Seasonal changes of macroinvertebrate communities in a Western Cape river, South Africa, receiving nonpoint‐source insecticide pollution

Abstract

A field study was conducted at three different sites along the Lourens River (South Africa) to assess aquatic macroinvertebrate abundances and community structures in relation to seasonal changes in rainfall and particle-associated organophosphorous (OP) insecticide contamination. Redundancy analysis indicated OP insecticide contamination (azinphos-methyl and chlorpyrifos) as the only significant variable determining the community composition. Principal response curves indicated that the invertebrate community dynamics of the Lourens River at the most-contaminated site, Lourens River 3 (38 +/- 23.0 microg total OP/ kg suspended particles), differed significantly from the less-contaminated site, Lourens River 2 (8.0 +/- 4.9 microg total OP/kg in suspended particles) during the dry season (October-December; pesticide application period), whereas no difference was found during the wet season (July-September). Ephemeroptera abundances increased significantly (p = 0.0021) at the control site, Lourens River 1, from the wet to dry season, whereas abundances significantly decreased (p = 0.0011) at Lourens River 3. Two-by-three factorial analysis of variance demonstrated a significant interaction of site and season for the three most abundant mayfly taxa, Baetis sp., Demoreptus sp., and Castanophlebia sp., confirming a possible OP effect. Lourens River 3, however, differed significantly from the other two sites in flow, ortho-phosphate, and algae growth, which may partly explain the lower abundance of sensitive species. Apart from the OP contamination, only flow velocities showed significant differences between the wet and dry season at some sites. In conclusion, the present study suggests that particle-associated OPs affected community structure in the Lourens River at levels greater than 30 microg total OP/kg, whereas levels less then 10 microg total OP/kg yielded no significant effects.