Showing papers in "Freshwater Biology in 2015"

Metacommunity organisation, spatial extent and dispersal in aquatic systems: patterns, processes and prospects

Abstract: Summary Metacommunity ecology addresses the situation where sets of local communities are connected by the dispersal of a number of potentially interacting species. Aquatic systems (e.g. lentic versus lotic versus marine) differ from each other in connectivity and environmental heterogeneity, suggesting that metacommunity organisation also differs between major aquatic systems. Here, we review findings from observational field studies on metacommunity organisation in aquatic systems. Species sorting (i.e. species are ‘filtered’ by environmental factors and occur only at environmentally suitable sites) prevails in aquatic systems, particularly in streams and lakes, but the degree to which dispersal limitation interacts with such environmental control varies among different systems and spatial scales. For example, mainstem rivers and marine coastal systems may be strongly affected by ‘mass effects’ (i.e. where high dispersal rates homogenise communities to some degree at neighbouring localities, irrespective of their abiotic and biotic environmental conditions), whereas isolated lakes and ponds may be structured by dispersal limitation (i.e. some species do not occur at otherwise-suitable localities simply because sites with potential colonists are too far away). Flow directionality in running waters also differs from water movements in other systems, and this difference may also have effects on the role of dispersal in different aquatic systems. Dispersal limitation typically increases with increasing spatial distance between sites, mass effects potentially increase in importance with decreasing distance between sites, and the dispersal ability of organisms may determine the spatial extents at which species sorting and dispersal processes are most important. A better understanding of the relative roles of species sorting, mass effects and dispersal limitation in affecting aquatic metacommunities requires the following: (i) characterising dispersal rates more directly or adopting better proxies than have been used previously; (ii) considering the nature of aquatic networks; (iii) combining correlative and experimental approaches; (iv) exploring temporal aspects of metacommunity organisation and (v) applying past approaches and statistical methods innovatively for increasing our understanding of metacommunity organisation.

Reconceptualising the beta diversity‐environmental heterogeneity relationship in running water systems

Abstract: SUMMARY 1. Beta diversity modelling has received increased interest recently. There are multiple definitions of beta diversity, but here, we focus on variability in species composition among sampling units within a given area. This facet can be described using various approaches. Some approaches ignore the spatial scale of the area considered (i.e. region limits), while some consider different region limits as a starting point for the analysis of beta diversity. 2. We focused specifically on the beta diversity–environmental heterogeneity relationship in running waters. First, we present two conceptual models, which assume either (1) strong environmental control among localities (riffle sites in our case) within each region unit (a region unit encompasses a species pool and can be a stream or a basin or an ecoregion) or (2) that the spatial level of a region unit affects the relative importance of mechanisms affecting variability in species composition among localities (i.e. among riffle sites) within each region unit. Second, we compared three recent studies that used similar methods to examine the beta diversity–environmental heterogeneity relationship, but which were based on different region units, comprising sets of streams or sets of basins or sets of ecoregions. 3. Our conceptual framework assumes that environmental control is not likely to be the sole mechanism affecting variability in community composition among localities within each region unit, but it is likely to be most important when dispersal rates are intermediate (i.e. among localities within a basin). In contrast, if dispersal rates are very high (i.e. among localities within a stream) or very low (i.e. among localities within an ecoregion), environmental control is in part masked by high dispersal rates or is prevented from occurring because not all species can reach all localities, respectively. Such scale dependency in the relative strength of environmental control might therefore transcend spatial scales from individual region units to the strength of the beta diversity–environmental heterogeneity relationship. We emphasise that the beta diversity–environmental heterogeneity relationship can only be tested across multiple region units. The results of three case studies are consistent with these predictions. Specifically, the beta diversity–environmental heterogeneity regression was highly significant across multiple basins, but not across multiple streams or across multiple ecoregions. 4. We suggest that researchers take spatial scale and region unit level explicitly into account when inferring the mechanisms structuring ecological communities and mapping variation in beta diversity. We also propose a unified terminology for studies examining the beta diversity–environmental heterogeneity relationship in running waters because inconsistent terminology is likely to hamper the progress of our science.

Functional classifications and their application in phytoplankton ecology

Abstract: SUMMARY 1. Ecologists often group organisms based on similar biological traits or on taxonomic criteria. However, the use of taxonomy in ecology has many drawbacks because taxa may include species with very different ecological adaptations. Further, similar characters may evolve independently in different lineages. 2. In this review, we examine the main criteria that have been used in the identification of nine modes of classifying phytoplankton non-taxonomically. These approaches are based purely on morphological and/or structural traits, or on more complex combinations including physiological and ecological features. 3. Different functional approaches have proved able to explain some fraction of the variance observed in the spatial and temporal distribution patterns of algal assemblages, although their effectiveness varies greatly, depending on the number and characteristics of functional traits used. The attribution of functional traits to single species or broad groups of species has allowed a few classifications (e.g. Functional Groups, FG) to be used in the assessment of ecological status. 4. We stress that the misuse of functional classifications (by applying them under conditions other than those intended) can have serious consequences for interpreting ecological processes. Assigning functional traits or groups cannot be considered a surrogate for the knowledge of species or ecotypes, and the use of specific traits must always be justified and circumscribed within the limits of ecological questions and hypotheses. 5. An important future challenge will be to integrate advances in molecular genetics, metabolomics and physiology with more conventional traits; this will form the basis of the next generation of functional classifications.

