The EU Nature Restoration Law represents an important opportunity for freshwater habitat restoration and, consequently, freshwater biodiversity protection. However, a number of challenges must be anticipated in its implementation, which may compromise its success. Some aspects, particularly those relating to freshwater ecosystems, require more clarification. We use riverine ecosystems to illustrate existing ambiguities in the proposed legislation and the potential consequences of leaving these aspects open to interpretation during the implementation process. We also discuss potential solutions to these problems which could help ensure that the law's objectives are met. We argue that river network structure and connectivity dimensions, which result into river meta‐ecosystems, must be explicitly considered. For that purpose, we ask for clear definitions of the critical terms “free‐flowing rivers,” “barriers,” and “reference areas.” In addition, we recommend developing methods for integrated assessment of connectivity across river networks. As a key property of river ecosystems, this must be used to prioritize actions to increase the length and number of free‐flowing rivers. Adequate restoration planning at larger spatial scales will benefit from a meta‐ecosystem perspective and accurate representation of aquatic‐terrestrial linkages, which will significantly improve the efficacy of restoration efforts. Furthermore, stakeholder and citizen engagement offer important opportunities at local, national, and European scales, and should be fostered to ensure inclusive decision‐making. The conservation challenges outlined here are particularly important for rivers, but they also have implications for other ecosystems. These considerations are useful for policymakers, conservationists, and other stakeholders involved in the Nature Restoration Law and related policy initiatives.This article is categorized under: Water and Life > Stresses and Pressures on Ecosystems Water and Life > Conservation, Management, and Awareness Human Water > Water Governance

The loss of longitudinal connectivity affects river systems globally, being one of the leading causes of the freshwater biodiversity crisis. Barriers alter the dispersal of aquatic organisms and limit the exchange of species between local communities, disrupting metacommunity dynamics. However, the interplay between connectivity losses due to dams and other drivers of metacommunity structure, such as the configuration of the river network, needs to be explored. In this paper, we analyzed the response of fish communities to the network position and the fragmentation induced by dams while controlling for human pressures and environmental gradients. We studied three large European catchments covering a fragmentation gradient: Upper Danube (Austrian section), Ebro (Spain), and Odra/Oder (Poland). We quantified fragmentation through reach-scaled connectivity indices that account for the position of barriers along the dendritic network and the dispersal capacity of the organisms. We used generalized linear models to explain species richness and Local Contributions to Beta Diversity (LCBD) and multilinear regressions on the distance matrix to describe Beta Diversity and its Replacement and Richness Difference components. Results show that species richness was not affected by fragmentation. Network centrality metrics were relevant drivers of beta diversity for catchments with lower fragmentation (Ebro, Odra), and fragmentation indices were strong beta diversity predictors for the catchment with higher fragmentation (Danube). We conclude that in highly fragmented catchments, the effects of network centrality/isolation on biodiversity could be masked by the effects of dam fragmentation. In such catchments, metapopulation and metacommunity dynamics can be strongly altered by barriers, and the restoration of longitudinal connectivity (i.e. the natural centrality/isolation gradient) is urgent to prevent local extinctions.

The freshwater pearl mussel Margaritifera margaritifera is a benthic organism sensitive to hydrological regime alterations and habitat degradation driven by excessive fine bed material deposit (FBMD). Both issues are potentially exacerbated by climate change. Understanding how climate change affects future mussel habitats and the dispersal among them (dependent on the brown trout as fish host) can support the planning of effective conservation actions. To project the impacts of climate change on the mussel, a semi‐mechanistic modelling cascade was implemented for the Aist catchment in Austria (630 km²), including a hydrological model, a hydraulic model, Random Forest Models for FBMD accumulation risk and Species Distribution Models. Two climate change models (RCPs 4.5 and 8.5) for two future horizons (2060 and 2090) were considered. A graph‐based assessment of the structural connectivity was used to measure the probability of successful dispersal. Results show a reduction of peak discharge that cascades into a widespread reduction in shear stresses during high flow. The mussel's habitats, defined by hydraulics (i.e. patches with low shear stresses during high flow), are predicted to be stable over the simulated scenarios. The pressure of FBMDs over the delineated habitat patches is predicted to increase in the future due to the reduced stream transport capacity, reducing up to 25% of the available patches in 2090 for RCP 8.5. Consequently, the mussel's dispersal probability decreases to 44.3%–75.6% of the maximum theoretical value, with the highest drops for short dispersal distances, impacting metapopulation dynamics. Synthesis and applications. The widespread issue of fine sediment deposition in the streambed will be exacerbated for those catchments where climate change reduces the stream transport capacity. The impacts on the freshwater pearl mussel include habitat loss due to the formation of a new unsuitable substrate, and a decrease in the potential dispersal among the residual habitats. Thus, conservation plans that aim to protect the mussel in the future should focus on the mitigation of fine bed material deposits, prioritizing those subreaches that offer the highest potential for preserving connectivity among suitable habitats.

The identification and prioritization of sites for conservation actions to protect biodiversity in lotic systems is crucial when economic resources or available areas are limited. Challenges include the incorporation of multi-scale interactions, and the application of species distribution models (SDMs) to rare organism with multiple life stages. To support the planning of conservation actions for the highly endangered Freshwater Pearl Mussel Margaritifera margaritifera (FPM), this paper aims at developing an ecohydrological modeling cascade including a hydrological model (SWAT) and a hydraulic model (HEC-RAS). Building on hydrology and hydraulics, Random Forest models for potential risk to juveniles due to sand accumulation, SDMs for adults habitat niche, and a landscape connectivity assessment of dispersal potential were developed. The feasibility of such models integration was tested in the Aist catchment (630 km2) in Austria. The potential FPM habitat and the sand accumulation risk for the whole catchment were predicted with good accuracy. Results show that while the potentially suitable habitats for adults FPM cover 34% of the river network, only few habitat patches can maximize the dispersal potential (4% of the river network) and even less are showing limited impact of accumulations (3.5% of river network). No habitat patch that meets all the three criteria is available, suggesting approaches that target the patch-specific critical life stage-factors are promising for conservation.

Increased coastal flooding caused by extreme sea levels (ESLs) is one of the major hazards related to sea level rise. Estimates of return levels obtained under the framework provided by extreme-event theory might be biased under climatic non-stationarity. Additional uncertainty is related to the choice of the model. In this work, we fit several extreme-value models to two long-term sea level records from Venice (96 years) and Marseille (65 years): a generalized extreme-value (GEV) distribution, a generalized Pareto distribution (GPD), a point process (PP), the joint probability method (JPM), and the revised joint probability method (RJPM) under different detrending strategies. We model non-stationarity with a linear dependence of the model's parameters on the mean sea level. Our results show that non-stationary GEV and PP models fit the data better than stationary models. The non-stationary PP model is also able to reproduce the rate of extremes occurrence fairly well. Estimates of the return levels for non-stationary and detrended models are consistently more conservative than estimates from stationary, non-detrended models. Different models were selected as being more conservative or having lower uncertainties for the two datasets. Even though the best model is case-specific, we show that non-stationary extremes analyses can provide more robust estimates of return levels to be used in coastal protection planning.