This study reports for the first time the occurrence of a field of large bathymetric ridges documented along the Moroccan Western Rifan shelf margin in the southern Gulf of Cadiz, southwest of Spartel Headland. These ridges extend along the outer shelf and the shelf break in a highly dynamic hydrological context. The dataset consists of swath bathymetric data, Sparker seismic profiles, and sediment samples. This study is primarily exploratory in nature and is not constrained by direct ground-truth data. Nevertheless, the characterization of these previously undocumented morphologies provides important insights into the evolution of the shelf margin and justify discussing their possible origin. Results highlight N-S trending, several kilometres long bathymetric ridges with heights reaching more than 20 m. Their internal architecture reveals four generations of buried preserved ridges. Several potential genetic mechanisms are discussed considering the overarching influence of Quaternary sea-level changes, shelf-margin physiography, and (paleo)oceanographic conditions. The ridge morphometrics, and the spatial relationship of the most recent ridges with current patterns suggest that they can be interpreted as submarine bedforms. They may correspond either to active submarine dunes formed by a branch of the North Atlantic Central Water or to sand ridges inherited from lower sea-level stages. Conversely, the long-term preservation of buried ridges is also consistent with an evolutionary scenario dominated by growth and demise of cold-water corals. While the available data do not allow us to conclusively determine the nature of these features, we suggest that their development may be explained by alternative hydro-sedimentary and biogenic processes.

Despite the rapid shifts in species distributions observed on intertidal rocky shores worldwide, which reportedly occur faster than in other ecosystems, the patterns and drivers of these changes remain poorly understood in many regions, notably due to a lack of data. In this context, citizen science (CS) initiatives are valuable tools for addressing data gaps, providing broad spatial, temporal and taxonomic coverage. However, CS data require quality validation. In this study, we used CS data to investigate the biogeographic patterns of rocky shore communities in Brittany, the westernmost region of France (a biogeographic transition zone) while addressing the quality of CS data by quantifying the impact of expertise levels on taxonomic detection and identification, and thus on community distributions observed. Using clustering, we highlight 7 community types matching reported biogeographic patterns in the region. In particular, we identified a biogeographic barrier at the Ushant Front, with an associated ecotone community showing the highest species richness. We then explored the relative influence of environmental variables, spatial predictors and expertise-related descriptors on recorded community composition using variance partitioning. Our results confirm an expertise-related bias in the CS data. However, its influence on community composition was limited, and largely independent of spatial and environmental factors, which emerged as the primary drivers. Both large-scale spatial gradients (latitudinal and longitudinal) and local environmental conditions (bays and estuaries) were found to shape community structure. These findings suggest that CS data can provide valuable insights into the biogeography of rocky shores when variability in volunteer expertise and sampling effort is accounted for.

Benthic ecosystems worldwide are increasingly exposed to multiple anthropogenic pressures. Ecological indices provide valuable tools for assessing the impacts of these activities and guiding mitigation strategies. While numerous indices have been developed, many show limitations in their applicability depending on habitat nature and pressure type. Here, we introduce the MUltivariate MArine Recovery INdEX (MUMARINEX), a new, generalist multi-metric index designed to assess benthic ecological quality. MUMARINEX is built on the assumption that ecological status can be described through the combined response of three components: preservation of species/taxonomic richness, preservation of community structure (no increase or decrease in abundances), and absence of change in opportunistic dominance. These three components are compared against a supposed unimpacted reference state. To evaluate its performance, MUMARINEX was compared with the widely used M-AMBI and two more recent and methodologically distinct indices, GPBI and EQA, using datasets from six case studies addressing distinct anthropogenic pressures: marine aggregate and carbonate sand extractions, eutrophication, experimental trawling, green tides and maerl bed exploitation. Across all case studies, MUMARINEX proved to be a simple, user-friendly (computable through an R package or a web interface), versatile, and sensitive tool capable of detecting impacts while providing insights into the ecological mechanisms underlying community changes. Its continuous scoring and explicit decomposition into complementary sub-indices make MUMARINEX a promising addition to the current toolbox for benthic monitoring. While no single index should be used alone, combining MUMARINEX with other dedicated indices offers a more robust framework for future environmental monitoring and impact assessment.