Resistance and resilience of invertebrate communities to seasonal and supraseasonal drought in arid‐land headwater streams

Abstract: SUMMARY 1. Climate change is expected to intensify drought in many regions, but ecological impacts on stream communities are poorly understood. Many arid-land streams are characterised by predictable seasonal cycles of wetting and drying, to which species are adapted, but unpredictable supraseasonal droughts may constitute extreme events that challenge resident biota. 2. In this article, we synthesise research conducted in arid-land streams of the Madrean Sky Islands (MSI) in Arizona, U.S.A, to evaluate the resistance and resilience of invertebrate communities to drying disturbances caused by normal seasonal drying and severe supraseasonal drought. We also highlight how spatial context (e.g. distance to perennial refuges) influences recovery patterns. 3. Invertebrate community structure changes predictably as habitat contraction progresses from loss of lateral connectivity to complete drying of MSI streams. When drying events are predictable (e.g. seasonal drying), post-drought community recovery is often rapid, since most MSI taxa possess life history traits conferring high resistance and/or resilience to stream drying. 4. Extreme supraseasonal droughts, in contrast, cause unprecedented transitions from perennial to intermittent flow in some MSI streams. While species richness may recover quickly following this flow regime shift, marked turnover in community structure can occur and may delay or preclude recovery to pre-drought conditions. In such cases, short-lived (<1 year) strong dispersers replace relatively long-lived (≥1 year) weak dispersers. As habitat isolation increases, the potential for community recovery from extreme drought decreases. 5. Many MSI aquatic species are threatened by extreme drought. Extinctions of endemic aquatic species due to habitat drying have already been observed in nearby deserts. Further studies are urgently needed to identify drought-sensitive species and understand how the loss of such species may affect stream ecosystem functioning.

The hippopotamus conveyor belt: vectors of carbon and nutrients from terrestrial grasslands to aquatic systems in sub-Saharan Africa

Abstract: Summary Hippopotami can play a significant role as ecosystem engineers and may play an important role as carbon and nutrient vectors from savanna grasslands to aquatic systems. We coupled the results of a feeding study of captive hippopotami, faeces leaching/mineralisation experiments, hippopotamus consumption estimates and the stoichiometry of savanna grasses to calculate excretion and egestion rates of hippopotami. We then used time budgets and population estimates to calculate nutrient loading by hippopotami in the Mara River, Kenya. In captivity, hippopotami consumed 11.0 g dry matter (DM) kg hippopotamus−1 day−1 (110.6 C; 6.8 N; 1.0 P) and egested 5.3 g DM kg−1 day−1. They excreted or egested 2.72 g C, 0.28 g N and 0.04 g P kg hippopotamus−1 day−1, and urine comprised 12% of C, 70% of N and 33% of P of total excretion and egestion. By integrating data from previously published work on hippopotamus digestion with the data we collected in the field, we estimated an average hippopotamus in the Mara River would excrete or egest 1.93–3.58 g DM, 0.78–1.47 g C, 0.13–0.19 g N and 0.01–0.02 g P kg hippopotamus−1 day−1, and that half of this excretion/egestion would enter the river. The hippopotamus population increased by 1500% inside the Maasai Mara National Reserve, Kenya, between 1959 and 2006. We estimate that hippopotami egest 36 200 kg faeces day−1 into the Mara River (wet mass). Daily loading into the river by excretion and egestion equals 8563 kg DM, 3499 kg C, 492 kg N and 48 kg P, which is equivalent to 670% of CPOM, 15% of DOC, 27% of TN and 29% of TP of loading from the upstream catchment. This research provides the first estimates for hippopotamus inputs to rivers that include both excretion and egestion and provides evidence that hippopotami are important resource vectors in sub-Saharan African rivers, even when compared to other sources of carbon and nutrients.

Biogeochemical hotspots: temporal and spatial scaling of the impact of freshwater mussels on ecosystem function

Abstract: SUMMARY 1. In streams, the creation of nutrient-transformation hotspots by aggregated organisms may have heterogeneous and strong cumulative influences on stream nutrient dynamics. Here, we examine the potential for aggregations of freshwater mussels to create such hotspots. 2. We measured nitrogen (N) and phosphorus (P) excretion rates of six mussel species and body tissue composition of eight. We combined these data with population densities of surveyed mussel beds in the Kiamichi River, OK, to estimate reach-scale and stream-scale nutrient recycling and storage. Additionally, we estimated the temporal variability in the magnitude of mussel nutrient recycling combining volumetric excretion at a reach scale with discharge and temperature data. 3. Mussel beds constituted 1.45% of the area of the Kiamichi River. Mussel nutrient remineralisation varied greatly across beds (11.1–699.5 lmol N m � 2 h � 1 and 0.8–53.0 lmol P m � 2 h � 1 ), because of varying mussel densities. The community-wide average excretion N:P (molar) of the mussel communities was 29.57, with higher excretion N:P significantly associated with higher abundances of Actinonaias ligamentina. Total nutrient storage per bed varied two orders of magnitude (6.3–631.7 kg N and 2.3–227.5 kg P) between mussel beds. Moreover, areal nutrient storage varied among the beds (11.2– 133.7 mg N m � 2 , 4.1–48.9 mg P m � 2 ) with the majority of nutrient storage in a long-term store, shell (c. 87% of total N storage, c. 95% of total P storage). 4. Freshwater mussels can be important to nutrient dynamics through nutrient regeneration and the creation of storage hotspots. However, the importance of nutrient remineralisation varies dramatically in response to organism patchiness, flow conditions and background nutrient concentrations.

Mixed effects of effluents from a wastewater treatment plant on river ecosystem metabolism: Subsidy or stress?

Abstract: The effluents of wastewater treatment plants (WWTPs) include a complex mixture of nutrients and pollutants. Nutrients can subsidise autotrophic and heterotrophic organisms, while toxic pollutants can act as stressors, depending, for instance, on their concentration and interactions in the environment. Hence, it is difficult to predict the overall effect of WWTP effluents on river ecosystem functioning. We assessed the effects of WWTP effluents on river biofilms and ecosystem metabolism in one river segment upstream from a WWTP and three segments downstream from the WWTP and following a pollution gradient. The photosynthetic capacity and enzymatic activity of biofilms showed no change, with the exception of leucine aminopeptidase, which followed the pollution gradient most likely driven by changes in organic matter availability. The effluent produced mixed effects on ecosystem-scale metabolism. It promoted respiration (subsidy effect), probably as a consequence of enhanced availability of organic matter. On the other hand, and despite enhanced nutrient concentrations, photosynthesis-irradiance relationships showed that the effluent partly decoupled primary production from light availability, thus suggesting a stress effect. Overall, WWTP effluents can alter the balance between autotrophic and heterotrophic processes and produce spatial discontinuities in ecosystem functioning along rivers as a consequence of the mixed contribution of stressors and subsidisers

