Physicochemical assessment of Unio crassus habitat quality in a small upland stream and implications for conservation

Abstract

The abundance of Unio crassus (Philipsson 1788) has declined over the last decades. Despite the high conservation status of this species, knowledge on its ecological requirements is scarce. The objective of this study was to identify key habitat characteristics in areas with recent recruitment of U. crassus in a small upland stream. Furthermore, we investigated stretches where the species is presently absent. Sediment deposition, redox potential, flow velocity, water depth, nitrogen and phosphorus load were investigated. Fine sediment deposition was high with 19.4 kg m−2 month−1 at colonized and 13.3 kg m−2 month−1 at non-colonized stretches of the Sallingbach. At all study stretches, redox potentials in the interstitial zone varied around 300 mV which constitutes the boundary value between oxic and anoxic conditions. Results of chemical water analyses indicated high nitrogen loads. Nitrate nitrogen concentrations averaged between 4.1 and 6.5 mg NO3–N/l at all study stretches, significantly exceeding the currently proposed threshold value of 2.0 mg/l for functional U. crassus streams. The results in this study suggest that U. crassus is more tolerant to eutrophic habitat conditions than previously expected. Our findings show that currently considered physicochemical parameters and high fine sediment deposition cannot mechanistically explain the occurrence of U. crassus in the stream.