A field study on the dependence of the fecundity of Daphnia spec. on food concentration.

The dependency of the fecundity of Daphnia spec. on the food concentration was studied in Lake Constance. To eliminate the influence of temperature the number of eggs per adult female was plotted against the concentration of particulate carbon (<50 μm) at the estimated time of the production of the eggs. The curve obtained is similar to the results of laboratory investigations. The minimum food concentration for egg production is slightly above 0.2 mg C/1; at 0.7 mg C/1 the curve reaches a plateau. The absolute values of the threshold concentration and the "incipient limiting level" are compared to laboratory results. It appears that the shift of the field curve to higher concentrations is caused by the imcomplete availability of the 50 μm particulate carbon fraction to the daphnids. Daphnia in Lake Constance seem to be food limited during most of the year.