Some effects of enclosure on the zooplankton in a small lake

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Abstract
Summary: Observations on the effect of enclosure on zooplankton by introducing two experimental tubes, each holding some 18 000 m3 of water, into a small lake, showed that a limnetic community could be maintained within tubes of this capacity throughout a period of 22 months. Two species, the copepod Diaptomus gracilis and the cladoceran Daphnia hyalina were dominant both in the open lake and in each tube, but Diaptomus gracilis was relatively more abundant in the tubes than in the lake, whereas Daphnia hyalina, especially in spring and autumn, was more abundant in the lake than in the tubes. Several scarce species became relatively more abundant in the tubes than in the lake. These changes are more probably associated with reduced predation, especially by larvae of Chaoborus, than with artificially induced changes in the phytoplankton, changes only poorly correlated with changes in the zooplankton.