Self-maintaining plankton: pelagic Cladocera in small microcosms with lake water

Abstract

Monospecific and mixed populations of pelagic cladocerans, Daphnia galeata, Bosmina coregoni, and Diaphanosoma brachyurum of Lake Glubokoe (Moscow Region) were isolated in 0.6 litre flasks with their natural medium — epilimnion water. In the laboratory they persisted for more than 100 days without external additions of food until the experiment was stopped. In three-species cultures Diaphanosoma was outcompeted, but in the presence of only Bosmina it persisted. Daphnia coexisted with Bosmina in two-and three-species cultures the whole period of observations in spite of competition. Birth and death rates of animals in microcosms depended on their density indicating that experimental conditions did not spoil natural population regulation. Demographic time lags underlay the observed oscillations in numbers. Because the time of Cladocera persistence in the cultures was comparable with lake vegetational period I conclude that summer zooplankton is capable of existing solely at the expense of nutrient recycling within epilimnion. Density regulation of cladocerans reinforces the stability of the system ‘herbivore — algae’, but it is weakened by the demographic time lags.