The practical importance of the interactions between fish, zooplankton and macrophytes in shallow lake restoration

Abstract

Zooplankton, using macrophytes as refuges from predation by zooplanktivorous fish, are believed to be important in maintaining the macrophyte-dominated state in shallow lakes. Their grazing upon phytoplankton is also believed to be instrumental in preserving water transparency in the establishment phase of macrophytes which follows an attempt to restore a shallow lake from the effects of eutrophication. This paper interprets the results of fish, zooplankton and macrophyte interactions from an intensive 3-year study in the Norfolk Broads of eastern England. In the presence of even a low density of 0+ fish (0.2 m−2), Daphnia spp. which typically dominate the cladoceran community in spring, and through grazing are responsible for producing clear water, are typically reduced to very low levels. This decline may become protracted, but not eliminated, by the presence of macrophytes, implying a refuge effect. The efficacy of any refuge effect appears to increase with an increasing proportion of the water column occupied by macrophytes (PVI). As macrophytes develop, the grazing role is taken over by Ceriodaphnia spp. and Simocephalus spp., and may be at a sufficient rate to maintain clear water at least within macrophyte stands and possibly in intervening open water areas through diel migration of the zooplankton. A macrophyte PVI of 30–40% may provide an adequate refuge for these species through the mechanism of predation-free space, although this depends on fish density and community structure. At high densities (1 m−2) of a suite of fish species, including efficient foragers in open water (roach, Rutilus rutilus) and within macrophytes (perch, Perca fluviatilis), any refuge effect is nullified. In stable macrophyte-dominated lakes, the shift in fish community structure towards a higher proportion of piscivorous compared to zooplanktivorous fish may have a role in promoting the refuge effect through changing the distribution of zooplanktivorous fish. Predation upon zooplankton may also be reduced through the provision of alternative prey in the form of macrophyte-associated macroinvertebrates for an alternative fish stock dominated by perch, rudd ( Scardinius erythrophthalmus) and tench (Tinca tinca).