Prehistoric shell middens have been studied since the 19th century in archaeology along the European Atlantic fa & ccedil;ade. These sites correspond to refuses of daily activities of human populations living near the sea. Amongst these, food remains are numerous and correspond to the exploitation of the marine environments, such as seashells. In order to understand the place of crustaceans in the daily life of past populations, this article reviews crab remains found in prehistoric shell middens in Atlantic Europe, from the Palaeolithic to the Neolithic. Data from the Mesolithic and Neolithic periods come from the online European Atlantic Prehistoric Shell-middens database (EAPSM). In addition, a synthesis of previous publications on crab remains from Palaeolithic sites in Atlantic Europe is provided. The main objective is to assess the presence and role of crab in the diet of prehistoric populations from the Palaeolithic to the Neolithic. These topics will be addressed by examining the impact of the excavation methods used to detect them and also the preservation of coastal sites linked to variations of the sea level over time. Analysis of published data shows a very uneven representation of sites across periods and spaces, with for example a high concentration in the Mesolithic and in southern regions (Iberian Peninsula). This first result can be explained by a lower level of the sea during the Palaeolithic. Except for the high cliffs of Spain or Portugal, Palaeolithic shell middens are not preserved if they ever existed. For all periods, crab remains are generally underrepresented. This result seems to be largely due both to their fragmentary state and the lack of systematic identification protocols. These gaps limit our vision of how human populations exploited crab. The article recommends a reassessment of archaeological material when sieved sediment existed and the implementation of standardized methods on new ones for the study of crabs in coastal subsistence economies during prehistory.

Despite the relatively small proportion of ocean surface they represent, continental shelf ecosystems are among the most productive in the world. Located at the interface between terrestrial and marine environments, these habitats are structured by strong environmental forcings, especially on the sea bottom. A clear understanding of the spatial distribution of these habitats, along with knowledge on the composition and functioning of their associated communities, is essential for fisheries management and ecosystem conservation. Here, we used data from yearly EVHOE otter trawl surveys (2008-2020) to characterize the spatial structuration of benthic communities of the entire continental shelf of the Bay of Biscay (France), and to investigate the potential environmental drivers of these patterns. Two separate biological components were studied: epibenthic megainvertebrates and bentho-demersal fish. Clustering analyses identified seven assemblages from species composition of stations. Each assemblage was denominated according to its geographical location. We detected a strong correlation between the spatial organization of the different assemblages identified for fish and megainvertebrates, providing evidence for broad-scale spatial structuration of benthic habitats-benthiscapes-in this shelf ecosystem. The most influential environmental variables were identified as bottom temperature, sediment type, and primary production. Patterns in certain structural parameters, such as biomass, revealed possible spatial differences in ecological functioning. For example, we observed a drop in biomass from the coast to the central part of the shelf, followed by an increase in biomass near the edge of the Armorican shelf. These patterns reflect major large-scale processes (river inputs versus shelf-break upwelling) structuring the entire Bay of Biscay ecosystem. A comparative analysis revealed that the biological features and functioning observed in this study are shared with other European continental shelves. In addition to improving our knowledge of benthic environments, studies such as this one can promote improvements in ecosystem-based management and marine spatial planning of a fast-changing ecosystem under multiple anthropogenic stresses.

