The Effects of Suspended Silt on Feeding and Reproduction of Daphnia pulex

Abstract

This paper reports the effect of suspended silt on the feeding and reproduction of Daphnia pulex and the impact of suspended silt and clay on freshwater zooplankton community structure. The effects of suspended silt and clay on the filtering and assimilation rates of Daphnia pulex were determined using a 14C radiotracer method. Both filtering and assimilation rates are severely depressed at even low concentrations of suspended silt and clay. Life table studies also showed population growth rate of zooplankton was significantly diminished by suspended silts and clays. The relative abundance of zooplankton varied markedly between two lakes of differing turbidity levels, the more turbid lake having a higher relative abundance of large zooplankton species. Suspended silt and clay reduced zooplankton feeding and production but probably influenced zooplankton community structure by impairing the ability of visually feeding planktivorous fish to locate their prey. INTRODUCTION Zooplankton play a pivotal role in the transfer of energy in food chains of ponds, lakes and oceans. These small herbivores feed on diffuse plant resources, thereby repackaging energy into a much more concentrated form which can be used by carnivores. There have been many studies of the feeding ecology of zooplankton (Ryther, 1954; Rigler, 1961; Burns and Rigler, 1967; Hall et al., 1976; Porter et al., 1982) and the forces which limit zooplankton populations. Still, there is debate on the relative importance of competition among zooplankton for limited food resources vs. planktivory as the dominant force limiting zooplankton population densities and structuring zooplankton communities (Brooks and Dodson, 1965; Hall et al., 1970; Lynch, 1979; Neill and Peacock, 1980; Zaret, 1980). This debate arises anew when considering turbid lakes characteristic of midwestern and southeastern North America. Such lakes often have large zooplankton in relatively high densities and such large zooplankton could result from interaction with suspended silts and clays in at least two ways. First, dissolved organic matter adsorbed to silt particles ingested by filter-feeding zooplankton could supply added nutrition sources (Marzolf, 1965). Second, suspended silt particles may serve as a refuge for zooplankton from planktivorous fish which locate prey visually (Vinyard and O'Brien, 1976; Gardner, 1981). Vinyard and O'Brien (1976) and Wright (1981) showed that increasing turbidity impairs the ability of planktivorous fish to locate prey. Buck (1956) reported production of redear sunfish (Lepomis microlophus) in low turbidity ponds was 300 times greater than in ponds with high turbidity. While the effects of turbidity on predaceous fish are documented, similar data for herbivorous zooplankton are not available. The purpose of this study is to determine the effects of suspended silt and clay on filtering rates, assimilation efficiencies, population growth and community structure of zooplankton. METHODS Algal cultures used in filtering rate and assimilation efficiency determinations were 1 Current address: Department of Environmental Science and Engineering, University of California at Los Angeles, Los Angeles, California.