Our understanding of how ecosystems function has changed from an equilibria-based view to one that recognizes the dynamic, fluctuating, nonlinear nature of aquatic systems. This current understanding requires that we manage systems for resilience. In this review, we examine how resilience has been defined, measured and applied in aquatic systems, and more broadly, in the socioecological systems in which they are embedded. Our review reveals the importance of managing stressors adversely impacting aquatic system resilience, as well as understanding the environmental and climatic cycles and changes impacting aquatic resources. Aquatic resilience may be enhanced by maintaining and enhancing habitat connectivity as well as functional redundancy and physical and biological diversity. Resilience in aquatic socioecological system may be enhanced by understanding and fostering linkages between the social and ecological subsystems, promoting equity among stakeholders, and understanding how the system is impacted by factors within and outside the area of immediate interest. Management for resilience requires implementation of adaptive and preferably collaborative management. Implementation of adaptive management for resilience will require an effective monitoring framework to detect key changes in the coupled socioecological system. Research is needed to (1) develop sensitive indicators and monitoring designs, (2) disentangle complex multi-scalar interactions and feedbacks, and (3) generalize lessons learned across aquatic ecosystems and apply them in new contexts.

Climate-change driven increases in water temperature pose challenges for aquatic organisms. Predictions of impacts typically do not account for fine-grained spatiotemporal thermal patterns in rivers. Patches of cooler water could serve as refuges for anadromous species like salmon that migrate during summer. We used high-resolution remotely sensed water temperature data to characterize summer thermal heterogeneity patterns for 11,308 km of 2- to 7-order rivers throughout the Pacific Northwest and northern California (USA). We evaluated (1) water temperature patterns at different spatial resolutions, (2) the frequency, size, and spacing of cool thermal patches suitable for Pacific salmon (i.e., contiguous stretches ≥0.25 km, ≤15°C and ≥2°C cooler than adjacent water), and (3) potential influences of climate change on availability of cool patches. Thermal heterogeneity was nonlinearly related to the spatial resolution of water temperature data, and heterogeneity at fine resolution (<1 km) would have been difficult to quantify without spatially continuous data. Cool patches were generally >2.7 and <13.0 km long, and spacing among patches was generally >5.7 and <49.4 km. Thermal heterogeneity varied among rivers, some of which had long uninterrupted stretches of warm water ≥20°C, and others had many smaller cool patches. Our models predicted little change in future thermal heterogeneity among rivers, but within-river patterns sometimes changed markedly compared to contemporary patterns. These results can inform long-term monitoring programs as well as near-term climate-adaptation strategies.

Mixotrophy seems to be widespread among phytoplankton, but whether this strategy is more relevant in oligotrophic lakes remains unclear. Here, we tested the hypothesis that the relative abundance of mixotrophic flagellates in lakes increases along an elevational gradient paralleling increasingly oligotrophic conditions. For this purpose, 12 lakes located between 575 and 2796 m above sea level were sampled in summer and fall to include two different seasonal windows in phytoplankton dynamics and environmental conditions. The degree of mixotrophy in phytoplankton was estimated in tracer experiments using fluorescently-labeled bacteria and done with composite samples collected in the euphotic zone and in samples obtained from the chlorophyll- maximum. The results indicated the existence of a positive trend particularly in summer in the relative abundance of mixotrophic flagellates with elevation, however, this trend was not linear, and exceptions along the elevational gradient were found. Changes in the relative abundance of mixotrophic flagellates were related with significant changes in water transparency, DOC and phosphorus concentrations, as well as in bacterial and flagellate abundance. Overall, our results reveal that the harsh growth conditions found in oligotrophic high mountain lakes favor a mixotrophic trophic strategy among phytoplankton.

