Innovative and practical tools for monitoring and assessing biodiversity status and impacts of multiple human pressures in marine systems.

Human activities at sea can produce pressures and cumulative effects on ecosystem components that need to be monitored and assessed in a cost-effective manner. Five Horizon European projects have joined forces to collaboratively increase our knowledge and skills to monitor and assess the ocean in an innovative way, assisting managers and policy-makers in taking decisions to maintain sustainable activities at sea. Here, we present and discuss the status of some methods revised during a summer school, aiming at better management of coasts and seas. We include novel methods to monitor the coastal and ocean waters (e.g. environmental DNA, drones, imaging and artificial intelligence, climate modelling and spatial planning) and innovative tools to assess the status (e.g. cumulative impacts assessment, multiple pressures, Nested Environmental status Assessment Tool (NEAT), ecosystem services assessment or a new unifying approach). As a concluding remark, some of the most important challenges ahead are assessing the pros and cons of novel methods, comparing them with benchmark technologies and integrating these into long-standing time series for data continuity. This requires transition periods and careful planning, which can be covered through an intense collaboration of current and future European projects on marine biodiversity and ecosystem health.

An integrated assessment of the Good Environmental Status of Mediterranean Marine Protected Areas.

Local, regional and global targets have been set to halt marine biodiversity loss. Europe has set its own policy targets to achieve Good Environmental Status (GES) of marine ecosystems by implementing the Marine Strategy Framework Directive (MSFD) across member states. We combined an extensive dataset across five Mediterranean ecoregions including 26 Marine Protected Areas (MPAs), their reference unprotected areas, and a no-trawl case study. Our aim was to assess if MPAs reach GES, if their effects are local or can be detected at ecoregion level or up to a Mediterranean scale, and which are the ecosystem components driving GES achievement. This was undertaken by using the analytical tool NEAT (Nested Environmental status Assessment Tool), which allows an integrated assessment of the status of marine systems. We adopted an ecosystem approach by integrating data from several ecosystem components: the seagrass Posidonia oceanica, macroalgae, sea urchins and fish. Thresholds to define the GES were set by dedicated workshops and literature review. In the Western Mediterranean, most MPAs are in good/high status, with P. oceanica and fish driving this result within MPAs. However, GES is achieved only at a local level, and the Mediterranean Sea, as a whole, results in a moderate environmental status. Macroalgal forests are overall in bad condition, confirming their status at risk. The results are significantly affected by the assumption that discrete observations over small spatial scales are representative of the total extension investigated. This calls for large-scale, dedicated assessments to realistically detect environmental status changes under different conditions. Understanding MPAs effectiveness in reaching GES is crucial to assess their role as sentinel observatories of marine systems. MPAs and trawling bans can locally contribute to the attainment of GES and to the fulfillment of the MSFD objectives. Building confidence in setting thresholds between GES and non-GES, investing in long-term monitoring, increasing the spatial extent of sampling areas, rethinking and broadening the scope of complementary tools of protection (e.g., Natura 2000 Sites), are indicated as solutions to ameliorate the status of the basin.

A microbial mandala for environmental monitoring: Predicting multiple impacts on estuarine prokaryote communities of the Bay of Biscay.

Routine monitoring of benthic biodiversity is critical for managing and understanding the anthropogenic impacts on marine, transitional and freshwater ecosystems. However, traditional reliance on morphological identification generally makes it cost-prohibitive to increase the scale of monitoring programmes. Metabarcoding of environmental DNA has clear potential to overcome many of the problems associated with traditional monitoring, with prokaryotes and other microorganisms showing particular promise as bioindicators. However, due to the limited knowledge regarding the ecological roles and responses of environmental microorganisms to different types of pressure, the use of de novo approaches is necessary. Here, we use two such approaches for the prediction of multiple impacts present in estuaries and coastal areas of the Bay of Biscay based on microbial communities. The first (Random Forests) is a machine learning method while the second (Threshold Indicator Taxa Analysis and quantile regression splines) is based on de novo identification of bioindicators. Our results show that both methods overlap considerably in the indicator taxa identified, but less for sequence variants. Both methods also perform well in spite of the complexity of the studied ecosystem, providing predictive models with strong correlation to reference values and fair to good agreement with ecological status groups. The ability to predict several specific types of pressure is especially appealing. The cross-validated models and biotic indices developed can be directly applied to predict the environmental status of estuaries in the same geographical region, although more work is needed to evaluate and improve them for use in new regions or habitats.

