Nutritional value of sediments for macroinvertebrate communities in shallow eutrophic waters

The role of the nutritional quality of non-polluted soft-bottom sediments as a factor structuring in situ macroinvertebrate communities was studied in shallow eutrophic waters in The Netherlands. Sediments from clean sites were collected and analyzed for general characteristics (e.g. grain-size distribution) and nutritional value (e.g. C, N, P, polyunsaturated fatty acids). Also, macroinvertebrates were collected from these sediments and identified. In laboratory bioassays, Chironomus riparius midge larvae were exposed to these sediments to assess the nutritional value of each sediment. The abundance of detritivore taxa was positively associated with polyunsaturated fatty acids, bacterial fatty acids, pigments, and mineralization rate. These variables represent newly produced organic matter originating from the pelagic part of watersystems. Therefore, there seems to be a strong connection between the organic matter production in the pelagic part of watersystems and sediment food quality. The growth response of C. riparius larvae measured in the bioassay showed strong correlations with the abundance of detritivorous Chironomidae taxa in the sediment samples. Herbivorous and carnivorous taxa did not correlate with the bioassay results. Highest growth of C. riparius was observed in sediments with abundant detritivorous Chironomidae taxa and lowest growth was observed in sediments with taxa with other modes of feeding. Therefore, growth of C. riparius seems to effectively indicate the nutritional value of sediments for sediment bulk feeders. The use of bioassays with midge larvae permits evaluation of the importance of organic matter as a determinant of community structure without the confounding effects of other factors such as oxygen regime or stability of the substrate, which tend to covary with organic content. It is postulated that such non-food variables select the species that dominate sites. Yet, nutritional value determines the overall density of detritivores and, therefore, is concluded to be a major factor influencing faunal communities in shallow eutrophic water