MEASURES OF FOOD QUALITY AS DEMOGRAPHIC-PREDICTORS IN FRESH-WATER COPEPODS

This study examines the ability of a number of short-term measures of algal food quality to predict longer term demographic parameters for t wo species of freshwater calanoid copepods, Diaptomus minutus and Epis chura lacustris. Food quality of two species of algae that are usually considered highly edible (Cryptomonas erosa var, reflexa and Chlamydo monas reinhardtii) are compared with respect to: (i) the biochemical c onstituents of the algae (total N, total C and protein); (ii) short-te rm foraging responses by D.minutus to either algal species (e.g. clear ance, ingestion and assimilation rates); (iii) longer term demographic responses (e.g. survivorship and reproduction) by both D.minutus and E.lacustris fed a diet of either algal species. Demographic responses of the two copepod species indicate that C.erosa is a higher quality f ood. In fact, survival and reproduction of both copepod species fed C. reinhardtii were not different from starved treatments. Cryptomonas er osa treatments also had greater C, N and protein. However, D.minutus i ngested five times more C.reinhardtii than C.erosa, indicative of 'com pensatory feeding' in the presence of poor-quality food. Based upon th ese higher ingestion rates, individuals fed C.reinhardtii actually ing ested greater amounts of C, N and protein. Hence, ingestion rates take n alone or coupled with biochemical parameters are not reliable predic tors of consumer demographic response. Assimilation rate, which was po sitive for C.erosa and zero for C.reinhardtii, was the single best sho rt-term predictor of food quality.