Molecular methods such as DNA/eDNA metabarcoding have emerged as useful tools to document the biodiversity of complex communities over large spatio-temporal scales. We established an international Marine Biodiversity Observation Network (ARMS-MBON) combining standardised sampling using autonomous reef monitoring structures (ARMS) with metabarcoding for genetic monitoring of marine hard-bottom benthic communities. Here, we present the data of our first sampling campaign comprising 56 ARMS units deployed in 2018-2019 and retrieved in 2018-2020 across 15 observatories along the coasts of Europe and adjacent regions. We describe the open-access data set (image, genetic and metadata) and explore the genetic data to show its potential for marine biodiversity monitoring and ecological research. Our analysis shows that ARMS recovered more than 60 eukaryotic phyla capturing diversity of up to ~5500 amplicon sequence variants and ~1800 operational taxonomic units, and up to ~250 and ~50 species per observatory using the cytochrome c oxidase subunit I (COI) and 18S rRNA marker genes, respectively. Further, ARMS detected threatened, vulnerable and non-indigenous species often targeted in biological monitoring. We show that while deployment duration does not drive diversity estimates, sampling effort and sequencing depth across observatories do. We recommend that ARMS should be deployed for at least 3-6 months during the main growth season to use resources as efficiently as possible and that post-sequencing curation is applied to enable statistical comparison of spatio-temporal entities. We suggest that ARMS should be used in biological monitoring programs and long-term ecological research and encourage the adoption of our ARMS-MBON protocols.

The Arctic and Subarctic seas are predicted to become hotspots for marine heatwaves (MHWs). High-latitude marine ecosystems face unique consequences from accelerated warming and sea ice loss, challenging species adapted to cold conditions. We review the literature on MHW characteristics and ecological impacts in the Arctic and Subarctic seas, and contrast MHW characteristics between the Bering Sea and Barents Sea. We uncover the pervasive impacts of MHWs across widely different organism groups, including benthic foundation species, phytoplankton, zooplankton, fish, seabirds, and marine mammals. MHWs in the Arctic marginal seas are especially prevalent in areas experiencing sea ice retreat, such as seasonal sea ice zones, highlighting the complex interplay between MHWs and sea ice dynamics. Overall, few studies have documented the ecological impacts of MHWs on high-latitude ecosystems, with the notable exception of the impacts from the Bering Sea and Chukchi Sea MHWs in 2017–2019. Many Arctic species, with their cold and narrow thermal preferences, appear vulnerable to MHWs, as they might not have access to cold climate refugia, while boreal species appear to benefit from Arctic and Subarctic MHWs. Sessile foundation species, such as kelp and seagrasses, are especially at risk during MHWs, although in the Arctic evidence of MHWs impacts remains limited. Reproductive failure and mass mortality events have been documented for several species in the Pacific Arctic (e.g., seabirds, fish, crabs). MHWs have been observed to have ecosystem-wide repercussions in the northern Bering Sea and Chukchi Sea with shifts in plankton communities affecting the entire food web. The ecological responses to MHWs in the Arctic and Subarctic ecosystems are still not fully understood, highlighting a need for further research to assess the direct and indirect impacts on various taxa and to improve predictive models for better management and conservation strategies. MHWs can also have large consequences for ecosystem services and socio-ecological systems, for example, closures of economically valuable and culturally important fisheries, as seen in Alaska, degradation of traditional ice-hunting practices, and compromised wellbeing of coastal communities. Large and abrupt ecosystem changes following MHWs underscore the urgent need for adaptive management strategies in the face of ongoing climate change.

IntroductionCoral reefs as key ecosystems in Zanzibar are exposed to several anthropogenic and natural stressors.MethodsThe benthic composition and coverage of coral reefs were investigated on three data sets involving ten coral reefs monitored from 1992 to 2016. Firstly, we investigated differences in the reef composition using data from seven reefs in 2015. Secondly, we analyzed communities on three distinctive reefs (2010 to 2012) to understand the importance of seasons and reef zones (slope, crest and flat) on species abundance. Finally, we investigated long-term changes (1992 to 2016) of five reefs.ResultsBranching Porites and Acropora, and soft coral order Corallimorpharia, characterized sheltered reef communities. Soft corals and algal species characterized the reef communities exposed to strong hydrodynamic conditions, which also displayed greater cover of rocks and rubbles. The average dissimilarity between reefs ranged between 60% and 75%. The seasonal changes in community structure for reefs near Stone Town were mostly associated with soft coral Corallimorpharia. Indeed, the bare rock and algae distinguished the northern exposed reef from more sheltered reefs. Acropora was a key genus for the sheltered Chumbe reef, which explained between 14% and 18% of the dissimilarities among the three reefs. Hard corals covered between 40% and 70% in most years, with severe declines following El Niño events in 1998 and 2016. The dominating genus Acropora showed a strong decline from the late 1990s’ with signs of recovery at remote reefs compared to reefs closer to human residence.DiscussionOur results highlight the importance of seasonality and spatial differences, reflecting differences in human impact and physical exposure and significant long-term changes in coral communities. Continued monitoring of reef health is essential to evaluate the success of ongoing management to sustain the reef services.

