Trends in Unionidae (Mollusca, Bivalvia) communities in Romania: an analysis of environmental gradients and temporal changes

Freshwaters are among the most imperilled habitats worldwide and Unionidae species are recognised as one of the most important and endangered parts of the communities they shelter. Understanding the changes in the structure of Unionidae communities and their responses to environmental variables are issues insufficiently studied and understood. We evaluated 35 naiad communities along geographical and ecological gradients in Romania, and measured 10 environmental factors at each sampling location. Using constrained ordination analysis, we learned that the most significant variables explaining the variance in species composition were flow, human impact and elevation. Among the levels of the habitat factorial variable the most explicative were the lower Danube habitats, which shelter the richest naiad assemblages. Unionids diversity decreases significantly with increases in elevation and human impact. By comparing some communities with those of two decades ago, a contrasting image emerges. In some rivers where the former heavy pollution has been greatly reduced, the naiads are recovering. However, the average diversity of the communities that we compared is statistically lower at present than two decades ago, indicating that in some areas the ecological state of the freshwater systems is slightly worsening. Main causes are pollution, hydrotechnical constructions, invasive species and climatic changes.