Gradient of fish predation alters body size distributions of lake benthos

We used normalized size spectra analysis (originally developed in the study of pelagic communities) and a fish bioenergetics model to examine whether predation by fishes affects the size structure of macrobenthos, We conducte d the study over five years in three north temperate lakes in which fish we re experimentally manipulated while allowing for natural recruitment and mo rtality in lakes with fish. The resulting variation in predator populations allowed us to address how prey community size structure responds to a grad ient of predation pressure. The slopes of normalized size spectra were used to characterize whether community biomass is relatively distributed as sma ller or larger individuals with regard to mass. We ask whether predator eff ects on prey are manifested through fish presence/absence or if a gradient of predation pressure is important. Tn addition, we examined whether consum ption rate of prey by fish is a better measure of predation effects on prey size structure than fish biomass alone. Our results suggest that benthos size structure responds to a gradient of f ish consumption, rather than a qualitative distinction of fish presence or absence. Consumption rates by fish on benthos explained more of the variati on in slopes of normalized size spectra of benthos than fish biomass alone. Slope values were more variable at lower consumption rates, which included no predation by fish. Conversely, high consumption rates by fish produced consistent, predictable body size distributions of benthos in which large i ndividuals were relatively underrepresented. However, these results may not be apparent through more traditional analyses based on predator or prey pr esence/absence, abundance, or taxonomic based measures. Generalizations add ressing patterns of body size distributions within and among ecosystems may be advanced by examining predation and other underlying size structuring m echanisms.