ABSTRACT Fucoid forests are areas dominated by marine brown seaweed in the taxonomic order Fucales that, like the better‐known marine foundation species—corals, kelps, seagrasses, salt marshes, and mangroves—are threatened by anthropogenic stressors. Fucoid forests are fabulous and important because they, like the better‐known marine foundation species (i) span large areas, bioregions, and ecosystems, (ii) provide ecological functions such as high productivity, biodiversity, and habitat for iconic and endemic species, and (iii) support a variety of ecosystem services, like commercial fisheries, regulation of nutrients and carbon, and cultural values. Fucoid forests are, based on a new citation analysis, forgotten worldwide, because they are described orders of magnitude less than the better‐known marine foundation species, in ecology and marine biology textbooks, in Google Scholar and Scopus databases over scientific literature, and in recent reports and reviews about seaweed forests. Fucoid forests would be less forgotten if more people acknowledge their biological importance and societal value more often and equate their importance to that of the better‐known marine foundation species. To decrease the knowledge gap between fucoids and the better‐known foundation species, researchers and science communicators could join forces under a broad “fucoid umbrella,” establish stronger online presences, coordinate and collaborate on publications, and produce free eye‐catching non‐technical materials for teachers, managers, politicians, grass‐root organizations, philanthropists, and funding agencies.

GuardIAS is a three-year Horizon Europe project starting in January 2025, uniting diverse expertise to address aquatic invasive alien species (IAS) management. This multidisciplinary initiative comprises seven interconnected work packages targeting all invasion stages (pre-border, border, post-border) to develop tools for disrupting invasions. GuardIAS will employ Artificial Intelligence and data workflows to enhance biodiversity databases with species distributions, environmental tolerances, traits, and genetic information, thereby improving the European Alien Species Information Network (EASIN) and harmonizing key datasets. The citizen science platform iNaturalist will be enriched with expert-verified images of aquatic IAS for early detection and monitoring of geographic spread. An Early Warning System focused on IAS of EU concern will be developed and integrated into EASIN. To prevent hull biofouling-a major IAS introduction pathway-GuardIAS will explore nanotechnology-based antifouling coatings. The project will also investigate recreational boat movements along European coastlines, an understudied factor in IAS secondary dispersal. An eDNA reference library and assay panel will be developed for effective IAS detection. Advanced models, such as the Nobel Prize winning Multi-Region Input-Output analysis, will assess IAS risks, including impacts on threatened species and critical habitats under current and future scenarios. Systematic conservation planning tools will prioritize IAS monitoring and management actions based on their impacts. GuardIAS will enhance data collection, monitoring, early detection, and public awareness through innovative citizen science initiatives like BioArtBlitz events-where arts serve as a communication vehicle- eDNA sampling, sound analysis projects on Zooniverse, and marina events for boaters. Stakeholder engagement will be fostered through applied games. Collaborating with environmental authorities, industry, and aquatic managers, the project will co-design and implement eradication and control efforts in marine and freshwater environments. By integrating Social Sciences and Humanities, GuardIAS will promote collaborative knowledge creation, understand public perceptions on IAS management, and facilitate exploitation of the project's outcomes.

This study investigated variability in ecosystem metabolism in the meromictic, oligotrophic, and deep Lake Tanganyika. A large buoy equipped with a weather station, oxygen and temperature sensors for every 10 m down to 102 m depth and an irradiance sensor at 0 and 22 m depth, provided a three-month data with one-minute frequency. These data enabled us to derive detailed description of water column mixing and light conditions along with daily depth specific rates of gross primary production, ecosystem respiration and net production over a 3-month period. We applied a mass balance approach which included dissolved oxygen exchange between depth layers driven by mixed-layer deepening and eddy diffusivity from a one-dimensional hydrodynamic model. The vertical extent of the upper mixed layer varied between 21-40 m and the extent of the metalimnion varied between 48-75 m, with the euphotic zone (20-38 m) extending into the metalimnion on several days, providing enough light for primary production to occur below the upper mixed layer. Vertical profiles of metabolism showed several periods with elevated primary production in the metalimnion around the deep chlorophyll maximum. This deep productivity may compensate for the decreasing primary production in the epilimnion caused by climate change induced reductions in nutrient inputs from deeper waters.

