The importance of macrophyte bed size for cladoceran composition and horizontal migration in a shallow lake

Abstract

Cladoceran composition and did horizontal migration were studied in 2,10 and 25 m diam- eter macrophyte exclosures established in the littoral zone of shallow Lake Stigsholm, Denmark. The exclosures were protected from waterfowl grazing, but open to fish. The macrophyte community com- prised Polamogeton pectinatus, Potamogeton pusillus and Callitriche hermaphroditica. Cladocerans were sampled randomly every third hour inside and outside the macrophyte exclosures during a 24 h period. In the 2 m exclosure, the pelagic species Ceriodaphnia spp. and Bosmina spp. dominated during the day, mean density being as high as 3430 indiv. h1. During the night, density decreased to 10-20% of the daytime density, thus indicating diel horizontal migration. In the 10 and 25 m exclosures, the daytime mean density of Ceriodaphnia spp. was 865 and 202 indiv. h1, respectively, and did not decrease at night. In contrast to the pelagic species, the density of macrophyte-as sociated species tended to be higher in the 10 and 25 m exclosure than in the 2 m exclosure. In the daytime, Eurycer- cus lamellatus density in the 2,10 and 25 m macrophyte exclosures was 7,28 and 16 indiv. H, respec- tively, while that of Simocephalus vetulus was 11, 171 and 92 indiv. I1, respectively. There was no day-night difference in the density of macrophyte-associated species. We conclude that cladoceran community composition varies with macrophyte bed size, and that the edge zone between the bed and open water is an important daytime refuge for potentially migrating pelagic cladocerans.