A critical analysis of regulated river ecosystem responses to managed environmental flows from reservoirs

Abstract: Summary 1.The flow regime of a river is fundamental in determining its ecological characteristics. Impoundment of rivers has been documented to severely impact the natural flow regime, resulting in abiotic and biotic changes in downstream ecosystems. Contemporary water legislation is driving increasing concern among environmentalists and water resource managers with respect to how these impacts can be mitigated. This has stimulated research aimed at assessing the relationship between reservoir outflow modification (i.e. managed environmental flows) and downstream ecosystem responses. 2.We carried out a critical review and synthesis of the global literature concerning post-impoundment reservoir outflow modification and associated downstream biotic and abiotic responses. Seventy-six studies published between 1981 and 2012 were analysed. In contrast to previous studies of this subject, we systematically assessed the methodological quality of research to identify strengths and weaknesses of the approaches. We also undertook a novel quantification of ecosystem responses to flow modification, thus enabling identification of priorities for future research. 3.We identified that: (i) there was a research bias towards North American and Western European studies; (ii) the majority of studies reported changes in flow magnitude (e.g. artificial floods) and primarily focused on traditionally monitored ecological groups (e.g. fish); (iii) relationships between flow, biota (e.g. macroinvertebrates) and water quality (e.g. electrical conductivity and suspended solids concentration) were evident, demonstrating the potential for managed environmental flows to manipulate river ecosystems; (iv) site-specific factors (e.g. location, climate) are likely to be important as some ecosystem responses were inconsistent between studies (e.g. fish movement in response to increases in flow magnitude); and (v) quality of study design, methodological and analytical techniques varied, and these factors may have contributed to the reported variability of ecosystem response. 4.To advance scientific understanding and guide future management of regulated flow regimes, we highlight a pressing need for: (i) diversification of study locations as well as flow modification and ecosystem response types assessed; (ii) a focus on understanding flow–ecosystem response relationships at regional scales; (iii) further quantitative studies to enable robust statistical analyses in future meta-analyses; and (iv) robust monitoring of flow experiments and the use of contemporary statistical techniques to extract maximum knowledge from ecological response data.

The ecological restoration of large rivers needs science‐based, predictive tools meeting public expectations: an overview of the Rhône project

Abstract: Summary Effective environmental management needs models that reliably predict quantitative ecological changes as a function of restoration effort (e.g. cost) and meet expectations of stakeholders. Principal threats to large rivers are linked to human-caused modifications of discharge and morphology of channels and floodplains. However, comprehensive large-scale tests of the reliability of models predicting ecological consequences of restoring these elements are still lacking. Following a governmental decision, water managers, local authorities and the ‘Compagnie Nationale du Rhone’ financed a scientific programme to develop, test and subsequently use predictive models to assess the restoration (particularly minimum flow increases and reconnections of floodplain channels with the main channel) of eight regulated reaches of the French Rhone River. This approach was fostered by (i) the existence of local initiatives aimed at the ecological improvement of the Rhone; (ii) a history of interactions based on trust among stakeholders; and (iii) knowledge provided by a large interdisciplinary research group that studied the Rhone for two decades before the programme started in 1998. This Special Issue synthesises the insights gained over recent decades of research during which four river reaches (total length 47 km) were restored since 1999. It contains 11 articles including this overview. One article relates physical habitats in the floodplain to river hydrology and morphology; five articles test predictive models linking changes in habitat conditions to changes in taxa abundance, community metrics and biological traits of macroinvertebrates and fish; and four articles address the effects of restoration in larger contexts (long-term community trends, optimisation of sampling strategies, social processes and bioindication). We describe the Rhone restoration project, explain the conceptual framework used to predict the effects of restoration on river biota and describe the contents of the Special Issue, the main results and their implications. The Rhone restoration led to more lotic and diverse aquatic communities and renewed social links with the river. When reliable pre-restoration data are available, simple habitat models can be used to predict quantitative ecological changes as a function of restoration effort. The project illustrates the need to describe changes in hydraulic conditions in studies of physical river restoration and shows the effort required for a powerful assessment of restoration effects.

Resetting the river template: the potential for climate‐related extreme floods to transform river geomorphology and ecology

Abstract: 1. Climate change is expected to alter the occurrence of extreme climatic events, including major floods. Future shifts in the frequency and intensity of extreme floods will vary by region and could modify the geomorphological character of riverine habitat. 2. The geomorphological structure of rivers determines the quality and quantity of habitat available for resident biota, and thus, changes to morphology from more extreme floods are likely to affect river ecology over and above the direct effects of the flood events themselves. 3. Extreme floods can exacerbate the effect of multiple anthropogenic stressors, with potentially dramatic effects on freshwater ecosystems. For instance, high flows mobilise nutrients, sediment and toxic chemicals, and aid dispersal of invasive species, whereas land-use change and channelisation impair flood refugia and constrain recolonisation pathways. 4. Extreme floods may also benefit riverine and riparian biota, overwhelming current anthropogenic constraints and infrastructure to increase habitat complexity and floodplain area. 5. Management to protect human life and infrastructure from severe river flooding can alter channel geomorphology, habitat quality and ecology. However, flood prevention engineering that incorporates the natural form of rivers could potentially mitigate anthropogenic flood damage, in turn restoring habitat and historical ecosystem functioning. 6. We examine potential changes in river channel and floodplain geomorphological characteristics resulting from altered extreme flooding regimes and consider likely implications for the management of the world’s freshwater biota.

