THE EFFECTS OF NYMPHAEID (NUPHAR LUTEA) DENSITY AND PREDATION BY PERCH (PERCA-FLUVIATILIS) ON THE ZOOPLANKTON COMMUNITIES IN A SHALLOW LAKE

1. The effects of addition of juvenile perch (Perca fluviatilis) on th e microcrustacean and rotifer communities associated with nymphaeid be ds were studied, at three different plant densities [high (normal), me dium (reduced by a half) and low (reduced to a third of normal)], in e ighteen 2 m x Im enclosures in a shallow lake. 2. At the low and mediu m densities of lilies, Daphnia densities were high in the absence of p erch but very low in the presence of perch. They increased, even in th e presence of perch, to high densities (comparable with those in the a bsence of perch) at the highest plant density. Body sizes of Daphnia h yalina were consistent with high predation by perch at low and medium plant densities but reduced predation at high plant densities. Pattern s of chlorophyll a concentration, in the presence of perch, inversely reflected those of D. hyalina density. 3. At naturally high densities but not at reduced densities, the plants appeared to act as refuges ag ainst predation for the Daphnia. Reductions in oxygen concentrations i n the plant beds were not responsible for the refuge effect, nor could there be avoidance of the beds by the fish. The mechanism of the refu ge effect must therefore lie in frustration of the process of capture of the Daphnia by the fish. 4. Numbers of other small Crustacea and ro tifers were mostly unaffected by fish predation. Numbers of Asplanchna sp., Chydorus sphaericus and copepodites were higher in the presence of fish and, although there was no main effect of fish on numbers of D iaphanosoma brachyurum, there was a plant-fish interaction, with this species being less abundant in the presence of fish at low plant densi ties but more abundant in the presence of fish at medium and high plan t densities. Main effects of plants were few, with only Asplanchna sp. and Keratella sp. decreasing in numbers with increasing plant density . Most taxa changed in numbers with time but interaction effects betwe en time and plants, and fish and plants, were few.