Ecosystem ecology meets adaptive management: food web response to a controlled flood on the Colorado River, Glen Canyon

Environmental warming increases the importance of high-turnover energy channels in stream food webs

Warming temperatures are altering communities and trophic networks across Earth's ecosystems. While the overall influence of warming on food webs is often context-dependent, increasing temperatures are predicted to change communities in two fundamental ways: (1) by reducing average body size and (2) by increasing individual metabolic rates. These warming-induced changes have the potential to influence the distribution of food web fluxes, food web stability, and the relative importance of deterministic and stochastic ecological processes shaping community assembly. Here, we quantified patterns and the relative distribution of organic matter fluxes through stream food webs spanning a broad natural temperature gradient (5-27°C). We then related these patterns to species and community trait distributions of mean body size and population biomass turnover (P:B) within and across streams. We predicted that (1) communities in warmer streams would exhibit smaller body size and higher P:B and (2) organic matter fluxes within warmer communities would increasingly skew toward smaller, higher P:B populations. Across the temperature gradient, warmer communities were characterized by smaller body size (~9% per °C) and higher P:B (~7% faster turnover per °C) populations on average. Additionally, organic matter fluxes within warmer streams were increasingly skewed toward higher P:B populations, demonstrating that warming can restructure organic matter fluxes in both an absolute and relative sense. With warming, the relative distribution of organic matter fluxes was decreasingly likely to arise through the random sorting of species, suggesting stronger selection for traits driving high turnover with increasing temperature. Our study suggests that a warming world will favor energy fluxes through "smaller and faster" populations, and that these changes may be more predictable than previously thought.

Road crossings change functional diversity and trait composition of stream-dwelling macroinvertebrate assemblages

Functional diversity is regarded as a key concept in understanding the link between ecosystem function and biodiversity, and is therefore widely investigated in relation to human-induced impacts. However, information on how the intersection of roads and streams (hereafter road crossings, representing a widespread habitat transformation in relation to human development), influences the functional diversity of stream-dwelling macroinvertebrates is still missing. The general aim of our study was to provide a comprehensible picture on the impacts of road crossing structures on multiple facets of the functional diversity of stream-dwelling macroinvertebrates. In addition, we also investigated changes in trait structure. Our research showed that road crossing structures had negative impacts on functional richness and dispersion; i.e., functional diversification. However, we found no significant impact on functional divergence and evenness components. We found a decrease in functional redundancy at road crossing structures. This indicates a reduced ability of the community to recover from disturbances. Finally, we found that road crossings drive stream habitat and hydrological changes in parallel with modification of the trait composition of stream-dwelling macroinvertebrate assemblages. All these results suggest that road crossings cause notable changes in the functional diversity of stream-dwelling macroinvertebrate assemblages.

Elements and omega-3 fatty acids in fishes along a large, dammed river

Damming of a river can trap and elevate levels of sediment-bound elements and alter food web dynamics in created reservoirs. It follows that dams may alter how elements and other nutrients, like the beneficial omega-3 fatty acids (n-3 FAs) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are accumulated in fish and thus the chemical composition of species above and below this barrier to migration. This study examined the spatial and species differences in contaminants and nutrients in fish from the Wolastoq | Saint John River (New Brunswick, Canada) in association with a large hydroelectric dam (Mactaquac Generating Station; MQGS), a river which supports both recreational fisheries and subsistence fishing by Indigenous communities. In 2020 and 2021, Smallmouth Bass, Yellow Perch, American Eel, and Striped Bass were collected from locations upstream (reservoir and river) and downstream of the MQGS and analyzed for mercury (Hg) and 30 other trace elements, n-3 FAs, δ15N, and δ13C. Fish from the reservoir were highest in the beneficial elements P, S, and K, while fish from upstream of the reservoir had lower levels of toxic elements, including Hg. The dam appeared to alter food web dynamics, as fish from the reservoir and immediately downstream of the dam had higher δ15N and reservoir fish were depleted in δ13C. DHA and Hg were positively corelated with δ15N, and EPA in Smallmouth Bass was higher in sites where fish had higher δ13C. Overall, this study suggests that the dam altered food web dynamics and the uptake of contaminants and nutrients by fish, and that location and species are important factors when examining the risks and benefits of consuming wild fish from a system impacted by a large dam.