Habitat complexity (HC) promotes species richness and abundance. Aquatic environments are faced with intense pressures that threaten the 3D structure of the seafloor, with cascading effects on ecosystem functioning and biodiversity. Maerl or rhodolith beds are marine biogenic habitats created by few species of free-living non-geniculate coralline algae that aggregate and form complex structures. Although their high biodiversity has been attributed to the HC provided by coralline algal nodules, the mechanisms through which HC modification affect associated communities remains uncertain in face of numerous confounding factors. Hence, we tested how changes in the extent and nature of maerl complexity drive changes in biodiversity. Using long-term monitoring data from ten maerl beds in Brittany (France) over 12 years, we investigated the links between structural complexity, environmental conditions and benthic macrofaunal communities. HC was quantified at the coralline algal nodule and bed level, through morphometrics and density, and its effects on local diversity and on communities spatial and temporal variability were evaluated. HC promoted species richness and density of most taxa regardless of other environmental factors. These relationships were linear and no limiting threshold of complexity was found at a regional scale. HC played a more important role in driving regional diversity patterns than other measured environmental constraints individually, and beds with relatively lower HC were the most distinct in terms of community composition and structure. Species replacement was the main component of temporal variability and HC promoted community stability. While overall facilitative, the effects of HC might be taxa and trait-dependent, justifying comprehensive trait-based approaches. Our results reiterate the need to protect complex biogenic habitats.

Located at the interface between the continent and the open ocean, continental shelves exhibit strong gradients in nutrients, organic matter, and primary production. Within these ecosystems, benthic invertebrates play a key role in transferring energy and matter from primary producers to bentho-demersal fish. Consequently, macrobenthic biomass can serve as an indicator of food resource availability for these species. This study focused on the northern part of the Bay of Biscay continental shelf to examine this relationship. Given the limited availability of field data, both empirical and biogeochemical models were employed to estimate macrobenthic biomass indices and to generate spatial distribution maps. Modeled biomass indices showed strong correlations with the few available field observations, although discrepancies were evident in deeper, poorly sampled areas. A pronounced coastal-offshore gradient was observed, with macrobenthic biomass decreasing towards offshore waters and locally elevated values occurring in deeper muddy sediments. Among the eight bentho-demersal fish species examined, the four identified as predominantly benthic-feeders exhibited positive relationships between body condition and macrobenthic biomass indices, particularly among smaller individuals. These findings suggest that modeled macrobenthic biomass indices and maps can serve as effective proxies for benthic resources gradients and provide insights into spatial variability in energy transfer within the food web. Nonetheless, improved field sampling, especially in deep circalittoral zones, remains essential to refine spatial trends, enhance our understanding of benthic food-web processes, and support the sustainable management of bentho-demersal fisheries.

Linguistic uncertainty is a prime source of uncertainty pervading ecology and conservation. Coralline algae are a widespread and diverse group of calcifying red macroalgae that underpin coastal ecosystem function and service provision. Recent increasing interest in coralline algae in the scientific literature has revealed a diverse but confusing terminology at organism to habitat scales. Coralline algal research and conservation are international and multidisciplinary, so there are geographic and disciplinary imbalances in research and conservation efforts. To reach consensus and reduce uncertainty, we propose a unified terminology. We review trends in cultural and scientific use of coralline algal terms and propose a system based on six morphologies: (1) attached, (2) free‐living geniculate, (3) encrusting and free‐living nongeniculate coralline algae, the latter either being (4) nucleated or (5) non‐nucleated thalli or (6) fragments. We take inspiration from other coastal systems that have achieved consensus through umbrella terms, such as ‘coral’ and ‘kelp’, to accelerate global progress in coralline algal research and conservation. We characterise 14 coralline algae–dominated habitat global types, falling within seven functional groups, four biomes and four realms: (1) freshwater coralline streams; (2) coralline tide pools; (3) intertidal coralline rims and (4) turf; (5) coralline sea caves; (6) coral–algal reefs; (7) algal ridges; (8) coralligenous reefs; subtidal (9) carbonate crusts, (10) coralline barrens and (11) turf; and (12) articulith, (13) maerl and (14) rhodolith beds, which fall into the coralline algal bed functional group. We hope this unified terminology promotes data comparison, enables cross‐boundary and cross‐sector sharing of best practices, develops capacity for meta‐analyses and improves conservation strategies.

