Aquatic Ecology is Aquatic ecology is an extraordinarily broad and interesting field. It investigates the interplay between aquatic organisms and their physical, chemical, and biological environment. Aquatic ecology encompasses all freshwater and marine ecosystems, including streams, rivers, lakes, wetlands, coastal environments, and the vast expanses of the open ocean. Aquatic ecology studies a wide diversity of different organisms, ranging from tiny bacteria to large whales, facing a myriad of different processes such as biogeochemical cycles, genetic differentiation, and climate change. Fundamental research in aquatic ecology adds new discoveries almost every day. Applied research makes major contributions to biotechnology, fisheries, water management, nature conservation, and environmental policy. Reassessments and syntheses in aquatic ecology are stimulating to the discipline as a whole, as well as enormously useful to students and researchers in ecological sciences.

River floodplains are among the most threatened ecosystems of the world and their protection and restoration is of key importance for river managers. In Europe, the Water Framework Directive (WFD) and the Habitats and Birds Directives (HBDs) provide a guideline for decision processes in floodplain restoration projects. While the WFD, however, represents an aggregated, multiple-species approach aiming at the restoration of the natural hydrological dynamics, the single-species focused HBDs regulate the protection of the existing fauna and flora with protection status. Thus, trade-offs between rheophilic and stagnophilic aquatic organisms may hamper the definition of a compromise solution between the ecological objectives of the restoration. We present an assessment scheme for the restoration of a degraded Danube floodplain near Vienna, which equally considers both WFD and HBDs objectives in a transparent, comprehensible, and objective way. In a first step, predictive hydrological and ecological models were generated for different hydrological scenarios considering the aquatic community composition (floodplain index according to WFD) as well as individual protected species of the taxonomic groups fish, amphibians, reptiles, and water birds (HBDs). Based on these models, we developed an assessment scheme which considered potential changes in the available habitats, the current conservation states, and priorities of the species. Thereby, we included experiences from other restoration projects. The results show that both the multiple-species and the single-species approach achieved a similar ranking of the hydrological scenarios, in which the "business-as-usual" alternative without any restoration measure was identified as the worst case. The multiple-species approach of the floodplain index provided a clear ranking of the hydrological scenarios and revealed a low potential of any target measure to restore the pre-regulation state of the floodplain. In contrast, the single-species approach required a much higher degree of decisions by experts, but provided a detailed insight into spatial effects of the measures on different species, thus revealing the potential for local compensation measures. Our study demonstrates that a combination of these two approaches can be an effective tool for river managers in the development of sustainable floodplain restoration schemes in accordance with the WFD, the HBDs, and national nature protection laws (in this case, the Nature Conservation Acts of Vienna and Lower Austria).

Biodiversity and environmental integrity of river systems in the Danube catchment is threatened by multiple human alterations such as channelization, fragmentation or the disconnection of floodplains. Multiple human activities, including the construction of hydropower plants, expansion of agricultural use, and large-scale river regulation measures related to navigation and flood protection, are resulting in an ongoing loss of habitat, biodiversity and ecosystem service provision. Conservation and restoration of the systems biodiversity and ecosystem service provisioning is a key task for management but is challenging because the diversity of human activities and policy targets, scarcity of data compared to the complexity of the systems, heterogeneity of environmental problems and strong differences in socio-economic conditions along the Danube River hampers coordinated planning at the scale of the whole river basin and along the whole river from source to mouth. We evaluated three different implementations of an Ecosystem-Based Management (EBM) approach, which aims to support management efforts. This was done following the principles for EBM related to the resilience of ecosystems, the consideration of ecological and socio-economic concerns, the inclusion of multi-disciplinary knowledge and data addressing the ecosystem scale independent of administrative or political boundaries. This approach has been developed in the H2020 project AQUACROSS.

