Suspended particulates decline along a dense mussel bed in a small Minnesota stream, U.S.A.

Water filtration by freshwater mussels (Bivalvia: Unionidae) is a valued ecosystem service; however, it has not been well studied in natural settings. To examine the potential influence of mussel filtration on suspended particulates, we measured the concentration of Escherichia coli, chlorophyll-a, and total suspended solids along a stream reach with a dense mussel assemblage (Mussel Site) and a stream reach with no mussels (Reference Site) within the Saint Croix watershed in Minnesota, U.S.A. We predicted that these particulates would exhibit a greater decline at the Mussel Site compared to the Reference Site because of mussel filtration. We collected three replicate water samples at upstream, midpoint, and downstream stations at both sites in August, September, and October 2022 to measure concentration values. In accordance with our predictions, concentrations of particulates declined more prominently from upstream to downstream at the Mussel Site than at the Reference Site. Linear mixed-effect models indicated that the interaction between mussel presence (Mussel Site, Reference Site) and sample location (upstream, midpoint, downstream) best explained these patterns. There was lower support for the total suspended solids interactive model compared to the other two particulates. Mussels selectively feeding on organic rather than inorganic particles may help to explain the lower support of the total suspended solids model. Our results suggest that mussels can appreciably reduce suspended particulate concentrations including harmful bacteria. This study provides a useful example of the ecosystem services mussels provide and why their conservation is needed.