The potential of artificial refugia for maintaining a community of large-bodied cladocera against fish predation in a shallow eutrophic lake

Abstract

The Norfolk Broads are a series of shallow lakes which are highly eutrophic and typified by dense populations of phytoplankton and an absence of submerged aquatic plants. The zooplankton community is subject to intense predation pressure by young fish and is dominated by small-bodied organisms which have a low potential for reducing phytoplankton populations through grazing. Various designs and densities of artificial refugia for zooplankton against fish predation were established in Hoveton Great Broad in order to enhance populations of large-bodied Cladocera. Initially some of the refuges contained higher densities and larger individuals of Daphnia and Ceriodaphnia than the surrounding open water. However, towards the end of the first season and throughout the subsequent two years, population densities and size-structure were similar both within and outside the refuges, although there was still evidence of enhanced body-size of Daphnia within the refuges compared with the open water. The provision of habitat structures designed as refugia from fish predation did not enhance large-bodied cladoceran populations enough to promote this restoration strategy as feasible for eutrophic and shallow lakes.