Importance of artificial high flows in maintaining the ecological integrity of a regulated river

Artificial high flows attempt to simulate natural flood pulses in flow-regulated rivers with the intent to improve their ecological integrity. The long-term use of such high flow events have shown beneficial ecological effects on various rivers globally. However, such responses are often non-linear and characterized by underlying feedback mechanisms among ecosystem components. The question arises as to what happens when such high flow releases are disrupted or even discontinued. Here, we used the long-term (22 years) monitoring dataset from the river Spöl to examine whether discontinuation (2016-2021) of the flood program (annual artificial high flows from 2000 to 2016) resulted in the ecological degradation of the river. We used monitoring data of physico-chemistry, periphyton, benthic organic matter, macroinvertebrates and fish (brown trout, Salmo trutta fario L.) in the analysis. The flood program had no long-term effect on water physico-chemistry with most parameters showing typical variations associated with season and inter-annual weather patterns. The floods were effective at mobilizing bed sediments that reduced periphyton biomass and benthic organic matter following each flood. Increases in periphyton biomass and benthic organic matter occurred between floods, but both parameters showed no significant increase with discontinuation of the flood program. Floods reduced macroinvertebrate densities, but with density increases occurring between floods. The pulsed disturbances, and the progressive change in the habitat template, resulted in shifts in community assembly by reducing densities of Gammarus fossarum, a dominant crustacean, which allowed other taxa to colonize the system. Macroinvertebrate densities remained low after discontinuation of the floods, although G. fossarum densities have increased substantially while other taxa, especially some stoneflies, remained low in abundance. Notably, community assembly returned to a pre-flood composition with discontinuation of the floods. The abundance of brown trout increased substantially during the flood program but returned to low pre-flood numbers with discontinuation of the floods. We conclude that the flood program was beneficial to the ecology of the river Spöl and discontinuation of the floods resulted in degradation of the system after a relatively short lag period. However, the system showed high resilience to an earlier perturbation, a sediment spill in 2013, suggesting a rapid positive response by biota with resumption of the flood program.

Simultaneous invasion decouples zebra mussels and water clarity

Species invasions are a leading threat to ecosystems globally, but our understanding of interactions among multiple invasive species and their outcomes on ecosystem properties is undeveloped despite their significance to conservation and management. Here we studied a large lake in Minnesota, USA, that experienced a simultaneous surge in invasive zebra mussel and spiny water flea populations. A long-term (2000-2018) dataset offered a rare opportunity to assess whole-ecosystem shifts following the co-invasion. Within two years, the native crustacean zooplankton community declined abruptly in density and productivity (-93% and -91%, respectively). Summer phytoplankton abundance and water clarity remained stable across the time series, an unexpected outcome given the high density of zebra mussels in the lake. Observational data and modeling indicate that removal of native herbivorous zooplankton by the predatory spiny water flea reduced zooplankton grazing pressure enough to compensate new grazing losses due to zebra mussels, resulting in a zero net effect on phytoplankton abundance and water clarity despite a wholesale shift in secondary production from the pelagic to the benthic food web. This study reveals the extent of direct and indirect effects of two aquatic invaders on food-web processes that cancel shifts in water clarity, a highly valued ecosystem service.

Experimental reductions in subdaily flow fluctuations increased gross primary productivity for 425 river kilometers downstream

Aquatic primary production is the foundation of many river food webs. Dams change the physical template of rivers, often driving food webs toward greater reliance on aquatic primary production. Nonetheless, the effects of regulated flow regimes on primary production are poorly understood. Load following is a common dam flow management strategy that involves subdaily changes in water releases proportional to fluctuations in electrical power demand. This flow regime causes an artificial tide, wetting and drying channel margins and altering river depth and water clarity, all processes that are likely to affect primary production. In collaboration with dam operators, we designed an experimental flow regime whose goal was to mitigate negative effects of load following on ecosystem processes. The experimental flow contrasted steady-low flows on weekends with load following flows on weekdays. Here, we quantify the effect of this experimental flow on springtime gross primary production (GPP) 90-to-425 km downstream of Glen Canyon Dam on the Colorado River, AZ, USA. GPP during steady-low flows was 41% higher than during load following flows, mostly owing to nonlinear reductions in sediment-driven turbidity. The experimental flow increased weekly GPP even after controlling for variation in weekly mean discharge, demonstrating a negative effect of load following on GPP. We estimate that this environmental flow increased springtime carbon fixation by 0.27 g C m^-2 d^-1, which is ecologically meaningful considering median C fixation in 356 US rivers of 0.44 g C m^-2 d^-1 and the fact that native fish populations in this river are food-limited.

Tributary effects on the ecological responses of a regulated river to experimental floods

Rivers regulated by dams display several ecosystem alterations due to modified flow and sediment regimes. Downstream from a dam, ecosystem degradation occurs because of reduced disturbance, mostly derived from limitations on flow variability and sediment supply. In the last decade, most flow restoration/dam impact mitigation was oriented towards the development of environmental flows. Flow variability (and consequent disturbance) can be reintroduced by releasing artificial high flows (experimental floods). Flow-sediment interactions during experimental floods represent strong ecosystem drivers, influencing nutrient dynamics, and metabolic and functional properties. In river networks, sediment and water inputs from tributaries generate points of discontinuity that can drive major changes in environmental conditions, affecting habitat structure and determining functional differences between upstream and downstream. However, despite the relevance for management, flow/sediment relations during environmental flows - and more importantly during experimental floods - remain poorly understood, mostly due to the lack of empirical evidence. In this study, we examined how a major tributary (source of water and sediments) modified the physical habitat template of a regulated river, thereby influencing ecological and geomorphological responses to experimental floods. Methods combined high-resolution drone mapping techniques with a wide range of biological samples collected in field surveys before, during, and after experimental floods in an alpine river. Data were used to quantify changes in relevant functional and structural ecosystem properties, relating ecological responses to geomorphological dynamics. Results highlight the importance of tributaries in restoring ecosystem properties lost after damming, enhancing the resilience of the system. In addition, we observed that disturbance legacy played a fundamental role in determining ecological conditions of a river prior to experimental floods, thus confirming that considering flow variability and sediment availability is crucial in adaptive dam management and environmental flows design.

