Toward a mechanistic understanding of contaminant-induced changes in detritus processing in streams: Direct and indirect effects on detritivore feeding

Macroinvertebrate feeding is a major rate-limiting step in the processing of detritus in streams. Environmental contaminants can reduce detritus processing by decreasing the abundance or feeding activity of detritivores. Contaminant-induced reductions in detritus processing can have important consequences for the structure and functioning of the stream community. Here we investigate the mechanistic basis for reduced leaf processing in a stream contaminated with motorway (superhighway) runoff. In situ and laboratory studies were used and the work focused on Gammarus pulex (Amphipoda), the dominant detritivore at the site. The in situ feeding rate of G. pulex was significantly reduced downstream of the motorway discharge. Reductions in feeding rate could be the result of contaminants in the discharge having a direct effect on the animal or the discharge may affect the animal's feeding indirectly, by reducing food quality. Food quality may he reduced by contaminant accumulation or changes in microbial conditioning. Laboratory studies demonstrated that the main mechanism responsible for the reduction in feeding was direct toxicity and that this was most severe when animals were in direct contact with contaminated sediments. Neither contaminant accumulation nor differences in microbial conditioning affected the choice or consumption of leaf material by G. pulex over short exposure periods (i.e., < 14 d). However, contaminant-induced reductions in microbial conditioning were important over longer exposure periods (i.e., greater than or equal to 27 d).