Zooplankton nearshore compared to offshore in historically fishless lakes of high elevation are influenced by the introduction of planktivorous fish and water residence time

Abiotic and biotic factors, such as water movement and planktivorous fish, have separately been shown to influence spatial distribution of zooplankton in lakes. The present study examines the independent and combined roles of fish and water residence time (WRT), which influences water movement, in the distribution of zooplankton. Greater zooplankton biomass and body size were expected nearshore as greater offshore water movement causes drift and avoidance of flowing water, and because offshore planktivores consume zooplankton and release kairomones that zooplankton avoid. Effects of WRT and fish on biomass varied depending on major taxonomic group. Total zooplankton biomass was greater nearshore across WRTs in lakes with fish. Alternatively, in lakes without fish, zooplankton biomass was greater nearshore when WRT was shorter and offshore when WRT was longer. Body size was greater nearshore and not influenced by WRT or fish. Species richness was greater nearshore in lakes with short WRT and offshore in lakes with long WRT. Each lake showed moderately unique communities that exhibited reasonable species turnover between near and offshore areas. The present study suggests that fish and water movement do not act independently in their influence on zooplankton distribution.