| dc.description.abstract | Benthic habitats and their benthic fauna, including the freshwater pearl mussel (Margaritifera margaritifera), have been investigated monthly in two small rivers in Southeast Norway from June to October 2023. Due to large variations in hydromorphological conditions in running water, 3 different stations were investigated in both rivers, River Hoenselva and River Skorgeelva.
By the Surber sampling method, we found 77 taxa in River Hoenselva and 79 taxa in River Skorgeelva. Dominant taxa groups were Chironomidae larvae (42%), particularly abundant in June, and Baetis rhodani (16%). We also performed surface counting of freshwater pearl mussel, as well as substrate digging to investigate potential recruitment.
Average water temperature was equal in both rivers, 14.3°C in River Hoenselva and 15.1°C in River Skorgeelva. There was a significant difference in water velocity between the two rivers, 0.31 and 0.36 m s-1 respectively, while highest velocities were observed in June, i.e. 0.59 m s- 1 in Hoenselva, and 0.49 m s-1 in Skorgeelva. During the investigated period, the water depth at the 3 stations in Hoenselva varied between 23.6 – 34.0 cm, compared with 19.0 – 24.0 cm in Skorgeelva. There was no significant difference in substrate index between the two rivers. All 6 investigated riverbed sites were dominated by gravel and cobble. As pH earlier has been documented to be ≥ 6.4 already more then 20 years ago, pH should not be critical for recruitment of freshwater pearl mussels in the rivers. Thus, the redox potential (EH) in sediments at 5 cm depth were measured and documented significant lower redox potential conditions in Skorgeelva (573 mV), compared to Hoenselva (609 mV), but redox values below 400 mV were never recorded.
According to the NMDS modeling, the temperature, redox potential in the substrate, and the substrate index contributed to grouping of the benthic macroinvertebrate community. Based on further correlation analysis, temperature was significantly negatively correlated with the Shannon index. We also revealed a weak significant positive correlation between redox potential and the Shannon index. Accordingly, the lower redox potential in River Skorgeelva sediments, might be a contributor to the lower diversity index in this river compared with River Hoenselva.
We were unable to identify a relationship between the density and/or diversity of benthic organisms and the density of freshwater pearl mussels. Due to data limitations, it was not possible to further test the interactions with more advanced statistical models. However, both rivers were classified with good ecological condition based on our results. To better assess the relationship between freshwater pearl mussels and the benthic community, more data would be necessary, preferably from multiple rivers and longer time series. A different approach, such as a stratified study design focusing on areas with high and low mussel densities, could also improve the results. | |