Pan-regional marine benthic cryptobiome biodiversity patterns revealed by metabarcoding Autonomous Reef Monitoring Structures.

Autonomous Reef Monitoring Structures (ARMS) have been applied worldwide to characterize the critical yet frequently overlooked biodiversity patterns of marine benthic organisms. In order to disentangle the relevance of environmental factors in benthic patterns, here, through standardized metabarcoding protocols, we analyse sessile and mobile (<2 mm) organisms collected using ARMS deployed across six regions with different environmental conditions (3 sites × 3 replicates per region): Baltic, Western Mediterranean, Adriatic, Black and Red Seas, and the Bay of Biscay. A total of 27,473 Amplicon Sequence Variants (ASVs) were observed ranging from 1,404 in the Black Sea to 9,958 in the Red Sea. No ASVs were shared among all regions. The highest number of shared ASVs was between the Western Mediterranean and the Adriatic Sea (116) and Bay of Biscay (115). Relatively high numbers of ASVs (103), mostly associated with the genus Amphibalanus, were also shared between the lower salinity seas (Baltic and Black Seas). We found that compositional differences in spatial patterns of rocky-shore benthos are determined slightly more by dispersal limitation than environmental filtering. Dispersal limitation was similar between sessile and mobile groups, while the sessile group had a larger environmental niche breadth than the mobile group. Further, our study can provide a foundation for future evaluations of biodiversity patterns in the cryptobiome, which can contribute up to 70% of the local biodiversity.

The recovery of estuarine quality and the perceived increase of cultural ecosystem services by beach users: A case study from northern Spain.

In Europe, the quality of coastal bathing waters improved considerably in the last decades, mainly due to the more demanding legislation and the adoption of water sanitation plans. In the Nerbioi estuary (North Spain), the Wastewater Treatment Plan implemented between 1990 and 2001 resulted on an abrupt decrease in microbial concentration; thus, complying with bathing waters legislation and allowing recreational activities again in the three beaches of the estuary. However, little is known about how improvements in bathing waters influences the provision of cultural ecosystem services and human well-being. A questionnaire was used to study beach users' behaviour and perceptions and compared with environmental time-series data (microbial concentration and water transparency). Most respondents perceived an improvement in bathing waters quality and linked it to the estuarine sanitation. Nerbioi beaches are important recreational areas, mainly for local visitors, and water quality improvement was found to be a critical factor for deciding to visit these beaches. Furthermore, most visitors answered that they would not return if water conditions deteriorate. Significant differences existed between beaches, with the most inner beach presenting worse environmental conditions than the other two beaches; and matching user's perceptions. Our findings highlight that water sanitation actions are important for the recovery of degraded coastal environments and for the maintenance of ecosystem services. Also, that multidisciplinary research is necessary to better comprehend the links between environmental recovery and the provision of ecosystem services.

Adaptation and application of multivariate AMBI (M-AMBI) in US coastal waters.