Large river-floodplain systems are hotspots of biodiversity and ecosystem services but are also used for multiple human activities, making them one of the most threatened ecosystems worldwide. There is wide evidence that reconnecting river channels with their floodplains is an effective measure to increase their multi-functionality, i.e., ecological integrity, habitats for multiple species and the multiple functions and services of river-floodplain systems, although, the selection of promising sites for restoration projects can be a demanding task. In the case of the Danube River in Europe, planning and implementation of restoration projects is substantially hampered by the complexity and heterogeneity of the environmental problems, lack of data and strong differences in socio-economic conditions as well as inconsistencies in legislation related to river management. We take a quantitative approach based on best-available data to assess biodiversity using selected species and three ecosystem services (flood regulation, crop pollination, and recreation), focused on the navigable main stem of the Danube River and its floodplains. We spatially prioritize river-floodplain segments for conservation and restoration based on (1) multi-functionality related to biodiversity and ecosystem services, (2) availability of remaining semi-natural areas and (3) reversibility as it relates to multiple human activities (e.g. flood protection, hydropower and navigation). Our approach can thus serve as a strategic planning tool for the Danube and provide a method for similar analyses in other large river-floodplain systems.

The capacity of ecosystems to supply ecosystem services is decreasing. Sustaining this supply requires an understanding of the links between the impacts of pressures introduced by human activities and how this can lead to changes in the supply of services. Here, we apply a novel approach, assessing ‘risk to ecosystem service supply’ (RESS), across a range of aquatic ecosystems in seven case studies. We link aggregate impact risk from human activities on ecosystem components, with a relative score of their potential to supply services. The greatest RESS is found where an ecosystem component with a high potential to supply services is subject to high impact risk. In this context, we explore variability in RESS across 99 types of aquatic ecosystem component from 11 realms, ranging from oceanic to wetlands. We explore some causes of variability in the RESS observed, including assessment area, Gross Domestic Product (GDP) and population density. We found that Lakes, Rivers, Inlets and Coastal realms had some of the highest RESS, though this was highly dependent on location. We found a positive relationship between impact risk and service supply potential, indicating the ecosystem components we rely on most for services, are also those most at risk. However, variability in this relationship indicates that protecting the supply of ecosystem services alone will not protect all parts of the ecosystem at high risk. Broad socio-economic factors explained some of the variability found in RESS. For example, RESS was positively associated with GDP and artificial and agricultural land use in most realms, highlighting the need to achieve balance between increasing GDP and sustaining ecosystem health and human wellbeing more broadly. This approach can be used for sustainable management of ecosystem service use, to highlight the ecosystem components most critical to supplying services, and those most at risk.

Aquatic ecosystems are under severe pressure. Human activities introduce an array of pressures that impact ecosystems and their components. In this study we focus on the aquatic domains of fresh, coastal and marine waters, including rivers, lakes and riparian habitats to transitional, coastal as well as shelf and oceanic habitats. In an environmental risk assessment approach, we identified impact chains that link 45 human activities through 31 pressures to 82 ecosystem components. In this linkage framework >22,000 activity-pressure-ecosystem component interactions were found across seven European case studies. We identified the environmental impact risk posed by each impact chain by first categorically weighting the interactions according to five criteria: spatial extent, dispersal potential, frequency of interaction, persistence of pressure and severity of the interaction, where extent, dispersal, frequency and persistence account for the exposure to risk (spatial and temporal), and the severity accounts for the consequence of the risk. After assigning a numerical score to each risk criterion, we came up with an overall environmental impact risk score for each impact chain. This risk score was analysed in terms of (1) the activities and pressures that introduce the greatest risk to European aquatic domains, and (2) the aquatic ecosystem components and realms that are at greatest risk from human activities. Activities related to energy production were relevant across the aquatic domains. Fishing was highly relevant in marine and environmental engineering in fresh waters. Chemical and physical pressures introduced the greatest risk to the aquatic realms. Ecosystem components that can be seen as ecotones between different ecosystems had high impact risk. We show how this information can be used in informing management on trade-offs in freshwater, coastal and marine resource use and aid decision-making.

EBM aims to recognize the full array of interactions within an ecosystem including humans, rather than considering single species, issues, or ecosystem services in isolation. Simulations and modelling techniques can be applied to study the complex interactions of biodiversity and drivers and pressures they are experiencing. They provide insight, allowing the modification of specific factors, e.g. conservation or management alternatives, while controlling for other factors within the simulation such as management costs and quantifying the uncertainty in the model predictions.

