Demography of a natural population of Daphnia retrocurva in a lake with low food quality

The hypothesis was tested that Daphnia exhibit low population recruitment in a lake with phosphorus (P)-deficient food, where Daphnia populations failed to increase in response to greatly lowered minnow density. Egg ratio analysis was performed on Daphnia retrocurva from L110 of the Experimental Lakes Area, Ontario, in 1994. L110 has high food abundance (seston carbon (C) ~ 100 μM C), but the seston is P deficient (C:P atomic ~ 600). Chrysophytes dominated the algal community. The fish community in this lake was manipulated in 1993 and 1994 by the addition of northern pike, Esox lucius. Zooplanktivorous minnows were greatly reduced after the piscivore addition, but the expected increase in abundance of the Daphnia to this altered predation regime was not observed. If food quality constrains the response of the Daphnia to changed predation, there should be low recruitment in the relatively sparse Daphnia population in this lake. Birth rates estimated by standard egg ratio analysis were <0.1 day^-1 for most of the sample period. In comparison to annual averages from the literature, recruitment in L110 was moderately, but not dramatically, low. Daphnia exhibited pronounced 'normal' vertical migration (up at night, down during the day) all year, and a strongly declining size at first reproduction through the season. Population density and birth rates seemed to show much less short-term change than previously observed in other lakes. Although the demographic predictions for food quality constraints seemed clear, in situ demography unfortunately neither supported nor strongly rejected the overall hypothesis of food quality limitation in this lake with very high C:P ratios. Possibly, direct demographic effects of low food quality will be difficult to observe in nature.