Impacts of environmental factors on the food web structure, energy flows, and system attributes along a subtropical urban river in southern China

Tropical and subtropical rivers are being subjected to multiple stressors from human disturbance (e.g., water pollution and habitat degradation). Understanding the relationship between environmental conditions and the river ecosystem is important for improving river management. We built 14 Ecopath models composed of 28 functional groups (trophic levels [TLs] of 1.0-3.8) along a subtropical urban river to explore the influence of environmental changes on system attributes. From headwaters to downstream, the model outputs showed that the transfer efficiency (TE), energy flow parameters, and ecosystem theory indices exhibited significant (P < 0.05) differences across a longitudinal gradient of disturbance, indicating heterogeneous attributes of local river segments. The high TE values of TLs I, II, and III separated the upper, middle, and lower reaches, respectively, which could be attributed to the shift in dominant consumption flows from upstream 'periphyton - aquatic insects - insectivorous fish' to midstream 'detritus - shrimp - crustaceavorous fish' and to downstream 'phytoplankton - filter-feeding invertebrates/fish'. Structural equation modelling was used to test the causal relationships among environmental variables and demonstrated that abiotic factors directly influenced biomass composition and indirectly influenced trophic networks. Water quality, including dissolved oxygen and flow velocity; habitat characteristics, such as riffles, cobble-gravel substrate, and seasonal floodplain; and biological indicators, including the relative contributions (%) of decapods, insectivorous fish, and insect scrapers to biomass composition, had significant (P < 0.05) positive impacts on system maturity (evaluated by omnivory, connectance, and cycling indices). In the future, it will be possible to evaluate the health of river ecosystems by monitoring representative environmental factors, which could be a cost-effective approach to system-level improvement.

Trophic niches of native and nonnative fishes along a river-reservoir continuum

Instream barriers can constrain dispersal of nonnative fishes, creating opportunities to test their impact on native communities above and below these barriers. Deposition of sediments in a river inflow to Lake Powell, USA resulted in creation of a large waterfall prohibiting upstream movement of fishes from the reservoir allowing us to evaluate the trophic niche of fishes above and below this barrier. We expected niche overlap among native and nonnative species would increase in local assemblages downstream of the barrier where nonnative fish diversity and abundance were higher. Fishes upstream of the barrier had more distinct isotopic niches and species exhibited a wider range in δ¹⁵N relative to downstream. In the reservoir, species were more constrained in δ¹⁵N and differed more in δ¹³C, representing a shorter, wider food web. Differences in energetic pathways and resource availability among habitats likely contributed to differences in isotopic niches. Endangered Razorback Sucker (Xyrauchen texanus) aggregate at some reservoir inflows in the Colorado River basin, and this is where we found the highest niche overlap among species. Whether isotopic niche overlap among adult native and nonnative species has negative consequences is unclear, because data on resource availability and use are lacking; however, these observations do indicate the potential for competition. Still, the impacts of diet overlap among trophic generalists, such as Razorback Sucker, are likely low, particularly in habitats with diverse and abundant food bases such as river-reservoir inflows.

Food web controls on mercury fluxes and fate in the Colorado River, Grand Canyon

Mercury (Hg) biomagnification in aquatic food webs is a global concern; yet, the ways species traits and interactions mediate these fluxes remain poorly understood. Few pathways dominated Hg flux in the Colorado River despite large spatial differences in food web complexity, and fluxes were mediated by one functional trait, predation resistance. New Zealand mudsnails are predator resistant and a trophic dead end for Hg in food webs we studied. Fishes preferred blackflies, which accounted for 56 to 80% of Hg flux to fishes, even where blackflies were rare. Food web properties, i.e., match/mismatch between insect production and fish consumption, governed amounts of Hg retained in the river versus exported to land. An experimental flood redistributed Hg fluxes in the simplified tailwater food web, but not in complex downstream food webs. Recognizing that species traits, species interactions, and disturbance mediate contaminant exposure can improve risk management of linked aquatic-terrestrial ecosystems.

