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

Abstract

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.

Stochastic processes and ecological connectivity drive stream invertebrate community responses to short‐term drought

“…small size) which evolved within a species’ lineage in response to other selective pressures may also confer an advantage against drought. For example, some of the same mechanisms that protect benthic invertebrates from desiccation in intermittent streams may also be beneficial in systems that periodically freeze (Tolonen et al., 2019). Boreal invertebrate communities may thus already possess some traits that could enable persistence despite drying.…”

Hydrological intermittency drives diversity decline and functional homogenization in benthic diatom communities

“…Under this scenario, Mediterranean freshwater ecosystems are expected to face a huge species loss (Chiu et al, 2017), due to the harsh environmental conditions induced by both hydrological alterations and habitat fragmentation, which act as a filter and select for species displaying specific traits that enhance their survival in these extreme environments (Datry et al, 2017c). In particular, drying up of the streambed causes longitudinal, lateral and vertical habitat fragmentation, limiting the recruitment of new individuals, which drift from upstream or colonizing the riverbed from lateral refugia, such as pools, or from the hyporheic zone, with consequences on species distribution and abundances (Tolonen et al, 2019). Focusing on diatom communities, it has been recently demonstrated that droughts in Mediterranean streams can led to a significant decrease of endangered species (Falasco et al, 2016a), thus reducing species diversity at both local and regional scale, and to changes in community functional traits (Elias et al, 2014;B-Béres, et al, 2019;Novais et al, 2020).…”

“…Flow intermittency also changes diatom community composition throughout the year, with shifts from lotic-to lentic-adapted species up to aerophilous ones during the lentification and the drying up of the riverbed respectively (Datry et al, 2017a). Thus, species surviving during the lentification phase can often represent both a taxonomic and functional subset of those typical of the wet phase (Tolonen et al, 2019). In addition, modifications in river physical elements (such as river banks, channel morphology, substrate composition and riparian canopy) induced by land use changes can play an important role as drivers of diatom composition (Kutka & Richards, 1996;Hill et al, 2000;Hlúbiková et al, 2014) and can furtherly lead to a decrease in species richness (Bona et al, 2008).…”

Abruptly and irreversibly changing Arctic freshwaters urgently require standardized monitoring

“…The losses will include local, regional or global extinctions of species that prefer cold-water environments and cannot migrate beyond the northern coasts to escape warming and competition with warm-adapted species. In addition, as many Arctic freshwater species have specific adaptations to withstand freezing conditions during the winter, such cold-adapted species may be particularly threatened by ongoing climate warming (Tolonen et al, 2019). Hence, cold-adapted species can be considered as 'climate change losers', resulting in an irreversible loss of unique Arctic biodiversity.…”