Regulation of cladoceran filtering rates in nature by body size, food concentration, and diel feeding patterns1

Abstract

Relationships of body length (L) and filtering rate (F) of four Daphnia species and Holopedium gibberum were examined in situ during the day and night. During the day filtering rates were low and variable. At night filtering rates were higher and more consistent, with large animals filtering at disproportionately high rates. This resulted in regressions of log F vs. log L with intercepts, slopes, and correlation coefficients that were generally lower in the day than at night. Feeding rates (f) of Daphnia were calculated with seston µm dry weight as a measure of food concentration (C). There were striking differences in the day and night response of feeding rates to food concentration. In the day Daphnia feeding rates increased slowly with increasing food level, reaching f_max at a relatively high incipient limiting food concentration (ILC), whereas at night feeding rates rose rapidly with food concentration to a maximum about twice the day f_max, at about half the day ILC. Multiple regression equations were developed to predict F from L and C. Exceptionally high filtering and feeding rates shortly after the upward migration of Daphnia may be due to a daily hunger cycle, although hunger alone does not account for the high filtering activity sustained throughout the night. Diel feeding patterns of large and small D. pulex were asynchronous, resulting in a reversal of the F:L relationship in the early morning when small animals filtered more rapidly than did large animals.