Eelgrass (Zostera marina) meadows are declining globally despite their critical role as foundational ecosystems and water quality indicators, e.g. under the EU Water Framework Directive (WFD). With the EU Nature Restoration Regulation mandating recovery targets, robust habitat distribution maps are urgently needed to support evidence-based restoration. Traditional surveys provide detailed local estimates but lack the spatial coverage required for comprehensive mapping. We developed a high-resolution (50 x 50 m) species distribution model (SDM) for Z. marina across Denmark's similar to 7000 km coastline using comprehensive national monitoring data (NOVANA, 2011-2022). A Random Forest model trained on similar to 30,000 field observations and validated on 7500 independent samples achieved strong predictive performance (accuracy: 80.3%, 95% CI: 79.4-81.2%), estimating current coverage at similar to 2000 km(2). Critically, the model moves beyond traditional habitat suitability mapping to also predict depth distribution limits (Zmax), achieving strong agreement with independent WFD field observations of Zmax for 85 of 109 water bodies (r = 0.779, slope = 0.99, mean error <1 m). This accuracy provides actionable restoration targets aligned with EU mandates. Accumulated Local Effects (ALE) analysis identified light availability at depth (Iz) as the most influential predictor (ALE range = 0.83), followed by Maximum Exposure Index (MEI, 0.70) and 95th percentile temperature (T95, 0.68), all showing nonlinear relationships with eelgrass presence. Beyond mapping current distribution, the model enables scenario-based assessments of restoration feasibility under improved water clarity and climate change. The methodology is scalable to other species and regions, offering a robust framework for adaptive management and supporting EU restoration goals.

Seamounts play a crucial role in shaping deep-sea ecosystem structure, influencing ocean circulation, enhancing biological productivity, and supporting diverse marine life. The Great Meteor Seamount (GMS), is the largest seamount in the North Atlantic and a key ecological feature in the regional network of the Protected Areas of the Azores Archipelago, but remains poorly understood in terms of small-scale physical-biological interactions. Using a new high-resolution setup of the FlexSem hydrodynamic model, we analyze ocean circulation and water mass properties around GMS over a 9-month period. The results reveal a persistent anticyclonic re-circulation along the upper seamount slopes and doming of isopycnals above the summit. Variations in density and temperature drive two distinct circulation and mixing regimes: A cold, dense period with bottom-intensified Taylor cap circulation and strong vertical coupling, and a warm, stratified period with enhanced stratification and reduced vertical mixing. The high resolution of the model makes it possible to compare habitat-suitability maps for benthic filter-feeding organisms with maps of internal wave slope characteristics, revealing a key role of internal wave induced mixing in supporting cold-water corals and other benthic filter-feeding communities. The study highlights the significance of biophysical interactions at the seamount and emphasizes the need for further research into small-scale processes that support biological growth.

Seamounts are underwater mountains with unique hydrodynamic properties that play a major role in supporting marine biodiversity and biological productivity. We explore how physical data, marine megafauna tracking, environmental parameters, and fisheries information can improve our understanding of these complex ecosystems. Abundant seabird tracking data from the Cabo Verde and Azores archipelagos make ideal case studies for exploring specific seamount characteristics that attract marine species, particularly seabirds, at fine scales. These case studies also provide a foundation for applying such approaches to identify areas of conservation importance. Multi-species predictive modeling is needed to identify the relationship between biological hotspots and seamount hydrodynamics, essential knowledge for developing effective marine conservation strategies. We propose six priority research directions to address knowledge gaps and inform conservation measures that protect these refuges of ocean biodiversity.

