Experimental mixing in a north temperate lake: examination of variability in spatial autocorrelation of fish and zooplankton populations

Abstract

In lakes, biotic and abiotic variables interact at multiple spatial and temporal scales, resulting in heterogeneous horizontal distributions of organisms. Although habitat heterogeneity is a vital aspect of ecosystem function and performance, few studies recognize spatial autocorrelation and scale dependence of biotic communities within their abiotic environment. Zooplankton and zooplanktivorous fish represent two trophic levels of pelagic lake food webs whose heterogeneous horizontal distributions may be affected by water movements, prey availability, predatory avoidance, swimming capacity, and thermal tolerance. The most active driver of these distributions depends heavily on scale of analysis. We used hydroacoustic surveys and variogram analysis in a small, north-temperate lake in Vilas County, WI to compare whole-lake horizontal patterns of fish and zooplankton separately, then examined patterns in their interactions. We tested the durability of these patterns in response to a whole-lake manipulation project in which we experimentally destratified the lake, and examined how horizontal patterns change in response to alteration in vertical structure. Lake manipulation decreased the scale of spatial autocorrelation for fish populations. Fish subsequently returned to pre-manipulation spatial patterns after mixing ceased, suggesting the manipulation induced a flight response by fish in response to removal of preferred coldwater habitat. We did not detect changes in zooplankton spatial patterning with mixing, but found support for the hypothesis that external factors, such as wind and wind-induced water movements, may drive large scale horizontal spatial patterns in zooplankton distributions. We also found that fish and zooplankton aggregations exhibit spatial structure at different scales, which may have implications for sampling designs measuring both fish and zooplankton. We detected bottom-up effects where fish aggregate in regions of high zooplankton biomass for at least a portion of the year, and did not detect top-down effects.