The multivariate AMBI (M-AMBI) is an extension of the AZTI Marine Biotic Index (AMBI) that has been used extensively in Europe, but not in the United States. In a previous study, we adapted AMBI for use in US coastal waters (US AMBI), but saw biases in salinity and score distribution when compared to locally calibrated indices. In this study we modified M-AMBI for US waters and compared its performance to that of US AMBI. Index performance was evaluated in three ways: 1) concordance with local indices presently being used as management tools in three geographic regions of US coastal waters, 2) classification accuracy for sites defined a priori as good or bad and 3) insensitivity to natural environmental gradients. US M-AMBI was highly correlated with all three local indices and removed the compression in response seen in moderately disturbed sites with US AMBI. US M-AMBI and US AMBI did a similar job correctly classifying sites as good or bad in local validation datasets (83 to 100% accuracy vs. 84 to 95%, respectively). US M-AMBI also removed the salinity bias of US AMBI so that lower salinity sites were not more likely to be incorrectly classified as impaired. The US M-AMBI appears to be an acceptable index for comparing condition across broad-scales such as estuarine and coastal waters surveyed by the US EPA's National Coastal Condition Assessment, and may be applicable to areas of the US coast that do not have a locally derived benthic index.

Quantitative criteria for choosing targets and indicators for sustainable use of ecosystems.

Wide-ranging, indicator-based assessments of large, complex ecosystems are playing an increasing role in guiding environmental policy and management. An example is the EU's Marine Strategy Framework Directive, which requires Member States to take measures to reach "good environmental status" (GES) in European marine waters. However, formulation of indicator targets consistent with the Directive's high-level policy goal of sustainable use has proven challenging. We develop a specific, quantitative interpretation of the concepts of GES and sustainable use in terms of indicators and associated targets, by sharply distinguishing between current uses to satisfy current societal needs and preferences, and unknown future uses. We argue that consistent targets to safeguard future uses derive from a requirement that any environmental state indicator should recover within a defined time (e.g. 30 years) to its pressure-free range of variation when all pressures are hypothetically removed. Within these constraints, specific targets for current uses should be set. Routes to implementation of this proposal for indicators of fish-community size structure, population size of selected species, eutrophication, impacts of non-indigenous species, and genetic diversity are discussed. Important policy implications are that (a) indicator target ranges, which may be wider than natural ranges, systematically and rationally derive from our proposal; (b) because relevant state indicators tend to respond slowly, corresponding pressures should also be monitored and assessed;

Challenges and difficulties in assessing the environmental status under the requirements of the Ecosystem Approach in North African countries, illustrated by eutrophication assessment.

Marine ecosystems provide many ecosystem goods and services. However, these ecosystems and the benefits they create for humans are subject to competing uses and increasing pressures. As a consequence of the increasing threats to the marine environment, several regulations require applying an ecosystem-based approach for managing the marine environment. Within the Mediterranean Sea, in 2008, the Contracting Parties of the Mediterranean Action Plan decided to progressively apply the Ecosystem Approach (EcAp) with the objective of achieving Good Environmental Status (GES) for 2018. To assess the environmental status, the EcAp proposes 11 Ecological Objectives, each of which requires a set of relevant indicators to be integrated. Progress towards the EcAp entails a gradual and important challenge for North African countries, and efforts have to be initiated to propose and discuss methods. Accordingly, to enhance the capacity of North African countries to implement EcAp and particularly to propose and discuss indicators and methods to assess GES, the aim of this manuscript is to identify the practical problems and gaps found at each stage of the environmental status assessment process. For this purpose, a stepwise method has been proposed to assess the environmental status using Ecologic Objective 5-Eutrophication as example.

Unveiling the ecological status of the Arabian Gulf's marine ecosystem: Insights from benthic community analysis.

This study assessed the ecological health of waters within the Saudi Arabian Exclusive Economic Zone, by utilizing benthic biotic indices with a marine monitoring dataset covering the years 2013 to 2018. This comprehensive evaluation covered a vast expanse, encompassing 67 distinctive sampling locations characterized by a wide range of depth and salinity gradients. The study examined spatial fluctuations in the benthic community and assessed potential correlations with environmental variables, including salinity, depth, sediment texture, total organic carbon, and other relevant factors. The macrobenthic density varied across the study sites, with an average density of 566 ± 120 ind.m-2. The Shannon diversity index ranged from 3.21 and 5.90, with an average of 4.70 ± 0.52. Based on the average AMBI values, all the locations were categorized as either slightly disturbed or undisturbed. Additionally, the M-AMBI analysis indicated that 95.5 % sites were in good or high ecological status.

