Growth response of a benthic detritivore to organic matter composition in sediments

The biochemical composition of lake and stream sediments was analyzed and compared to growth and survival of detritivorous larvae of the midge Chironomus riparius (Meigen) to determine which biochemical parameters correlated most strongly with sediment food quality. Sediments were collected from soft bottoms of 41 water systems and fed to midge larvae. These sediments were analyzed for organic matter (OM) content, total C, N, and P, carbohydrates, proteins, fatty acids, pigments, and grain-size distribution. A microbial assay was used as an indicator of the fraction of easily biodegradable OM. Positive correlations of larval growth or survival with polyunsaturated fatty acids, pigments, and labile OM were found when these biochemical variables were standardized based on dry mass. When variables were standardized based on mass of OM, additional significant positive correlations between larval growth and P, carbohydrates, proteins, and fatty acids of bacterial origin were detected. Similarly, multivariate analyses revealed stronger correlations between larval growth and survival and biochemical variables standardized by OM compared to those standardized by dry mass. We postulate that dilution of OM by mineral particles caused the difference between the standardization methods. Organic matter content of sediments, particularly labile organic matter, appeared to strongly influence detritivore growth.