A coupled hydrodynamic-ecosystem model (GOTM-FABM-ERGOM) was applied to test the hypothesis that primary production in the upper mixed layers of Lake Tanganyika is primarily controlled by internal nutrient inputs. The model was calibrated (data: May 2015–April 2016) and validated (data: May 2016–April 2017) against monthly field data of water temperature, dissolved oxygen, nutrients (nitrate, ammonium, phosphate) and chlorophyll a collected from Kigoma Bay in the northern part of the lake. Data of nutrients and discharge from the rivers (Ruzizi and Malagarasi) and atmospheric dry and wet deposition were derived from the literature. The model generally showed good agreement with the observed data for water temperature, dissolved oxygen and nutrients during the calibration and validation periods. The model satisfactorily reproduced the lake’s seasonal dynamics (dry and wet seasons) induced by the lake’s hydrodynamic processes. We found that both internal and external sources contribute importantly to total nutrient loading in the lake. Our results indicate that nutrient supply from rivers into Lake Tanganyika is more important than previously known. However, we call for further studies to investigate the contribution of other sources of regenerated nutrients (e.g. N2-fixation) to the overall primary productivity of Lake Tanganyika.

Identifying the scaling rules describing ecological patterns across time and space is a central challenge in ecology. Taylor's law of fluctuation scaling, which states that the variance of a population's size or density is proportional to a positive power of the mean size or density, has been widely observed in population dynamics and characterizes variability in multiple scientific domains. However, it is unclear if this phenomenon accurately describes ecological patterns across many orders of magnitude in time, and therefore links otherwise disparate observations. Here, we use water clarity observations from 10,531 days of high-frequency measurements in 35 globally distributed lakes, and lower-frequency measurements over multiple decades from 6342 lakes to test this unknown. We focus on water clarity as an integrative ecological characteristic that responds to both biotic and abiotic drivers. We provide the first documentation that variations in ecological measurements across diverse sites and temporal scales exhibit variance patterns consistent with Taylor's law, and that model coefficients increase in a predictable yet non-linear manner with decreasing observation frequency. This discovery effectively links high-frequency sensor network observations with long-term historical monitoring records, thereby affording new opportunities to understand and predict ecological dynamics on time scales from days to decades.

Non-indigenous species (NIS) are of concern for biodiversity conservation and ecosystem functioning. We present an updated list of NIS, including cryptogenic species, from Danish marine waters containing 123 species. Benthic invertebrates (36%) and phytoplankton (28%) dominate the list, but fish (15%) and macroalgae (13%) are also important. The Limfjord in Northern Jutland emerges as a hotspot for the introduction of NIS. Data from multiple sources were included, i.e., the National Monitoring Program (NOVANA), the National Fish Atlas project, the citizen science project Arter.dk, research articles, and annual national reports of the ICES working group ITMO. Forty-six NIS species were subject to expert judging using a modified Harmonia protocol; 19 were found to fulfil the four selected criteria identifying a species as being ‘invasive’. Additionally, 38 species, not yet recorded in Danish waters, were evaluated using the same method, and 31 were found to fulfil the ‘invasive’ criteria. For nine selected species, introduction history, distribution maps, and time-series diagrams are presented. Our data document that the national monitoring efforts should be expanded to record macrozooplankton, coastal fish, and mobile epibenthic species. Furthermore, the national data repository, Arter.dk, should be expanded to enable more detailed documentation of new NIS records.

A rapid increase in population size along with expansion in hotel investment have been identified as key drivers of marine pollution in Zanzibar coastal waters. A validated high-resolution hydrodynamic tracer model was used to estimate the dispersal and impact range of pollutants from the main hotel areas along the western coastline of Unguja Island. The model showed the highest impact of pollutants from land during the northeast monsoon season due to the weakening of the main current. Marine protected areas with short distances to hotel areas and weak tidal currents were more likely to be impacted by pollutants from land than the more remote areas with higher flushing in agreement with water quality and seagrass health measurements. Cumulative effects of pollutants instead of single sources are important to consider for integrated wastewater treatment plans and management strategies to reduce pollution for the protection of biodiversity and guide future monitoring.

