Effects of beaver dams on cellulose decomposition and water quality

Abstract Beavers are keystone species that modify ecosystem structure and function through dam building, thereby creating a complex mosaic of lotic and lentic ecosystems. We assessed the influence of Eurasian beavers ( Castor fiber ) on cellulose decomposition using a cotton-strip assay and compared physical and chemical conditions between upstream reference reaches and beaver ponds in five low-order streams in Hungary. Standardized cotton strips were deployed in both aquatic and shoreline habitats at each site. Cotton-strip decomposition, measured as tensile-strength loss, was significantly faster in upstream reference reaches than in beaver ponds and faster in aquatic habitats than in shoreline habitats. Turbidity, pH, dissolved oxygen, and oxygen saturation were higher in reference reaches than in beaver ponds, whereas most measured chemical variables did not differ significantly. Additional multivariate analysis indicated that decomposition was associated with several physical and chemical gradients rather than with any single environmental variable. Overall, our results suggest that beaver dams modify habitat conditions and reduce cellulose decomposition rates in ponded reaches. These changes may influence organic matter processing and carbon retention pathways, although direct measurements of carbon pools and greenhouse-gas fluxes are needed to evaluate whether beaver-modified habitats function as net carbon sinks.