Ecological effects of extreme climatic events on riverine ecosystems: insights from Australia

Abstract: Climate extremes and their physical impacts - including droughts, fires, floods, heat waves, storm surges and tropical cyclones - are important structuring forces in riverine ecosystems. Climate change is expected to increase the future occurrence of extremes, with potentially devastating effects on rivers and streams. We synthesise knowledge of extremes and their impacts on riverine ecosystems in Australia, a country for which projected changes in event characteristics reflect global trends. Hydrologic extremes play a major structuring role in river ecology across Australia. Droughts alter water quality and reduce habitat availability, driving organisms to refugia. Extreme floods increase hydrological connectivity and trigger booms in productivity, but can also alter channel morphology and cause disturbances such as hypoxic blackwater events. Tropical cyclones and post-cyclonic floods damage riparian vegetation, erode stream banks and alter water quality. Cyclone-induced delivery of large woody debris provides important instream habitat, although the wider ecological consequences of tropical cyclones are uncertain. Wildfires destroy catchment vegetation and expose soils, increasing inputs of fine sediment and nutrients to streams, particularly when followed by heavy rains. Research on the impacts of heat waves and storm surges is scarce, but data on temperature and salinity tolerances, respectively, may provide some insight into ecological responses. We identify research gaps and hypotheses to guide future research on the ecology of extreme climate events in Australia and beyond. A range of phenomenological, experimental and modelling approaches is needed to develop a mechanistic understanding of the ecological impact of extreme events and inform prediction of responses to future change.

Extreme water quality degradation following a catastrophic forest fire

Abstract: SUMMARY 1. Global change is impacting the forests of the western United States through rising temperatures, earlier snowmelt, more rain and less snow, greater vapour pressure deficits in spring and autumn, forest dieback and increasing forest fire frequency and severity. 2. A catastrophic forest fire (Las Conchas fire) occurred in central NM, USA, in 2011 burning c. 634 km 2 with c. 46% of the fire being of severe or moderate intensity. 3. National Oceanic and Atmospheric Administration (NOAA) next-generation radar data (NEXRAD) were used to link precipitation events occurring in the burn scar to extreme water quality excursions observed in the Rio Grande. At four sites, in situ sensors captured the response of water temperature, specific conductance, pH, turbidity and dissolved oxygen to flood events following the fire. 4. Runoff from burn scars caused turbidity peaks (to 2500 NTU), dissolved oxygen sags (to 0.0 mg L -1 ), pH sags (up to 0.75 units) and conductivity changes (both increases and decreases). These water quality excursions extended at least 50 km downstream, with significant implications for the ecosystem health of this crucial river that supplies water to cities and agriculture. 5. Sudden, dramatic changes to forested catchments from severe forest fires and forest dieback are very likely to be among the strongest impacts of global change on stream and river ecosystems throughout the western United States.

Extreme events in running waters

Abstract: Summary Extreme climatic events are a natural feature of climate variability and a major organising force in running waters. Climate change is shifting the occurrence of extremes, and understanding the far-reaching consequences for river and stream ecosystems is a research priority. The aim of this special issue is to unite research on the wide array of contrasting extreme events affecting river and stream ecosystems globally, including heat waves and hot days, fires, droughts, heavy rainfall and floods, tropical cyclones, storm surges and coastal flooding. This issue contains reviews as well as observational and experimental case studies that assess the ecological impacts of extreme events at multiple levels of biological organisation, from individuals to ecosystems and food webs, in river and stream ecosystems from the poles to the tropics. Together, these papers synthesise knowledge of extreme events and their effects, identify research gaps and propose new hypotheses and approaches to guide future research. The contributions reveal that impacts of single events such as catastrophic floods, droughts and heat waves are highly context dependent, ranging from deleterious to beneficial, and are contingent upon event magnitude, extent and timing relative to life cycles of constituent species. Not all extreme events generate extreme ecological impacts, but combinations of events that are cumulative, compound or cross key biological or physical thresholds may have the most adverse ecological consequences. Long-term monitoring programmes and sensor networks are essential in describing rare and usual events. Other approaches, including experiments, will be needed to gain a stronger mechanistic understanding of extreme events, and to predict and manage future events that may be more frequent and intense.

Ecological engineering and aquatic connectivity: a new perspective from beaver‐modified wetlands

Abstract: SUMMARY 1. Habitat fragmentation and wetland loss due to anthropogenic causes are usually attributed to physical modifications of the environment; however, the loss of key species can compound these impacts and further reduce the connectivity of aquatic ecosystems. 2. Ecosystem engineers can play a critical role in modifying aquatic systems by altering the bed of ponds and streams, increasing water coverage and influencing biogeochemical processes within and adjacent to freshwater habitats. However, there is a paucity of research on how these organisms enhance connectivity among aquatic habitats, especially in otherwise isolated wetland systems. 3. In this study, we collected field data at natural and agriculturally impacted sites to quantify physical alterations to otherwise isolated, morainal wetlands modified by beavers, and to determine how these modifications might enhance connectivity. For finer-scale analysis, we collected and modelled bathymetric data for 16 wetlands, eight of which were occupied by beavers and eight abandoned by beavers. 4. We demonstrated that beavers actively increase the volume-to-surface area ratio of wetlands by almost 50% and that their digging of foraging channels increases average wetland perimeters by over 575%. Some channels were 200–300 m long, which enhanced the interface between the riparian zone and upland forests. A coarse estimate of soil displacement due to the digging of channels by beavers exceeded 22 300 m 3 within the total 13 km 2 natural area. Additional measures of wetland depth, basin complexity and basin circularity revealed other dramatic differences between wetlands with beavers and those without in both natural and agricultural landscapes. 5. Exclusion or removal of beavers could limit ecosystem processes and resilience, especially in areas with otherwise isolated aquatic habitats and limited connectivity. Conversely, reintroduction of such an ecosystem engineer into areas targeted for restoration could result in significant increase in habitat heterogeneity and connectivity.

