Natural filtration rates of zooplankton in a closed system: the derivation of a community grazing index

Measurements of individual filtration rates by natural suspensions of zooplankton are presented. The products filtration rate x animal concentration, calculated for each of several species-size categories, are summated to estimate the total volume of water filtered by the zooplankton per unit time, here defined as the “Community Grazing Index” (CGI). Seasonally, CGI varies over nearly three orders of magnitude. The Daphnia hyalina individuals present frequently contributed >90% of CGI. Individual filtration rates were highest when the concentrations of suitable foods were limiting (equivalent to 0.1–0.2 μg C ml −1 ), but were depressed after long (3-week) periods of low food availability and when large Microcystis colonies dominated the phytoplankton. Approximately one order of magnitude separated these extremes. Fluctuations in CGI owe relatively more to change in the concentration, size and species distribution of the animal population than to changes in individual filtration rate.