The effects of sediment types on the structure of bottom invertebrate communities in a small European lowland dam reservoir

The study compares the structure of invertebrate communities inhabiting two types of bottom sediments in Krzynia Reservoir, known also as Lake Krzynia (on the Słupia River, northern Poland). The lake is prone to intensive sedimentation because it is limnic. This limnicity is due to the impact of the river, rather than to surface runoff from the catchment area, where the heavy predominance of natural habitats over transformed ones slows the transport of mineral suspensions from adjacent terrain. In the organic-poor sediments dominated by sand and gravel, the mean density of benthic invertebrates was ten-fold higher than in silty, organic-rich sediments, while invertebrate biomass was only slightly and non-significantly higher in organic-poor sediments than in organic-rich sediments. Simultaneously, the mean invertebrate body weight was markedly lower in organic-poor sediments (0.03 g) than in organic-rich sediments (0.18 g). No significant differences in taxonomic diversity (number and Shannon index of taxa) were found between the studied sediment types in the analysed communities. Among the major invertebrate groups, the Oligochaeta and Mollusca significantly differed in density between sediment types, while the Oligochaeta and Chironomidae differed in biomass. An increased percentage of silt fraction in bottom sediments resulted in a drastic decrease in abundance of benthic fauna, but their biomass declined only slightly. Thus, plans to construct new reservoirs should aim to reduce the rate of silt accumulation in them, in order to preserve their primary function and support biodiversity in transformed river valleys.