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1. MERLIN produced a mapping exercise using European datasets on the status of freshwater habitats, freshwater-related species, bird species, ecological state and pressures. 2. An innovative spatial aggregation of European river networks was developed to integrate all input data at the same resolution, and discretizing Europe into River Restoration Units (R2Us) for small and large rivers, with wetlands integrated into R2Us. 3. River restoration needs throughout the EU were assessed by identifying R2Us that failed to abide by the Habitats and Water Framework Directives. 4. Ecosystem Services (ES) assessment was made at the R2U level throughout the EU by combining information on 5 ES delivered by freshwater ecosystems. 5. Limitations to restoration were evaluated at the R2U level using the Human Footprint Index as a proxy to restoration constraints. 6. Areas where restoration actions might be facilitated or passively enabled were determined by identifying the amount of freshwater protected areas (N2K) by R2U. 7. Restoration potential of each R2U was determined as the combination of ES, restoration constraints, and restoration enablers. 8. Restoration needs and restoration potential were integrated to determine the R2Us in need of restoration that had the greater potential to be restored. 9. This identification provides valuable insights for an upscaling analysis of restoration potential across the EU that serves to effectively guide large-scale restoration, management, and conservation plans.

Streams generally are affected by multiple stressors acting at different timescales. Periphyton, often the most important primary producer in these ecosystems, may respond to short-term impacts as well as to different long-term environmental conditions with potentially various changes in community structure. Here, we experimentally investigated the effects of sudden flow pulses on periphyton communities as a way to mimic extreme precipitation events in lowland streams that are predicted to occur more often with climate change in some regions. Using outdoor flumes, we allowed periphyton to colonise nutrient-diffusion substrates under two light conditions (50% shade and fully open) and nutrient availabilities (control, with access only to stream nutrients, and N–P-enriched) along a gradient in baseflows (0.43 to 2.17 L/s). After one month, we exposed the communities to a flow pulse (two-fold peak flow increase to simulate conditions of a potentially high disturbance) and analysed the responses of biomass and taxonomic composition. Flow pulse promoted periphyton growth in the lowest range of the baseflow but led to biomass reduction in the highest range. Light was the second major driver of biomass accrual, whereas nutrient enrichment had a strong effect on community composition both before and after the pulse (i.e., diatom dominance vs. green algae dominance in scenarios without and with enrichment, respectively). In all treatments, the flow pulse promoted a higher taxonomic richness, suggesting a partial reset of the succession of the periphyton communities. However, independent of flow and resources, periphyton communities showed low ecological resistance against the pulse with changes in chlorophyll a, biovolume and taxonomic richness to the pulse. We demonstrated that the effects of pulses on periphyton are similar in terms of biomass but varied strongly regarding composition depending on their initial structure, which is in turn mediated by the baseflow normally experienced by the systems, and on light and nutrient availability. Our results highlight the importance of testing multiple stressors, such as an increase in extreme events, under a wide range of environmental conditions (i.e., flow regime, trophic state, light availability). This approach allows us to detect potential interaction effects and non-linear responses, and highlights that the environmental settings ultimately determine the net effects of flow pulses on community structure and probably also on several important ecosystem processes.

1. Half of the funding in the EU MERLIN project – almost 10 million € - will be used to realise freshwater restoration measures across Europe. 2. Restoration measures will be implemented at sixteen case studies within the lifetime of MERLIN (2021–2025). 3. This deliverable comprises the implementation plans of all MERLIN case studies, describing the hands-on restoration activities that are to be implemented in MERLIN. 4. The implementation plans support the case study leads in their planning and implementation process, by detailing their goals, listing necessary work steps, planning budget and time, distributing tasks and considering optimisation potential. 5. Being aware of anticipated or potential risks is crucial in the implementation process. Strategies for mitigating these risks are developed to avoid delays or hinderances in restoration implementation. 6. Implementation planning is carried out on the basis of accomplished tasks, including: SWOT analysis, optimisation planning, self-assessment of the IUCN criteria for nature-based solutions, stakeholder screening, and monitoring of indicators for the European Green Deal goals. 7. The implementation planning process thus offers a basis for the participatory and transparent implementation of freshwater restoration measures.

