Submerged macrophytes as refuges for grazing Cladocera against fish predation: Observations on seasonal changes in relation to macrophyte cover and predation pressure

The provision of a refuge from fish predation for large-bodied Cladocera among stands of submerged macrophytes is thought to be an important stabilising mechanism against nutrient-induced phytoplankton increases in clear water shallow lakes. The occurrence of any refuge effect in relation to the seasonal impact of both macrophyte development and recruitment of zooplanktivorous fish was monitored over the summer months (May to September) in three lakes (Cromes Broad, Hoveton Little Broad/Pound End, and Upton Broad) of variable trophic status, fish community structure and nature and extent of macrophyte cover, in the Norfolk Broads in Eastern England. At all sites, Daphnia spp. exhibited a early summer peak of abundance but had declined rapidly by July probably as a result of predation from underyearling fish. In extensive macrophyte stands (Cromes Broad) Daphnia spp. persisted after its elimination in open water, indicating some refuge effect. At sites with macrophyte cover and/or low fish predation pressure, Daphnia spp. was replaced by Ceriodaphnia spp., both with and without Simocephalus sp., thus maintaining large populations of grazing Cladocera, apparently capable of exerting a high grazing pressure on phytoplankton. This occurred, even under high predation pressure from a relatively high density of zooplanktivorous 0+ fish (Cromes Broad), consistent with the refuge hypothesis. In addition, at this site, significant positive associations of cladoceran abundance with increasing macrophyte cover were apparent throughout the summer. From information on the distribution of fish it is suggested that dense macrophytes offer the most suitable refuge through provision of predator-free space.