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Abstract

A well-structured metalimnetic community, composed mainly of phototrophic purple bacteria, Cryptomonas, and a few species of ciliates and rotifers, was regularly found between 1984 and 1991 in Lake Ciso. These populations appeared during spring, reached high concentrations and remained in the metalimnion until winter mixing. We had previously postulated that these metalimnetic populations could persist in such high numbers because they were free of efficient predation, since the oxygen-sulfide interface provided a refuge from predation. In September 1992, an abundant Daphnia pulex population developed in the lake which resulted in drastic changes in the formerly stable community. The presence of D.pulex induced an extreme clear-water phase in the epilimnion and the metalimnetic populations were severely reduced. Feeding experiments with Cryptomonas and heterotrophic and phototrophic bacteria as food for D.pulex resulted in clearance rates around 0.7 ml individual(-1) h(-1). The impact was very high on the microaerophilic populations, whereas the anaerobic populations were least affected, since sulfide limited the vertical distribution of D.pulex. We conclude from this evidence that the metalimnetic populations in previous years were mostly food limited (bottom-up control) and that their high abundances were in great part due to a lack of efficient predation.