Relationships between lotic macroinvertebrate traits and responses to extreme drought

Abstract: Summary The prospect of increasing drought intensity in many river basins under climate change threatens the persistence of vulnerable freshwater species. Understanding how the traits of each species affect its resistance and resilience to drought may help to identify those species at most risk and elucidate the mechanisms by which impacts occur. I analysed macroinvertebrate monitoring data collected from rivers across Australia's Murray–Darling Basin (>106 km2) during the middle and later stages of the recent decade-long Millennium Drought and the initial post-drought period. I tested the ability of eight traits, expressing aspects of life history, diet and environmental tolerance, to explain changes in the broad-scale prevalence (the proportion of sites with observed presence) of macroinvertebrate families during and after the drought. The rate of basin-scale change in the riverine macroinvertebrate assemblage was least in the final stages of the drought. Immediately after the drought, the assemblage did not shift back towards its mid-drought state but instead moved further away. Eleven families that had a statistically significant fall in prevalence during the drought did not increase afterwards. Negative responses to drought were associated with slower maturation, absence of atmospheric respiration, high rheophily and low thermophily. Positive responses to cessation of drought were associated with having a holometabolous life cycle, greater requirements for dissolved oxygen, high rheophily and low thermophily. Because several traits were related to drought vulnerability, management to mitigate the adverse ecological effects of future droughts should consider a number of mechanisms by which drought has an effect. These include a loss of flowing water that supports rheophilous species, inadequate duration of wetting for species with a long aquatic phase, and effects of high temperature and hypoxia on species requiring cool conditions and well-aerated water. A revival of research on the life histories of freshwater invertebrate species and more information on oxygen requirements and temperature and desiccation tolerance are needed to improve our ability to predict the effects of drought.

Strong impact of nitrogen loading on submerged macrophytes and algae: a long-term mesocosm experiment in a shallow Chinese lake

Abstract: Summary Excess loading of phosphorus (P) and nitrogen (N) triggers a shift in the trophic structure of shallow lakes from a clear-water, macrophyte-dominated state to an algal-dominated turbid state. However, the role of N in the shift is debated, and experimental evidence is, with a few exceptions, based on short-term studies (days to a few months). We studied the effect of N loading on macrophytes (dominated by Potamogeton lucens and Cabomba caroliniana), periphyton, filamentous algae and phytoplankton in mesocosms over 10 months (starting in October) in subtropical China (Wuhan). There were three N treatments: controls (CN) without nitrogen addition (mean TN = 1.9 mg L−1), low nitrogen (LN) addition (mean TN = 3.5 mg L−1) and high nitrogen (HN) addition (mean TN = 5.5 mg L−1). Total phosphorus (TP) concentration in the water column remained moderate (0.05–0.07 mg L−1) during the experiment in all treatments. Macrophyte abundance declined in the LN and HN treatments in the first 6 months, but not in controls, followed by a partial recovery in the LN treatments. They disappeared completely in the HN treatments the following summer. Periphyton (biofilm on plastic) and phytoplankton biomass remained unaffected during the first 6 months but increased over the summer by two or three times, compared with controls, in low and high nitrogen treatments, respectively. By contrast, the abundance of filamentous algae increased over winter but declined during the summer with no obvious relationship to the N treatments. There was no difference in the TN or nitrate concentrations or soluble protein, soluble sugar and Chl-a content of P. lucens leaves and stems with increasing N load. Macrophyte populations are partially resilient to abrupt increases in N loading at moderate TP concentrations, but, after prolonged exposure, a complete collapse occurs. Our results further indicate that macrophyte loss is exacerbated by shading by filamentous algae during the winter, and by phytoplankton and periphyton in the summer, while there was no indication of direct N toxicity.

Microbial methane cycling in the bed of a chalk river: oxidation has the potential to match methanogenesis enhanced by warming

Abstract: SUMMARY 1. Many rivers are oversaturated in methane (CH4) and carbon dioxide (CO2) relative to the atmosphere, but we know little about the biological controls on the balance between these two important greenhouse gases and how they might respond to warming. 2. We characterise the potential response to temperature in the biological production of CO2 and CH4 and the subsequent microbial oxidation of that CH4, that is the sink and source components of the CH4 cycle, in contrasting river bed sediments: fine sediments, which are largely anoxic, and oxic, coarse gravels. 3. In the fine sediments, anaerobic production of both CH4 and CO2 increased with temperature, with apparent activation energies for each being 0.51 eV and 0.24 eV, respectively. The difference between the two resulted in a 4% increase in the ratio of CH4:CO2 production for a 1 °C increase in temperature. 4. In the coarse gravels, microbial CH4 oxidation showed no response to temperature at CH4 concentrations characteristic of these gravel beds (30–200 nmol CH4 L 1 ), due to strong substrate limitation. In contrast, at higher (although still rate limiting) CH4 concentrations, more characteristic of the fine sediment patches (2–4 lmol CH4 L 1 ), CH4 oxidation exhibited an increasingly strong response to temperature, eventually exceeding that for CH4 production. 5. In the fine sediment, the surface layers had a CH4 oxidation capacity over 100 times greater than the gravels and the kinetic response to differing pore water CH4 concentrations meant CH4 was oxidised some 2000 times faster in the fine sediment patches compared with the coarse gravels. 6. The calculated kinetic and temperature responses showed that with warming, methanogenesis is unlikely to outstrip methanotrophy and the ratio of CO2 to CH4 emitted could be conserved. Consequently, any changes in the efflux ratio of CH4 to CO2 are unlikely to be due to the incapacity of methanotrophy to respond to CH4 production, but rather to a physical bypassing of the methanotrophic community (e.g. through ebullition or transport via plant stems) or contraction of the oxic layer.

Unifying research on the fragmentation of terrestrial and aquatic habitats: patches, connectivity and the matrix in riverscapes

Abstract: Summary While there is an increasing emphasis in terrestrial ecology on determining the influence of the area that surrounds habitat patches (the landscape ‘matrix’) relative to the characteristics of the patches themselves, research on these aspects in running waters is still rather underrepresented. Here we outline conceptual foundations of matrix ecology for stream and river ecosystems (‘riverscapes’). We discuss how a hierarchical, patch-based perspective is necessary for the delineation of habitat patches and the surrounding matrix, through which we may identify two classes of habitat edges in riverscapes (i.e. edges between the terrestrial–aquatic interface and edges within streams). Under this conceptual framework, we discuss the role of the matrix in influencing between-patch movement, and resource quality and quantity within and among habitat patches in riverscapes. We also review types of empirical and modelling approaches which may advance our understanding of fragmentation effects in these systems. We identify five key challenges for understanding fragmentation and matrix effects more completely: (i) defining populations and their status (i.e. quantifying the demographic contribution of habitat patches to metapopulation dynamics), (ii) scaling from metapopulations to metacommunities (particularly searching for generalities in species responses to landscape heterogeneity), (iii) scaling from metacommunities to metaecosystems (i.e. exploring the interactive role of the terrestrial–aquatic and within-stream matrix effects on the flow of material and energy at the network scale), (iv) understanding temporal dynamics in matrix permeability and (v) revealing the utility of different patch and matrix representations for modelling connectivity relationships. Fragmentation of habitats is a critical issue in the conservation and management of stream networks across spatial scales. Although the effects of individual barriers (e.g. dams) are well documented, we argue that a more comprehensive patch–matrix landscape model will improve our understanding of fragmentation effects and improve management in riverscapes.

