Food quantity and quality regulation of trophic transfer between primary producers and a keystone grazer (Daphnia) in pelagic freshwater food webs

Abstract

The transfer of energy and nutrients from plants to animals is a keyprocess in all ecosystems. In lakes, inefficient transfer of primaryproducer derived energy can result in low animal growth rates,accumulation of nuisance phytoplankton blooms and dissipation of energyfrom the ecosystem. Most research on carbon transfer efficiency inpelagic food webs has focused on either food quantity or food quality,with the latter considered separately as either elemental stoichiometryor biochemical composition. The natural occurrence and magnitude ofthese types of growth limitations and their combined effects on Daphnia,a keystone grazer in pelagic freshwater ecosystems, are largely unknown.Our empirical models predict that the strength and nature of foodquantity and quality limitation varies greatly with lake trophic state(total phosphorus, TP) and that Daphnia growth rates and thus energy andnutrient transfer efficiency are highest in lakes with intermediatetrophic status (TP 10-25 mu g l(-1)). We predict that food availabilityplace the greatest constraint on Daphnia growth in nutrient poor lakes(TP