Most ecoregions lack data on maerl distribution and ecological status, so this needs fundamental research for conservation. Brittany, NW France, is an exception and has extensive research on maerl species, associated biodiversity, human-induced impacts and protection efforts. Breton maerl habitats host exceptionally high species richness and functional diversity, surpassing all other coastal habitats in the region. The meiofauna and microflora of maerl beds and the role of these habitats as carbon stores remain poorly known. Bans on direct exploitation in Europe have led to environmental improvements, although maerl extraction has begun in other regions of the world. In Europe, serious maerl conservation problems persist, particularly due to scallop and clam dredging, eutrophication and mariculture impacts. Not enough has been done to curb these issues, which are proven to severely degrade maerl, its biodiversity and ecosystem functions. Conservation measures for maerl beds should be strengthened and codesigned with local stakeholders as these habitats take millennia to form and are inadequately protected by current strategies.

Rhodolith beds are diverse and globally distributed habitats. Nonetheless, the role of rhodoliths in structuring the associated species community through a hierarchy of positive interactions is yet to be recognised. In this review, we provide evidence that rhodoliths can function as foundation species of multi-level facilitation cascades and, hence, are fundamental for the persistence of hierarchically structured communities within coastal oceans. Rhodoliths generate facilitation cascades by buffering physical stress, reducing consumer pressure and enhancing resource availability. Due to large variations in their shape, size and density, a single rhodolith bed can support multiple taxonomically distant and architecturally distinct habitat-forming species, such as primary producers, sponges or bivalves, thus encompassing a broad range of functional traits and providing a wealth of secondary microhabitat and food resources. In addition, rhodoliths are often mobile, and thus can redistribute associated species, potentially expanding the distribution of species with short-distance dispersal abilities. Key knowledge gaps we have identified include: the experimental assessment of the role of rhodoliths as basal facilitators; the length and temporal stability of facilitation cascades; variations in species interactions within cascades across environmental gradients; and the role of rhodolith beds as climate refugia. Addressing these research priorities will allow the development of evidence-based policy decisions and elevate rhodolith beds within marine conservation strategies.

Abstract Foundation species such as seagrasses modulate critical ecosystem processes, promote biodiversity, and structure community spatial and temporal dynamics. Hence, they play a key role in mediating the response of biodiversity to environmental changes. The breadth of their contribution to biodiversity maintenance and the potential cascading effects of their alteration remain unclear as we lack a comprehensive understanding of the intricate mechanisms governing their response to environmental changes and that of their associated fauna. Indeed, the mechanisms involved are often studied in isolation or at scales that only provide incomplete representations of the complex functioning of these ecosystems. This study aimed to clarify the direct and indirect relationships linking the environment, seagrass, and their associated faunal assemblages, using structural equation modeling (SEM). To this end, we review the literature to derive theoretical models of the functioning of seagrass ecosystems and test them using long‐term monitoring data covering 14 years of nine different Zostera marina seagrass beds across 500 km of coastline. We show that contradictory relationships and ambiguities regarding seagrass–biodiversity relationships emerge from the currently available literature (covering experimental, observational, and modeling studies). The SEM approach allowed us to clarify these direct and indirect relationships and resolve most ambiguities. In particular, we show that seagrass mediates the effect of the environment on its associated communities. However, this mediating effect is different, both qualitatively and quantitatively, for epifauna and infauna. Unexpectedly, the diversity of benthic macrofauna appeared to be controlled and promoted by the biomass rather than by the shoot density of the seagrass beds. We also provide quantitative estimates for the direct and cascading pathways linking seagrass biodiversity to environmental changes. Overall, by synthesizing, clarifying, and quantifying the multiple relationships linking a foundation species such as seagrass to its environment and associated biodiversity, we contribute to a better understanding of seagrass meadows functioning and help predict the potential consequences of foundation species alteration on their associated fauna.