Freshwater biodiversity is severely threatened, and despite many efforts to halt biodiversity loss the decline continues. Here, we used a transplantation approach to explore whether failure to reverse this trend for freshwater plants reflects that restoration measures applied to streams are inadequate for restoring suitable habitat conditions for plant species in decline. We identified declining species within the plant genus Potamogeton and introduced five of these (Potamogeton compressus L., Potamogeton gramineus L., Potamogeton lucens L., Potamogeton praelongus Wulfen., and Potamogeton pusillus L.) into five restored lowland stream reaches in three different habitat types to examine species establishment and growth. We found that plant establishment varied among streams and species, but we did not see any effect of habitat type within streams on either species growth or survival. Potamogeton compressus and P. pusillus failed to overwinter, while P. gramineus, P. lucens, and P. praelongus were all able to establish viable stands and to overwinter successfully in several of the experimental streams. Our results suggest that suitable habitats are available for declining Potamogeton species in restored stream reaches in Denmark, but their continued growth may be challenged by limited overwintering success. Consequently, we find it likely that dispersal constraints prevent declining Potamogeton species from naturally recolonising restored stream reaches. Understanding the importance of life history traits, such as overwintering and dispersal, can be crucial for the recovery of plant diversity in streams. Here we show that species that survived during winter were those with extensive growth of roots and rhizomes making them more adapted to withstand high flows than species relying on seeds and turions for overwintering. The creation of calmer backwater habitats can therefore be crucial when restoring stream reaches since these habitats may facilitate overwintering of species lost from the main channel during winter.

1. A SWOT analysis of the IUCN criteria for naturebased solutions to support European Green Deal goals yielded a broad spectrum of information on the MERLIN freshwater restoration case studies. 2. This information provides the necessary ingredients to draft an optimisation strategy for the MERLIN case studies on themes including multiple goals, society, restoration techniques, economy, policy and regulations. 3. This information offers a basis for peer-to-peer learning between the case studies and those in other regions. 4. The optimisation strategies provide new insights and broaden perspectives on restoration actions in individual case studies. 5. Identifying the potential for multiple Green Deal goal achievements will initiate broader dialogues with stakeholders and provide new incentives for restoration with multiple benefits for local people. 6. Capacity building on financial arrangements will create new opportunities for restoration actions. 7. Clear and agile policy arrangements are needed to enhance stakeholder participation in restoration actions. 8. The visioning, planning and evaluation of restoration actions are long-term activities that benefit from adaptive approaches and sound longterm monitoring programs on multiple aspects of biophysical and socio-ecological systems.

Restoration has been increasingly applied over the last decades as a way to improve the ecological conditions in stream ecosystems, but documentation of the impact of restoration on ecosystem functions is sparse. Here, we applied a space-for-time approach to explore effects of stream restoration on metabolism and organic matter decomposition in lowland agricultural streams. We included stream reaches that were restored >10 years ago and compared ecosystem functioning in these streams with those in channelized and naturally meandering stream reaches from the same geographical region. Specifically, we tested the following hypotheses: 1) rates of stream metabolism (gross primary production, GPP, and ecosystem respiration, ER) and organic matter decomposition in restored reaches resemble rates in naturally meandering reaches more than rates in channelized stream reaches and 2) higher resemblance in ecosystem metabolism and organic matter decomposition between restored reaches and meandering reaches can be attributed to the improved physical habitat conditions in the restored stream reaches. Overall, we did not find that stream metabolism or organic matter decomposition differed among restored, channelized and naturally meandering stream reaches even though habitat conditions differed among the three stream types. Instead, we found a large variation in ecosystem function characteristics across all sites. When analyzing all stream types combined, we found that GPP increased with increasing plant coverage and that ER increased with increasing stream size and with the coverage of coarse substratum on the stream bottom. Organic matter decomposition, on the other hand, only slightly increased with the number of plant species and declined with increasing concentrations of nutrients. Overall, our findings suggest that physical habitat improvements in restored stream reaches can affect ecosystem functions, but also that the restoration outcome is context-dependent since many of the physical characteristics playing a role for the measured functions were only to some extent affected by the restoration and/or clouded by interference with factors operating at a larger-scale. Restoration has been increasingly applied over the last decades as a way to improve the ecological conditions in stream ecosystems, but documentation of the impact of restoration on ecosystem functions is sparse. Here, we applied a space-for-time approach to explore effects of stream restoration on metabolism and organic matter decomposition in lowland agricultural streams. We included stream reaches that were restored >10 years ago and compared ecosystem functioning in these streams with those in channelized and naturally meandering stream reaches from the same geographical region. Specifically, we tested the following hypotheses: 1) rates of stream metabolism (gross primary production, GPP, and ecosystem respiration, ER) and organic matter decomposition in restored reaches resemble rates in naturally meandering reaches more than rates in channelized stream reaches and 2) higher resemblance in ecosystem metabolism and organic matter decomposition between restored reaches and meandering reaches can be attributed to the improved physical habitat conditions in the restored stream reaches. Overall, we did not find that stream metabolism or organic matter decomposition differed among restored, channelized and naturally meandering stream reaches even though habitat conditions differed among the three stream types. Instead, we found a large variation in ecosystem function characteristics across all sites. When analyzing all stream types combined, we found that GPP increased with increasing plant coverage and that ER increased with increasing stream size and with the coverage of coarse substratum on the stream bottom. Organic matter decomposition, on the other hand, only slightly increased with the number of plant species and declined with increasing concentrations of nutrients. Overall, our findings suggest that physical habitat improvements in restored stream reaches can affect ecosystem functions, but also that the restoration outcome is context-dependent since many of the physical characteristics playing a role for the measured functions were only to some extent affected by the restoration and/or clouded by interference with factors operating at a larger-scale.

