Developing environmental flow recommendations for freshwater mussels using the biological traits of species guilds

North American freshwater mussels (Unionidae) are a diverse and imperilled fauna that are very sensitive to flow alterations. Previous attempts to develop environmental flows for mussels have struggled to accommodate their varied habitat requirements and complex life histories. We review what is known about the habitat requirements of mussels, how they can vary among species within a community, and how this variation influences the effectiveness of different environmental flow methodologies. We propose a trait-based environmental flow method that addresses the needs of mussel guilds differentiated by their thermal tolerances and reproductive strategies. Used previously for fish, plants and macroinvertebrates, the guild approach groups species by traits and identifies flow requirements of guilds as opposed to entire communities. We apply the guild approach to the mussel fauna of the Kiamichi and Little Rivers in south-eastern Oklahoma, U.S.A., and illustrate how changes in the hydrograph can be made to better meet the seasonal flow needs of differing mussel guilds and their host fish. The guild environmental flow method can provide flow recommendations that cater to the life history and habitat requirements represented within diverse mussel communities. In addition, the method can be adapted among regions where trait data are available or combined with rating models that estimate trait data for understudied species.