Turnover time and production of planktonic crustacea in limed and reference portion of a bog lake

Bog lakes may be compared to engines which have operated on low-quality fuel and have, as a consequence, lost efficiency in the conversion of energy. Treatment of such lakes with hydrated lime provides at least a measure of temporary cor rection as inferred by the appearance of algal blooms (Waters 1957) and larger standing crops of zooplankton (Johnson and Hasler 1954, Waters and Ball 1957). The mechanisms by which alkalization augments production have been reviewed elsewhere (Stross and Hasler 1960). The observed increases in standing crops of primary consumers following treatment is presumed to reflect an increase in production at this trophic level. If the assumption is correct, there is, as yet, no good evidence that the increase is transferred to the plankton consumer. Johnson and Hasler found no increase in production of rainbow trout, a secondary consumer in early life, in a lake treated with lime despite the observed increase in the standing crop of Daphnia, the chief source of food for the trout in the lake. They attributed a part of the failure to reduced utilization resulting from dilution of food following a lime-induced increase in depth of the aerobic zone of the lake. The susceptibility of trout to stress when competing with other members of the same species (Brown 1957) also may have been responsible for the failure of the trout to respond to the presumed increase ill production of food. In view of the inconclusive circumstantial evidence, it be came of interest to attempt a direct measurement of production at the level of primary consumers. Measuring production of the planktonic Crustacea, the primary consumers, was made feasible with the help of 2 simplifications. It was assumed first, that all average standing crop was being re placed at a constant rate, and, second, that the rate of replacement of all species represented was approximately the rate of replacement of the dominant species. Because Daphnia dominated both portions of the lake, it was selected for this determination. The choice was facilitated by the fact that growth is proportional to the quantity of food available (Slobodkin 1954, 1959).