Diel vertical movement of zooplankton in Douglas Lake as a mechanism of predator avoidance and optimal foraging.

Abstract

Diel vertical movement (DVM) of zooplankton has been shown to follow a pattern of evening ascent and morning descent in response to light levels, amount of food, and risk from visually dependent predators. Since zebra mussels have colonized Douglas Lake, they have potentially altered the typical zooplankton migratory patterns by affecting phytoplankton abundance, productivity and water transparency. We hypothesized that zooplankton migration would be consistent with optimal foraging theory and influenced by fish predation. We predicted higher concentrations of zooplankton where there are higher concentrations of phytoplankton and a stronger correlation at night than during the day. Diel samples of zooplankton and chlorophyll were collected and further analyzed to determine the strength of their linear relationship. There was not a significant linear relationship observed between concentrations of zooplankton and chlorophyll during the day or at nighttime. DVM of zooplankton was observed in Douglas Lake prior to the colonization of zebra mussels, and our results suggest that zebra mussels may have altered the previously described DVM of zooplankton. A trend was observed in our results that, on average, there was a stronger relationship between zooplankton and chlorophyll during the night than during the day. This trend is statistically insignificant, but more sampling may increase the certainty of this relationship. Our results provide support for studies that describe how zebra mussels affect lake ecosystems and might indirectly affect zooplankton behavior.