Effects of elevated water temperature on physiological responses in adult freshwater mussels

Abstract: SUMMARY 1. Freshwater mussels (order Unionoida) face multiple environmental stressors, which pose serious conservation challenges to this diverse assemblage of aquatic invertebrates. Of these stressors, elevated water temperature from global climate change and other anthropogenic sources may be the most ubiquitous and could be placing many mussel populations dangerously close to their thermal maxima. 2. We tested the hypothesis that elevated water temperatures (20, 25, 30 and 35 °C) adversely affected physiological responses in adults of four North American species of mussels (Amblema plicata, Elliptio complanata, Fusconaia flava and Lampsilis cardium) in 21-d laboratory tests. 3. Oxygen consumption rates were directly affected by temperature in E. complanata and L. cardium, and indirectly affected by temperature in A. plicata and F. flava. Rates of O2 consumption were generally positively correlated with water temperature. Ammonium excretion rates varied significantly with temperature in E. complanata and generally increased with temperature. The amount of O2 consumed relative to nitrogen excreted (O : N ratio), varied significantly with temperature in A. plicata, E. complanata and F. flava. The tissue condition index varied among temperatures and species. 4. These data suggest that elevated temperatures can alter metabolic rates in native mussels and may decrease the amount of energy that is available for key biological processes, such as survival, growth and reproduction.

Land-use effects on terrestrial consumers through changed size structure of aquatic insects

Abstract: We assessed the influence of agricultural land use on aquatic-terrestrial linkages along streams arising from changes in the emergence of aquatic insects. We expected that terrestrial predators wou ...

Upstream and downstream responses of fish assemblages to an eastern Amazonian hydroelectric dam

Abstract: Major hydroelectric dams are proliferating in tropical regions such as Amazonia, where extensive new hydropower developments are planned despite potentially severe ecological and social impacts. The status of freshwater biota in the vicinity of existing dams could be valuable to predict the effects of such developments, but detailed ecological monitoring before and after dam construction is frequently lacking. In the absence of these data, we used a space-for-time replacement to compare a key component of the aquatic biota at upstream and downstream sites, with the latter more closely resembling river channel conditions prior to the dam. We assessed the fish assemblages upstream and downstream of the Coaracy Nunes Dam in Amapa, Brazil, the first ecological study at this site since this dam was constructed 42 years earlier. We used gillnets during eight bimonthly field campaigns, covering both wet and dry seasons, and sampled the ichthyofauna (1819 individuals, 81 species) at four sites: Downstream Channel, Reservoir, Upstream Lake and Upstream Channel. We found clear negative impacts on the abundance, biomass, species richness, alpha diversity and species dominance upstream of the dam. The physical subdivision of the river channel and the upstream channel conversion from lentic to lotic habitats were strongly associated with differences in the composition and structure of fish communities. Notable changes include an increased contribution of small-bodied fish in the reservoir and an absence of long-distance migrants upstream of the dam. Downstream channel habitats, in particular, retained their fish diversity with high conservation value typical of eastern Amazonia, yet these areas now face the threat of new hydropower development farther downstream. The long-term impacts on aquatic biodiversity highlighted in this study are especially relevant in the face of burgeoning new hydroelectricity development plans for rivers across lowland Amazonia.

Resilience of a stream community to extreme climatic events and long‐term recovery from a catastrophic flood

Abstract: SUMMARY 1. Floods and droughts are predicted to increase in intensity, duration and frequency in many future climate scenarios, yet long-term data that track before-and-after responses of natural communities remain scarce. We explored the impacts of a series of extreme events, including a particularly catastrophic flood, over 13 years in the Glenfinish River in Ireland. 2. Overall, seasonal cycles of absolute and relative abundance were the strongest temporal signal in the data, and the community as a whole was relatively persistent. At interannual scales, a core of c. 15 taxa were present throughout most of the time series, which spanned >10 generations for most taxa, whereas extreme events had negligible or weak effects, with two notable exceptions. The catastrophic 1986 summer flood triggered a 10-fold decline in abundance: although most populations returned to their pre-disturbance state in <3 years, some took up to a decade to recover. In 1988– 1989, two contrasting extreme low-rainfall and storm events occurred within a year and were associated with far more marked compositional and functional shifts than were seen in the 1986 flood, although abundance was not so strongly affected. 3. Contrasting extreme events that occur close to one another in time and/or at atypical times of year (e.g. floods in summer, when base flow is low) can have particularly strong effects, whereas at other times even the most extreme events may leave little longer-term footprint. Therefore, both the temporal context and the magnitude of events determine the biota’s response. 4. Effects of the events also varied across taxa and organisational levels: the more r-selected taxa (e.g. Chironomidae) were relatively unaffected compared with the larger, more K-selected taxa. Community-level responses were mostly driven by shifts in relative abundances, rather than species loss or turnover, and the functional diversity of species traits was more resilient still. The apparent stability in relative abundances of functional groups in the face of significant variation at the species level is suggestive of high functional redundancy, but without exhaustive further analyses using null-model approaches, this remains unproved. In turn, relatively high levels of redundancy and the reshuffling of species with similar traits could confer resilience in the face of all but the most extreme events.