Thermal niche diversity and trophic redundancy drive neutral effects of warming on energy flux through a stream food web

Climate warming is predicted to alter routing and flows of energy through food webs because of the critical and varied effects of temperature on physiological rates, community structure, and trophic dynamics. Few studies, however, have experimentally assessed the net effect of warming on energy flux and food web dynamics in natural intact communities. Here, we test how warming affects energy flux and the trophic basis of production in a natural invertebrate food web by experimentally heating a stream reach in southwest Iceland by ~4°C for 2 yr and comparing its response to an unheated reference stream. Previous results from this experiment showed that warming led to shifts in the structure of the invertebrate assemblage, with estimated increases in total metabolic demand but no change in annual secondary production. We hypothesized that elevated metabolic demand and invariant secondary production would combine to increase total consumption of organic matter in the food web, if diet composition did not change appreciably with warming. Dietary composition of primary consumers indeed varied little between streams and among years, with gut contents primarily consisting of diatoms (72.9%) and amorphous detritus (19.5%). Diatoms dominated the trophic basis of production of primary consumers in both study streams, contributing 79-86% to secondary production. Although warming increased the flux of filamentous algae within the food web, total resource consumption did not increase as predicted. The neutral net effect of warming on total energy flow through the food web was a result of taxon-level variation in responses to warming, a neutral effect on total invertebrate production, and strong trophic redundancy within the invertebrate assemblage. Thus, food webs characterized by a high degree of trophic redundancy may be more resistant to the effects of climate warming than those with more diverse and specialized consumers.

Connectivity Dynamics in Dryland Litter Cycles: Moving Decomposition beyond Spatial Stasis

Drylands (arid and semiarid ecosystems) cover nearly half of Earth's terrestrial surface, but biogeochemical pools and processes in these systems remain poorly understood. Litter can account for a substantial portion of carbon and nutrient pools in these systems, with litter decomposition exerting important controls over biogeochemical cycling. Dryland decomposition is typically treated as a spatially static process in which litter is retained and decomposed where it is initially deposited. Although this assumption is reasonable for mesic systems with continuous plant canopy cover and a stable subcanopy litter layer, dryland pools generally reflect discontinuous inputs from heterogeneous canopy cover followed by substantial litter transport. In the present article, we review horizontal and vertical transport processes that move litter from the initial deposition point and retention elements that influence litter accumulation patterns. Appreciation of the spatially dynamic litter cycle, including quantitative assessment of transport patterns, will improve estimates of the fate and distribution of organic matter in current and future drylands.

River food webs: an integrative approach to bottom-up flow webs, top-down impact webs, and trophic position

The majority of food web studies are based on connectivity, top-down impacts, bottom-up flows, or trophic position (TP), and ecologists have argued for decades which is best. Rarely have any two been considered simultaneously. The present study uses a procedure that integrates the last three approaches based on taxon-specific secondary production and gut analyses. Ingestion flows are quantified to create a flow web and the same data are used to quantify TP for all taxa. An individual predator's impacts also are estimated using the ratio of its ingestion (I) of each prey to prey production (P) to create an I/P web. This procedure was applied to 41 invertebrate taxa inhabiting submerged woody habitat in a southeastern U.S. river. A complex flow web starting with five basal food resources had 462 flows >1 mg·m^-2 ·yr^-1 , providing far more information than a connectivity web. Total flows from basal resources to primary consumers/omnivores were dominated by allochthonous amorphous detritus and ranged from 1 to >50,000 mg·m^-2 ·yr^-1 . Most predator-prey flows were much lower (<50 mg·m^-2 ·yr^-1 ), but some were >1,000  mg·m^-2 ·yr^-1 . The I/P web showed that 83% of individual predator impacts were weak (<10%), whereas total predator impacts were often strong (e.g., 35% of prey sustained an impact >90%). Quantitative estimates of TP ranged from 2 to 3.7, contrasting sharply with seven integer-based trophic levels based on longest feeding chain. Traditional omnivores (TP = 2.4-2.9) played an important role by consuming more prey and exerting higher impacts on primary consumers than strict predators (TP ≥ 3). This study illustrates how simultaneous quantification of flow pathways, predator impacts, and TP together provide an integrated characterization of natural food webs.

Designing flows to resolve human and environmental water needs in a dam-regulated river

Navigating trade-offs between meeting societal water needs and supporting functioning ecosystems is integral to river management policy. Emerging frameworks provide the opportunity to consider multiple river uses explicitly, but balancing multiple priorities remains challenging. Here we quantify relationships between hydrologic regimes and the abundance of multiple native and nonnative fish species over 18 years in a large, dryland river basin in southwestern United States. These models were incorporated into a multi-objective optimization framework to design dam operation releases that balance human water needs with the dual conservation targets of benefiting native fishes while disadvantaging nonnative fishes. Predicted designer flow prescriptions indicate significant opportunities to favor native over nonnative fishes while rarely, if ever, encroaching on human water needs. The predicted benefits surpass those generated by natural flow mimicry, and were retained across periods of heightened drought. We provide a quantitative illustration of theoretical predictions that designer flows can offer multiple ecological and societal benefits in human-altered rivers.

