Differential contribution of concentration and exposure time to sediment dose effects on stream biota

Anthropogenically increased sediment loads are one of the most pervasive pollution pressures on surface waters and carry major ecological implications for biota. We conducted an experiment in artificial streams to quantify the response of 4 common macroinvertebrate taxa (Rithrogena semicolorata, Baetis rhodani, Asellus aquaticus, and Glossosoma boltoni) and benthic algae to doses of inorganic sediment that differed in maximum concentration and exposure time. Relatively minor disturbances (maximum concentration 250 mg/L) decreased % organic content of epilithon significantly after the first 24 h of exposure. However, algal biomass (measured as chlorophyll a) was reduced significantly only in the most prolonged exposure time (7 d). Epilithic nutrient stoichiometry (as indicated by molar organic ON ratios) was not affected by sediment addition. Invertebrate drift was affected strongly by exposure time, and all taxa exhibited statistically significant responses within the first day under all concentrations. However, concentration interacted significantly with exposure time in its effects on the drift of both mayfly species. Macroinvertebrate survival was high or complete for all treatments. Therefore, the differing relative and interactive effects of concentration and exposure time suggest that the common prediction of severity of effect based on a constant relationship between these factors can produce considerable overestimates or underestimates of environmental risk. Biological effects derived from increased sediment loads should be assessed on the basis of their overall dose and on the basis of the specific relationship between the concentration and exposure time that make up the dose.