The response ofDaphniato 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.

Optimal Method for Biomass Estimation in a Cladoceran Species, Daphnia Magna (Straus, 1820): Evaluating Length–Weight Regression Equations and Deriving Estimation Equations Using Body Length, Width and Lateral Area

“…It is the main predator of phytoplankton, causing the clear water phase phenomenon in spring, as well as a food source for fish, especially visual-dependent predators, because of the large size of individuals. Thus, determining the biomass of Daphnia, located in the intermediate position between phytoplankton and fish communities, is important for understanding the material and energy transfer processes of the grazing food web [21][22][23]. As Daphnia is too small for direct measurements of either wet or dry weight, its biomass is generally estimated indirectly using known regression equations obtained from the relationship between body length and weight [4,24].…”