The use of temporary pond microcosms for aquatic toxicity testing: direct and indirect effects of endosulfan on community structure

Replicate aquatic microcosms can be established using sediment from the basin of dried temporary ponds. When the sediment is flooded with water, the resting stages or eggs of zooplankton, phytoplankton, macrophytes and filamentous algae are activated and a complex community develops in a few weeks. The aim of this study was to lest the utility of using temporary pond microcosms for aquatic toxicity testing; by dosing a set of microcosms with the organochlorine pesticide, endosulfan and measuring physical, chemical and biological responses over a period of 10 weeks. A total of 20 3-1 microcosms were established and after 6 weeks dosed with endosulfan at 0, 1, 10 or 50 mu g l(-1). Th-application was repealed after 3 weeks. The pH, conductivity, dawn and dusk oxygen levels, ammonia, nitrite, orthophosphate, chlorophyll-a, zooplankton and phytoplankton were measured at weekly intervals. After 10 weeks the total composition of each microcosm was determined and endosulfan residues in the macrophytes and sediments measured. The half-life of endosulfan in the water column was approximately 24 h. After 10 weeks between 6 and 12% of the initial dose could be recovered from the sediment in a toxic form (alpha or beta endosulfan or endosulfan sulfate). Total mean endosulfan residues were 0.57, 27 and 61 ng g(-1) dry weight in macrophytes and 1.49, 7.0 and 36 ng g(-1) dry weight in the sediments for the 1, 10 and 50 mu g l(-1) treatments, respectively. At 10 and 50 mu g l(-1) endosulfan, populations of ostracods and calanoid copepods were eliminated and the cladoceran Ceriodaphnia sp. significantly reduced. There were significant increases in Simocephalus sp., tardigrades, filamentous algae, chlorophyll-a and macrophytes. Orthophosphate concentrations were correlated with macrophyte biomass and were lower in the treated microcosms. Nitrite levels were also lower in the treated microcosms. There was quantal shift in composition of the animal community between 1 and 10 mu g l(-1) endosulfan which could be visualized using the ordination technique multi-dimensional scaling. The mechanisms underlying this quantal shift in community structure are analyzed and the advantages and limitations of using temporary pond microcosms for toxicity testing are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.