An attack on two fronts: predicting how changes in land use and climate affect the distribution of stream macroinvertebrates

Abstract: Global environmental change entails not only climatic alterations, but also changes in land use. Freshwater ecosystems are particularly sensitive to both of these changes, and their sustainable management requires better information on likely responses. To examine the effects of climate and land use on the freshwater community, the distributions of stream macroinvertebrates of the Changjiang catchment in south-east China were modelled. The present distributions of 72 taxa were predicted using environmental variables generated by regional climate, land-use and hydrological models. Hydrological predictors, sensitive to both climate and land use, were the most relevant predictors in the species distribution models (SDMs), followed by land use. The stream macroinvertebrates' distributions were then projected for the period 2021 to 2050 using three different future scenarios: (i) climate change, (ii) land-use change and (iii) climate and land-use change combined. Land-use change was predicted to have the strongest negative impact on the community, with reductions in local richness (-20%), predicted diversity (-0.3%) and range size (-25%) and a general shift towards higher altitudes (+12%). The climate-change scenario had a negative effect on predicted diversity (-0.1%) and resulted in a moderate altitudinal shift (+3%) along with increased richness (+15%) and range size (+19%). In the combined scenario, climate and land-use changes counterbalanced each other to a certain degree, but had an overall detrimental effect. The results underscore the high relevance of land-use change in future distribution predictions, exemplify the possible effect of interactions between land use and climate on hydrology and indicate how such responses can vary among freshwater taxa. The model also allows the detection of key environmental variables, the identification of vulnerable species and the definition of their potential distributions. This information is essential to establishing effective management and conservation strategies and gives a more comprehensive insight into the possible effects of global environmental change on freshwater ecosystems.

Elements of metacommunity structure and community- environment relationships in stream organisms

Abstract: SUMMARY 1. Most metacommunity studies aim to explain variation in community structure using environmen-tal and spatial variables. An alternative is to examine patterns emerging at the level of an entiremetacommunity, whereby six models of metacommunity structure (i.e. random, chequerboards, nest-edness, evenly spaced, Gleasonian gradients and Clementsian gradients) can be examined.2. We aimed to test the fit of six competing models of metacommunity structure to extensive surveydata on diatoms, bacteria, bryophytes and invertebrates from three drainage basins in Finland, alonga latitudinal gradient from 66°Nto70°N.3. Species were mainly distributed independently of one another (following the Gleasonian model) inthe southernmost drainage basin (66°N), whereas there were discrete community types, with sets ofspecies responding similarly along environmental gradients (following the Clementsian model), inthe northernmost drainage basin (70°N). The patterns found were not directly related to an expectedrelationships between environmental heterogeneity and metacommunity structures, but rather to thegeographical location of the drainage basin.4. There is evidently among-region variation in the best-fit models of metacommunity structure ofstream organisms. These metacommunity patterns may show some similarities among biologicallydisparate organismal groups sampled at the set of the same sites, although the underlying environ-mental drivers of those patterns may vary between the groups.Keywords: bacteria, biological communities, bryophytes, diatoms, fresh waters, invertebrates

Modelling nutrient transport and transformation by pool-breeding amphibians in forested landscapes using a 21-year dataset

Abstract: SUMMARY 1. Migrations of animals can transfer energy and nutrients through and among terrestrial and aquatichabitats. Pool-breeding amphibians, such as the wood frog (Lithobates sylvaticus), make annual breed-ing migrations to ephemeral wetlands in forest habitats in the eastern and midwestern United Statesand Canada.2. To model the influence of wood frogs on nutrient transport and transformation through time, wecoupled long-term population monitoring data (1985–2005) from a wood frog population with esti-mates of the elemental composition of wood frog egg masses and emerging juveniles.3. Over the 21-year study period, 8.8 kg carbon (C), 2.0 kg nitrogen (N) and 0.20 kg phosphorus (P)were transported from the terrestrial to the aquatic habitat and approximately 21 kg C, 5.5 kg N and1.2 kg P were exported to the surrounding terrestrial habitat by wood frogs.4. During the study period, the average net flux of C, N and P was from aquatic to terrestrial habi-tats, but the magnitude and direction of the net flux was element dependent. Thus, the net flux of C,N and P did not always flow in the same direction.5. Predicting long-term trends in nutrient and energy flux by organisms with biphasic life cyclesshould rely on long-term population data to account for temporal variability. This is especially truefor organisms that are sensitive to long-term shifts in temperature and precipitation patterns, such asamphibians that breed in ephemeral pools.Keywords: animal migrations, nutrient dynamics, resource subsidies, vernal pool, wood frog

Effectiveness of a large reserve network in protecting freshwater biodiversity: a test for the Iberian Peninsula

Abstract: SUMMARY 1. Although coverage by protected areas is increasing worldwide, their capacity to protect freshwater biodiversity is uncertain. This may be the case of the European Natura 2000, the world’s largest reserve network, given its primary focus on terrestrial biodiversity. We tested this hypothesis by assessing the effectiveness of Natura 2000 in protecting freshwater biodiversity and associated key ecological processes in the Iberian Peninsula, where more than 25% of land is under some kind of protection. 2. We compiled distributional data on 91 species of freshwater fish, amphibians and aquatic reptiles, at the subcatchment scale (c. 20 km 2 resolution), for the whole Iberian Peninsula, and assessed the proportion of their ranges covered by Natura 2000. We also explored the coverage of the main environmental gradients, and the capacity of Natura 2000 to offer protection against human impacts. To do so, we evaluated land-use intensity and the human footprint within and upstream of protected areas, both of which might compromise the protection of biodiversity. Finally, we also analysed the incidence of reservoirs within and downstream of protected areas. 3. Natura 2000 broadly covers the main environmental gradients in the Iberian Peninsula, but fails to provide sufficient coverage of freshwater biodiversity, with <20% of the range of species covered on average. This would be insufficient to achieve a modest conservation target of 25% of the range for more than 80% of species, including most of species specifically listed in the legislation. Moreover, although the network tends to comprise areas with the least human impact, it seems to be vulnerable to the propagation of upstream effects (intensive land use downstream) and the loss of downstream connectivity (reservoirs). 4. Large reserve networks, primarily designed to protect terrestrial biodiversity, may not offer adequate protection for freshwater biodiversity. We recommend revisiting the design of Natura 2000 to improve the representation of freshwater biodiversity and enhance its capacity to address threats and particular ecological needs, such as for migration. Given the high pressure on land and the poor condition of the remaining unprotected habitat, conservation efforts must minimise the conflict of interest and management costs.

