Riverine endosulfan concentrations in the Namoi River, Australia: Link to cotton field runoff and macroinvertebrate population densities

Population densities of six dominant macroinvertebrate tars (mayfly nymphs Jappa kutera, Atalophlebia sp., Tasmanocoenis sp., Baetis sp. and the caddi sfly larvae Cheumatopsyche sp. and Ecnomus sp.) were negatively correlated to total endosulfan concentrations in the Namoi River (New South Wales, Aus tralia) in the 1995/96 and 1997/98 cotton growing seasons (November-March). Total endosulfan concentrations measured in solvent-filled polyethylene ba gs (passive samplers) at the exposed sites correlated with rainfall, sugges ting that endosulfan entered the riverine environment in runoff from land. At the start of both surveys, in November 1995 and November 1997, there was no significant difference (p < 0.05) between the reference and exposed sit es for both total endosulfan concentrations and mean population densities o f the combined study tare. This indicates distance downstream, which was ne gatively correlated with exposure, was not an important variable in explain ing the significantly higher population densities in other months at the re ference sites compared with those at sites subsequently exposed to increase d (10-25-fold) total endosulfan concentrations. The multivariate analysis o f the 1997/98 data with the principal response curves (PRC) method indicate d that endosulfan explained a significant proportion (25%) of the variation in the total macroinvertebrate community. Principal components analysis (P CA) indicated other covariables were involved, including river discharge. R iver discharge was positively correlated to increased densities of the mayf ly tars at the reference sites in the 1995/96 survey, but these correlation s were reduced to near zero, except for Baetis sp., in the 1997/98 survey.