When rivers run dry: Perennial pools as ecological refuges for freshwater mussels during drought

Abstract

Abstract As the intensity and frequency of drought and drying events are predicted to increase globally with climate warming and increasing human water usage, there is an urgent need to understand what kind of habitats may serve as refuges for organisms to withstand adverse effects. When flows decrease in rivers, disconnected pools may provide critical habitat for survival of aquatic organisms such as freshwater mussels, a highly imperilled group for which drought and dewatering pose a major threat. The objective of this study was to (1) examine whether pools serve as ecological refuges for freshwater mussels (Bivalvia: Unionidae) in drought‐stricken streams by surveying mussel distribution in an intermittent section (i.e., a section that regularly goes dry) of the San Saba River, Texas, and (2) examine the habitat characteristics that influence whether a pool functions as a refuge. We found that pool dimensions, thermal conditions, past dryness (number of years a pool was dry between 2004 and 2020), underlying geology, and aquatic and riparian vegetation influence the abundance and species richness of freshwater mussels in pools that may serve as important refuges for mussels to avoid desiccation and lethal high temperatures. Despite this, abundances and richness of mussels were low at all sites. Our results suggest that pools that retain water during drought conditions (i.e., perennial pools) may serve as ecological refuges for globally imperilled organisms such as freshwater mussels on shorter timescales (e.g., days to years). Protection and prioritisation of maintaining suitable conditions in refuge pools may help mussels persist as drought and drying issues become more pervasive in aquatic ecosystems. However, if there is a limited number of isolated pools that only allow the survival of a small number of mussels, then these pools cannot prevent overall declines of mussels in drying rivers. Better management of surface water and groundwater in anthropogenically impacted systems could ensure the flowing conditions that are needed to protect mussels in the face of climate change.