An empirical analysis of zooplankton community size structure across lake trophic gradients’

The hypothesis was tested that zooplankton community size structure shifts toward an increased relative biomass of microzooplankton with increased lake trophy at 12 sites in Quebec, The seasonal mean abundance and biomass of ciliates, rotifers, nauplii, cladocerans, and cyclopoid copepods were significantly (P < 0.1) related to lake trophy, but Calanoid copepod abundance and biomass varied independently of lake trophy. Regressions of microzooplankton and macrozooplankton biomass with total phosphorus (TP) were highly significant (P < 0.000 l), and TP explained a large proportion of the total variation (microzooplankton: Y* = 0.72; macrozooplankton: r2 = 0.86). The regression models for microzooplankton and macrozooplankton were not significantly different, refuting the hypothesis that relative biomass changes with lake trophy. Further analysis with a community structure index (the slope of the log weight-log abundance relationship) and mean lengths of various taxa indicated that zooplankton community size structure was not correlated with either TP or chlorophyll. On average, about 40% of the total zooplankton biomass is accounted for by microzooplankton in the Quebec lakes. The inverse relationship between body size and specific flux rates suggests that microzooplankton account for the major portion of zooplankton community rates. Zooplankton biomass and its distribution among organisms of different size are important determinants of grazing (Haney 1973; Gulati et al. 1982; Bogdan and Gilbert 19 82), nutrient regeneration (Hargrave and Geen 1968; Peters and Rigler 1973; Lehman 1980), and production (Banse and Mosher 1980; Morgan et al. 1980; Downing 1984). Although empirical theories can predict zooplankton biomass from indices of lake trophy, existing theories provide only qualitative suggestions about changes in zooplankton community structure with lake trophy. The relation of community structure to community rates ofgrazing, nutrient regeneration, and production among lakes of different trophy is, therefore, not known. The principal generalization concerning size structure and lake trophy is that oligotrophic lakes are often dominated by Calanoid copepods, eutrophic lakes by smaller cyclopoid copepods, rotifers, and cladocerans (e.g. Brooks 1969; McNaught 1975). Although microzooplankton may account for a large portion of the total zooplankton biomass, ’ A contribution to the McGill Limnology Research Centre. 2 Present address: Inst. Ecosystem Studies, The New York Botanical Garden, Carey Arboretum, Box AB, Millbrook, N.Y. 12545. few studies have considered how these forms may vary across lake trophic gradients as a proportion of the total zooplankton community. Microzooplankton are defined as those heterotrophic organisms in the size range of 20-200 pm (Sieburth et al. 1978) and in freshwater consist primarily of ciliates, rotifers, and copepod nauplii. Because of the smaller body size of microzooplankton and consequent higher specific rates of metabolism, it has been argued that oligotrophic environments may contain insufficient energy to support small bacterivorous ciliates and rotifers (Hall et al. 1976; Fenchel 1980; Beaver and Crisman 1982; Pace 1982). It has also been shown that the relative proportion of inedible net phytoplankton increases with lake trophy (Watson and Kalff 198 l), suggesting that more eutrophic lakes would support higher concentrations of detritus and thus enhance bacterial and microzooplankton biomass and productivity (Gliwicz 1969; Hillbricht-Ilkowska 1977; Sprules 1980). These considerations lead to the hypothesis that the biomass of microzooplankton increases relative to that of macrozooplankton as lake trophy increases. Bays and Crisman (1983) found in 35 Florida lakes an increase in the percentage of microzooplankton relative to total zoo-