Thibault Datry

Natural variation in immersion and emersion affects breakdown and invertebrate colonization of leaf litter in a temporary river

Flow pulses that alternately immerse and expose benthic habitats are widely recognized as key determinants of biodiversity and ecosystem functioning in rivers. Terrestrial leaf litter input, colonization, and breakdown are also key processes in river ecosystems, but little is known about the effects of alternating immersion and emersion on these processes. We used litterbags to examine breakdown, microbial activity, and colonization of Populus sp. leaves by invertebrates along a natural gradient in immersion and emersion (i.e., submergence and exposure to air) in a temporary river. Rates of leaf litter mass loss, microbial activity and colonization by invertebrates differed among litterbags that were permanently immersed, intermittently immersed and permanently emersed, and breakdown rate coefficients (k) decreased with increasing cumulative emersed duration (the total number of day of emersion during the experiment). In contrast, the frequency of emersed periods had no detectable effects on these variables. k was positively correlated with the density of invertebrate shredders in immersed litterbags, with microbial activity and shredder density in intermittent litterbags, and with microbial activity in emersed litterbags. These correlations suggest that the relative importance of microbial activity on k increases with emersed duration, due to the periodic elimination of aquatic shredders and the scarcity of terrestrial detritivores. The fact that leaf litter breakdown was detectable under permanently emersed conditions indicates that mechanisms other than shredding by invertebrates, such as leaching and photodegradation, are dominant in dry river habitats.

Invertebrate communities in gravel-bed, braided rivers are highly resilient to flow intermittence

In naturally disturbed systems, harsh environmental conditions act as filters on the regional species pool, restricting the number of taxa able to form a local community to those with traits promoting resistance or resilience. Thus, communities in highly disturbed ecosystems may be less sensitive to a given disturbance than those in less disturbed ecosystems. We explored this idea by examining the response of aquatic invertebrate communities to flow intermittence in gravel-bed, braided rivers (BRs). Flow intermittence is considered a major driver of communities in rivers, but its influence on communities in BRs, which are recognized as naturally highly disturbed environments, is relatively unexplored. We used a multisite Before-After–Control-Impact (BACI) design to quantify the effects of drying events of different durations (moderate: 2–3 wk, severe: 1–3 mo) on invertebrate communities in 8 BRs in southeastern France. As predicted, no effects of flow intermittence were detected 1 to 4 mo after flow resumption on taxonomic richness, composition, or functional diversity of communities facing moderate drying events. Communities subjected to severe drying events were similar to those in perennial reaches as few as 19 d after flow resumption. Moreover, communities showed functional redundancy and no loss of functional diversity after drying events. These results differ from those of studies in other river systems, where persistent effects of flow intermittence on communities generally have been found, and highlight the need for cross-system comparisons that explore the effects of drying on communities. Identifying the processes (e.g., niche selection, cotolerance) and habitat features (e.g., hyporheic zone refugia) that promote community resilience in BRs will advance our understanding of how anthropogenic stressors and climate change may affect communities in freshwater ecosystems.

Gammarus pulex (Crustacea: Amphipoda) avoids increasing water temperature and intraspecific competition through vertical migration into the hyporheic zone: a mesocosm experiment

The saturated interstices below and adjacent to the riverbed (i.e., the hyporheic zone) can be a refuge for biota during low flows, flow cessation and river drying. Prior to complete drying, organisms are constrained by abiotic and biotic factors (e.g., water temperature, competition) and may respond through vertical migration into the hyporheic zone. However, it remains unclear when temperature and competition become harsh enough to trigger migration. Furthermore, potential consequences of using the hyporheic zone, which is often food-limited, on the survival, effects on ecosystem function and physiology of organisms are unknown. We tested the hypotheses that (1) Gammarus pulex, a widespread detritivore, migrates into the hyporheic zone to avoid increasing surface water temperature and intraspecific competition and (2) that these factors would reduce their survival, leaf mass consumption and energy stores. Using 36 mesocosms, three temperature (15, 20, 25xa0°C) and species density levels (low, medium, high) were manipulated in a factorial design over 15xa0days. Increasing temperature to 25xa0°C and a threefold increase in density both caused G. pulex to vertically migrate, and the interaction of these factors was additive, rather than antagonistic or synergistic. Importantly, survival, leaf consumption and glycogen content were reduced in high temperature and density treatments, suggesting tradeoffs between tolerating harsh surface conditions and limitations of inhabiting the hyporheic zone.xa0Identifying that the hyporheic zone is used by G. pulex to avoid high water temperature and intraspecific competition is a key finding considering the global-scale increases in temperature and flow intermittence.

New records of water mites (Acari: Hydrachnidia, Halacaroidea) from Patagonia (Chile)

Abstract New records of water mite species (Acari: Hydrachnidia, Halacaroidea) from Patagonia (Southern Chile) are reported. Four species, Anisitsiellides australis Smit, 2002, Peregrinacarus falklandensis Bartsch, 2001, Lobohalacarus weberi (Romijn & Viets, 1924) and Soldanellonyx monardi Walter, 1919 are new for the fauna of Chile. The first two species are previously known only from the type locality in Argentina and the Falkland Islands, respectively.

Habitat Fragmentation and Metapopulation, Metacommunity, and Metaecosystem Dynamics in Intermittent Rivers and Ephemeral Streams

Abstract Intermittent rivers and ephemeral streams (IRES) are highly heterogeneous systems, where habitat conditions range from fully aquatic to fully terrestrial. The dynamics of these habitats culminate in variation in connectivity between sites, which has multiple consequences for populations, communities, and ecosystems. IRES can thus be best understood in the context of metasystems, where local dynamics are influenced by regional processes. In this chapter, we synthesize recent work describing how metapopulation, metacommunity, and metaecosystem dynamics are driven by the hydrological variability of IRES. To date, very few studies have considered metapopulation issues in IRES, and the same is largely true for metacommunity and metaecosystem research. IRES are excellent arenas to study metasystems in highly dynamic systems. Research on metasystem issues is important for developing and testing ecological theory and also necessary to manage and conserve biodiversity and ecosystem services in IRES in the face of global change.

