Organic matter and fine grains as possible determinants of spatial and seasonal variability in bed sediment fauna: A case study from a Hercynian gravel stream

The spatial and seasonal distributions of organic matter and fine grains were tested as possible determinants of fauna distribution in bed sediment of a Hercynian gravel stream. Invertebrate densities and the amounts of fine grains and organic carbon were assessed in freeze-core samples taken along 70 cm depth profiles at three different positions in the stream channel. Sampling was conducted on five occasions of low discharge over two years. The variability in invertebrate community composition was analysed using Detrended Correspondence Analysis with posterior projection of explanatory variables; Variation Partitioning was used to estimate the independent and shared effects of the explanatory variables. We found that the best predictors of the invertebrate community were spatial variables (depth, position in the channel) and then variables influenced by seasonal patterns (surface water temperature and discharge). The influence of organic matter and fine grain content was significant only after eliminating spatial autocorrelation. High amounts of organic matter, randomly accumulated in the sediment, improved the model by explaining high fauna densities. The fine grain content was not a limiting factor to fauna at our study site. It is possible that the large amount of mica flakes in the sediment has caused the arrangement of grains with a pore space sufficient for fauna even when fine grain content was high.