Spatial and Temporal Variability in Zooplankton-Fish Interactions in Freshwater Communities

Abiotic and biotic factors interact at multiple scales to create heterogeneity in the distribution of zooplankton and zooplanktivorous fish in the pelagic area of freshwater lakes. In this dissertation, I explored the predator-prey relationship between these trophic levels in four studies conducted at a variety of spatial and temporal scales. The objective was to identify the processes that may structure spatial heterogeneity in these populations. At the smallest scales in the laboratory, I simulated light and temperature conditions similar to those found in an oligotrophic lake and observed interactions between age-0 lake trout (Salvelinus namaycush) and mysids (Mysis diluviana), and determined that the intake rate of age-0 lake trout (mg min-1) could be modeled as a function of mysid biomass (mg m-2). I then applied this model to field data collected at intermediate scales at a spawning shoal in Lake Superior and determined that age-0 lake trout distributed in a spatially predictable pattern based on maximizing their growth rate potential. I also explored the more general relationship between pelagic zooplankton biomass and fish density at intermediate spatial scales in three north temperate lakes. In all six depth strata that I analyzed, there was consistent and significant autocorrelation in the distribution of zooplankton biomass, but spatial structure in the distribution of fish density was weaker and more variable. I also detected a significant bottom-up influence of zooplankton biomass on the spatial structure of fish density in three of the six depth strata that I analyzed, but I did not detect any top-down influences of fish density on the spatial structure of zooplankton biomass. Finally, I explored the influence of surface temperature and mesoscale eddies on the distribution of epilimnetic zooplankton biomass and fish density at large scales in Lake Superior. Circulation patterns associated with prevailing wind conditions could explain some of the spatial patterns in zooplankton biomass, but epilimnetic fish distributions showed no pattern during both years of the study.