Cladoceran composition and diel horizontal migration were studiedin 2, 10 and 25 m diameter macrophyte exclosures establishedin the littoral zone of shallow Lake Stigsholm, Denmark. Theexclosures were protected from waterfowl grazing, but open tofish. The macrophyte community cornprized
Potwnogeton pectinatus,Potamogeton pusillus and
Callitriche hemaphroditica. Cladoceranswere sampled randomly every third hour inside and outside themacrophyte exclosures during a 24 h period. In the 2m exclosure,the pelagic species
Ceriodaphnia spp. and
Bosmina spp. dominatedduring the day, mean density being as high as 3430 indiv. l
1During the night, density decreased to 1020% of the daytimedensity thus indicating diel horizontal migration. In the 10and 25 m exciosures, the daytime mean density of
Ceriodaphniaspp. was 865 and 202 indiv. l
1, respectively, and didnot decrease at night. In contrast to the pelagic species, thedensity of macrophyte-associated species tended to be higherin the 10 and 25 m exclosure than in the 2 m exclosure. In thedaytime,
Eurycercus lamellatus density in the 2, 10 and 25 rnmacrophyte exclosures was 7, 28 and 16 indiv. l
1 respectively,while that of
Simocephalus vetulus was 11, 171 and 92 mdiv.l
1, respectively. There was no thy-night difference inthe density of macrophyte-associated species. We conclude thatcladoceran community composition varies with macrophyte bedsize, and that the edge zone between the bed and open wateris an important daytime refuge for potentially migrating pelagiccladocerans.
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