It is well recognized that river-floodplain systems contribute significantly to riverine ecosystem metabolism, and that bacteria are key players in the aquatic organic carbon cycle, but surprisingly few studies have linked bacterial community composition (BCC), function and carbon quality in these hydrologically highly dynamic habitats. We investigated aquatic BCC and extracellular enzymatic activity (EEA) related to dissolved organic carbon quality and algae composition, including the impact of a major flood event in one of the last remaining European semi-natural floodplain-systems. We found that surface connectivity of floodplain pools homogenizes BCC and EEA, whereas low connectivity led to increased BCC and EEA heterogeneity, supported by their relationship to electrical conductivity, an excellent indicator for surface connection strength. Hydrogeochemical parameters best explained variation of both BCC and EEA, while the algal community and chromophoric DOM properties explained only minor fractions of BCC variation. We conclude that intermittent surface connectivity and especially permanent isolation of floodplain pools from the main river channel may severely alter BCC and EEA, with potential consequences for nutrient cycling, ecological services and greenhouse gas emissions. Disentangling microbial structure-function coupling is therefore crucial, if we are to understand and predict the consequences of human alterations on these dynamic systems.

Freshwater ecosystems represent a significant natural source of methane (CH). CH produced through anaerobic decomposition of organic matter (OM) in lake sediment and water column can be either oxidized to carbon dioxide (CO) by methanotrophic microbes or emitted to the atmosphere. While the role of CH oxidation as a CH sink is widely accepted, neither the magnitude nor the drivers behind CH oxidation are well constrained. In this study, we aimed to gain more specific insight into CH oxidation in the water column of a seasonally stratified, typical boreal lake, particularly under hypoxic conditions. We used CH incubations to determine the active CH oxidation sites and the potential CH oxidation rates in the water column, and we measured environmental variables that could explain CH oxidation in the water column. During hypolimnetic hypoxia, 91% of available CH was oxidized in the active CH oxidation zone, where the potential CH oxidation rates gradually increased from the oxycline to the hypolimnion. Our results showed that in warm springs, which become more frequent, early thermal stratification with cold well-oxygenated hypolimnion delays the period of hypolimnetic hypoxia and limits CH production. Thus, the delayed development of hypolimnetic hypoxia may partially counteract the expected increase in the lacustrine CH emissions caused by the increasing organic carbon load from forested catchments.

Mountain lakes are especially vulnerable to climate change, but are also increasingly exposed to local anthropogenic development through winter and summer tourism. In this study, we aimed to tease apart the influence of tourism from that of climate in a mountain lake located within one of the largest French ski resorts, by combining paleolimnological and present ecological data. The reconstructed long-term ecological dynamics highlighted an increase in lake biological production from the end of the Little Ice Age up to the 1950s, suggesting a historical dominance of climate control. Afterward, a major drop in pelagic production occurred at the same time as the watershed erosion increased and peaked in the 1990s, concomitant with massive digging for the ski resort expansion. The benthic invertebrates collapsed in the 1980s, concomitantly with the onset of massive salmonid stocking and recent warming. Stable isotope analyses identified benthic invertebrates as the major salmonid diet resource and suggested a possible direct impact of salmonid stocking on benthic invertebrates. However, habitat use may differ among salmonid species as suggested by the way fish DNA was preserved in surficial sediment. The high abundances of macrozooplankton further confirmed the limited reliance of salmonids on pelagic resources. The variable thermal tolerance of benthic invertebrates suggested that the recent warming may mostly affect littoral habitats. Our results indicate that winter and summer tourism may differently affect the biodiversity of mountain lakes and could collectively interfere with the ecological impacts of recent warming, making local management of primary importance to preserve their ecological integrity.

The main objective of this paper is to investigate how archaeological fish remains and written historical records can contribute to the reconstruction of long-term developments of fish communities along the Austrian and Hungarian Danube. Although such approaches are sensitive to various factors, the chronological subdivision and relative quantification of proxy data demonstrate environmental and faunal changes from Prehistory onwards. Intensification of fisheries, decline of large specimens and massive exploitation of small and young fish point to increasing pressure along the chronological sequence towards Early Modern times. One result of this impact was the establishment of regulations and laws to protect such fish. At the same time, the rise of aquaculture and common carp cultivation can be viewed as another upshot of human impact on the Danube's environment. Finally, the massive import of salted marine fish reflects a compensation for the undersupply caused by overexploitation of the Danube fish fauna and points to the growing demand for fish as food in late medieval and Early Modern times.