Cold-water corals (CWCs) thrive in areas with complex and rough topography favoring the development of highly diverse benthic communities. Several biotic and abiotic factors including organic matter supply, temperature, bottom roughness and currents are important drivers of ecosystem structure and functioning in deep-sea environments at different spatial and temporal scales. Little is known, however, how basin-scale changes in the ocean climate affect these drivers at local scales. Here, we use high-resolution implementations of the hydrodynamic model ROMS-AGRIF for estimating characteristic spatial and temporal scales of local hydrodynamics in response to variations of basin-scale currents imposed by distinct changes of the Atlantic Meridional Overturning Circulation (AMOC) in the past century. We focus on two CWC communities on the SE Rockall Bank slope and at Condor Seamount. We considered two contrasting AMOC states that were identified from the 1958-2009 hindcast of the 1/20 degrees resolution VIKING20 North Atlantic basin-scale ocean circulation model and used as boundary conditions for the high-resolution local area models. At SE Rockall Bank, variability of near-bottom currents in both regions was largely dominated by tidal dynamics, but strongly modified by AMOC induced basin-scale variations of water mass properties and bottom currents. During strong AMOC years, waters in the main CWC depth corridor (600-1200 m) were cooler and less saline but were dominated by stronger bottom currents when compared with conditions during weak AMOC years. At Condor Seamount, bottom currents were largely unaffected by AMOC related changes close to the summit at water depths < 400 m. Kinetic energy dissipation rates derived from the 3D near -bottom velocity field appeared to positively relate with the in-situ CWC distribution. Kinetic energy dissipation is therefore proposed as a mechanistic descriptor of CWC presence as it provides a more mechanistic view of hydrodynamics driving organic matter supply to filter and suspension-feeding communities.

We investigated the use of eDNA metabarcoding for supplementing traditional diver-based monitoring of biodiversity of marine boulder reefs within the photic zone. The applied sampling design made it possible to evaluate the usefulness of eDNA monitoring as a supplement for traditional monitoring. Specifically, this study aimed to (1) assess the local influence of boulder reefs on biodiversity across the North Sea to Baltic Sea transition zone and (2) investigate the importance of environmental gradients for patterns in community structure. On samples collected during August 2020, we compared the composition and abundance of species associated with nine reefs, representing an environmental gradient of salinity (16–33 psu), water temperature (16–21°C) and water depth (6–29 m). At each reef site, water was sampled near the bottom just above the reef and on average 2.6 km upstream and downstream (location) and sequenced with metabarcoding using COI, 18S and 12S rDNA primers. eDNA identified 400 species, diver-based observations identified 184 with an overlap of 70 species (12%) and 81 genera (18%). While eDNA identified many infaunal species, it did not detect several macroalgal species which dominated in the diver-based observations. Multivariate analysis of eDNA and diver-based community structure both distinguished between reef communities, with a significant match between patterns observed by the two methods ( r = 0.37, p = 0.02). Furthermore, the eDNA approach made it possible to identify significant differences in species composition between upstream, above-reef and downstream locations, suggesting that eDNA leaves a local footprint in benthic habitats. Patterns in both eDNA and diver-based species composition and richness were significantly related to geographical distance, salinity, water temperature and water depth. Despite of low detection of macroalgae, the eDNA sampling provided a substantial supplement to traditional diver-based monitoring of biodiversity around benthic hotspots in the Danish marine waters and therefore we recommend to add eDNA methods to conventional monitoring programs in the future.

We assess the trends and influences of non-indigenous and cryptogenic species (hereafter simply referred to as "NIS") on Danish marine community compositions using three decades of quantitative monitoring data. Since the initiation of the Danish marine monitoring programmes in the 1980s, the number of marine MS recorded in Denmark increased from 30 to 77 in 2014. Thus, of the total 77 marine MS known from Denmark, 56 (73%) were captured in the standardized monitoring program, while the remaining 21 species (27%) were not detected (in particular gelatinous zooplankton, shallow water fish., parasitic invertebrates, and littoral angiosperms) because of limited spatial-temporal sampling efforts as well as methodological limitations. Significant exponential increases in records of non-native phytoplankton, benthic invertebrates and macroalgae (only in one region) relative to the total species records in the database were observed. Multivariate analyses of presence-absence data, indicated that the contribution of MS to total community similarity increased over time, highlighting that MS is becoming an increasingly important component of Danish marine communities. While the presence of MS generally explained less than 10% of long-term changes in the community similarity across large regions, NIS presence showed local dominance. A correlation analysis indicated that changes in overall species composition within functional groups (phytoplankton. zooplankton, macroalgae and benthic fauna) were related to changes in NIS contribution but more strongly influenced by salinity, highlighting a well-described general positive relationship between salinity and species richness in Danish waters. The process described within this study could form the basis for the analysis of impact of NIS in marine water as required by the Marine Strategy Framework Directive (MSFD).

