Shading of River Channels as an Important Factor Reducing Macrophyte Biodiversity

Studies of the impact of river shading on biological diversity of aquatic plants were carried out in central and western parts of the Polish Lowlands. Data was collected during field surveys in 2003 to 2010. Studied rivers represented watercourses with small and average watershed areas according to the Water Framework Directive, and sand was the dominant substrate of the river bottom. To evaluate the impact of shading on aquatic plants, biodiversity metrics such as number of taxa, Shannon-Wiener index, Simpson index, and evenness index were used. For statistical analyses, pairs of studied sites were compared and it was found that increased shading was limiting biodiversity, enhancing growth of bryophytes, and had minor influence on macrophyte indices describing ecological status. In the case of most of the diversity indices, high statistical significance was observed for pairs of sites with greater differences of shading between each other.