The temporal dynamic of riverine ecosystems and their fish communities and populations has been addressed in ecological theory and management for several decades. A growing number of case studies on the historic development especially of European and North American rivers have been published. Nonetheless, a theoretical debate about the contributions and limits of historical approaches and interdisciplinary co-operation is lacking. This article presents a brief overview of the role of history in river and fish ecology and suggests historical ecology as a scientific field that can offer a framework for future research. Based on case studies compiled in this special issue on the "Historical ecology of riverine fish in Europe", we draw conclusions on long-term changes of fish communities, on fisheries, aquatic ecosystem management and past habitat alterations and the potential of archaeological remains and written sources to study them. We discuss how modelling of historical fish data can help elucidate the effects of climate change and human influences on rivers and fish. Finally, we account for the necessity to consider appropriate spatial and temporal scales. In conclusion we call for future comparative studies on continental and global scales and methodological development, which can benefit especially from recent advances in marine historical ecology. We suggest that future interdisciplinary studies of ecologists, hydrologists, historians and archaeologists can reveal the history of riverine ecosystems as socio-ecological systems, addressing both their natural dynamics and human dimension. Such an endeavor can also support developing management plans for habitat restoration and conservation against the background of global change.

Environmental factors and dispersal can influence the structure of biological communities. Their effects can depend on the functional features of the species in the community. Since species belonging to the same trophic level, such as phytoplankton, may show functional differences, we investigated whether the effects of environment and dispersal differ among phytoplankton species from different functional groups. We analyzed data from a rainy and a dry period in 30 reservoirs in a subtropical region. In both periods, the environment as well as high and limited dispersal influenced the metacommunity structure. The functional groups had a low correspondence in their response to both dispersal and environment. Our results showed that the influence of the processes underlying the structure of the metacommunities, such as species sorting (environment influence), mass effect (high dispersal), and neutral dynamics (limited dispersal), depended on the functional characteristics of the organisms and could vary even among species of the same trophic level. These findings suggested that species at the same trophic level could not be considered ecological equivalents. This paper includes a Portuguese and Spanish version of the abstract in the online resources.

We described the spatial and temporal dynamics of aquatic macrophytes in a Neotropical coastal estuarine river, and identified the negative effects associated to the presence and dominance of the invasive tanner grass. We compared macrophyte beds along the Guaraguaçu River (South Brazil) over four years, using taxonomic and functional dimensions. Biodiversity descriptors were higher in the driest periods compared to the rainiest, although this difference seems to be decreasing over the studied years. Moreover, the spatial organization of biodiversity and community structure slightly changed over time. Such spatial community stability was highlighted by the dominance of the highly invasive tanner grass. In beds dominated by the invasive species, the biodiversity was reduced. As aquatic macrophytes represent an important group in water bodies, the long-term loss of seasonal differences in community structuring is of concern, mainly regarding its potential to impact other groups and ecosystem functioning. By analysing data from standardized monitoring, we were able to identify a poorly discussed facet of biotic homogenisation-the seasonal homogenisation. We also discussed the impact of massive development of invasive species and its consequences for biodiversity in a Neotropical river of outmost importance for biological conservation.

Common carp, , are a non-native species that established within the Laurentian Great Lakes more than a century ago and are abundant in some locations. Common carp have negatively impacted freshwater ecosystems, including in the Great Lakes, by increasing turbidity and uprooting vegetation through foraging and/or spawning activities. Knowledge of spatial ecology is necessary to effectively manage non-native species and aid in the development of remediation strategies. The aim of this study was to examine the spatial ecology of common carp across multiple spatial scales within Lake Ontario using passive acoustic telemetry. First, Residency Index (RI), as a metric for habitat preference, was calculated for common carp in Toronto Harbour (TH) and Hamilton Harbour (HH). Linear mixed modelling revealed that season, as well as the interaction between season and physical habitat conditions significantly affected RI. Specifically, during spring and summer common carp had significantly higher RI at sites with increased submerged aquatic vegetation, which could be associated with spawning activities. All common carp tagged in HH were resident, compared to half of individuals tagged in TH. Larger individuals tagged in TH were more likely to be absent from the array during summer. Non-resident common carp tagged at TH made extensive movements in spring and summer along the nearshore of Lake Ontario and were detected throughout the entire basin. Knowledge of spawning habitat could inform efforts to exclude common carp from these specific locations. Based on our findings, common carp should be managed at a regional level, as opposed to single sites, owing to their extensive movements.

