Dynamics of Recruitment Into a High‐Density Mussel Assemblage Below a Small Dam

Freshwater mussels are important components of stream ecosystems and are threatened worldwide. Understanding the factors that influence the abundance and structure of mussel assemblages is essential for their conservation and often requires long‐term data sets to detect gradual change. While many factors related to their survival have been studied, the potential roles of competition among these organisms remain poorly characterized. In this study, we examined the structure of a high‐density mussel assemblage (up to an average of 135 mussels/m 2 ) below a small dam in Minnesota, USA, over a 24‐year period. Between 1998 and 2010, total mussel density declined by 42%; the dominant species, Actinonaias ligamentina , consisted primarily of large adults, and we observed little evidence of recruitment. By 2022, total mussel density declined another 35%, including a decline in large A. ligamentina . This was accompanied by an increase in the density of recruits. The structure of the assemblage also shifted over the study period, resulting in a higher proportion of equilibrium strategists among both adult mussels and recruits, especially A. ligamentina . We suggest that competition for food, space or both may have limited recruitment, and with the loss of large A. ligamentina , these resources became available for juveniles to establish. This research illustrates both the utility and need for long‐term data sets to understand how mussel assemblages may respond to environmental change.