Diel Vertical Migration of Zooplankton in Response to Hypoxia in Lacamas Lake, Washington
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Zooplankton, microscopic aquatic animals, are often distributed vertically in the water column in unequal densities, sometimes exhibiting diel vertical migration behavior, typically residing in surface waters at night and at depth during the day. In the past, the availability of food resources and the risk of predation have been shown to influence these behaviors; however, more recently the role of hypoxia and anoxia in the water column has come into question. I hypothesized that zooplankton will alter their vertical distribution and diel vertical migration patterns as a response to seasonally variable hypoxia in Lacamas Lake, a highly modified and managed reservoir in Southwest Washington. During three periods of varying hypoxia - June (mild hypoxia), August (strong hypoxia), and September (moderate hypoxia) - I collected vertically discrete zooplankton samples at six depths at mid-day and mid-night. Statistical analysis shows that, in contrast to our original hypothesis and contrary to some extant literature, zooplankton are indeed occupying the hypoxic zone of the water column. Additionally, we found that some genera (copepods and cladocera) occupy the entire water column, and exhibit little or no diel vertical migration, whereas some other taxa (rotifers) were more heterogeneously distributed. Our results suggest the need for further investigation, specifically, i) observing additional taxa, such as microzooplankton and phytoplankton, for similar behaviors, ii) analyzing which environmental variables may be affecting these behaviors, and iii) analyzing other lakes in the region with similar and different biogeochemical profiles. This will help to generate a clearer picture of how naturally occurring and anthropogenically induced geochemical processes (e.g., hypoxia) affect the community ecology and ecosystem dynamics of freshwater lakes.