Effect of Endosulfan Runoff from Cotton Fields on Macroinvertebrates in the Namoi River

Of the several pesticides used in the pest management strategy for cotton, endosulfan is ranked as having the greatest impact on the riverine ecosystem. A survey of changes in the densities of six abundant macroinvertebrate taxa (ephemeropteran nymphsJappa kutera, Atalophlebia australis, Tasmanocoenissp., andBaetissp. and two trichopteran larvae,Cheumatopsychesp. andEcnomussp.) between upstream and downstream zones of the cotton-growing region in the Namoi River was conducted between November 1995 and February 1996. In November and December 1995, there were few differences in population densities between all sites. In January and February 1996, population densities of the study taxa increased 7- to 10-fold higher at the two reference sites, with low concentrations of endosulfan in sediment and in passive samplers placed in the water column. In contrast, densities of these taxa at sites with exposure to 25-fold higher concentrations of endosulfan remained static and were between one and two orders of magnitude lower than densities at the reference sites in January and February. Population densities ofBaetissp., a mobile ephemeropteran, did not indicate any inverse relationship with endosulfan concentrations. Multivariate redundancy analysis indicated that endosulfan concentrations were the leading environmental predictor of changes in density of the five benethic taxa. Laboratory 48-h LC50values of technical endosulfan in river water were 0.6, 1.3, and 0.4 ppb for early-instar nymphs ofA. australisandJ. kutera, and larvae ofCheumatopsychesp., respectively. Endosulfan sulfate formed a large proportion of the total endosulfan concentrations measured fromin situpassive samplers, indicating that its main route of entry into the river is through surface runoff during storm events.