Food limitation, predation and allelopathy in a population of Daphnia carinata

Major factors influencing the population dynamics of zooplankton are food availability and predation. Also, especially in shallow lakes, zooplankton may be affected by benthic macrophytes, some species of which produce allelopathic substances, and by suspended sediments which may constitute an additional food resource, or inhibit feeding. We estimated the influence of resource availability, predation by larval fish, and potential allelopathic substances on the population dynamics of Daphnia carinata in Tomahawk Lagoon, a shallow, coastal lake, over 14 months. Zooplankton and fish were sampled regularly, and estimates of food availability were obtained from biomasses of plant pigments and suspended sediments, and from clearance rates, fecundity and lipid-ovary indices of Daphnia. The effects of allelopathic substances in solution on the clearance rates of Daphnia were assessed regularly by comparison with rates in a 'standard' water. D. carinata underwent large fluctuations in abundance (0-258 l(-1)), with peaks of abundance in early spring. The population appears to have been severely food limited in late spring and early summer of both years; predation by juvenile fish (Gobiomorphus cotidianus) is likely to have contributed to a catastrophic decline of the Daphnia population in mid summer and to have effectively suppressed the adult daphniid breeding stock until late autumn. Substances in solution severely inhibited daphniid feeding only once, and not at a time when macrophytes (Nitella hookeri) were abundant. The possible effects of allelopathy from macrophytes on zooplankton warrant further study in other ecosystems.