A framework for evaluating food-web responses to hydrological manipulations in riverine systems

Environmental flows are used to restore elements of the hydrological regime altered by human use of water. One of the primary justifications and purposes for environmental flows is the maintenance of target species populations but, paradoxically, there has been little emphasis on incorporating the food-web and trophic dynamics that determine population-level responses into the monitoring and evaluation of environmental flow programs. We develop a generic framework for incorporating trophic dynamics into monitoring programs to identify the food-web linkages between hydrological regimes and population-level objectives of environmental flows. These linkages form the basis for objective setting, ecological targets and indicator selection that are necessary for planning monitoring programs with a rigorous scientific basis. Because there are multiple facets of trophic dynamics that influence energy production and transfer through food webs, the specific objectives of environmental flows need to be defined during the development of monitoring programs. A multitude of analytical methods exist that each quantify distinct aspects of food webs (e.g. energy production, prey selection, energy assimilation), but no single method can provide a basis for holistic understanding of food webs. Our paper critiques a range of analytical methods for quantifying attributes of food webs to inform the setting, monitoring and evaluation of trophic outcomes of environmental flows and advance the conceptual understanding of trophic dynamics in river-floodplain systems.

Modelling food-web mediated effects of hydrological variability and environmental flows

Environmental flows are designed to enhance aquatic ecosystems through a variety of mechanisms; however, to date most attention has been paid to the effects on habitat quality and life-history triggers, especially for fish and vegetation. The effects of environmental flows on food webs have so far received little attention, despite food-web thinking being fundamental to understanding of river ecosystems. Understanding environmental flows in a food-web context can help scientists and policy-makers better understand and manage outcomes of flow alteration and restoration. In this paper, we consider mechanisms by which flow variability can influence and alter food webs, and place these within a conceptual and numerical modelling framework. We also review the strengths and weaknesses of various approaches to modelling the effects of hydrological management on food webs. Although classic bioenergetic models such as Ecopath with Ecosim capture many of the key features required, other approaches, such as biogeochemical ecosystem modelling, end-to-end modelling, population dynamic models, individual-based models, graph theory models, and stock assessment models are also relevant. In many cases, a combination of approaches will be useful. We identify current challenges and new directions in modelling food-web responses to hydrological variability and environmental flow management. These include better integration of food-web and hydraulic models, taking physiologically-based approaches to food quality effects, and better representation of variations in space and time that may create ecosystem control points.

River doctors: Learning from medicine to improve ecosystem management

Effective ecosystem management requires a robust methodology to analyse, remedy and avoid ecosystem damage. Here we propose that the overall conceptual framework and approaches developed over millennia in medical science and practice to diagnose, cure and prevent disease can provide an excellent template. Key principles to adopt include combining well-established assessment methods with new analytical techniques and restricting both diagnosis and treatment to qualified personnel at various levels of specialization, in addition to striving for a better mechanistic understanding of ecosystem structure and functioning, as well as identifying the proximate and ultimate causes of ecosystem impairment. In addition to applying these principles, ecosystem management would much benefit from systematically embracing how medical doctors approach and interview patients, diagnose health condition, select treatments, take follow-up measures, and prevent illness. Here we translate the overall conceptual framework from medicine into environmental terms and illustrate with examples from rivers how the systematic adoption of the individual steps proven and tested in medical practice can improve ecosystem management.

Growth of Endangered Humpback Chub in Relation to Temperature and Discharge in the Lower Colorado River

Assessments of growth can provide information needed to understand how fish populations respond to changing environmental conditions and management actions, including ecosystem experimentation. We estimated growth rates and parameter uncertainty from otoliths of endangered Humpback Chub Gila cypha from the Colorado River in Grand Canyon, Arizona. We then compared growth of Humpback Chub &lt; age 2 that were 1) occupying the mainstem Colorado River during a period of variable discharge and cooler water temperatures (1980–1998; epoch 1), 2) occupying the Colorado River during a period of moderate discharge variability and warmer water (2001–2011; epoch 2), and 3) occupying the unregulated Little Colorado River. Because growth rates of juvenile Humpback Chub (&lt; age 2) may be more sensitive to changes in environmental conditions than adult fish, we used analysis of covariance and linear models to compare growth of juvenile fish (slopes) between epochs and capture sites (mainstem Colorado River vs. Little Colorado River). Our analysis of covariance results were ambiguous (age × epoch × site interaction; P = 0.06). However, individual linear regressions of size and age by epoch and site suggest biologically important differences in growth, as evidenced by slower growth in the Colorado River in epoch 1 than in epoch 2, and slower growth in the Colorado River compared with the Little Colorado River for all time periods. Overall our results 1) provide information on growth and growth variability useful for parameterizing models to assess population viability and 2) provide empirical information on how growth of juvenile and adult Humpback Chub growth may respond to changing environmental conditions.

