A review and evaluation of the effects of hydrodynamic variables on freshwater mussel communities

Abstract

Abstract Riverine species are governed by the physical (i.e. hydrodynamic) forces generated by flowing water. Freshwater species are also disproportionately imperilled compared to terrestrial and marine species in large part due to widespread anthropogenic alteration of the natural flow regimes to which organisms are adapted. Sedentary species such as freshwater mussels (Bivalvia: Unionoida) are especially vulnerable because they are unable to move quickly in response to novel flow patterns created by impoundments and water withdrawals. We evaluated the effects of hydrodynamic variables on mussel communities. We reviewed the literature to synthesise the relationships between three categories of hydrodynamic variables (hydrologic, simple hydraulic, and complex hydraulic) and mussel community parameters. We evaluated which variables are most useful in predicting mussel presence, abundance, and species richness, provided recommendations to help standardise their use in characterising mussel habitat and reviewed the reciprocal influence of mussels on hydraulic forces at the sediment–water interface. Hydrologic variables such as stream size and anthropogenic flow alteration were useful in predicting basin‐ and landscape‐scale changes in mussel communities. At finer scales, complex hydraulic variables—mainly shear stress and Reynolds number—were more useful at predicting mussel community responses. Simple hydraulic variables (flow velocity, depth, discharge) had mixed success as predictors of mussel community parameters and are likely to be indirectly correlated to mussel responses because they are used to calculate more complex variables. More emphasis should be placed on species‐ and basin‐specific responses of freshwater mussels when considering the effects of the hydrologic regime on mussels, with an eye towards sustainable water management (e.g. flow restoration, environmental flows). Simple hydraulic variables should be assessed across the range of discharge at a site, in combination with channel and substrate characteristics. This allows the calculation of complex hydraulic variables that may be more directly limiting to mussel communities. Mussels also alter near‐bed hydraulic characteristics, resulting in feedbacks between mussel community parameters and local hydraulic variable s. Researchers should consider that mussel beds have a reciprocal influence on near‐bed flow characteristics and should continue to explore the effects of such ecological engineering for possible interspecific facilitation benefits between mussel species.