ECOTOXICITY OF 3,4-DICHLOROANILINE IN ENCLOSED FRESH-WATER PLANKTON COMMUNITIES AT DIFFERENT NUTRIENT LEVELS

The sensitivity of Daphnia magna to 3,4-dichloroaniline (DCA) was stud ied in a multispecies mesocosm study and its response was compared to those of naturally occurring zooplankton species. A series of DCA conc entrations was tested during two experiments conducted in 1.2 m(3) pla stic enclosures, which contained water and natural plankton from Lake Markermeer (The Netherlands) and laboratory-cultured D. magna. Low con centrations of 2.7 and 27 mu gl(-1) were tested during the first exper iment and additional nutrients were supplied to half of the enclosures . Cladocerans were found to be the zooplankton group most sensitive to DCA. The addition of nutrients resulted in a more marked reduction of cladoceran populations at 27 mu gl(-1). In the second experiment, add itional DCA concentrations (27, 90 and 270 mu gl(-1)) were tested unde r nutrient-enriched conditions only. The EC50 for D. magna, calculated from the integrated density over the first 21 days, was 3.2 (1.9-5.5) mu gl(-1) DCA when the results of the two experiments were combined. Two naturally occurring dominant cladoceran species were more sensitiv e to DCA than was D. magna. Less sensitive rotifer and copepod species profited from the reduced competition. The opportunisitic response of the less sensitive species could not prevent an increase in phytoplan kton densities at high nutrient supply. The results show that both the effects of DCA on zooplankton and the diminished control exerted by z ooplankton on phytoplankton are intensified under nutrient-enriched co nditions.