The ability of seagrass meadows to filter nutrients and capture and store CO2 and nutrients in the form of organic carbon (OC) and nitrogen (N) in their sediments may help to mitigate local eutrophication as well as climate change via meadow restoration and protection. This study assesses OC and N sediment stocks (top 50 cm) and sequestration rates within Danish eelgrass meadows. At four locations, eelgrass-vegetated and nearby unvegetated plots were studied in protected and exposed areas. The average OC and N sediment 50 cm stocks were 2.6 +/- 0.3 kg OC m(-2) and 0.23 +/- 0.01 kg N m(-2), including vegetated and unvegetated plots. In general, OC and N stocks did not differ significantly between eelgrass meadows and unvegetated sediments. Lack of accumulation of excess Pb-210 suggested sediment erosion or low rates of sediment accumulation at most sites. OC accumulation rates ranged from 6 to 134 g m(-2) yr(-1) and N from 0.7 to 14 g m(-2) yr(-1). Generalized additive models showed that >= 80% of the variation in sediment OC and N stocks was explained by sediment grain size, organic matter source, and hydrodynamic exposure. Long cores, dated with Pb-210, showed declining OC and N densities toward present time, suggesting long-term declines in eelgrass OC and N pools. Estimates of potential nation-wide OC and N accumulation in eelgrass sediments show that they could annually capture up to 0.7% +/- 0.5% of CO2 emissions and 6.9% +/- 5.2% of the total terrestrial N load.

Biological invasions, resulting from human activities, exert substantial impacts on ecosystems worldwide. This review focuses on marine invasive alien species (IAS) in Europe, examining the current state, proposing strategies to address the problem, and offering recommendations for enhanced management. Effective management of biological invasions relies on accessible, accurate data to inform decision-making. Information systems such as the European Alien Species Information Network (EASIN), Aquatic Non-Indigenous and Cryptogenic Species (AquaNIS), and World Register of Introduced Marine Species (WriMS) provide comprehensive databases on IAS, but their sustainability requires long-term maintenance, continuous updates, and support. Most countries lack specific monitoring programs for marine IAS, and standardization and improvement of monitoring methods are needed. Port monitoring plays a vital role in the early detection of new arrivals, and recent advancements in molecular techniques show promise for effective IAS monitoring. Risk screening tools are commonly employed to rank taxa based on their invasiveness potential in European regions, but variations in protocols can yield inconsistent results. European impact assessments highlight resource competition, novel habitat creation, and predation as primary mechanisms for negative impacts on biodiversity, while the creation of novel habitats represents a key mechanism for positive impacts. Preventing IAS introductions is critical, and measures such as ballast water treatment systems are implemented to reduce the likelihood of marine introductions. However, understanding introduction pathways remains uncertain for many IAS. Eradication and control efforts for marine IAS have limited success, emphasizing the need for enhanced biosecurity measures. Climate change, especially ocean warming, can intensify IAS impacts on native species and ecosystems. In climate change hotspots, some tropical aliens may, however, compensate for the loss of thermally sensitive natives with similar traits. Therefore, it is imperative to consider the interactions between climate change and IAS in developing effective management and conservation strategies. Enhancing IAS management in Europe entails i) securing adequate funding, ii) expanding the list of IAS of Union Concern to adequately cover marine invasions, iii) learning from countries with successful biosecurity practices, iv) sustaining information systems, v) improving monitoring and early warning systems with innovative technologies, vi) enhancing prediction models, vii) conducting integrated impact assessments and mapping cumulative IAS impacts, and vii) considering the potential benefits of IAS in ecosystem functioning and services.

The introduction-rate of non-indigenous species (NIS) to coastal water bodies has accelerated over the last century. We present a model study assessing the fate of NIS released in likely point sources of the Danish Wadden Sea. We show that NIS-particles released in the deep North Sea are generally transported away from the Wadden Sea, while those released in the coastal North Sea and the Wadden Sea show large variability in track pattern and settlement location. Consequently, the introduction of NIS from ships entering the port of Esbjerg pose a threat to the Wadden Sea through primary and secondary spreading, while transport of species from sources in the south likely causes a slow and steady settling of NIS in the Wadden Sea and coastal North Sea. The study points to the importance of enforcing an efficient monitoring system to ensure early detection of changes to the species composition of the Wadden Sea.