Marine poikilotherm organisms have evolved several physiological strategies to acclimate to different thermal conditions occurring in coastal environments. This study aimed to describe the acclimation capacity of sand dollars Echinarachnius parma found in the highly seasonal sub-Arctic archipelago of Saint Pierre and Miquelon. Fatty acid composition was quantified in cell membranes and energy reserve of sand dollars collected at specific stations according to depth and seasons. We hypothesized that perceived temperatures, seasonal thermal conditions and the trophic environment may considerably influence E. parma fatty acid profiles. In the polar fraction, E. parma showed high relative contributions of eicosapentaenoic acid (20:5 omega 3; 28.5%) followed by 16:0 (10.83%), 14:0 (6.67%) and 18:4 omega 3 (3.49%). Whereas variations in 20:5 omega 3 relative contributions seemed to be mainly related to spawning events, docosahexaenoic acid (22:6 omega 3; 2.48%) and arachidonic acid (20:4 omega 6; 2.49%) varied according to the thermal gradient of the water column. Furthermore, the seasonal thermal internal wave acting throughout the archipelago appeared to lack sufficient intensity to trigger modifications in the polar concentration in individuals. In the neutral fraction, the strong dominance of diatom markers (20:5 omega 3; 24.2% and 16:1 omega 7; 21.9%) suggests that the diet of sand dollars mostly reflects the composition of the organic matter available over the sediment. Our study (1) highlights specific fatty acid functions in the metabolic regulation of echinoids, (2) defines the feeding ecology of sand dollars and (3) describes the effects of different thermal conditions on the physiology of locally adapted poikilotherm animals.

Although knowledge of Arctic benthic biodiversity has increased considerably in recent decades, some regions, such as Northeast Greenland, remain poorly studied. The aim of this study was to complement a previous macrofaunal inventory carried out in Young Sound, a High-Arctic fjord in this region (74°N). We sampled shallow benthic assemblages along a small inner/outer fjord gradient, including one station previously prospected two decades ago and three new stations. This sampling strategy revealed highly diversified benthic assemblages (166 species identified on a total sampling area of 1.32 m 2 ), which considerably increases the number of species recorded for the fjord (i.e. 225 species vs 100 previously recorded). The outermost station was dominated in abundance by various assemblages of bivalves, while the middle stations showed greater species evenness, including numerous species of polychaetes, bivalves and crustaceans. The innermost station was dominated by ostracods, gammarid amphipods and tube-dwelling polychaetes. Overall, benthic assemblages varied little between the four stations and the transect as a whole exhibited characteristics typical of outer fjord habitats, reflecting the rather moderate impact of meltwater inputs in this part of the fjord. Finally, trophic plasticity and omnivory were observed in most of the recorded macrobenthic species, highlighting the adaptability of these species to low trophic availability in the ecosystem. Future biodiversity studies will need to explore the innermost and deepest areas to provide a more comprehensive inventory and understanding of the influence of environmental conditions on the structure and functioning of Young Sound benthic habitats.

Although knowledge of sandy beaches has increased recently, the benthic diversity of macrotidal sandy beaches in the northeastern Atlantic Ocean (more specifically, the western Channel and the northern Bay of Biscay) is poorly known. Here, we present a regional-scale account of the species diversity, both observed and estimated, of the macrobenthic fauna of Brittany based on an analysis of a species-level data set from 18 sandy beaches in Brittany that were sampled annually for 13 yr on the lower shore. In total, 526 species were identified, including 210 Annelida, 167 Arthropoda, 103 Mollusca, 19 Echinodermata and 27 species of other phyla. Four distinct habitats were distinguished based on their benthic communities and characterised using environmental variables. Sediment heterogeneity appeared to enhance diversity, as heterogeneous muddy sand harboured significantly more species than (muddy) fine sand. The role of environmental variables as structuring factors of benthic communities was investigated using redundancy analysis and variance partitioning. Beach morphodynamics and sediment structure explained most diversity variations (25.40 and 24.91%, respectively) followed by wave characteristics (13.46%). Finally, we offer some habitat-specific reference values regarding species richness and the Shannon index for M-AMBI computation during Water Framework Directive and Marine Strategy Directive evaluations for a more reliable characterisation of the ecological status of sandy beaches.