Development of a biotic index using stream macroinvertebrates to assess stress from deposited fine sediment

Abstract: 1. Detrimental impacts of excessive fine-grained sediment inputs to streams and rivers are well established. What is less well understood is the susceptibility of different elements of the freshwater biota to such perturbations and how such knowledge of their susceptibility could aid in identifying where excessive fine-grained sediment is impairing ecological condition. 2. Following the collection of biological and sediment data from 179 streams across England and Wales, representative of a range of river types over a gradient of fine sediment loading, objective statistical approaches were applied to establish relationships between the macroinvertebrate assemblage and fine-grained sediment inputs to river channels. 3. Having factored out that portion of the biological variation associated with natural environmental gradients, a model comprising mass of organic sediment in erosional areas of the stream bed [predominantly associated with the first axis of the partial canonical correspondence analysis (pCCA)], and mass of fine-grained sediment in the surface drape of depositional areas and % organic content in erosional areas (associated with the second axis of the pCCA) as explanatory variables best accounted for the residual variation in the macroinvertebrate assemblage. 4. The relative position of taxa along both axes of the pCCA, provided a ranking of taxa in relation to the two gradients of fine-grained sediment and provided the basis for a new empirically derived diagnostic index for fine-grained sediment stress in rivers. Two sub-indices were derived to capture the assemblage responses to both the gradient of organic sediment in erosional areas and the gradient of total fines in depositional areas. The two sub-indices were then combined to derive the new combined fine sediment index (CoFSIsp). 5. The index was tested on an independent test data set (comprising 127 samples from 83 sites) and was found to provide a robust indication of benthic fine-grained sediment conditions (Spearman's rank correlations ρ = −0.519 to −0.703). The strength of correlation with the total fine-grained sediment gradient was always greater than that for other routinely used indices, confirming that CoFSIsp offered additional explanatory power when assessing this stressor of aquatic environments.

Responses of cyanobacteria to herbivorous zooplankton across predator regimes: who mows the bloom?

Abstract: 1. The massive growth of large, toxic cyanobacteria in eutrophic waters has traditionally been explained by their evolution of defences to herbivorous zooplankton. These conclusions come mostly from studies using the large-bodied cladoceran Daphnia as a grazer model. In contrast, very little is known about the effects of other zooplankters such as copepods and small cladocerans that, unlike Daphnia, often coexist with cyanobacterial blooms and under high levels of fish predation. We performed a series of grazing experiments during a cyanobacterial bloom in a eutrophic lake, subject to decreasing predation on zooplankton (removal of cyprinid fish). We also used long-term observational data to analyse the response of cyanobacteria to changes in abundance, body size and biomass of the naturally, coexisting zooplankton community across different predator regimes. A natural grazer community, dominated by selective herbivores like calanoid and cyclopoid copepods, positively affected cyanobacterial growth in early summer at low cyanobacterial densities. However, cyclopoid copepods and small cladocerans suppressed the growth of Anabaena, Microcystis and Planktothrix species under bloom conditions in late summer, with the exception of the highly toxic Microcystis botrys. Long-term observational data support the results from the grazing experiments regarding the potential capacity of the natural zooplankton community to suppress cyanobacterial growth, especially of the dominant taxon Microcystis (including the most toxic species, M.botrys). In particular, these results suggest that direct grazing may contribute significantly to the observed 72% decrease in Microcystis biomass through a 111% increase in abundance of cyclopoid copepods, and an 18% increase in body size of cyclopoid copepods and a 31% increase in body size of the generalist feeder Daphnia, during the biomanipulation. Together, these results suggest that, within the complex interactions brought about by trophic cascades, direct grazing by the coexisting zooplankton community is a strong driver regulating cyanobacterial growth in eutrophic lakes and that this can display contrasting effects, both seasonally and under different levels of fish predation. Importantly, we show that herbivory may be enhanced along a gradient of decreasing fish predation by the combined action of copepods (selective herbivores) and Daphnia (generalist herbivores). (Less)

Climate change modifies the size structure of assemblages of emerging aquatic insects

Abstract: Climate change is expected to not only raise water temperatures, but also to cause brownification of aquatic ecosystems via increased inputs of terrestrial dissolved organic matter. While efforts h ...

The effects of predation by juvenile fish on the meiobenthic community structure in a natural pond

Abstract: Summary Benthic communities in aquatic ecosystems include an abundant and functionally important meiofaunal component. The effects of predation by fish on zooplankton, macroinvertebrates and periphyton in freshwater habitats have been clearly demonstrated, whereas impacts on meiofaunal assemblages are poorly understood. In this study, the predation effects of benthivorous juvenile carp (Cyprinus carpio) on the abundance, biomass, secondary production, size classes and community structure of meiofaunal assemblages and on nematode diversity in the littoral zone of a natural freshwater pond were examined over 125 days using field enclosures and exclosures. Fish predation altered the abundance, biomass and secondary production of nematodes, oligochaetes and microcrustaceans (cyclopoids, harpacticoids, ostracods and cladocerans) but not of rotifers. These changes were consistent with top-down control of the freshwater meiofaunal assemblages in the natural pond over time. By contrast, small-bodied meiofauna were more abundant in the presence of fish, suggesting indirect facilitation. Both species richness and species composition but not the diversity of the nematode community were affected by fish predation. Removal of the fish from the enclosures resulted in the thorough recovery of the density, biomass and secondary production of nematodes, oligochaetes and microcrustaceans within 45 days. The results indicate that predation by juvenile freshwater fish depresses the overall abundance, biomass and secondary production of meiofaunal assemblages, except in the case of rotifers, and alters the size structure of the meiofaunal community. Our study provides insight into the food-web ecology of lakes and the first evidence of freshwater fish predation effects on meiofaunal assemblages in a natural habitat.