Assessment of the eutrophication status at Mediterranean sub-basin scale, within the European Marine Strategy Framework Directive.

The aim of this work is to define harmonized reference conditions and assessment thresholds for selected criteria elements of the Marine Strategy Framework Directive (MSFD) Descriptor 5 (Eutrophication) in the Eastern Mediterranean Sea and to test if a tool for integrated assessment of the status of marine systems can be used as a common methodological approach. In this frame, we tested two statistical approaches in order to set threshold values for four criteria of Descriptor 5: nutrients, chlorophyll a, transparency and dissolved oxygen in the bottom waters. It is noteworthy that this work revealed the need to apply common procedures in data treatment and assessment evaluation. This is the first attempt to set common methods for the assessment of eutrophication in the Eastern Mediterranean, which is essential in marine environments, especially those shared by several countries. To this end, we have applied common criteria and metrics and established thresholds "Good" and "Moderate" for nutrients, chlorophyll a, transparency and dissolved oxygen in the bottom waters for the different Water Types of the Adriatic and Aegean Seas (I, II, IIIW, IIIE), based on datasets provided by Italy, Slovenia, Croatia and Greece. The selected criteria elements were common for all countries, providing a unified approach to GES assessment of two case study areas: the Adriatic Sea and the Saronikos Gulf. Dissolved Inorganic Nitrogen (DIN) threshold values of 15.6, 6.85, 1.61 and 2.11 µmol L-1 were set for the Water Types I, II, IIIW and IIIE, respectively. We also tested if an aggregation tool for GES assessment, such as Nested Environmental status Assessment Tool (NEAT), could be used as a common methodological approach. The comparison of NEAT with TRIX showed good comparability. In this end, NEAT can be used as a useful and much needed assessment tool for assessing eutrophication status of the marine.

Cross-basin and cross-taxa patterns of marine community tropicalization and deborealization in warming European seas.

Ocean warming and acidification, decreases in dissolved oxygen concentrations, and changes in primary production are causing an unprecedented global redistribution of marine life. The identification of underlying ecological processes underpinning marine species turnover, particularly the prevalence of increases of warm-water species or declines of cold-water species, has been recently debated in the context of ocean warming. Here, we track changes in the mean thermal affinity of marine communities across European seas by calculating the Community Temperature Index for 65 biodiversity time series collected over four decades and containing 1,817 species from different communities (zooplankton, coastal benthos, pelagic and demersal invertebrates and fish). We show that most communities and sites have clearly responded to ongoing ocean warming via abundance increases of warm-water species (tropicalization, 54%) and decreases of cold-water species (deborealization, 18%). Tropicalization dominated Atlantic sites compared to semi-enclosed basins such as the Mediterranean and Baltic Seas, probably due to physical barrier constraints to connectivity and species colonization. Semi-enclosed basins appeared to be particularly vulnerable to ocean warming, experiencing the fastest rates of warming and biodiversity loss through deborealization.

A dataset and template for assessing the ecological status of marine sediments and waters, based on microbial taxa.

Microbes have often been overlooked as indicators of how the ecological status is affected by human pressures. Recently, the biotic index microgAMBI was proposed to assess the status of marine sediments and waters, and it has been tested under different pressures and biogeographical areas. This index is based on the assignation of microbial taxa to one of two ecological groups: sensitive or tolerant to pollution or disturbance. The resulting taxa list has grown significantly since its first publication. Given the growing use of microgAMBI, it is crucial to make it more FAIR: Findable, Accessible, Interoperable and Reusable. Hence, this work provides the calculation template, the updated taxa list (1,974 taxa currently), and instructions on how to access and use them for assessing marine microbial ecological status.

Assessing the ecological quality status of macrobenthic communities in a marine terminal of liquefied natural gas in Peru.

