River ecosystems are highly biodiverse, influence global biogeochemical cycles, and provide valued services. However, humans are increasingly degrading fluvial ecosystems by altering their ...streamflows. Effective river restoration requires advancing our mechanistic understanding of how flow regimes affect biota and ecosystem processes. Here, we review emerging advances in hydroecology relevant to this goal. Spatiotemporal variation in flow exerts direct and indirect control on the composition, structure, and dynamics of communities at local to regional scales. Streamflows also influence ecosystem processes, such as nutrient uptake and transformation, organic matter processing, and ecosystem metabolism. We are deepening our understanding of how biological processes, not just static patterns, affect and are affected by stream ecosystem processes. However, research on this nexus of flow-biota-ecosystem processes is at an early stage. We illustrate this frontier with evidence from highly altered regulated rivers and urban streams. We also identify research challenges that should be prioritized to advance process-based river restoration.
Measuring Earth's rivers Palmer, Margaret; Ruhi, Albert
Science (American Association for the Advancement of Science),
2018-Aug-10, 2018-08-10, 20180810, Letnik:
361, Številka:
6402
Journal Article
Recenzirano
Satellite images enable global tally of freshwater ecosystems and resources
A recent communication to corporate leaders warned that “Water is connected to every global risk we face” (
1
). For the ...past 7 years, water crises have ranked among the top five global risks according to the World Economic Forum (
2
). The reason for this concern is that water is linked to environmental, societal, and economic risks—from biodiversity loss and ecosystem collapse to food crises, and from failures of urban planning to energy price shocks. Mitigating water-related risks to humans and ecosystems requires reliable accounting of freshwater resources. An essential first step is knowing where to find Earth's fresh water. On page 585 of this issue, Allen and Pavelsky (
3
) make an important step in this direction by improving estimates of the global surface area of rivers using satellite observations.
Despite its far-reaching implications for conservation and natural resource management, little is known about the color of environmental noise, or the structure of temporal autocorrelation in random ...environmental variation, in streams and rivers. Here, we analyze the geography, drivers, and timescale-dependence of noise color in streamflow across the U.S. hydrography, using streamflow time series from 7504 gages. We find that daily and annual flows are dominated by red and white spectra respectively, and spatial variation in noise color is explained by a combination of geographic, hydroclimatic, and anthropogenic variables. Noise color at the daily scale is influenced by stream network position, and land use and water management explain around one third of the spatial variation in noise color irrespective of the timescale considered. Our results highlight the peculiarities of environmental variation regimes in riverine systems, and reveal a strong human fingerprint on the stochastic patterns of streamflow variation in river networks.
As climate change unfolds, changes in population dynamics and species distribution ranges are expected to fundamentally reshuffle communities worldwide. Yet, a comprehensive understanding of the ...mechanisms and extent of community reorganization remains elusive. This is particularly true in riverine systems, which are simultaneously exposed to changing temperature and streamflow, and where land-use change continues to be a major driver of biodiversity loss. Here, we use the most comprehensive compilation of fish abundance time series to date to provide a global synthesis of climate- and LU-induced effects on riverine biota with respect to changes in species thermal and streamflow affinities. We demonstrate that fish communities are increasingly dominated by thermophilic (warm-water) and limnophilic (slow-water) species. Despite being consistent with trends in water temperature and streamflow observed over recent decades, these community changes appear largely decoupled from each other and show wide spatial variation. We further reveal a synergy among climate- and land use-related drivers, such that community thermophilization is heightened in more human-modified systems. Importantly, communities in which species experience thermal and flow regimes that approach or exceed their tolerance thresholds (high community sensitivity), as well as species-poor communities (low community resilience), also display faster rates of compositional change. This research illustrates that quantifying vulnerability of riverine systems to climate change requires a broadening from a narrower thermal focus to more integrative approaches that account for the spatially varying and multifaceted sensitivity of riverine organisms to the interactive effects of water temperature, hydrology, and other anthropogenic changes.