The biota of intermittent rivers and ephemeral streams: terrestrial and semiaquatic invertebrates

Intermittent rivers and ephemeral streams (IRES) can support a diverse and often abundant terrestrial and semiaquatic invertebrate (TSAI) fauna. This fauna can inhabit the shoreline, the surface of exposed gravel bars, unsaturated gravels, dry riverbeds, riparian zones, and floodplains. TSAI communities are typically dominated by ground-dwelling beetles, ants, and spiders, but many other taxonomic groups are represented. Much less is known about the species composition and ecological roles of TSAIs of IRES than their aquatic counterparts. In this chapter, we discuss the habitat requirements of TSAI fauna, as well as their taxonomic and functional diversity. We describe approaches to sampling them and the collection equipment required, the adaptations of IRES invertebrates to flow intermittence, and their roles in processing organic matter and nutrients. Finally, we identify threats to TSAI communities, outline existing knowledge gaps, and suggest how to manage IRES ecosystems to conserve or restore TSAI biodiversity. The explicit inclusion of IRES in management and legislation will help safeguard the diversity of TSAIs.

Genetic, Evolutionary, and Biogeographical Processes in Intermittent Rivers and Ephemeral Streams

Intermittent rivers and ephemeral streams (IRES) are strong drivers of evolution because they experience spatially variable and recurrent disturbances and succession processes. Alternating wet and dry phases exert strong selection that imposes trade-offs and leads to multiple strategies for spreading risk through space and time. IRES populations range from isolated to connected across landscapes, with contrasting effects on genetic structure and speciation, immigration, and extinction processes. IRES communities are characterized by continuously changing niche- and neutral-related factors, and the potential for allopatric diversification is high for species resistant to drying with low-dispersal ability. Flow intermittence determines the relative roles of local and regional factors controlling IRES community composition and diversity, which in turn influences ecosystem functions. Habitat contraction resulting from intermittence may also change the relative effects of different evolutionary processes acting through time. Consequently, IRES are ideal systems to advance our understanding of evolutionary ecology.

Ecosystem Services, Values, and Societal Perceptions of Intermittent Rivers and Ephemeral Streams

Ecosystem services (ES) are the benefits to human society that are directly attributable to the ecological functioning of ecosystems. To date, ES and their values in intermittent rivers and ephemeral streams (IRES) have been largely overlooked. This chapter discusses ES provided by IRES, the economic values attached to them, and how current understanding of these services affects societal perception of IRES. Principal ES are identified and predictions are made about the level of their provision during different hydrological phases of IRES. Considering the benefits accruing from IRES, their ES are explored in terms of use and nonuse values. Examples are presented to demonstrate the use of economic techniques to estimate the economic value of a number of ES in IRES. Last, this chapter addresses how societal perceptions affect the valuation of IRES and their ES compared to perennial rivers and how these societal perceptions influence the management of IRES.

Conclusions: Recent Advances and Future Prospects in the Ecology and Management of Intermittent Rivers and Ephemeral Streams

Abstract Intermittent rivers and ephemeral streams (hereafter, IRES) abound on every continent in every climate. Arguably the most globally abundant flowing waterway, their distribution is expanding in many parts of the world through climate change and intensifying human demand for fresh water. Yet IRES remain poorly studied, undervalued, and often mismanaged compared with perennially flowing waters. A flow regime that includes intermittence, the defining feature of all IRES, governs their sediment dynamics, biogeochemical processes, water chemistry and the diversity, ecology and evolution of their biota across multiple spatial and temporal scales. Each chapter in this book highlights the central role of intermittence in governing different facets of the physical, chemical, and biological structure and function of IRES ecosystems and their management; this final chapter synthesizes the main findings (presented as fourteen “themes”) and future research directions described in these chapters. Future successful management of IRES relies on using these findings to guide effective strategies while retaining the adaptive capacity to incorporate new information as it emerges.

Hydrological Connectivity in Intermittent Rivers and Ephemeral Streams

Abstract In intermittent rivers and ephemeral streams (hereafter, IRES), hydrological connectivity mediated by either flowing or nonflowing water extends along three spatial dimensions—longitudinal, lateral, and vertical—and varies over time. Flow intermittence disrupts this connectivity, operating through complex hydrological transitions (e.g., between flowing and nonflowing phases). These transitions occur concurrently and interact along all three spatial dimensions, primarily driven by flow regime and catchment geomorphology, modified by human activities. Longitudinally, streamflow cessation and drying interrupt hydrological connectivity, contributing to physicochemical patchiness, habitat isolation, and fragmentation of metapopulations and metacommunities. Laterally, hydrological connectivity established during overbank flows is lost when water levels fall, reducing water-mediated transfers of energy, materials, and organisms from the floodplain and riparian zone. Vertically, flow cessation impairs exchange of surface and shallow groundwater, severely altering hydrological, chemical, and microbial gradients within the sediments. Concurrent interactions and physical discontinuities in hydrological connectivity along these three dimensions produce complex mosaics of physicochemical patches at different scales whose boundaries fluctuate over time in response to the flow regime. This complex patchiness underpins the characteristic physical, chemical, and biological diversity at multiple scales along longitudinal, lateral, and vertical hydrological dimensions in IRES.