Parallels and contrasts between intermittently freezing and drying streams: From individual adaptations to biodiversity variation

Intermittently freezing and drying lotic systems are common in many parts of the world. These ecosystems provide dynamic habitats for biota, as both freezing and drying processes result in the loss of water flow along stream and river channels. However, research into the ecological effects of intermittent freezing on stream biota has remained relatively scarce compared to the recent increase in studies exploring the ecology of intermittent streams that dry. Climate change is predicted to alter the distribution and dynamics of intermittently freezing stream types. A better understanding of the responses of stream biota to intermittent freezing would inform predictions of the effects of ongoing climate change on these ecosystems. In this review, we compare the effects of both freezing and drying events on stream biota, ranging from individual-level adaptations to population effects, community-level biodiversity variation and food webs. Despite major contrasts in environmental conditions (e.g. the drying stream channels are exposed to a direct interaction with the riparian zone during the dry phases and the freezing stream channels are enclosed during the freezing phases), we show that both freezing and drying phenomena have pervasive effects on the structure and dynamics of aquatic stream biota. At the individual organism level, aquatic taxa use a range of physiological, morphological, life-history, and behavioural responses to persist despite changing habitat conditions in both freezing and drying streams. The variety of organisms' responses can result in both biodiversity increases and decreases, which may affect population and community dynamics in both ecosystem types. Future research should further explore the population and community-level effects of freezing in stream ecosystems. Due to the ecological parallels between intermittently drying and freezing streams, approaches developed in research into drying streams may also be applied in freezing stream ecosystems. This could improve our understanding of climate change effects on freezing stream ecosystems.