The relationship between riparian vegetation buffer size and unionid mussel habitats

The effectiveness of riparian vegetation buffers at conserving hyporheic habitats used by freshwater unionid mussels is not well understood. A comparison of sites with intact vs. fragmented vegetation buffers in the east branch of the Sydenham River (Ontario, Canada) revealed differences. Higher pore water habitat quality, including high hydraulic conductivity, dissolved oxygen concentration (DO), diatom and chlorophyte abundance, and low concentrations of ammonia and cyanobacteria, was found in sites with intact buffers; differences were not significant in the north branch of the river due to the high proportion of fine sediments in the surficial geology of the subwatershed. Surprisingly, the concentration of nutrients was similar among branches of the rivers and types of riparian buffers. Results from a partial least square path analysis indicated a strong association between riparian buffer width and hydraulic conductivity, the latter of which was positively associated with pore-water chemical characteristics necessary for mussel existence (e.g., high DO and abundance of diatoms) and negatively associated with pore-water chemical characteristics associated with mussel toxicity (e.g., high ammonia and abundance of cyanobacteria). The abundance of adult mussels mirrored these differences especially in the east branch of the Sydenham River, however, the number of observations on juvenile mussels was limited. The relationship between habitat variables and mussel abundance was supported by the results of a Canonical Correspondence Analysis. Riparian buffers appear to be effective at maintaining unionid mussel habitats provided they are sufficiently large to preclude excess fine sediment loads, because fine sediment affects the rate of hyporheic exchange, i.e., hydraulic conductivity, in the hyporheic zone. The effectiveness of riparian buffers, however, can vary because of factors outside the riparian zone such as surficial geology of the watershed. These results are relevant to the conservation of unionid mussels and other benthic organisms in rivers affected by changes in land-use practices.