Understanding how nutrient cycles and freshwater mussels (Unionoida) affect one another

Nutrient loads and nutrient cycling, especially of phosphorus and nitrogen, are among the most important controls on the character of freshwater ecosystems and have been greatly affected by human actions. Despite the widespread importance of nutrients in freshwater ecosystems, the varied linkages between nutrient cycling and freshwater mussel populations have not been thoroughly described. Here, I explore three of these linkages. First, I suggest that nutrient loads are related to the well-being of mussel populations through several mechanisms, probably producing a nonlinear and non-monotonic relationship between nutrient loads and mussel populations. Second, I discuss the ability of mussels to spatially focus nutrients from the overlying water onto the sediments, which has not been fully appreciated, perhaps because nutrient cycling has been viewed chiefly from the viewpoint of the well-mixed water column rather than the patchy sediments. Third, I discuss the ability of mussel populations to accumulate and release nutrients, introducing time lags into nutrient dynamics and stoichiometry (nutrient capacitance). Finally, I propose a speculative analysis of the role of freshwater mussels in the nutrient cycles of pristine river systems, which must have been much greater than in modern rivers, with their high nutrient loads and depleted mussel populations.