MESOCOSM EXPERIMENTS ON THE INTERACTION OF SEDIMENT INFLUENCE, FISH PREDATION AND AQUATIC PLANTS WITH THE STRUCTURE OF PHYTOPLANKTON AND ZOOPLANKTON COMMUNITIES

1. Little Mere, U.K., received large quantities of sewage effluent unt il 1991, when the effluent was diverted. Experiments, carried out in m esocosms in 1992 and 1993, were designed to predict the effects of: (i ) reduced external nutrient loading; (ii) reduced internal loading fro m the sediment; and (iii) recolonization by fish of the better aerated water. Treatments included isolation of the water from the underlying sediment or exposure to the sediment (which lacked plants in 1992, bu t was covered by Potamogeton berchtoldii in 1993) crossed with differe nt population densities of Rutilus rutilus in 1992 and of Perca fluvia tilis in 1993. 2. Exposure to sediment (as opposed to isolation from i t) resulted in no net change in the biovolumes of most major algal gro ups, but this masked major complementary effects on individual species . The experiments showed a decreasing influence of the sediment, betwe en 1992 and 1993, in determining water chemistry, and an increasing pH between years but no increase in cyanophyte dominance. This had been anticipated because a lake upstream provides abundant inocula, and con ditions in Little Mere after diversion of effluent were expected to fa vour cyanophytes. 3. Roach and perch additions to the mesocosms result ed in reductions in Daphnia populations but increases in numbers of sm all Cladocera and copepods. Plant-associated Cladocera were unaffected by fish. The presence of submerged plants to some extent reduced fish predation effects on Daphnia hyalina. 4. The experimental results in general accurately predicted subsequent events in the open lake.