Research on how intermittent water releases from hydropower plants affect the early life stages of fish has advanced in the last years, focusing not only on the direct impacts of rapid flow changes (hydropeaking), but also on the short-term fluctuations in water temperature (thermopeaking). Flow and thermal fluctuations caused by hydropeaking may affect fish movement patterns and migration at critical stages of a species' life cycle, e.g., by inducing passive downstream drift. Using two experimental outdoor channels, we investigated how nase (, Cypriniformes) larvae respond to a rapid drop in water temperature during hydropeaking (simulating a cold thermopeaking event), reaching on average 5.5 °C under peak flow (maximum discharge) conditions, in comparison with a hydropeaking treatment with a constant water temperature regime. Responses of fish larvae were analyzed during acclimation, up-ramping (increase in discharge), peak flow and down-ramping (decrease in discharge) phases. Fish drift increased during peak flow in the cold thermopeaking treatment compared to hydropeaking. Higher drift rates were also negatively associated with pronounced water temperature drops during peak flow conditions. In addition, the starting temperature of the experiment influenced drift during up-ramping. Overall, the results suggest that cold thermopeaking may increase drift in the early life stages of cypriniform fish compared with hydropeaking with stable water temperature. Hence, monitoring and active water temperature adjustments following hydropower releases should be adopted as strategies to mitigate power plant-related impacts on aquatic organisms.

Disturbance can strongly influence ecosystems, yet much remains unknown about the relative importance of key processes (selection, drift, and dispersal) in the recovery of ecological communities following disturbance. We combined field surveys with a field experiment to elucidate mechanisms governing the recovery of aquatic macroinvertebrates in habitats of an alluvial floodplain following flood disturbance. We monitored macroinvertebrates in 24 natural parafluvial habitats over 60 days after a major flood, as well as the colonization of 24 newly-built ponds by macroinvertebrates over 45 days in the same floodplain. We examined the sources of environmental variation and their relative effects on aquatic assemblages using a combination of null models and Mantel tests. We also used a joint species distribution model to investigate the importance of primary metacommunity structuring processes during recovery: selection, dispersal, and drift. Contrary to expectations, we found that beta diversity actually decreased among natural habitats over time after the flood or the creation of the ponds, instead of increasing. This result was despite environmental predictors showing contrasting patterns for explaining community variation over time in the natural habitats compared with the experimental ponds. Flood heterogeneity across the floodplain and spatial scale differences between the experimental ponds and the natural habitats seemingly constrained the balance between deterministic and stochastic processes driving the ecological convergence of assemblages over time. While environmental selection was the dominant structuring process in both groups, biotic interactions also had a prominent influence on community assembly. These findings have profound implications towards understanding metacommunity structuring in riverscapes that includes common linkages between disturbance heterogeneity, spatial scale properties, and community composition.

The global demand for food and healthcare products based on natural compounds means that the industrial and scientific sectors are on a continuous search for natural colored compounds that can contribute to the replacement of synthetic colors. Natural pigments are a heterogeneous group of chemical molecules, widely distributed in nature. Recently, the interest in marine organisms has increased as they represent the most varied environment in the world and provide a wide range of colored compounds with bioactive properties and biotechnological applications in areas such as the food, pharmaceutical, cosmetic, and textile industries. The use of marine-derived pigments has increased during the last two decades because they are environmentally safe and healthy compounds. This article provides a comprehensive review of the current knowledge of sources, applications, and sustainability of the most important marine pigments. In addition, alternatives to protect these compounds from environmental conditions and their applications in the industrial sector are reviewed.

Assessing the impacts of invasive organisms is a major challenge in ecology. Some widespread invasive species such as crayfish are potential competitors and reciprocal predators of ecologically and recreationally important native fish species. Here, we examine the effects of signal crayfish () on the growth, diet, and trophic position of the chub () in four rivers in Britain. Growth rates of 0+ chub were typically lower in sympatric populations with signal crayfish compared with allopatric populations, and this effect could be traced through to 2+ chub in one river. However, growth rates of older chub (5+ to 6+) were typically higher in the presence of crayfish. Sympatry with crayfish resulted in lower chub length-at-age and mass-at-age in half of the rivers sampled, with no change detected in the other rivers. Stable isotope analyses (δC and δN) revealed that both chub and crayfish were omnivorous, feeding at multiple trophic levels and occupying similar trophic positions. We found some evidence that chub trophic position was greater at invaded sites on one river, with no difference detected on a second river. Mixing models suggested crayfish were important food items for both small and large chub at invaded sites. This study provides evidence that invasive species can have both positive and negative effects on different life stages of a native species, with the net impact likely to depend on responses at the population level.