Aquatic ecosystems are rich in biodiversity and home to a diverse array of species and habitats, providing numerous economic and societal benefits to people. Despite progress in defining conservation goals to protect ecosystems and their biodiversity (e.g. the EU 2020 Biodiversity Strategy), biodiversity is declining due to strong anthropogenic activities and over-exploitation of natural goods and services. Evaluating the consequences of anthropogenic impacts on biodiversity, and subsequently the ecosystem functioning and ecosystem services provided by aquatic ecosystems, is an important step within ecosystem-based management (EBM) approaches. Therefore, it is crucial to better understand the interplay and dependencies between biodiversity, ecosystem functioning and ecosystem services. Based on this understanding, alternative pathways in terms of management scenarios can be explored, which evaluate the impacts and feedbacks to biodiversity, ecosystem functioning and ecosystem services (as described in the AQUACROSS Assessment Framework). The potential outcome of management scenarios can be assessed using simulations and modelling techniques, allowing the modification of specific factors, e.g. conservation or management targets and management costs, while controlling for other factors within the simulation and quantifying the uncertainty in the model predictions.

Global initiatives have been increasingly focusing on mainstreaming the values of biodiversity and ecosystem services into decision-making at all levels. Due to the accelerated rate at which biodiversity is declining and its consequences for the functioning of ecosystems and subsequently, the services they provide, there is need to develop comprehensive assessments of the services and the benefits nature delivers to society. Based on expert evaluation, we identified relevant flow linkages in the supply-side of the socio-ecological system, i.e. from biodiversity to ecosystem services supply for eight case studies across European aquatic ecosystems covering freshwater, transitional, coastal and marine waters realms. Biological mediated services were considered, as well as those reliant on purely physical aspects of the ecosystem, i.e. abiotic outputs, since both have implications for spatial planning, management and decision-making. Due to the multidimensional nature of ecosystems and their biodiversity, our approach used ecosystem components such as habitats and biota as proxies for biodiversity and as the focal point for linkage identification. Statistical analysis revealed the importance of considering mobile biota in the spatial assessment of habitats. Contrary to literature evidences so far, our results showed significantly different and complementary ecosystem services supply patterns across the continuum of aquatic realms. The implemented score of ecosystem services supply has a high potential for integrated aquatic ecosystem service supply assessments in the context of ecosystem-based management.

Floodplains are highly diverse landscape elements within river systems and among the most endangered ecosystems worldwide. In this paper we complement indices developed to assess the ecological status of floodplain systems, compliant with the EU Water Framework Directive, to an overall "Benthic Invertebrate Floodplain Index" (BIFI). With the addition of taxa (mainly oligochaetes, chironomidae and amphipoda) to the floodplain index (FI), caddisfly (CHI), and dragon fly (OHI) indices a new extended BIFI can be calculated. We provide values for the calculation of the index derived from a comprehensive dataset of Austrian floodplain waters complemented by literature data. Values are given for those taxonomic groups which are abundant in the Austrian Danube and determinable in reasonable time. The new index was compared to published floodplain indices and tested with an independent data set at two floodplain segments along the Austrian Danube. The newly classified benthic invertebrates (NCB') showed a good performance in comparison to the so far published indices and extend these to a better coverage of dynamic water bodies. Further the inclusion of abundant and species rich taxa improves the robustness of calculated values already with a low sampling effort. Altogether it is a promising tool for the integrated assessment of the ecological status of river-floodplain systems according to the EU Water Framework Directive.

