Filtering activity of Daphnia in low concentrations of a pesticide

A concentration of lindane (y-hexachlorocyclohexane) of 0.05 mg liter-‘, much lower than the 48-h lethal concentration (LC,,) of 3.8 mg liter-‘, resulted in a 25% depression in the frequency of movements of filtering limbs and mandibles of Daphnia pulex in a food-free medium. Different instars (from 2nd to 8th) were affected to the same extent. Animals 6-l 2 h after molting were more sensitive than those 30-36 h after molting. Due to the greater number of molts before reproduction in larger Daphnia species, the effect of low pesticide concentrations may be size-selective and, therefore, responsible in part for the disappearance of large cladocerans from eutrophic lakes enriched from agriculture. It is believed that pesticides may amplify the expression of eutrophication in lakes by reducing the abundance of filter-feeding zooplankton that otherwise might keep algal populations under control (Hurlbert et al. 1972; Hurlbert 1975; Shapiro 1980). Lakes enriched with nutrients from agricultural land in their drainage basins are also frequently enriched with pesticides that are used along with fertilizers. The zooplankton populations of eutrophic lakes enriched from agriculture are therefore more frequently exposed to pesticides than those of oligotrophic lakes. Zooplankters in eutrophic lakes are also exposed to more intense predation by planktivorous fishes (Shapiro et al. 1975) and to severe food limitation that results from high densities of inedible, indigestible, interfering, and toxic algae (Gliwicz 1975; Porter 1977). Assimilation rates and reproduction are affected more in large cladocerans because filtering rates are more depressed by interfering (Gliwicz 1977) and toxic (Lampert 1982) algae in larger species than in smaller ones. Any factor causing a further decrease in filtering rate may result in decreasing reproduction and, therefore, in the extinction of the large cladocerans which are more efficient in keeping algal blooms under control (Shapiro 198 1). Pesticides are rarely present in lake water in quantities lethal to planktonic herbivores. Yet, much lower sublethal concentrations may reduce feeding rate and reproduction in animals (e.g. see Koeman and Strik 1975). A concentration of dichlobenil or atrazin a tenth of the 48-h LC50 depressed the feeding rate of Daphnia by half (Kersting and Honing 198 1; Pott 1980). Even though all individuals may survive the effect of a diluted pesticide, an animal population may become extinct soon afterward if its reproduction is halted or, mortality being high for other reasons, only slightly reduced. We determine here the lowest concentration of a commonly used pesticide (lindane) to produce in 48 h a detectable decrease in filtering rate of a large cladoceran (Daphnia pulex Leydig) and at which stage of its growth the cladoceran is affected most. We estimated the filtering rate of Daphnia as the frequency of the movements of the filtering limbs (thoracic appendages) in animals exposed to the slow flow of a foodfree medium containing various concentrations of the pesticide (O-5 mg liter-‘). The mesh size of its filtering apparatus does not allow D. pulex to retain small particles which pass 0.45~pm Millipore filters (Gophen and Geller 1984; Brandelberger 1985). We selected this approach deliberately to avoid aggregation of the pesticide on food particles and subsequent poisoning of animals with uncontrolled doses of the pesticide assimilated with ingested food. From pilot experiments we found that such poisoning depends first, on the adsorption rate of a pesticide on food particles (i.e. on the kind