ENVIRONMENTAL CONTROL OF THE ABUNDANCE AND DISTRIBUTION OF THE CHYDORID CLADOCERA1

Littoral populations of chydorid Cladocera were studied in 1968 and 1969 at 3 collection areas in Lake Lacawac, Pennsylvania. The station on the sandy leeward margin of the lake had peak populations in spring, the one on the detritus rich windward shore peaked in late summer and fall, and the one at the interface between the sandy and detritus-rich shores had peak populations in both spring and fall. At all arcas the chy-dorids were least abundant in summer. There appeared to be ample food available in summer; chydorids were still able to reproduce after water temperatures had reached the summer maximum; and potential competitor species did not increase as chydorids decreased. Observations indicated that the summer population minimum was largely caused by predaceous tanypodinc midges. The midges were most common, and the number of chydorids in their guts increased, just before major chydorid population declines. The midges were rare or absent in spring and fall when chydorids wcrc most abundant. Eight species of tanypodines were found in the lake. Three were common from mid-May to June; the others occurred in July. The first pulse of midges in June devastated the chydorid populations. The second pulse appeared only after the chydorids had recov-ercd and began to develop large populations