from harmful UV radiation, while recent studies have disputed this assumption, arguing that UV radiation reacts with DOM to produce reactive oxygen species

(ROS) capable of damaging zooplankton DNA (Solomon 2017). It is therefore difficult to determine if DOC changes the effect of UV radiation on DVM.

While there are lingering questions about the influence of DOC on DVM, even less is known about these effects with respect to different zooplankton taxa. In this study, zooplankton were collected during the day and night from six lakes, three with high DOC and three with low

DOC, to determine if DVM differed between copepods and cladocerans. Chaoborus density was also measured at day and night for each lake to establish if a link exists between predation and

DVM. I hypothesized that both copepods and cladocerans would both exhibit normal DVM in all lakes, but that migration would not be as detectable in low DOC lakes. In addition, I predicted that DVM would also be found in Chaoborus and that it would also be less apparent in low DOC lakes since they are also consumed by fish and may follow the zooplankton migratory pattern in order to feed.

4 Methods

All sampling and analysis were conducted at the University of Notre Dame

Environmental Research Center (UNDERC). The three lakes chosen for the low-DOC group were Bay (5.99 mg/L), Crampton (5.49 mg/L), and West Long (8.09 mg/L), while those selected for the high-DOC group were Hummingbird (25.9 mg/L), Morris (17.49 mg/L, and Tenderfoot

(12.88 mg/L). DOC data for these lakes were taken from previous DOC records provided by

Kelly et al. (2014), Craig et al.(2015), and Weidel et al (2017).

For each lake, all measurements were conducted at the deepest part of each lake based on bathymetric maps provided by UNDERC. For lakes with multiple deepest areas, the one closest to the dock was chosen for convenience (Figure 1). For each lake, a temperature vs depth curve plot was created for the first 4 meters in order to determine the depth of the epilimnion before sampling (Figure 2). Four zooplankton samples per lake were collected via a 12-inch diameter zooplankton tow, two during the day and two at night. These tows were lowered to the depth marking the end of the epilimnion and were raised to ensure as much verticality as possible.

Daytime samples were conducted between 11am and 1pm, while nighttime samples were performed between 11pm and 1am. All samples were taken during the same week and in similar weather conditions (clear to partly cloudy, no precipitation).

These samples were filtered using 35 m mesh, and the zooplankton were preserved in pee cups with ethanol. For counting, samples were removed from ethanol and mixed with deionized water. For each sample, the mass of the empty cup and the cup with sample were recorded. The sample was then stirred, and a subsample was taken using a pipette and placed onto a petri dish with grid. Zooplankton were subsequently identified with a dissecting microscope and counts were recorded. The taxa used for counting purposes were Daphnia,

5 calanoids, cyclopoids, Holopedium, Bosmina, and copepod nauplii, along with Chaoborus.

Identification followed the Practical Guide to Identifying Freshwater Crustacean Zooplankton

(Witty et al. n.d). Additional subsamples were taken until 200 total zooplankton were counted or until no sample remained. The sample was then weighed again to determine the mass of the subsample.

Zooplankton counts were pooled to the level of order for each sample, where nauplii, calanoids, and cyclopoids were grouped as copepods and Daphnia, Holopedium, and Bosmina were classified as cladocerans. Zooplankton and Chaoborus densities were calculated by first multiplying recorded counts by the number of summed subsamples in the sample and then dividing by the volume of the tow. Replicate samples were averaged to generate one density per lake per time of day.

To determine if DVM was present for each of the three taxa, a one-sided (day < night)

Wilcoxon signed-rank test was performed on the six paired day/night densities. A one-sided (low

DOC < high DOC) Mann-Whitney U test was conducted on the difference between night and day densities to ascertain whether DVM was greater in the three high DOC lakes than in the three low DOC lakes for each taxon. These non-parametric tests were chosen due to the lack of normality in the density data. A p-value less than 0.1 was deemed significant for all tests.

Results

Crampton, Bay, and Hummingbird lakes were characterized by the dominance of calanoids and cyclopoids in night samples, while Daphnia composed the majority of daytime zooplankton in the latter two lakes (Figure 3). In Tenderfoot Lake, calanoid and Daphnia

6 densities were considerably greater in nighttime samples, while no sizable difference in densities were present between day and night samples in Long and Morris Lakes (Figure 3).

Both copepods (p = 0.03) and Chaoborus (p = 0.02) showed significance for the

Wilcoxon signed-rank test, suggesting DVM is present for these taxa in the selected lakes

(Figure 4). Chaoborus densities were higher in nighttime samples for all six lakes. No significance was found for cladocerans (p = 0.16), though cladoceran density was considerably greater in night samples in Tenderfoot Lake.