During the last century, eutrophication significantly reduced the depth distribution and density of the habitat forming eelgrass meadows (Zostera marina) in Danish coastal waters. Despite large reductions in nutrient loadings and improved water quality, Danish eelgrass meadows are currently not as widely distributed as expected from improvements in water clarity alone. This point to the importance of other environmental conditions such as sediment quality, wave exposure, oxygen conditions and water temperature that may limit eelgrass growth and contribute to constraining current distributions. Recently, detailed local models have been set up to evaluate the importance of such regulating factors in selected Danish coastal areas, but nationwide maps of eelgrass distribution and large-scale evaluations of regulating factors are still lacking. To provide such nationwide information, we applied a spatial habitat GIS modeling approach, which combines information on six key eelgrass habitat requirements (light availability, water temperature, salinity, frequency of low oxygen concentration, wave exposure and sediment type) for which we were able to obtain national coverage. The modeled potential current distribution area of Danish eelgrass meadows was 2204 km2 compared to historical estimates of around 7000 km2, indicating a great potential for further distribution. While validating the modeled eelgrass distribution area in three areas (83-111 km2) that hold large eelgrass meadows, we found an agreement of 67% with in situ monitoring data and 77% for eelgrass areas as identified from summer orthophotos. The GIS model predicted higher coverage especially in shallow waters and near the depth limits. Areas of disagreement between GIS-modelled and observed coverage generally exhibited higher exposure level, mean summer temperature and salinity compared to areas of agreement. A sensitivity analysis showed that the modeled area distribution of eelgrass was highly sensitive to light conditions, with 18 to 38% increase in coverage following an increase in light availability of 20%. Modelled coverage of eelgrass was also sensitive to wave exposure and temperature conditions while less sensitive to changes in oxygen and salinity conditions. Large regional differences in habitat conditions suggest spatial variation in the factors currently limiting the recovery of eelgrass and, hence, variations in actions required for sustainable management.

Modem ecosystem-based forms of marine management such as Marine Spatial Planning (MSP) deal with various complex systems and often with huge amounts of data. Software-based simulative and analytical tools are therefore frequently mentioned in the scientific literature on marine management approaches. But in addition to the evolution of management approaches, the requirements for more integrated tools are also progressing. MSP, for instance, comes with different spatial resolutions, an increased need to consider multiple interdepencies, and increased requirements for validity than most of the previous marine management questions. We reviewed seven well-known Decision Support Tools (DSTs) by asking 59 MSP practitioners from at least 25 countries worldwide about their experience with these tools. The results revealed that, while respondents were mostly positive about the use of DSTs in MSP processes, DSTs are still mainly used in the academic realm and have not yet found their way into everyday MSP practice. There is a broad range of reasons for not using DSTs, including the complexity of these tools, the resources required to operate them, low stakeholder confidence in DST outcomes, and the lack of additional value in using DSTs.

The role of tools and approaches is currently much debated in maritime spatial planning (MSP). Past evaluation has mainly concentrated on decision support tools and the tangible outputs these can provide for MSP, but little attention has so far been been given to the soft or indirect benefits tool use can have in MSP. This paper assesses the potential benefits of tool use in the context of four common integration challenges in MSP. Drawing on case study material from the Baltic Sea region, the paper reviews the potential contribution of five selected tools and approaches to multi-level and transboundary, policy and sector, stakeholder and knowledge integration. Specific end points are defined for each integration challenge, including general desired outcomes of integrated MSP processes as a template for assessment. Our review shows that the selected tools play different roles in moving towards the various end points of MSP integration. There is an important difference between the potential of each tool, or its inherent capacity, and how it is applied, e.g. in a participative or non-participative setting. Another lesson is that some integration benefits can be achieved by the tools alone, while others – often secondary benefits - depend on how the outcomes of tool use are taken up by the subsequent MSP process. Although the nature of a tool does restrict its potential contribution to MSP integration challenges, the secondary "soft" benefits that can be achieved through certain styles of application and good links to the MSP process can add important integration benefits up and beyond the tool itself. The results presented here may also be relevant to other types of spatial planning and conservation management.