Macrobenthic organisms are useful bioindicators to assess ecological quality status. On the south-central coast of Peru (13°15.15'S, 76°18.5'W), a Liquefied Natural Gas (LNG) marine terminal has been operating since 2010. We investigated the macrobenthic communities and sediment parameters from 2011 to 2020 to evaluate the ecological quality status in the surrounding area of the marine terminal, using the AZTI Marine Biotic Index (AMBI) and its multivariate version (M-AMBI). We analyzed the diversity and community composition of macrobenthic invertebrates and the physico-chemical parameters of the sediment from 29 sampling sites, ranging from 0 to 15m depth. The sampling design considered: the direct influence zone ("DIZ", surroundings of the marine terminal), and northern (NCZ) and southern (SCZ) control zones. Our results indicated that abundance was high at SCZ and decreased with depth. Species richness and diversity were high at DIZ and NCZ, respectively, and increased up to 10m but dropped at 15m. High sand content was recorded in shallow depths, while in deeper areas and DIZ, mud and organic matter increased and redox potential was negative. AMBI indicated a "slightly disturbed" status in general, while M-AMBI indicated "good" or "moderate" status at depths ≤ 12m, and "poor" status at 15m. Overall, the season/year factor was not important, and variables were mostly significantly different across depths. Redox potential and organic matter were correlated with M-AMBI at 15m. In general, our results indicate an acceptable ecological quality surrounding the marine terminal, likely because the study area is not influenced by an important input of an anthropogenic stressor. This study highlights the importance of monitoring benthic communities in the surroundings of human-made structures and the use of ecological quality indices for understanding potential impacts.

Impacts of human activities on the supply of marine ecosystem services: A conceptual model for offshore wind farms to aid quantitative assessments.

Increased pressures from human activities may cause cumulative ecological effects on marine ecosystems. Increasingly, the study of ecosystem services is applied in the marine environment to assess the full effects of human activities on the ecosystem and on the benefits it provides. However, in the marine environment, such integrated studies have yet to move from qualitative and score-based to fully quantitative assessments. To bridge this gap, this study proposed a 4-tiered method for summarizing available knowledge and modelling tools to aid in quantitative assessments of ecosystem services supply. First, the ecosystem functioning mechanisms underlying the supply of services are conceptually mapped. Second, the impacts of the human activity of interest are summarized and linked to the first conceptual model in a case-specific model of ecosystem services supply. Third, indicators are selected that would best represent changes in the most important parameters of the conceptual model in a quantitative manner. Fourth, the knowledge gained in the previous steps is used to select models that are most useful to quantify changes in ecosystem services supply under the human pressure of interest. This approach was applied to the case study of offshore wind energy in the Belgian part of the North Sea, which is one of the most rapidly expanding industries in the marine environment globally. This study provides a useful tool to proceed towards quantification of marine ecosystem services, highlighting the need for a fully integrated approach to developing environmental impact assessment tools.

Surfing the waves: Environmental and socio-economic aspects of surf tourism and recreation.

Marine ecosystems contribute to human well-being, e.g. through the promotion of nature-based recreational activities such as surfing, which is a benefit obtained from Cultural Ecosystem Services (CES). Our research objective is to identify the benefits and impacts associated to surfing, and who are the main affected subjects and/or objects, achieving a better understanding of the sustainability status of this recreational activity. To this end, a bibliometric study and systematic review was carried out for the period 1965-2021. Benefits and impacts were collated and grouped according to their dimensional focus and type of effects in 6 groups (3-dimensional focus × 2 type of effects). The results revealed that since the beginning of 21st century surfing research topics are growing and diversifying. This review shows that implications of surfing go beyond direct users (i.e., surfers) and has consequences in diverse dimensions (environmental, socio cultural and economic), involving many stakeholders (e.g., scientific, and local communities). Most of the pieces of evidence collated in this research were related with the people who practice the activity and its social implications (psychological benefits as main benefit and injuries as main impact). Following an interdisciplinary approach, we obtained a holistic understanding of the surfing activity, not only in terms of the different dimensions addressed but on the sectors of the society that obtain benefits or are impacted by the activity. All of them should be considered and integrated to guarantee the sustainable management of this CES benefit.

