A statistical approach for the assessment of the toxic influences on Gammarus pulex (Amphipoda) and Asellus aquaticus (Isopoda) exposed to urban aquatic discharges

A statistical procedure has been developed to quantify the mortality response of caged macroinvertebrates in terms of measured environmental parameters. Populations of Gammarus pulex and Asellus aquaticus were caged in urban receiving waters downstream of a combined sewer overflow (CSO) and a surface water outfall (SWO) for 36 d. During this period of exposure, the mortality and heavy metal bioaccumulation responses of populations of both species, as well as seventeen different chemical and hydrological characteristics of the receiving watercourse were monitored. Multivariate statistical techniques, comprising principal component analysis (with VARIMAX rotation) and multiple regression analysis were used to determine the measured variables that influenced mortality. Certain parameters (BOD5, total aqueous copper concentration, flow rate and suspended solids) influenced the mortality responses of both species. Ammonia, total aqueous lead concentrations and dissolved aqueous concentrations of zinc and copper additionally affected the response of Gammarus pulex. Body concentrations of zinc, lead, cadmium and copper and dissolved aqueous concentrations of cadmium were found to be influential upon the mortality response of caged Asellus aquaticus. The relationships between the mortality responses of each species and the selected independent variables are expressed in the form of stable quantitative equations by regressing the dependent variable (ie mortality) against the principal components of the independent parameters. This statistical approach represents an important tool for interpreting large intercorrelated sets of environmental data obtained in situ.