Incorporating food web dynamics into ecological restoration: a modeling approach for river ecosystems

Restoration is frequently aimed at the recovery of target species, but also influences the larger food web in which these species participate. Effects of restoration on this broader network of organisms can influence target species both directly and indirectly via changes in energy flow through food webs. To help incorporate these complexities into river restoration planning, we constructed a model that links river food web dynamics to in-stream physical habitat and riparian vegetation conditions. We present an application of the model to the Methow River, Washington, USA, a location of on-going restoration aimed at recovering salmon. Three restoration strategies were simulated: riparian vegetation restoration, nutrient augmentation via salmon carcass addition, and side channel reconnection. We also added populations of nonnative aquatic snails and fish to the modeled food web to explore how changes in food web structure mediate responses to restoration. Simulations suggest that side channel reconnection may be a better strategy than carcass addition and vegetation planting for improving conditions for salmon in this river segment. However, modeled responses were strongly sensitive to changes in the structure of the food web. The addition of nonnative snails and fish modified pathways of energy through the food web, which negated restoration improvements. This finding illustrates that forecasting responses to restoration may require accounting for the structure of food webs, and that changes in this structure, as might be expected with the spread of invasive species, could compromise restoration outcomes. Unlike habitat-based approaches to restoration assessment that focus on the direct effects of physical habitat conditions on single species of interest, our approach dynamically links the success of target organisms to the success of competitors, predators, and prey. By elucidating the direct and indirect pathways by which restoration affects target species, dynamic food web models can improve restoration planning by fostering a deeper understanding of system connectedness and dynamics.

Characterizing Growth and Condition of Endangered Humpback Chub in the Lower Colorado River

This paper is a preface to the two papers that follow in this issue. The two papers (by Pine et al. and Hayes et al.) use long-term fish sampling data from ongoing Humpback Chub monitoring efforts and archival otolith samples (from museums) collected in the lower Colorado and Little Colorado Rivers during periods of both cold- and warm-water conditions to assess whether Humpback Chub growth characteristics may have responded to changes in water temperature. Growth patterns are often of interest to resource managers because growth integrates a large range of environmental and ecological factors, including habitat conditions. Together, these papers contribute information to a large collection of recent studies, developing a line of evidence designed to inform management decisions related to water releases, dam operations, and management actions that could be taken to aid recovery of native fish populations in regulated river systems around the world.

Addressing the local aspects of global change impacts on stream metabolism using frequency analysis tools

Global change, as a combination of climate change, human activities on watersheds and the river flow regulation, causes intense changes in hydrological cycles and, consequently, threatens the good ecological status of freshwater biological communities. This study addresses how and whether the combination of climatic drivers and local human impacts may alter the metabolism of freshwater communities. We identified a few factors modulating the natural water flow and quality in 25 point spread within the Ebro river Basin: waste water spills, industrial spills, reservoir discharges, water withdrawals, agricultural use, and the presence of riparian forests. We assessed their impacts on the freshwater metabolism as changes in the annual cycle of both gross primary production-GPP - and ecosystem respiration-ER -. For this purpose, daily data series were analyzed by continuous wavelet transformation, allowing for the assessment of the metabolic ecosystem Frequency Spectrum Patterns (FSPs). Changes in the behavior of ecosystem metabolism were strongly associated with local characteristics at each sampling point, however in 20 out of 25 studied points, changes in metabolic ecosystem FSP were related to climatic change events (the driest period of the last 140years). The changes in FSP indicate that severe impacts on how biological communities use carbon sources occur as a result of the human water management - too much focus on human needs - during intense climatic events. Results show that local factors, and specially the flow regulation, may modulate the impact of global change. As example those points exposed to a more intense anthropization showed a clear disruption - and even disappearance - of the annual FSP. This information may help managers to understand the action mechanisms of non-climatic factors at ecosystem level, leading to better management policies based on the promotion of ecosystem resilience. The method here presented may help on improving the calculation of ecological flows to maintain the river metabolic annual cycles as close as possible to the natural ones.

Application of Effective Discharge Analysis to Environmental Flow Decision-Making

Well-informed river management decisions rely on an explicit statement of objectives, repeatable analyses, and a transparent system for assessing trade-offs. These components may then be applied to compare alternative operational regimes for water resource infrastructure (e.g., diversions, locks, and dams). Intra- and inter-annual hydrologic variability further complicates these already complex environmental flow decisions. Effective discharge analysis (developed in studies of geomorphology) is a powerful tool for integrating temporal variability of flow magnitude and associated ecological consequences. Here, we adapt the effectiveness framework to include multiple elements of the natural flow regime (i.e., timing, duration, and rate-of-change) as well as two flow variables. We demonstrate this analytical approach using a case study of environmental flow management based on long-term (60 years) daily discharge records in the Middle Oconee River near Athens, GA, USA. Specifically, we apply an existing model for estimating young-of-year fish recruitment based on flow-dependent metrics to an effective discharge analysis that incorporates hydrologic variability and multiple focal taxa. We then compare three alternative methods of environmental flow provision. Percentage-based withdrawal schemes outcompete other environmental flow methods across all levels of water withdrawal and ecological outcomes.

