Experimental evidence for density-dependent effects and the importance of algal production in determining population growth rates of riverine zooplankton

Food limitation effects on life history traits of lake zooplankton have been well documented but few studies have examined linkages between population growth rates and food resources in riverine environments. In rivers, allochthonous inputs of particulate organic matter may mitigate food limitation effects allowing density-independent mechanisms associated with washout (discharge) and feeding interference (turbidity) to assume greater importance. We experimentally manipulated densities of commonly occurring riverine zooplankton (Bosmina longirostris and cyclopoid copepods) within 20001 mesocosms containing ambient or algal-enriched food resources. The experiment was repeated through time (July, August, September) to represent the range of zooplankton densities and food resource levels observed in the Ohio River during warm-water, low-flow conditions. High growth rates and low sensitivity to density-dependent effects were observed during July when particulate organic carbon (POC) and chlorophyll concentrations were highest. Lower growth rates and stronger response to density-dependent effects were observed during August and September experiments when POC and chlorophyll concentrations were lower. Direct manipulations of algal abundance resulted in higher growth rates when gains in chlorophyll were accompanied by increases in the edible size fraction (September experiment). Algal C concentrations were found to be a significant predictor of variation in population growth rates for Bosmina but not cyclopoids. Algal C concentrations in the Ohio River rarely fell below experimentally derived minimum food thresholds but were often below saturation thresholds suggesting that population growth rates were constrained by autochthonous food resources despite the prevalence of allochthonous carbon. Copyright (c) 2005 John Wiley & Sons, Ltd.