Food Resources, Survivorship, and Reproduction of the Omnivorous Calanoid Copepod Epischura Lacustris

Abstract

Previous studies have shown that many if not most copepods are omnivores and that all zooplankton experience substantial fluctuation in the abundance of food resources. We attempt to integrate these two observations by determining the implications of fluctuations in both the abundance and type of available food resources for the survival and reproduction of the omnivorous calanoid copepod Epischura lacustris. Egg production and survivorship were measured in laboratory experiments as food type (zooplankton and phytoplankton) and concentration were experimentally manipulated. In addition, the reproductive activity of a field population was monitored as the abundance and composition of the food resources varied through the year. There are three main results. (1) The alga Cryptomonas erosa and the rotifer Euchlanis dilatata both, when offered alone, serve as adequate foods to support survival and reproduction of Epischura. (2) However, survival and reproduction of individuals feeding on Cryptomonas vary temporally, even when resource availability is held constant. Extended survival and high egg production rates in the spring are followed by poor survival and minimal egg production rates later in the year. In contrast, survival and reproduction of individuals given the rotifer do not vary significantly across the seasons. (3) Field measurements suggest that Epischura lacustris reproduction in situ may depend upon relatively large or evasive animal prey, such as the cladoceran Holopedium gibberum and the rotifer Polyarthra remata.