Mercury and selenium accumulation in the Colorado River food web, Grand Canyon, USA

Mercury (Hg) and selenium (Se) biomagnify in aquatic food webs and are toxic to fish and wildlife. The authors measured Hg and Se in organic matter, invertebrates, and fishes in the Colorado River food web at sites spanning 387 river km downstream of Glen Canyon Dam (AZ, USA). Concentrations were relatively high among sites compared with other large rivers (mean wet wt for 6 fishes was 0.17-1.59 μg g(-1) Hg and 1.35-2.65 μg g(-1) Se), but consistent longitudinal patterns in Hg or Se concentrations relative to the dam were lacking. Mercury increased (slope = 0.147) with δ(15) N, a metric of trophic position, indicating biomagnification similar to that observed in other freshwater systems. Organisms regularly exceeded exposure risk thresholds for wildlife and humans (6-100% and 56-100% of samples for Hg and Se, respectfully, among risk thresholds). In the Colorado River, Grand Canyon, Hg and Se concentrations pose exposure risks for fish, wildlife, and humans, and the findings of the present study add to a growing body of evidence showing that remote ecosystems are vulnerable to long-range transport and subsequent bioaccumulation of contaminants. Management of exposure risks in Grand Canyon will remain a challenge, as sources and transport mechanisms of Hg and Se extend far beyond park boundaries.

Threats and opportunities for freshwater conservation under future land use change scenarios in the United States

Freshwater ecosystems provide vital resources for humans and support high levels of biodiversity, yet are severely threatened throughout the world. The expansion of human land uses, such as urban and crop cover, typically degrades water quality and reduces freshwater biodiversity, thereby jeopardizing both biodiversity and ecosystem services. Identifying and mitigating future threats to freshwater ecosystems requires forecasting where land use changes are most likely. Our goal was to evaluate the potential consequences of future land use on freshwater ecosystems in the coterminous United States by comparing alternative scenarios of land use change (2001-2051) with current patterns of freshwater biodiversity and water quality risk. Using an econometric model, each of our land use scenarios projected greater changes in watersheds of the eastern half of the country, where freshwater ecosystems already experience higher stress from human activities. Future urban expansion emerged as a major threat in regions with high freshwater biodiversity (e.g., the Southeast) or severe water quality problems (e.g., the Midwest). Our scenarios reflecting environmentally oriented policies had some positive effects. Subsidizing afforestation for carbon sequestration reduced crop cover and increased natural vegetation in areas that are currently stressed by low water quality, while discouraging urban sprawl diminished urban expansion in areas of high biodiversity. On the other hand, we found that increases in crop commodity prices could lead to increased agricultural threats in areas of high freshwater biodiversity. Our analyses illustrate the potential for policy changes and market factors to influence future land use trends in certain regions of the country, with important consequences for freshwater ecosystems. Successful conservation of aquatic biodiversity and ecosystem services in the United States into the future will require attending to the potential threats and opportunities arising from policies and market changes affecting land use.

Response of an invasive native wetland plant to environmental flows: implications for managing regulated floodplain ecosystems

The natural flow regimes of rivers underpin the health and function of floodplain ecosystems. However, infrastructure development and the over-extraction of water has led to the alteration of natural flow regimes, resulting in the degradation of river and floodplain habitats globally. In many catchments, including Australia's Murray-Darling Basin, environmental flows are seen as a potentially useful tool to restore natural flow regimes and manage the degradation of rivers and their associated floodplains. In this paper, we investigated whether environmental flows can assist in controlling an invasive native floodplain plant in Barmah Forest, south-eastern Australia. We experimentally quantified the effects of different environmental flow scenarios, including a shallow (20 cm) and deeper (50 cm) flood of different durations (12 and 20 weeks), as well as drought and soil-saturated conditions, on the growth and survival of seedlings of Juncus ingens, a native emergent macrophyte that has become invasive in some areas of Barmah Forest following river regulation and alteration of natural flow regimes. Three height classes of J. ingens (33 cm, 17 cm and 12 cm) were included in the experiment to explicitly test for relationships between treatments, plant survival and growth, and plant height. We found that seedling mortality occurred in the drought treatment and in the 20-week flood treatments of both depths; however, mortality rates in the flood treatments depended on initial plant height, with medium and short plants (initial heights of ≤17 cm) exhibiting the highest mortality rates. Both the 20 cm and 50 cm flood treatments of only 12 weeks duration were insufficient to cause mortality in any of the height classes; indeed, shoots of plants in the 20 cm flood treatment were able to elongate through the water surface at rapid rates. Our findings have important implications for management of Barmah Forest and floodplain ecosystems elsewhere, as it demonstrates the potential for using environmental flows to limit the spread of invasive plants by targeting a life-stage that is particularly sensitive to prolonged submergence. However, there may be narrow thresholds between the conditions that provide effective control of an invasive species, and those that instead facilitate growth and may promote further invasion.