Responses of the benthic environment to reduction in anthropogenic nutrient loading in the Seto Inland Sea (Japan), based on M-AMBI assessment.

Deterioration of the sediment environment and benthic ecosystem is an undesirable effect of eutrophication, but little is known about the response of macrobenthic communities to eutrophication and their long-term recovery. In the present study, temporal changes in benthic ecological status, associated with reductions in anthropogenic impacts on a largest semi-enclosed sea in Japan, were determined using long-term monitoring data from water and sediment quality based on the multivariate AZTI Marine Biotic Index (M-AMBI), focusing on spatial differences in anthropogenic impacts. Several sub-areas were classified based on Chlorophyll a (Chl.a) concentrations in surface water during the 1980s. Chl.a concentrations decreased in all sub-areas except the sub-area with <2 µg Chl.a L-1 from the 1990s-2010s. On the other hand, total organic carbon contents in sediment decreased and M-AMBI values increased in all sub-areas during this period may be due to reduced lateral organic matter advection from surrounding areas.

Microbial diversity alteration reveals biomarkers of contamination in soil-river-lake continuum.

Microbial communities inhabiting soil-water-sediment continuum in coastal areas provide important ecosystem services. Their adaptation in response to environmental stressors, particularly mitigating the impact of pollutants discharged from human activities, has been considered for the development of microbial biomonitoring tools, but their use is still in the infancy. Here, chemical and molecular (16S rRNA gene metabarcoding) approaches were combined in order to determine the impact of pollutants on microbial assemblages inhabiting the aquatic network of a soil-water-sediment continuum around the Ichkeul Lake (Tunisia), an area highly impacted by human activities. Samples were collected within the soil-river-lake continuum at three stations in dry (summer) and wet (winter) seasons. The contaminant pressure index (PI), which integrates Polycyclic aromatic hydrocarbons (PAHs), alkanes, Organochlorine pesticides (OCPs) and metal contents, and the microbial pressure index microgAMBI, based on bacterial community structure, showed significant correlation with contamination level and differences between seasons. The comparison of prokaryotic communities further revealed specific assemblages for soil, river and lake sediments. Correlation analyses identified potential "specialist" genera for the different compartments, whose abundances were correlated with the pollutant type found. Additionally, PICRUSt analysis revealed the metabolic potential for pollutant transformation or degradation of the identified "specialist" species, providing information to estimate the recovery capacity of the ecosystem. Such findings offer the possibility to define a relevant set of microbial indicators for assessing the effects of human activities on aquatic ecosystems. Microbial indicators, including the detection of "specialist" and sensitive taxa, and their functional capacity, might be useful, in combination with integrative microbial indices, to constitute accurate biomonitoring tools for the management and restoration of complex coastal aquatic systems.

Antibiotics in the Basque coast (N Spain): Occurrence in waste and receiving waters, and risk assessment (2017-2020).

The study of the presence of antibiotics in the aquatic environment is a preliminary step to analyse their possible harmful effects on aquatic ecosystems. In order to monitor their occurrence in the aquatic environment, the European Commission established in 2015, 2018, and 2020 three Watch Lists of substances for Union-wide monitoring (Decisions (EU) 2015/495, 2018/840, and 2020/1161), where some antibiotics within the classes of macrolides, fluoroquinolones and penicillins were included. In the Basque coast, northern Spain, three macrolide antibiotics (erythromycin, clarithromycin, azithromycin) and ciprofloxacin were monitored quarterly from 2017 to 2020 (covering a period before and after the COVID19 outbreak), in water samples collected from two Waste Water Treatment Plants (WWTPs), and three control points associated with receiving waters (transitional and coastal water bodies). This work was undertaken for the Basque Water Agency (URA). The three macrolide antibiotics in water showed a frequency of quantification >65 % in the Basque coast, with higher concentrations in the WWTP emission stations than in receiving waters. Their frequency of quantification decreased from 2017 to 2020, as did the consumption of antibiotics in Spanish primary care since 2015. Ciprofloxacin showed higher frequencies of quantification in receiving waters than in wastewaters, but the highest concentrations were observed in the WWTP emission stations. Although consumption of fluoroquinolones (among which is ciprofloxacin) in primary care in the Basque Country has decreased in recent years, this trend was not observed in the waters sampled in the present study. On the other hand, concentrations of clarithromycin, azithromycin, and ciprofloxacin in receiving waters exceeded their respective Predicted No-Effect Concentrations, so they could pose an environmental risk. These substances are widely used in human and animal medicine, so, although only ciprofloxacin is included in the third Watch List, it would be advisable to continue monitoring macrolides in the Basque coast as well.

