A Survey of Diel-Vertical Migration of Freshwater Zooplankton at Pinchot Lake Eric Holtzapple Department of Biological Sciences, York College of Pennsylvania Introduction Diel Vertical Migration of Zooplankton in Lakes DVM is a pattern of movement that organisms such as zooplankton undertake over a twenty-four hour time span. Two major reasons that zooplankton exhibit DVM are predator avoidance and for metabolic advantages. The Predator Avoidance Hypothesis states that visual predators such as fish feed near the surface at shallow depths during the day. In response, zooplankton migrate to deeper, darker depths to avoid being seen by the fish (Bollens & Frost, 1989). At night, zooplankton migrate toward the surface, where food is found in greater abundance (Lampert, 1989; Loose & Dawidowics, 1994). The Metabolic Advantage Hypothesis states that zooplankton migrate to deeper waters during the day where the water is cooler. The cooler the water, the less energy zooplankton must exert (Loose & Dawidowicz, 1994). They Come to the surface at night to feed when the surface layer is cooler. In some lakes, DVM is not observed. Zooplankton tend to live in deeper waters both day and night when food is abundant at these deeper depths. Also, stratification and oxygen profile can limit the depth to which zooplankton can migrate. Anoxic conditions observed in these stratified waters do not favor DVM as zooplankton are unable to migrate into the water lacking oxygen. In shallow waters, there is little benefit for DVM as light intensities, oxygen, and temperature do not drastically fluctuate between surface and deeper waters. RESEARCH QUESTION Can we locate two populations that would show different DVM patterns within a single lake? HYPOTHESES Null H0: There will be no difference in DVM of freshwater zooplankton between shallow and deep areas of a single lake. Alternate H1: Freshwater zooplankton will exhibit more pronounced diel vertical migration in deeper areas of the lake. Methods Samples Were Collected from three depths at two locations in the lake Location 1 Location m m m 6.096m m 2 replicates/time 06:00, 12:00, 18:00, 24:00 Three plankton nets with 153μm mesh were attached (at designated sample collection depths) to a telescopic fiberglass pole anchored to the front of a boat. Dissolved oxygen and temperature were also collected. Samples were collected over a distance of 60 feet at a rate of 1 foot per second which was tracked by GPS. Zooplankton Samples were Preserved in 5% formalin and were later counted in their entirety in the lab. Two-way ANOVA’s with Bonferroni post tests were used to analyze the data. Results Location 1 Zooplankton Location 2 Zooplankton Figure 1 shows data of relative abundance of copepods and daphnia at location 1 with respect to time. +/- Standard error accounting for the differences between the replicates are displayed with each data point. Figure 2 shows data of relative abundance of copepods and daphnia at location 2 with respect to time. +/- Standard error accounting for the differences between the replicates are displayed with each data point. Copepods – (B) and (D) at 0m – (C) and (D) at 6m Daphnia –No Significant differences between the separate time plots Copepods – (C) and (D) at 3m Daphnia – No Significant differences between the separate time plots Figure 3 shows data of dissolved oxygen and temperature at location 1 with respect to depth. Dissolved oxygen within the individual graphs above, was considered to be a significant drop with depth. Dissolved Oxygen – No Significant differences between the separate time plots Temperature –No Significant differences between the separate time plots Figure 4 shows data of dissolved oxygen and temperature at location 2 with respect to depth. Temperature and dissolved oxygen within the individual graphs above, was found to drop significantly with depth. Dissolved Oxygen – No significant difference between the separate time plots Temperature –No Significant differences between the separate time plots Conclusions Copepods at location 1 show an increase in abundance at both surface and bottom at 24:00. Although this may indicate migration, it is not the expected trend of vertical migration. Although Daphnia do appear to decrease at midday at the surface in this experiment, this change is not significant. Perhaps this trend would become significant if the amount of replicates were increased. A future study could be conducted to determine where the zooplankton are migrating, since it isn’t vertically within the water column. Copepods did show a change in abundance at Location 1, but not at Location 2. This can be expected because light intensities at Location 2 are not as variable, thus the benefits of DVM are not as great. Oxygen and temperature profiles for each Location indicate that the waters are indeed stratified, though the deeper depths were not anoxic to the point of being uninhabitable. Acknowledgements I would like to thank Dr. Nolan for dedicating her time and expert advice into this study. Location 1 Environmental Factors Location 2 Environmental Factors Literature Cited Bollens & Frost, B (1989). Zooplanktivorous Fish and Variable Diel Vertical Migration in the Marine Planktonic Copepod Calanus pacificus. Limnology and Oceanography. 34, Lampert, W (1989).The adaptive significance of diel vertical migration of zooplankton. Functional Ecology. 3, Loose, C, & Dawidowicz, P (1994). Trade-Offs in Diel Vertical Migration by Zooplankton: The Costs of Predator Avoidance. Ecology. 75, Location 1 Location 2