Diel vertical migration (DVM) is the daily up and down movement of diverse populations in both freshwater lakes and oceans. These long migrations cause daily, local fluctuations in zooplankton density, therefore directly affecting grazing rates on phytoplankton and energy transfer to fish populations. Field research and numerical simulations have identified both the ecological conditions under which zooplankton production can be maximized, along with the constraints on survival, growth and reproduction imposed on alternative DVM behaviors. Accordingly, it appears that variation in DVM, observed in the field, is adaptive. However, mechanisms causing such variation in DVM are unknown, largely because little is known about the importance of genetic variation relative to environmental change. The objectives of this study are: (1) to determine whether genetically different individuals in a population have different DVM distributions in the field (2) the laboratory, determine whether genetically different individuals respond in the same way to experimental environments designed to simulate field conditions, and whether those responses are similar in different environments. The study objectives will provide an understanding of the causes of variability in stratification behavior of the zooplankter Daphnia galeata mendotae. Since stratification behavior largely determines habitat range, this study offers insight into not only mechanisms of DVM but also data on basic zooplankton ecology, determinants of habitat choice, along with maintenance of polymorphisms. Finally, understanding mechanisms of zooplankton DVM means a better understanding of behaviors influencing food availability for fish and grazing rates on phytoplankton.