A Bayesian Network model to identify suitable areas for offshore wave energy farms, in the framework of ecosystem approach to marine spatial planning.

The production of energy from waves is gaining attention. In its expansion strategy, technical, environmental and socioeconomic aspects should be taken into account to identify suitable areas for development of wave energy projects. In this research we provide a novel approach for suitable site identification for wave energy farms. To achieve this objective, we (i) developed a conceptual framework, considering technical, environmental and conflicts for space aspects that play a role on the development of those projects, and (ii) it was operationalized in a Bayesian Network, by building a spatially explicit model adopting the Spanish and Portuguese Economic Exclusive Zones as case study. The model results indicate that 1723 km2 and 17,409 km2 are highly suitable or suitable for the development of wave energy projects (i.e. low potential conflicts with other activities and low ecological risk). Suitable areas account for a total of 2.5 TWh∙m-1 energy resource. These areas are placed between 82 and 111 m water depth, 18-30 km to the nearest port, 21-29 km to the nearest electrical substation onshore, with 143-170 MWh m-1 mean annual energy resource and having 124-150 of good weather windows per year for construction and maintenance work. The approach proposed supports scientists, managers and industry, reducing uncertainties during the consenting process, by identifying the most relevant technical, environmental and socioeconomic factors when authorising wave energy projects. The model and the suitability maps produced can be used during site identification processes, informing Strategic Environmental Assessment and ecosystem approach to marine spatial planning.

Perceived multiple stressor effects depend on sample size and stressor gradient length.

Multiple stressors are continuously deteriorating surface waters worldwide, posing many challenges for their conservation and restoration. Combined effect types of multiple stressors range from single-stressor dominance to complex interactions. Identifying prevalent combined effect types is critical for environmental management, as it helps to prioritise key stressors for mitigation. However, it remains unclear whether observed single and combined stressor effects reflect true ecological processes unbiased by sample size and length of stressor gradients. Therefore, we examined the role of sample size and stressor gradient lengths in 158 paired-stressor response cases with over 120,000 samples from rivers, lakes, transitional and marine ecosystems around the world. For each case, we split the overall stressor gradient into two partial gradients (lower and upper) and investigated associated changes in single and combined stressor effects. Sample size influenced the identified combined effect types, and stressor interactions were less likely for cases with fewer samples. After splitting gradients, 40 % of cases showed a change in combined effect type, 30 % no change, and 31 % showed a loss in stressor effects. These findings suggest that identified combined effect types may often be statistical artefacts rather than representing ecological processes. In 58 % of cases, we observed changes in stressor effect directions after the gradient split, suggesting unimodal stressor effects. In general, such non-linear responses were more pronounced for organisms at higher trophic levels. We conclude that observed multiple stressor effects are not solely determined by ecological processes, but also strongly depend on sampling design. Observed effects are likely to change when sample size and/or gradient length are modified. Our study highlights the need for improved monitoring programmes with sufficient sample size and stressor gradient coverage. Our findings emphasize the importance of adaptive management, as stress reduction measures or further ecosystem degradation may change multiple stressor-effect relationships, which will then require associated changes in management strategies.