AN IN-SITU TEST OF THE EFFECTS OF FOOD QUALITY ON DAPHNIA POPULATION-GROWTH

In years of low planktivory by fish, midsummer declines and low popula tion abundances of Daphnia pulicaria in Lake Mendota, WI, USA have bee n attributed to poor food quality and low food abundance. This hypothe sis has been proposed because of midsummer blooms of blue-green algae in this eutrophic urban lake. We tested the hypothesis by performing i n situ food manipulations during the midsummer decline of the D. pulic aria population, and during the early autumn low population period. In July, animals held in clear plastic vials containing GF/F filtered la ke water exhibited poor survivorship, ceased producing offspring after three days, and had low lipid reserves. Daphnids fed whole-lake water or filtered water enriched with Chlamydomonas survived at a significa ntly higher rate and produced abundant offspring. In September, there were no significant differences in survivorship among treatments, but animals fed water enriched with Chlamydomonas produced significantly m ore offspring than animals in the other treatments. The hypothesis of poor food quality causing the midsummer decline of the Daphnia populat ion is not supported by our experiments. Because predation by juvenile fish does not appear to be important in regulating the midsummer Daph nia population during these years, alternate hypotheses for the midsum mer decline are suggested. These include increased predation by invert ebrate predators such as Leptodora kindtii, a life history shift by da phnids during midsummer to production of resting eggs, and a combinati on of low levels of planktivory and deteriorating feeding conditions a cting together to cause the population decline.