Results of the Mann-Whitney U test showed significance for Chaoborus and copepods, suggesting greater DVM for these taxa in lakes with high DOC concentrations. No such significance was found for copepods (Figure 5).

Discussion

The purpose of this study was to first establish if DVM was present among the selected taxa and also to determine if the degree of migration differed between high-DOC and low-DOC lakes. These findings would then help ascertain how the drivers of DVM are affected by varying concentrations of DOC.

In general, the results of the study were as expected, though some findings were a bit surprising. I found that copepods and Chaoborus exhibited DVM, while cladocerans and

Daphnia generally did not. Daphnia only migrated in one of the three lakes, Tenderfoot Lake.

Since only the epilimnion was towed for zooplankton, it is impossible to determine with any certainty whether cladocerans are residing primarily in the surface waters or at greater depths. If the former hypothesis is true, it may be due to the high degree of DVM in Chaoborus. The cladocerans in this case may be avoiding greater depths during the day due to high Chaoborus

7 densities, instead taking their chances with planktivorous fish near the surface. If cladocerans are staying in deeper waters, however, then it is possible that nutrient quality is in fact greater deeper in the lakes, which would raiser further questions about why copepods still exhibited DVM. For copepods, it is uncertain whether UV radiation or visual predation is the primary driver of migration. The reason for DVM in Chaobrous is equally ambiguous, though reasonable hypotheses include the pursuit of migrating copepods and predator avoidance.

In Morris and Tenderfoot lakes, the size of zooplankton may be an important factor to

DVM or lack thereof. Though zooplankton lengths were not measured, there was a clear size dependent trend in these two lakes. In Morris Lake, where DVM was not present, zooplankton were extremely small in both day and night samples. This observation suggests that migration may not be necessary since visual predation is already so difficult, especially considering the high DOC content in the lake. Tenderfoot Lake, on the other hand, contained primarily small zooplankton in day samples and a more varied population in terms of size in night samples. This trend indicates that only the larger zooplankton are migrating, most likely due to visual predation. Further investigations into the size of zooplankton and DVM as well as their interactions may shed further light on this in the future. Another interesting finding is that cladocerans and chaborus exhibited greater DVM in lakes with high DOC, while copepods did not. In general, the copepods in samples were smaller than cladocerans and much smaller than

Chaobrous, so it is possible these zooplankton do not benefit as much from the reduction in visual predation.

In future field studies, additional measurements would help elucidate the drivers of the trends found in this study. Firstly, additional tows at greater depths would help clarify the level of migration achieved by zooplankton species. Next, the degree of influence of zooplankton size

8 on DVM would be better established if size measurements of individuals were taken. In addition,

UV penetration and fish density data would be invaluable to pinpointing the primary drivers of

DVM in lakes. Finally, year-long studies that record how DVM changes with the seasons would help shed light on the role of thermal stratification on migration patterns.

Mesocosm experiments also provide insight into zooplankton DVM behavior. In such studies, researchers can better focus on specific factors (e.g. nutrients, fish density, UV) than in lakes which are significantly more complex systems. For researchers seeking to determine the direct effects of increases in DOC on DVM, lake division and mesocosm experiments are the two best options. Sampling lakes of differing DOC levels is helpful, but it is much more difficult to determine drivers of DVM when the starting conditions are so different. However, this study adds to the growing literature on zooplankton DVM and proposes theories for differences in migration among taxa and in varying DOC conditions.

Acknowledgements

I would like to first thank my wonderful mentor, Amaryllis Adey, for all the guidance and support throughout the summer. I also thank all the people who helped with the fieldwork, especially those who came to help in the middle of the night: Matty Auborg, Katherine Franz

Alexandra Lugo-Arroyo, Matt Millado, Harrison Wojtas, and Hunter Wojtas. A big thanks to

Matt Gregory, Jasper Leavitt, and Shannon Jones for their help with statistical analyses, as well as to Dr. Cramer and Dr. Belovsky for making UNDERC-East such a great experience. Finally, I would like to thank the Bernard J. Hank Family Endowment for funding and supporting this project.

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13 Figures and Graphs

X X

Figure 1. Sampling locations for Morris (top left), Bay (top right), Tenderfoot (middle left), Hummingbird (center), Crampton (middle right), and Long (bottom) Lakes are marked with an “X”. Each site represents the deepest area of the respective lake.

