The response of Daphnia to nutrient additions and kokanee abundance in Dworshak Reservoir, Idaho

Abstract

Daphnia are a keystone species in lentic systems worldwide and an important prey source for planktivorous fishes, such as kokanee (lacustrine Oncorhynchus nerka). Managers have added nutrients, nitrogen (N) and (or) phosphorus (P), in an attempt to improve the growth or carrying capacity for fish by increasing available prey, such as Daphnia. To be successful, this strategy requires that fish growth be prey limited, and prey availability be nutrient limited. In this study, kokanee in Dworshak Reservoir, Idaho fed preferentially on Daphnia ≥ 0.8 mm TL. The biomass of consumable Daphnia decreased with increasing kokanee abundance, indicating top-down control. However, the biomass of consumable Daphnia was greater at any given level of kokanee abundance when N was added to the reservoir, indicating bottom-up control. These results provide an example where Daphnia biomass was simultaneously controlled by both bottom-up and top-down forces, and that N addition alone can be an effective tool for increasing Daphnia biomass.