As introduced species constitute a major threat to biodiversity, it is crucial to properly monitor their spread to new regions. The present study reports new records of four species: 1) the amphipod Ampithoe valida S.I. Smith, 1873 and 2) the polychaete Polydora colonia Moore, 1907, both new records for Brittany (North-West part of France); 3) the polychaete Bispira polyomma Giangrande & Faasse, 2012, a first finding in the North-East Atlantic; and 4) the flatworm Prostheceraeus moseleyi Lang, 1884 with a confirmed northernmost record in North-East Atlantic. These species were recorded within two semi-enclosed bays: the Morbihan Gulf on the southern coast of Brittany and the Bay of Brest in the North-West part of Brittany. Both localities are already known to host numerous non-indigenous species as well as benthic macrofaunal distributional range limits. Morphological variations and inconsistencies are discussed and reported for A. valida, revealing missing characters and errors in the literature. This work adds three non-indigenous species to Brittany and two nonindigenous species to French waters, for which we discuss potential introduction vectors.

Coastal zones are biodiversity hotspots and deliver essential ecosystem functions and services, yet they are exposed to multiple and interacting anthropogenic and environmental constraints. The individual and cumulative effects of these constraints on benthic communities, a key component of coastal ecosystems, and their variability across space and time, remains to be thoroughly quantified to guide conservation actions. Here, we explored how the presence of biogenic habitats influences the response of benthic communities to natural and anthropogenic constraints. We investigated this effect in both intertidal and subtidal habitats exposed to different pressures. We used data collected in the North-East Atlantic over 15 years (2005-2019) as part of the REBENT monitoring program, covering 38 sites of bare sediments, intertidal seagrass beds and maerl beds. We collected a range of environmental variables and proxies of anthropogenic pressures and used variation and hierarchical partitioning with redundancy analyses to estimate their relative effect on macrobenthic communities. We used descriptors modeling spatial and temporal structures (dbMEMs) to explore the scale of their effects and potential missing predictors. The selected variables explained between 53 % and 64 % of macrobenthic beta diversity depending on habitat and depth. Fishing pressures, sedimentary and hydrodynamics variables stood out as the most important predictors across all habitats while proxies of anthropogenic pressures were overall more important in intertidal habitats. In the intertidal, presence of biogenic habitat strongly modulated the amount of explained variance and the identity of the selected variable. Across both tidal levels, analysis of models' residuals further indicated that biogenic habitats might mitigate the effect of extreme environmental events. Our study provides a hierarchy of the most important drivers of benthic communities across different habitats and tidal levels, emphasizing the prominence of anthropogenic pressures on intertidal communities and the role of biogenic habitats in mitigating environmental changes.

Arctic and sub-Arctic marine ecosystems are experiencing some of the highest sea surface warming in the world, which has intensified water column stratification and subsequently reduced phytoplankton production and particulate organic matter quality. However, the effects of these changes on benthic food webs and the transfer of organic matter to higher trophic levels are still poorly understood. This study examines the spatial and temporal variability of food web structure in a sub-Arctic benthic community exposed to contrasting thermal stratification conditions. The study hypothesizes that during stratified periods, oceanographic conditions would have a limited effect on benthic invertebrates located above/at the thermocline due to their direct access to surface/subsurface primary production. On the other hand, organisms below the thermocline may be more sensitive to increased stratification because they do not have direct access to these food resources. To test this hypothesis, we sampled benthic invertebrates and several fish species on the Newfoundland Shelf along a cross-shore transect (2 shallow stations versus 2 deep stations above and below the thermocline, respectively) over two seasons. We used isotopic analyses (delta 13C and delta 15N) to study the structure of the food web and the transfer of organic matter. No temporal variation and little spatial variability in food web structure was observed, resulting in a 73.2% overlap between isotopic niches of shallow and deep stations. At all stations, most primary consumers were characterized by high trophic plasticity, feeding on both phytoplankton and benthic organic matter (mean dependence on benthic sources = 46.7%). In the context of global warming and increased thermal stratification, we hypothesize that benthic primary production may be less vulnerable to nutrient depletion than phytoplankton. We suggest that an increased contribution of benthic primary producers to organic matter fluxes in shallow coastal food webs could significantly enhance the resilience of the benthic food web to stratification intensification.