Storing and managing river flows through reservoirs could dampen or increase climate‐induced fluctuations in streamflow, but interactions between the effects of dams and climate are poorly ...understood. Here, we examined how dam properties control different facets of flow alteration across the coterminous United States (CONUS), and compared alteration trends between dam‐affected and reference stream gages. We quantified departures from the natural flow regime using 730 stations with long‐term daily discharge data. Dam size and purpose explained high variation in flow alteration, and alteration was particularly severe in water‐stressed regions. Importantly, regulation of river flows by dams often dampened climate‐driven alteration (48% of the flow metrics), particularly in watersheds with positive flow trends; while worsening climatic impacts in other cases (44%), or even having dual effects (8%). Our results show that dam and climate impacts on streamflow need to be assessed jointly, and based on a diverse range of flow regime facets (e.g., event magnitude and duration, frequency, and timing) instead of mean annual flows only. By pairing the USGS streamflow records available from upstream and downstream of 209 dams across the CONUS, we advance the notion that dams amplify flow alteration, but also ameliorate some flow alteration metrics. Understanding such potential and limitations is important in light of climate non‐stationarity and a new wave of damming in developing economies, and will be key to further advancing environmental flow science into the future.
Key Points
Dams and climate trends jointly altered river flow regimes across the United States
Flow alteration was particularly severe in water‐stressed regions, sensitive to the flow metric being analyzed, and largely explained by dam size and purpose
Via regulation and associated human activities, dams ameliorated climate‐induced alteration on some flow metrics, exacerbated climate impacts in other cases, or had dual effects
Summary
Due to their ubiquity and highly efficient passive dispersal, benthic diatoms are an ideal group in which to explore the drivers of community spatial structure and the ultimate links between ...such patterns and species autoecology. However, studies addressing these issues have largely overlooked unicellular taxa to date.
We examined the spatial order (i.e. nestedness) of a diatom meta‐community of 122 lotic sites in the north‐east Iberian Peninsula and the relationship between the contributions of individual species to nestedness and their niche breadths measured using outlying mean index (OMI) analyses.
We observed a significantly nested pattern in the meta‐community (T = 14.05, NODF = 22.57), although idiosyncratic species accounted for a substantial fraction (34%) of the global species pool. Hydrological stability was identified as the main driver of nestedness. Highly stable, moderately stable and intermittent sites differed in nestedness and in alpha diversity, with intermittent sites being the most species poor and the least nested.
We also observed a significant association between nestedness and niche attributes, with idiosyncratic species exhibiting wider distributions and larger niche breadths compared with nested species. Overall, our results indicate that diatom communities inhabiting hydrologically stable rivers present a higher level of order in community spatial pattern and a higher proportion of specialist taxa than communities in intermittent streams.
Because recent hydroclimatic models predict an increase in stream intermittency in the Mediterranean region, our findings may help anticipate future diatom biodiversity patterns resulting from global change. We suggest that the current need to protect a diverse network of sites (as opposed to a few species‐rich sites) will be even greater in the near future because local communities are not only probably to become taxonomically and functionally poorer but also less predictable, making it more difficult to implement efficient programmes for biodiversity conservation at the regional scale.
We analysed taxa lists from 447 individual wetlands from several ecoregions across the world using nestedness and similarity‐based multivariate analyses. We examined how similar wetland assemblages ...are across regions, whether variation in assemblages is ordered (nested) or unpredictable (idiosyncratic), whether individual taxa occur predictably or unpredictably across wetland habitats, and if any of these patterns differed between temporary‐ and permanent water habitats. We found that macroinvertebrate assemblages were highly nested (N = 0.947), but unexpectedly 37 of the 40 most widespread taxa (>10% occurrence) were idiosyncratic. Of the 447 wetlands, we identified 277 that shared more than 40% similarities in assemblages, were mostly nested, and clustered together in ordination space, and thus could be considered a core set of wetlands in terms of assemblage structure. Assemblages in the 170 wetlands outside this core (mostly idiosyncratic) tended to be depauperate sites in arid or high elevation areas, or alternatively taxonomically rich sites supporting numerous lotic or lacustrine organisms. The ‘Core’ itself split into two main parts, one comprised of wetlands from semi‐arid or mild climate areas dominated by strong flying insects, and the second comprised of wetlands from wetter, more northerly areas where non‐insects with passive dispersal were very prevalent. Climate and geology appear to be major controls on macroinvertebrate distributions across the set of 447 wetlands. Hydrology (temporary versus permanent) of wetlands was a lesser control on assemblage structure over the set of 447 wetlands. That wetlands are dominated by about 40 widely‐occurring macroinvertebrate taxa, and those taxa tend to occur idiosyncratically, suggests that overall assemblages across wetlands may share many similarities, but some of widespread taxa may still be missing from many individual wetlands. Why these otherwise fairly ubiquitous taxa do not occur in specific wetlands may shed important light on how those wetlands are controlled ecologically; in other words, do sites lack specific factors required by these taxa?
