The Relative Importance of Protozoans, Rotifers, and Crustaceans in a Freshwater Zooplankton Community

A complete analysis of the macroand microzooplankton of a warm monomictic lake indicates that Protozoa dominate the community numerically. During winter mixis, ciliates are found at densities of 1-8x 103.1iter-' and constitute up to 32% of the zooplankton community biomass. With summer stratification crustaceans decline, while both the relative and absolute abundance of protozoans and rotifers increase. Densities of protozoans are highest (1-2 x l o 5liter-') in the metalimnion where scuticociliates bloom in zones of intense bacterial activity. During the period of these blooms (July-October), Protozoa account for 15-62% of the zooplankton biomass. This suggests that Protozoa make a significant contribution to rates of grazing, nutrient regeneration, and secondary productivity and should not be overlooked in zooplankton community studies. Protozoa are rarely included in studies of freshwater zooplankton communities. This is primarily because cladocerans, copepods, and rotifers are considered to be most important in terms of density, biomass, production, grazing, and nutrient regeneration (Hutchinson 1967; Haney 1973; HrbhZek 1977; Porter 1977; Makarewicz and Likens 1979). The Protozoa also require methods of sampling not normally included in zooplankton studies. They are not quantitatively sampled by standard macrozooplankton nets with mesh openings of 64 pm or larger or by the 35-pm-mesh nets recommended for rotifers (see Likens and Gilbert 1970). As a consequence, information about planktonic Protozoa is fragmentary and incomplete. Previous quantitative studies have considered Protozoa over only brief intervals of the year (Sorokin and Paveljeva 1972), at a few depths (Bamforth 1958; Wilbert 1969), without regard to species composition (Rigler e t al. 1974), or exclusive of the total zooplankton community (Bamforth 1958; Hecky et al. 1978). We report results from an annual study of a macroand microzoo'This research was supported by National Science Foundation grant DEB 77-245354 and DEB 8005582 to K. G. Porter and is Lake Oglethorpe Limnological Association Contribution 4. Present address: Dept. Biol., McGill Univ., 1205 Ave. Dr. Penfield, Montreal, Quebec H3A 1B1. plankton association which shows that Protozoa (here used to refer to ciliates and amoebae) dominate numerically and are present at standing stocks comparable to the Crustacea and Rotifera. We describe methods of sampling and preservation for microzooplankton, including protozoans and small rotifers, and provide accurate biomass determinations for crustacean zooplankton. The zooplankton community of Lake Oglethorpe, a 30-ha manmade lake (x,,, = 8.5 m), was sampled over 13 months. The site was chosen for its convenience and because we felt it was typical of the small lakes in the southeast used primarily for recreational fishing. Lake Oglethorpe is a warm monomictic lake with a winter mixis period from November to March followed by the development of stratification in spring. During summer the hypolimnion becomes anaerobic and remains so until turnover in October or November, The surface temperature ranged from 4" to 28"C, the bottom temperature from 3" to 13°C during 1979. Lake Oglethorpe is a eutrophic system based on chemical parameters (J. Meyer pers. comm.) and algal and bacterial densities (Porter and Feig 1980). Primary productivity, however, is below that normally found in eutrophic situations, due primarily to high turbidity (T. Jacobsen pers. comm.). We acknowledge members of the Lake Oglethorpe study group. Y. Feig helped 823 Importance of planktonic Protozoa with the fieldwork; K. Porter provided advice and encouragement. M. B. Csutor shared her unpublished method for preserving Protozoa. The criticisms of an anonymous reviewer, K. Porter, and J. Gerritsen improved earlier versions of this paper.