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Abstract

This study examined the functional response to seston concentration of filter-feeding zooplankton in several freshwater lakes in British Columbia. A simple plankton biomass model was used to develop the hypothesis that zooplankton feeding strategies should be of two distinct types - one characterized by an ability to obtain maximal food intake at high food densities and the other by an ability to harvest food efficiently in very low food densities. A radioactive tracer technique was used to measure feeding rates: zooplankton were allowed to graze for short periods of time on natural seston to which small amounts of ³²P-labelled yeast cells had been added. A preliminary series of experiments checked for possible effects of container size, rinsing technique, kind of food, yeast dosage level, length of grazing time, and acclimation time. A total of 53 functional responses, representing 8 species from 6 lakes, were measured. Functional responses were compared by examining two parameters -maximum feeding rate (asymptote of the functional response), and maximum filtering rate (slope of the functional response near the origin). Some of the species occasionally exhibited a sigmoid functional response, but in general most of the responses were of the Disc or Michaelis-Henten type. Most of the species studied had a very similar range of functional response parameters, and essentially identical maximum filtering rates-Maximum filtering rates were independent of temperature from 8° to 20° C. Maximum feeding rates, on the other hand, were functions of temperature, dividing the zooplankton into "Harm" and "cold" species vita highest measured maximum feeding rates at 20° and 8° C respectively. The similarity of functional response parameters over the range of species and lakes studied indicates that there may in fact be a small number of general feeding strategies underlying the observed complexity of agnatic ecosystems.