Weltweit zählen Fließgewässersysteme und Auenlandschaften zu den gefährdetsten Ökosystemen. Flussregulierungs- und Verbauungsmaßnahmen haben diese Systeme von ihren natürlichen Wasserstandsschwankungen bzw. dem Wasseraustausch mit dem Fluss entkoppelt und führen damit zu einer Verlandung ehemaliger hochdynamischer Habitate. Gezielte Managementmaßnahmen können diesen Entwicklungen entgegenwirken, müssen vor Umsetzung aber auf ihre Auswirkungen hin überprüft werden. Grundsätzlich initiieren alle Restaurationsmaßnahmen grundlegende Prozesse im Fluss-Auensystem, auf die Arten mit unterschiedlichen Habitatansprüchen und unterschiedlichem Schutzstatus verschieden reagieren können. Eine Möglichkeit einer wissenschaftlich fundierten Prognose von hydrologischen Managementmaßnahmen wird in diesem Artikel am Beispiel der Verwendung von Habitatmodellen für eine Studie in der Unteren Lobau vorgestellt. Die Untere Lobau ist eine Auenlandschaft im Osten von Wien, nördlich der Donau. Sie ist Teil des Nationalparks Donau-Auen und geschützter FFH-Lebensraum. Vor der Donauregulierung Ende des 19. Jahrhunderts war die Untere Lobau ein dynamisches Auengebiet, aber die heute vorherrschenden Verlandungsprozesse bedrohen dieses in Europa selten gewordene Ökosystem. Für dieses Gebiet wurden drei Managementvarianten untersucht: 1) Nullvariante – Das Ausbleiben jeglicher Maßnahmen mit einer Fortsetzung der Verlandungsprozesse; 2) Dotation – Die Zufuhr einer gerade ausreichenden Menge an Wasser, um Wasserflächen gemäß dem Status quo zu erhalten; 3) Anbindung – Das Herstellen einer stromaufwärtigen Verbindung zum Hauptstrom der Donau, was eine Rheophilisierung des Systems und zumindest eine Annäherung an historische Verhältnisse darstellt. Auf Basis modellierter Umweltdaten und Habitatpräferenzen wurden mittels binär-logistischer Regressionen von ausgewählten Arten verschiedenster Gruppen potenzielle Habitatflächen (Weighted Usable Areas) berechnet. Die Qualität der Modellergebnisse konnte unterstreichen, dass die Modellierungen die hydrologischen Maßnahmen mit ihrem Einfluss auf die Biozönose gut beschreiben und damit eine wertvolle Hilfe in der Entscheidungsfindung im Gewässermanagement sein können.

Flüsse benötigen für eine langfristige ökologische Stabilität – Bereiche, die bei verschiedenen Abflusssituationen als hoch produktive Lebensräume verfügbar sind und entsprechend mit dem Fluss in Austausch stehen. Diese Retentionsbereiche oder bioaktive Zonen sind wichtig für die flusseigene Tierwelt als Refugialräume (z. B. Makrozoobenthos und Fische) oder bestimmte Entwicklungsstadien (z. B. Jungfische), dienen benthischen Algengemeinschaften zum Aufbau von Biomasse und sind damit Grundlage für das Nahrungsnetz des Flusses sowie den Stoffrückhalt und Umbau (z. B. Nährstoffaufnahme). Vor allem während Niederwasser sind die Uferbereiche und die Stromsohle selbst von wesentlicher Bedeutung, aber in ihrer Ausprägung durch Regulierungsmaßnahmen deutlich eingeschränkt. Ein grundlegendes Verständnis der Abläufe und funktionalen Zusammenhänge ist erforderlich, damit im Rahmen von wasserbaulichen Maßnahmen negative Konsequenzen hintangehalten werden können bzw. der ökologische Zustand wieder verbessert werden kann. Im Rahmen des Pilotprojekts Bad Deutsch-Altenburg an der Donau östlich von Wien ist daher nicht nur ein Monitoring der flussbaulichen Maßnahmenumsetzung erforderlich, sondern auch ein wissenschaftlicher Erkenntnisgewinn über grundlegende Zusammenhänge und biologische Abläufe, um optimierte Maßnahmen zum Einsatz bringen zu können. In diesem Artikel werden Ergebnisse mehrjähriger Forschungsarbeiten in den Tiefenzonen der Donau und typischen Uferzonenlebensräumen für die Organismengruppen benthische Algen, Makrozoobenthos und Fische präsentiert.