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

The hypothesis was tested that Daphnia exhibit low populationrecruitment in a lake with phosphorus (P)-deficient food, whereDaphnia populations failed to increase in response to greatlylowered minnow density. Egg ratio analysis was performed onDaphnia 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 fishcommunity in this lake was manipulated in 1993 and 1994 by theaddition of northern pike, Esox lucius. Zooplanktivorous minnowswere greatly reduced after the piscivore addition, but the expectedincrease in abundance of the Daphnia to this altered predationregime was not observed, If food quality constrains the responseof 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 <0.1day^–1 for most of the sample period. In comparison toannual averages from the literature, recruitment in L110 wasmoderately, but not dramatically, low. Daphnia exhibited pronounced‘normal’ vertical migration (up at night, down duringthe day) all year, and a strongly declining size at first reproductionthrough the season. Population density and birth rates seemedto show much less short-term change than previously observedin other lakes. Although the demo graphic predictions for foodquality constraints seemed clear, in situ demography unfortunatelyneither supported nor strongly rejected the overall hypothesisof food quality limitation in this lake with very high C:P ratios.Possibly, direct demographic effects of low food quality willbe difficult to observe in nature.