Using historical sources from the turn of the 19th to the 20th century, we investigated the long-term evolution of the fish community in a mountainous river network and the influence of different human uses and management measures. Within the alpine Salzach catchment, historical presence was reconstructed for 26 fish species, abundance classes for 19 species. Due to channelization, flood protection and dam erections, the spatial distribution of fish species was reduced during the 20th century. Many rheophilic and eurytopic fish species historically inhabited river reaches along a wide longitudinal profile and were present in more upstream river reaches than nowadays. The decrease of species diversity in the headwater sections is a consequence of lost lateral connectivity. Strongest effects are reported for sensitive species requiring different habitat types during their life cycles (especially pike, nase, Danube salmon). One of the most important shifts from the historical fish community to the present one reflects the deliberate introduction of fish species for fisheries. Rainbow trout and brook trout, absent from the historical fish assemblage, today represent up to 29 % of the total number of fish occurrences. In contrast, log driving, one of the most common historical pressures in European mountainous rivers, did not show significant negative effects on the past fish ecological situation. This result strongly differs from the impacts of log driving and deforestation demonstrated for recent times, and could be related to the change in log driving practices during the 20th century and to the high societal value of fish before the industrialization period along with other historical pressures affecting fish in rivers without log driving. In general, our results can be valid for a large number of European mountainous rivers. They highlight the usefulness of such detailed historical studies for our understanding of the long-term evolution of fish communities and their present functioning, and point the way for future river management strategies to restore fish biodiversity.

Submerged macrophytes are key components of shallow lake biological communities, and their presence has been associated with a predominantly clear-water state. Conversely, lakes lacking macrophytes are often turbid with elevated phytoplankton abundance. One main mechanism that influences the presence or absence of submerged macrophytes is turbidity that reduces the light available to macrophytes. Increases in turbidity can be caused by increased phytoplankton abundance and by increased concentrations of suspended inorganic sediment and understanding the relative contributions of these two factors can inform efforts to manage the effects of increased turbidity on macrophyte occurrence. Here, a continental scale data set is analyzed to quantify the effects of macrophytes on turbidity that originates from phytoplankton and from non-phytoplankton sources (e.g., inorganic sediment). Effects of phytoplankton assemblage composition on turbidity are also estimated. Based on this model, illustrative examples of chlorophyll concentrations needed to maintain or restore macrophytes to shallow lakes are calculated, and the difference in the magnitude of these concentrations illustrates the stabilizing effect of macrophytes on lake condition.

Species distributions models (SDM) that rely on estimated relationships between present environmental conditions and species presence-absence are widely used to forecast changes of species distributions caused by global warming but far less to reconstruct historical assemblages. By compiling historical fish data from the turn to the middle of the twentieth century in a similar way for several European catchments (Rhône, Danube), and using already published SDMs based on current observations, we: (1) tested the predictive accuracy of such models for past climatic conditions, (2) compared observed and expected cumulated historical species occurrences at sub-catchment level, and (3) compared the annual variability in the predictions within one sub-catchment (Salzach) under a future climate scenario to the long-term variability of occurrences reconstructed during an extended historical period (1800-2000). We finally discuss the potential of these SDMs to define a "reference condition", the possibility of a shift in baseline condition in relation with anthropogenic pressures, and past and future climate variability. The results of this study clearly highlight the potential of SDM to reconstruct the past composition of European fish assemblages and to analyze the historical ecological status of European rivers. Assessing the uncertainty associated with species distribution projections is of primary importance before evaluating and comparing the past and future distribution of species within a given catchment.