Bay Lake Hummingbird Lake 0 0 0.5 0.5 Epilimnion Epilimnion 1 1 1.5

) 1.5 )

(

t 2

p p 2.5

2.5 D 3 3 3.5 3.5 4 4 4.5 5 10 15 20 25 30 5 10 15 20 25 30 Temperature (°C) Temperature (°C) Crampton Lake Morris Lake 0 0 0.5 0.5 Epilimnion Epilimnion 1 1 1.5

)

1.5 m

(

t 2 p 2.5

p e

D 2.5 3 3 3.5 3.5 4 4.5 4 5 10 15 20 25 30 5 10 15 20 25 30 Temperature (°C) Temperature (°C) Long Lake Tenderfoot Lake 0 0 0.5 0.5 Epilimnion Epilimnion 1 1 1.5 1.5 )

)

m (

( 2

h t

p 2.5

2.5 e

D D 3 3 3.5 3.5 4 4 4.5 4.5 5 10 15 20 25 30 5 10 15 20 25 30 Temperature (°C) Temperature (°C) Figure 2. Temperature curves for the first 4 meters of each lake. Measurements were taken at 0.5-meter intervals with a standard temperature probe. The epilimnion was determined to end at a depth of 3.5 meters for Bay, Crampton, and Tenderfoot Lakes, 2.5 meters for Morris and Long Lakes, and 1.5 meters for Hummingbird Lake.

Bay Lake Hummingbird Lake 35 50 30

) )

L L 40

/ /

s s l 25 l

a a

u u

d d i i 30

v 20 v

i i

d d

n n

i i

( 15 ( 20

y y

t t

i i s 10 s

n n

e e 10 D 5 D

0 0 Daphnia Calanoids Cyclopoids Holopedium Bosmina Nauplii Daphnia Calanoids Cyclopoids Holopedium Bosmina Nauplii

Day Night Day Night

Crampton Lake Morris Lake 30 250

) 25 )

L L

/ / 200

s s

l l

a a

u 20 u

d d i i 150

v v

i i

d 15 d

n n

i i

( ( 100

y y

t t i 10 i

s s

n n e e 50 D 5 D 0 0 Daphnia Calanoids Cyclopoids Holopedium Bosmina Nauplii Daphnia Calanoids Cyclopoids Holopedium Bosmina Nauplii

Day Night Day Night

Long Lake Tenderfoot Lake 100 120

) )

L L 100

/ 80 /

s s

l l

a a

u u 80

d d i 60 i

v v

i i

d d 60

n n

i i

( ( 40

y y

t t i i 40

s s

n n

e e

D 20 D 20 0 0 Daphnia Calanoids Cyclopoids Holopedium Bosmina Nauplii Daphnia Calanoids Cyclopoids Holopedium Bosmina Nauplii

Day Night Day Night

Figure 3. Averaged densities for all zooplankton taxa for each lake and time of day. Low-DOC lakes are presented on the left while high-DOC lakes appear on the right.

Cladocera 120

)

s l 90 a

d i V = 5, p = 0.156

n 60

(

n 30

0 Bay Crampton Long Hummingbird Morris Tenderfoot Day Night

Copepod 250

)

/ 200

u V = 1, p = 0.031

d i 150

( 100

n e 50

0 Bay Crampton Long Hummingbird Morris Tenderfoot Day Night

Chaoborus 5

)

/ 4

u V = 0, p = 0.015

d i 3

( 2

e 1

0 Bay Crampton Long Hummingbird Morris Tenderfoot Day Night Figure 4. Results of a one-sided (day < night) Wilcoxon signed-rank test performed on the six paired, averaged day/night densities showed significance for copepods and Chaoborus, suggesting DVM is present for these taxa in the selected lakes. No such significance was found for cladocerans.

17 Chaoborus

s W = 9, p = 0.05

D 3

)

i a 2

N u

e d

(

b 1

f i 0 D Low DOC High DOC Cladocera

45 s

i s W = 9, p = 0.05

)

t /

i a 15

N u

e i

(

b 0

f i -15 D Low DOC High DOC

Copepod

s W = 4, p = 0.65 n

D 30

)

i a 20

N u

( e b 10

e r

i 0

D Low DOC High DOC Figure 5. Results of a one-sided (low DOC < high DOC) Mann-Whitney U test performed on the averaged differences between night and day densities showed significance for Chaoborus and copepods, suggesting DVM is more pronounced for these taxa in lakes with high DOC concentrations. No such significance was found for copepods.

18 Appendix

Table 1. Raw zooplankton count, sample/subsample mass, and tow data for high-DOC lakes. Four samples were taken and counted per lake: two during the day and two at night.

Table 2. Raw zooplankton count, sample/subsample mass, and tow data for low-DOC lakes: Four samples were taken and counted per lake: two during the day and two at night.

Table 3. Individual and condensed zooplankton densities for each high-DOC lake sample.

Table 4. Individual and condensed zooplankton densities for each low-DOC lake sample.

Table 5. Summary table of averaged zooplankton densities for all lakes.