Response of a macro-invertebrate community to insecticide application in replicated freshwater microcosms with emphasis on the use of principal component analysis.

The effects of chlorpyrifos on a macro-invertebrate community were studied in indoor freshwater ecosystems. A single dose of the insecticide was applied to achieve nominal concentrations of 5 (low dose) and 35 (high dose) μg/1. Acute primary effects of the treatment consisted of the death of most Arthropoda with only some isopods surviving in the low-dose systems. Effects considered as secondary included a decrease in numbers of Turbellaria and Tubificidae in both low- and high-dose systems, and an increase in the abundance of Gastropoda in the high-dose systems. Principal Component Analysis (P.C.A.) revealed that macro-invertebrate communities in all systems were much alike in the pre-treatment period. Similarity between treatments decreased considerably as a result of insecticide application. Similarity between controls decreased from 8 to 10 weeks post application onwards. High dose replicates remained more or less similar until the end of the experiment (week 26). It is concluded that, in combination with basic statistics, multivariate techniques such as P.C.A. are a useful tool to evaluate effects of pesticides on community responses in freshwater microcosm