Periphyton is a key primary producer in shallow lakes, sensitive to global warming and changes in nutrient balances. Reduced nitrogen availability due to accelerated denitrification at higher temperatures or in response to reduced N loadings aimed to reduce the eutrophication may affect periphyton biomass and composition, to compensate for the low N availability (e.g. promoting N2-fixing). We analysed periphyton responses to N decline in 12 eutrophic shallow lake mesocosms during one year of low N compared to high N, under three temperature scenarios: ambient, A2 IPCC scenario and A2 increased by 50%. We used two submerged macrophytes (Potamogeton crispus and Elodea canadensis) and artificial imitations of these as substrates for periphyton growth. Nitrogen decline increased periphyton biomass and induced compositional changes irrespective of season, plant type, and temperature. Periphyton biomass was negatively associated to phytoplankton and positively to plant complexity. Warmer scenarios negatively affected periphyton exclusively at high N loadings. Low N conditions were associated with lower periphyton taxonomic richness, lower N2-fixing cyanobacteria biovolume and increased biovolume of large-sized chlorophytes and non-N2-fixing cyanobacteria. Our results suggest that low N conditions promoted periphyton due to a more efficient use of nutrients and improved light conditions resulting from lower phytoplankton biomass and contrasting effects of temperature.

Robust demonstration case-studies are needed to evaluatewhether improvements in ecosystem condition are translated into improvements in ecosystem services. This research is essential for effectively scaling-up nature-based solutions across Europe and providing the evidence to support transformation agendas in society and industries, and ultimately supporting the ambitions of the European Green Deal. Clear guidance and standardised indicators help monitor the impact of freshwater and wetland restoration measures on European Green Deal goals. These monitoring indicators should include environmental indicators (e.g. for biodiversity, greenhouse gas emissions and water storage capacity) as well as socio-economic indicators such as stakeholder representation, private finance mobilisation and job creation. A combined Before-After-Control-Intervention (BACI) monitoring design is recommended to provide robust evidence and attribute change to the restoration measures. Benchmark (cost or effort) and local/regional context data (e.g. land use, governance) are also needed alongside monitoring of impacts to not just measurewhathas happened at a site but to understandwhyit has happened. &nbsp

The invasive freshwater dinoflagellate Ceratium furcoides is extending its distribution in South America with increasing environmental impacts associated with its bloom. We here report two events related to C. furcoides distribution expansion in Uruguay: (1) the main environmental drivers (physical and chemical factors, extreme wind events and zooplankton composition) of the first appearance and bloom of C. furcoides in 2012 in a subtropical eutrophic shallow lake (Lake Blanca, Uruguay); and (2) a fish kill event of Prochilodus lineatus observed during a bloom of C. furcoides in 2016 in a deep eutrophic lake (Puente de las Americas, Uruguay). The bloom of C. furcoides in Lake Blanca started in spring 2012 (December) after a clear water period with high phytoplankton species replacement after a cyanobacterial bloom of Raphidiopsis raciborskii. Extreme wind during this period may have initiated the bloom of C. furcoides by enhanced cysts resuspension from the sediments. High nutrient availability and low zooplankton grazing, further allowed C. furcoides to expand and reach over 96% of the phytoplankton biomass. In Lake Puente de las Americas, we registered the fish kill of the large-sized benthic Prochilodus lineatus during a bloom of C. furcoides. This bloom caused the oxygen depletion in the hypolimnion and the gills from these fish exhibited massive accumulations of C. furcoides cells compared to the ones collected in non-bloom conditions. Concurring to other studies our results suggest that the C. furcoides bloom likely caused the fish kill of P. lineatus by asphyxia. Our study is the earliest register of a bloom of C. furcoides in Uruguay (in 2012) and discusses the potential environmental effects of its bloom in subtropical lakes.