Flow regimes are a major driver of community composition and structure in riverine ecosystems, and flow regulation by dams often induces artificially-stable flow regimes downstream. This represents a ...major source of hydrological alteration, particularly in regions where biota is adapted to strong seasonal and interannual flow variability. We hypothesized that dam-induced hydrological stability should increase the availability of autochthonous resources at the base of the food web. This, in turn, should favour herbivorous over detritivorous strategies, increasing the diversity of primary consumers, and the food-web width and length. We tested this hypothesis by studying the longitudinal variation in food-web structure in a highly-seasonal Mediterranean river affected by an irrigation dam. We compared an unregulated reach to several reaches downstream of the dam. Hydrological and sedimentological stability increased downstream of the dam, and altered the type and quantity of available resources downstream, prompting a change from a detritus-based to an algae-based food web. The fraction of links between top and intermediate species also increased, and the food web became longer and wider at the intermediate trophic levels. Food-web structure did not recover 14km downstream of the dam, despite a partial restitution of the flow regime. Our results advance the notion that hydrologic alteration affects riverine food webs via additions/deletions of taxa and variation in the strength and distribution of food-web interactions. Thus, flow regulation by dams may not only impact individual facets of biodiversity, but also food-web level properties across river networks.
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•Flow regulation increased flow stability in a highly-variable Mediterranean river.•The detritus-based food web shifted into an algae-based food web.•Changes in consumers and their interactions widened and lengthened the food chain.•Flow regime alteration may impact river food webs beyond their individual components.
Impacts of environmental stressors on food webs are often difficult to predict because trophic levels can respond in divergent ways, and biotic interactions may dampen or amplify responses. Here we ...studied food‐web‐level impacts of urban wastewater pollution, a widespread source of degradation that can alter stream food webs via top‐down and bottom‐up processes. Wastewater may (1) subsidize primary producers by decreasing nutrient limitation, inducing a wide‐bottomed trophic pyramid. However, (2) wastewater may also reduce the quality and diversity of resources, which could decrease energy transfer efficiency by reducing consumer fitness, leading to predator starvation. Additionally, (3) if higher trophic levels are particularly sensitive to pollution, primary consumers could be released from predation pressure. We tested these hypotheses in 10 pairs of stream sites located upstream and downstream of urban wastewater effluents with different pollutant levels. We found that wastewater pollution reduced predator richness by ∼34%. Community size spectra (CSS) slopes were steeper downstream than upstream of wastewater effluents in all except one impact site where predators became locally extinct. Further, variation in downstream CSS slopes were correlated with pollution loads: the more polluted the stream, the steeper the CSS. We estimate that wastewater pollution decreased energy transfer efficiencies to primary consumers by ∼70%, limiting energy supply to predators. Additionally, traits increasing vulnerability to chemical pollution were overrepresented among predators, which presented compressed trophic niches (δ15N‐δ13C) downstream of effluents. Our results show that wastewater pollution can impact stream food webs via a combination of energy limitation to consumers and extirpation of pollution‐sensitive top predators. Understanding the indirect (biotically mediated) vs. direct (abiotic) mechanisms controlling responses to stress may help anticipating impacts of altered water quantity and quality, key signatures of global change.
Unveiling the patterns of amphibian phylogenetic diversity across broad spatial scales is extremely timely, given the need to optimize conservation strategies for this group. Here we analyzed the ...distributions of 51 amphibians in the Western Palearctic, and we explored the variation in phylogenetic structure and beta diversity across space. We predicted that the incidence of phylogenetic clustering in local assemblages would follow a latitudinal gradient, since only a few families would be able to occupy most of the available climate space whereas many families would be restricted to the mid-latitudinal range. For the same reason, we predicted that we would observe a latitudinal pattern in the relative contributions of turnover and nestedness, the two components of overall beta diversity. We observed a decline in species richness at both geographic extremes, with these areas presenting relatively higher levels of phylogenetic clustering. As expected, a few families (Bufonidae, Ranidae, and Salamandridae) occupied one or both extremes, whereas the rest were confined to the mesothermal belt. Also as predicted, the nestedness mechanism prevailed in explaining overall beta diversity in both the northern and southernmost regions of the ecozone, but not in the temperate regions. We therefore illuminate a conservation challenge: in northern Europe and the arid regions of Northern Africa, the relatively high contribution of nestedness to overall beta diversity allows conservation efforts to prioritize the few areas with the greatest species richness. However, this pattern does not hold in southern Europe and the mesic regions of Northern Africa, where conservation efforts should be focused on identifying phylogenetically-diverse areas rather than focusing on species-rich sites.
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