In situ Chl-a data were used to perform empirical calibration and validation of Sentinel-3 level 2 product in Danish marine waters. Comparing in situ data with both same-day and & PLUSMN;5 days moving averaged Sentiel-3 Chl-a values yielded two similar positive correlations (p > 0.05) with rpearsonvalues of 0.56 and 0.53, respectively. However, as the moving averaged values resulted in significantly more available data than daily matchups (N = 392 vs. N = 1292) at a similar quality of correlation with similar model parameters (slope (1.53 and 1.7) and intercept (-0.28 and -0.33) respectively), which were not significantly different (p > 0.05), the further analyses were focused on & PLUSMN;5 days moving averaged values. A thorough comparison of seasonal and growing season averages (GSA) also showed a very good agreement, except for a few stations characterized by very shallow depth. Overestimation by the Sentinel-3 occurred in shallow coastal areas and was attributed to the interferences from benthic vegetation and high levels of Colored Dissolved Organic matter (CDOM) interfering with the Chl-a signals. Underestimation observed in the inner estuaries with shallow Chl-a rich waters, however, seen as a result of self-shading at high Chl-a concentrations, reducing effective absorption by phytoplankton. Besides the observed minor disagreements, there was no significant difference when the GSA values from in situ and Sentinel-3 were compared for all three water types (p > 0.05, N = 110). Analyzing Chla estimates along a depth gradient showed significant (p < 0.001) non-linear trends of declining concentrations from shallow to deeper waters for both in situ (explaining 15.2 % of the variance (N = 109)) and Sentinel-3 data (explaining 36.3 % of the variance (N = 110)), with higher variability in shallow waters. Furthermore, Sentinel-3 enabled full spatial coverage of all 102 monitored water bodies providing GSA data at much higher spatial and temporal resolutions for good ecological status (GES) assessment compared to only 61 through in situ sampling. This underlines the potential of Sentinel-3 for substantially extending the geographical coverage of monitoring and assessment. However, the systematic over- and underestimation of Chl-a in shallow nutrient rich inner estuaries through Sentinel-3 requires further attention to enable routine application of the Sentinel-3 level 2 standard product in the operational Chl-a monitoring in Danish coastal waters. We provide methodological recommendations on how to improve the Sentinel-3 products' representation of in situ Chl-a conditions. Continued frequent in situ sampling remains important for monitor-ing as these measurements provide essential data for empirical calibration and validation of satellite based estimates to reduce possible systematic bias.

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 investigated the use of multisensory satellite data to determine long-term changes in surface chlorophyll concentrations using a 19-year (1998–2016) time series of chlorophyll data in the Danish Kattegat region of the Baltic Sea. Merged satellite estimates (SeaWiFS-MODIS/Aqua-MERIS-VIIRS) were compared with in situ ship based time series from four monitoring stations situated with increasing distance from land and nutrient sources. In situ and satellite derived estimates showed similar trend in chlorophyll with several fold higher values closer to land. Satellites aligned very well with in situ estimates in the open water stations but showed significant differences in magnitude and inter-annual variability, in particular in shallow coastal waters. Some systematic deviation was observed with satellite underestimating the growing season average for the earlier periods (1998–2002) and overestimating for the later period (2012–2016) compared to in situ estimates. Comparing growing season chlorophyll means over the 19 year period showed increasing magnitude and variability in nearshore and shallower areas, most pronounced for the satellite derived chlorophyll. Satellites overestimated chlorophyll in nearshore areas 2–4 fold, despite excluding shallow nearshore areas with possible benthic interferences from the analyses. This bias needs further validation and requires correction to improve the overall applicability of satellites for long-term monitoring of chlorophyll in the Kattegat region. From analysis of normalized data, we developed a simple correction model, which reduced deviations considerably between methods, underlying the importance of in situ data for application of satellite observations. While significant deviations were observed from in situ data, satellites are clearly advantageous in the much higher temporal and high spatial coverage they provide. Multisensory satellites can, however, not be used currently as a standalone technique for long-term assessment of chlorophyll. They require validation with in situ measurements, which provide essential data for calibration, validation and correction of satellite based estimates. A complementary use of multisensory satellite and in situ measurements therefore remains essential to assess trends in the ecological status of optically complex waters such as the Kattegat region of the Baltic Sea.

The UN Decade of Ecosystem Restoration is a response to the urgent need to substantially accelerate and upscale ecological restoration to secure Earth’s sustainable future. Globally, restoration commitments have focused overwhelmingly on terrestrial forests. In contrast, despite a strong value proposition, efforts to restore seaweed forests lag far behind other major ecosystems and continue to be dominated by small‐scale, short‐term academic experiments. However, seaweed forest restoration can match the scale of damage and threat if moved from academia into the hands of community groups, industry, and restoration practitioners. Connecting two rapidly growing sectors in the Blue Economy—seaweed cultivation and the restoration industry—can transform marine forest restoration into a commercial‐scale enterprise that can make a significant contribution to global restoration efforts.