Macroalgal communities are essential to coastal ecosystems, yet increasing effects of global change and anthropogenic pressures are leading to their global decline. Investigating the long-term dynamics of these communities across different localities appears crucial to better understand their responses to such pressures, as our knowledge of spatial heterogeneities in macroalgal trajectories remains elusive. To fill this gap, the community trajectory analysis framework provides a set of innovative multivariate metrics to characterize and quantitatively compare the temporal dynamics of different communities. Using long-term monitoring data (2004-2022), this method was applied to intertidal macroalgal communities across 10 locations distributed over more than 500 km of coastline in Brittany, France. Three distinct temporal dynamics were identified. High-shore communities exhibited minimal changes over time, while low-shore communities were characterized by a fluctuating understorey species composition but a general stability pattern. In contrast, the mid-shore community dominated by Ascophyllum nodosum underwent conspicuous changes in composition and structure. Further analysis of the latter community unveiled clear spatial patterns, with a significant deterioration of the structural state attributed to canopy loss in eastern Brittany, negatively impacting understorey species. This decline may ultimately lead to massive changes in coastal ecosystem functioning and services. This study emphasizes the importance of maintaining long-term ecological monitoring as well as the pertinence of temporal trajectories methods to identify and understand community changes at various spatial scales.

Effluents resulting from fish farming activities can alter the structure and functioning of benthic ecosystems. In Norway, recent technological advances within the salmon industry have facilitated the expansion of aquaculture into shallower locations, leading to additional pressures upon coastal habitats. Maerl/rhodolith beds are important bioengineers in coastal waters-supporting highly diverse associated macrofaunal communities-and are likely to be particularly at risk. This study aims to investigate the environmental changes induced by fish farms in their vicinity and their impact on the structure and functioning of maerl bed macrofaunal communities. High dissolved nutrients and organic enrichment levels were observed close to the cages, associated with lower live maerl cover. Overall, the most substantial impact of salmon farms on macrofaunal communities was observed within 100 m from the cages. Nevertheless, changes in the structure and functioning of assemblages were evidenced up to 300 m. In the vicinity of the cages, communities were characterized by the dominance of endobenthic subsurface deposit feeders and opportunistic species, notably the polychaetes Chaetozone sp., Capitella sp. and Scoloplos armiger. Conversely, sites situated farther away from the cages exhibited higher abundances of epibenthic taxa, including crawlers, grazers and species sensitive to organic enrichment. Sites unaffected by the presence of fish farms were described by high abundances of the ophiuroid Ophiura robusta, the polychaete Proclea graffi and ostracods, reflecting more favourable environmental conditions. Such changes in macrofaunal communities highlight the vulnerability of maerl beds to the impacts of aquaculture, thus emphasizing the importance of rigorous site selection when establishing new fish farming facilities.

The abundances of some macronutrients, and trace elements (K, Al, P, V, Mn, Co, Ni, Cu, Zn, Rb, Sr, Mo, Cs, Ba, Pb, Th, U, Y and REE) were determined in a series of coralline algae (Lithothamnion corallioides) samples (n = 101) collected alive in the Bay of Brest and the Iroise Sea (Western Brittany, NW France), in order to assess the potential of these algae as archives of seawater chemistry and potential metal pollution. REE and Y (REY) patterns are similar in shape to those of local seawater, exhibiting similar La, Ce and Y anomalies, but with abundances ranging between 4 and 5 orders of magnitude higher than seawater values. Variations in La anomalies (La/La* = 1.29-2.08), Y anomalies (Y/Ho = 38.6-55.8), and heavy rare earth enrichments (Prsn/Ersn = 0.22-0.52) are consistent with mixing of seawater with rivers flowing into the Bay of Brest. The behavior of other trace elements, such as Al and Cs, also reflects this mixing. Other parameters and processes can control the abundances of the other elements measured. For example, the presence of organic matter in studied samples controls the abundances of K and Rb. The abundances of base metals (e.g., Co, Ni, Cu) are highly sensitive to the various pollutants present in the Bay of Brest. In particular, the Pb content of coralline algae clearly reflects the pollution caused by mining of a nearby Pb deposit from the 18th to the early 20th century. Our results demonstrate the potential of coralline algae not only for tracing water masses, but also for studying metal pollution.