Freshwater lakes are essential hotspots for the removal of excessive anthropogenic nitrogen (N) loads transported from the land to coastal oceans. The biogeochemical processes responsible for N removal, the corresponding transformation rates and overall removal efficiencies differ between lakes, however, it is unclear what the main controlling factors are. Here, we investigated the factors that moderate the rates of N removal under contrasting trophic states in two lakes located in central Switzerland. In the eutrophic Lake Baldegg and the oligotrophic Lake Sarnen, we specifically examined seasonal sediment porewater chemistry, organic matter sedimentation rates, as well as 33-year of historic water column data. We find that the eutrophic Lake Baldegg, which contributed to the removal of 20 ± 6.6 gN m year, effectively removed two-thirds of the total areal N load. In stark contrast, the more oligotrophic Lake Sarnen contributed to 3.2 ± 4.2 gN m year, and had removed only one-third of the areal N load. The historic dataset of the eutrophic lake revealed a close linkage between annual loads of dissolved N (DN) and removal rates (NRR = 0.63 × DN load) and a significant correlation of the concentration of bottom water nitrate and removal rates. We further show that the seasonal increase in N removal rates of the eutrophic lake correlated significantly with seasonal oxygen fluxes measured across the water-sediment interface (R = 0.75). We suggest that increasing oxygen enhances sediment mineralization and stimulates nitrification, indirectly enhancing denitrification activity.

Spectroscopic techniques and extracellular enzyme activity measurements were combined with assessments of bacterial secondary production (BSP) to elucidate flood-pulse-linked differences in carbon (C) sources and related microbial processes in a river-floodplain system near Vienna (Austria). Surface connection with the main channel significantly influenced the quantity and quality of dissolved organic matter (DOM) in floodplain backwaters. The highest values of dissolved organic carbon (DOC) and chromophoric DOM (CDOM) were observed during the peak of the flood, when DOC increased from 1.36 to 4.37 mg l and CDOM from 2.94 to 14.32 m. The flood introduced DOC which consisted of more allochthonously-derived, aromatic compounds. Bacterial enzymatic activity, as a proxy to track the response to changes in DOM, indicated elevated utilization of imported allochthonous material. Based on the enzyme measurements, new parameters were calculated: metabolic effort and enzymatic indices (EEA 1 and EEA 2). During connection, bacterial glucosidase and protease activity were dominant, whereas during disconnected phases a switch to lignin degradation (phenol oxidase) occurred. The enzymatic activity analysis revealed that flooding mobilized reactive DOM, which then supported bacterial metabolism. No significant differences in overall BSP between the two phases were detected, indicating that heterogeneous sources of C sufficiently support BSP. The study demonstrates that floods are important for delivering DOM, which, despite its allochthonous origin, is reactive and can be effectively utilized by aquatic bacteria in this river-floodplain systems. The presence of active floodplains, characterized by hydrological connectivity with the main channel, creates the opportunity to process allochthonous DOC. This has potential consequences for carbon flux, enhancing C sequestration and mineralization processes in this river-floodplain system.

Shallow littoral areas in lakes are productive and highly diverse ecotonal zones, providing habitats for both invertebrate and vertebrate species. We developed a Bayesian modeling framework to elucidate the relationships between environmental drivers (lake typology, habitat, water chemistry, and latitude) and taxon richness, abundance, as well as the content of polyunsaturated fatty acids (PUFAs) in littoral macroinvertebrate communities in 95 boreal lakes. PUFAs, particularly arachidonic acid (ARA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), are critical micronutrients to maintain normal physiological functions in consumers. Lake typology was a significant predictor for PUFA content in the invertebrate assemblages, which was connected to taxon richness and/or abundance. Benthic communities in large humus-poor or nutrient-rich lakes displayed higher abundance, taxon richness, and more PUFA-rich taxa, whereas those in medium- and large-sized humic (color 30-90 mg Pt/L) and humus-rich lakes (color >90 mg Pt/L) were characterized by decreased abundance and subsequently low PUFA content. The abundance, taxon richness, and nutritional quality of the communities were also strongly related to latitude. Lakes with lower pH were characterized by lower benthic invertebrate diversity and low frequency of taxa with high somatic EPA and DHA content. The complexity of littoral habitats dominated by various macrophyte assemblages creates an environment that favors higher benthic abundance and increased presence of taxonomic groups with high PUFA content. Nutritional quality of benthic invertebrates for upper trophic levels can be modulated by a complex interplay between external stressors and abiotic factors that typically shape the structure of littoral benthic communities.