As a response to increasing human pressures on marine ecosystems, the legislation aimed at improving the conservation and management of marine coastal areas in European and Contiguous Seas (ECS) underwent crucial advances. ECS, however, still remain largely affected by increasing threats leading to biodiversity loss. Here, by using emblematic case studies and expert knowledge, we review current conservation tools, comparing their application in different areas to assess their effectiveness, potential for synergies, and contradictions. Despite regional differences in their application, the existing legislative frameworks have the potential to regulate human activities and to protect marine biodiversity. However, four challenges remain to be addressed to fully achieve environmental policy goals: (1) Lack of shared vision representing a limitation in transboundary collaboration. Although all EU countries are committed to fulfil EU Directives and other binding international legislative acts, a remarkable heterogeneity exists among countries in the compliance with the common legislation on conservation and in their degree of implementation. (2) Lack of systematic procedures for the selection of protected marine sites. Regional and national approaches in designating Natura 2000 sites and nationally designated marine protected areas (MPAs) reflect varying conservation targets and importance of conservation issues in political agendas. (3) Lack of coherent ecological networks. Natura 2000 sites and other MPAs are still far from reaching the status of effective networks in all considered case studies. (4) Hotspot of conflicts with private economic interests prevailing over conservation aims. Recommendations are given to overcome the fragmented approach still characterizing the conservation and management of coastal marine environments. Holistic, integrated, ecosystem-based, cross-cutting approaches can avoid conflicts among institutions so as to provide effective and timely solutions to current and future challenges concerning the conservation and management of marine ecosystems and associated goods and services.

The development of offshore wind energy and other competing interests in sea space are a major incentive for designating marine and coastal areas for specific human activities. Maritime Spatial Planning (MSP) considers human activities at sea in a more integrated way by analysing and designating spatial and temporal distributions of human activities based on ecological, economic and social targets. However, specific tools supporting spatial decisions at sea incorporating all relevant sectors are rarely adopted. The decision support tool Marxan is traditionally used for systematic selection and designation of nature protection and conservation areas. In this study, Marxan was applied as a support tool to identify suitable sites for offshore wind power in the pilot area Pomeranian Bight / Arkona Basin in the western Baltic Sea. The software was successfully tested and scenarios were developed that support the sites indicated in existing national plans, but also show options for alternative developments of offshore wind power in the Pomeranian Bight / Arkona Basin area.

Fucoidans are sulphated fucose-rich polysaccharides predominantly found in the cell walls of brown algae. The bioactive properties of fucoidans attract increasing interest from the medico-pharmaceutical industries and may drive an increase in demand of brown algae biomass. In nature, the biochemical composition of brown algae displays a seasonal fluctuation driven by environmental factors and endogenous rhythms. To cultivate and harvest kelps with high yields of fucoidans, knowledge is needed on seasonal variation and impact of environmental conditions on the fucoidan content of brown algae. The relations between the fucoidan content and key environmental factors (irradiance, nutrient availability, salinity and exposure) were examined by sampling natural populations of the common North Atlantic kelps, Saccharina latissima and Laminaria digitata, over a full year at Hanstholm in the North Sea and Aarhus in the Kattegat. In addition, laboratory experiments were carried out isolating the effects of the single factors. The results demonstrated that (1) seasonal variation alters the fucoidan content by a factor of 2–2.6; (2) interspecific differences exist in the concentrations of crude fucoidan (% of dry matter): L. digitata (11%) > S. latissima (6%); and (3) the effects of single environmental factors were not consistent between species or between different conspecific populations. The ambiguous response to single environmental factors complicates prospective directions for manipulating an increased content of fucoidan in a cultivation scenario and emphasizes the need for knowledge on performance of local kelp ecotypes.