Midsummer decline of a Daphnia population attributed in part to cyanobacterial capsule production

A midsummer decline in a Daphnia population has been documented in Lake Winnebago, Wisconsin, USA, similar to that which typically occurs in other freshwater lakes throughout the world. We investigated the role of two products of the cyanobacterium Microcystis aeruginosa (microcystin and polysaccharide capsular matrix) in contributing to this population decrease by utilizing laboratory and lake studies. Feeding on either the microcystin-producing, minimally encapsulated M. aeruginosa strain PCC 7820 or the highly encapsulated, non-microcystin-producing strain C3-40, resulted in decreased survival when compared with animals fed chlorophytes. The survivorship of Daphnia fed C3-40 cells washed to remove the capsule increased by 30% over that seen in animals fed encapsulated C3-40. Feeding purified microcystin or capsule alone decreased Daphnia survival to a greater degree than did starvation. Physiological studies conducted while Daphnia were exposed to these food sources showed that ingestion of capsular material resulted in increased post-abdominal claw movements and decreased mandibular movements. Concurrently, elevated respiration rates were measured in Daphnia, implying that capsule increased the energy expended by these animals through increased attempts to reject the material and decreased food intake. Lake studies reflected the results of the laboratory experiments. The midsummer decline of the Daphnia population occurs as the Microcystis biovolume increases and both microcystin and capsular matrix levels rise. While both cyanobacterial products may contribute to the midsummer decline in Daphnia pulicaria, laboratory studies suggest that encapsulation may play the greater role.