The floodplain food web mosaic: a study of its importance to salmon and steelhead with implications for their recovery

Although numerous studies have attempted to place species of interest within the context of food webs, such efforts have generally occurred at small scales or disregard potentially important spatial heterogeneity. If food web approaches are to be employed to manage species, studies are needed that evaluate the multiple habitats and associated webs of interactions in which these species participate. Here, we quantify the food webs that sustain rearing salmon and steelhead within a floodplain landscape of the Methow River, Washington, USA, a location where restoration has been proposed to restore side channels in an attempt to recover anadromous fishes. We combined year-long measures of production, food demand, and diet composition for the fish assemblage with estimates of invertebrate prey productivity to quantify food webs within the main channel and five different, intact, side channels; ranging from channels that remained connected to the main channel at low flow to those reduced to floodplain ponds. Although we found that habitats within the floodplain had similar invertebrate prey production, these habitats hosted different local food webs. In the main channel, 95% of total prey consumption flowed to fishes that are not the target of proposed restoration. These fishes consumed 64% and 47% of the prey resources that were found to be important to fueling chinook and steelhead production in the main channel, respectively. Conversely, in side channels, a greater proportion of prey was consumed by anadromous salmonids. As a result, carrying capacity estimates based on food were 251% higher, on average, for anadromous salmonids in side channels than the main channel. However, salmon and steelhead production was generally well below estimated capacity in both the main and side channels, suggesting these habitats are under-seeded with respect to food, and that much larger populations could be supported. Overall, this study demonstrates that floodplain heterogeneity is associated with the occurrence of a mosaic of food webs, all of which were utilized by anadromous salmonids, and all of which may be important to their recovery and persistence. In the long term, these and other fishes would'likely benefit from restoring the processes that maintain floodplain complexity.

Long-term changes in community assembly, resistance, and resilience following experimental floods

This study examined the long-term changes in community assembly, resistance, and resilience of macroinvertebrates following 10 years of experimental floods in a flow regulated river. Physico-chemistry, macroinvertebrates, and periphyton biomass were monitored before and sequentially after each of 22 floods, and drift/seston was collected during six separate floods over the study period. The floods reduced the density and taxon richness of macroinvertebrates, and a nonmetric dimensional scaling (NMDS) analysis distinguished temporal shifts in community assembly. Resistance (measured as the relative lack of loss in density) tofloods varied among taxa, and the abundance of resistant taxa was related to the temporal changes in community assembly. Community resistance was inversely related to flood magnitude with all larger floods (> 25 m3/s, > 16-fold over baseflow) reducing densities by > 75% regardless of flood year, whereas smaller floods (< 20 m3/s) reduced taxon richness approximately twofold less than larger floods. No relationship was found between flood magnitude and the relative loss in periphyton biomass. Resilience was defined as the recovery slope (positive slope of a parameter with time following each flood) and was unrelated to shifts in community assembly or resistance. Macroinvertebrate drift and seston demonstrated hysteresis (i.e., a temporal response in parameter quantity with change in discharge) during each flood, although larger floods typically had two peaks in both parameters. The first peak was a response to the initial increases in flow, whereas the second peak was associated with streambed disturbance (substrate mobility) and side-slope failure causing increased scour. Drift density was 3-9 times greater and that of seston 3-30 times greater during larger floods than smaller floods. These results demonstrate temporal shifts in macroinvertebrate community assembly toward a pre-dam assemblage following sequential floods in this flow regulated river, thus confirming the ecological role of habitat filtering in organism distribution and abundance. Community resistance and resilience were unrelated to shifts in community assembly, suggesting that they are mostly evolutionary properties of ecosystems as populations adapt to changing environmental (disturbance regimes) and biotic (novel colonists) conditions. As these systems show behaviors similar to dispersal-limited ecosystems, a long-term perspective is required for management actions targeted toward regulated and fragmented rivers.

Food webs: reconciling the structure and function of biodiversity

The global biodiversity crisis concerns not only unprecedented loss of species within communities, but also related consequences for ecosystem function. Community ecology focuses on patterns of species richness and community composition, whereas ecosystem ecology focuses on fluxes of energy and materials. Food webs provide a quantitative framework to combine these approaches and unify the study of biodiversity and ecosystem function. We summarise the progression of food-web ecology and the challenges in using the food-web approach. We identify five areas of research where these advances can continue, and be applied to global challenges. Finally, we describe what data are needed in the next generation of food-web studies to reconcile the structure and function of biodiversity.