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.

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.

Intertidal macroalgae define complex habitats and play a key role in structuring coastal areas. While, they are primarily studied during field campaigns, remote sensing acquisitions are becoming increasingly prevalent. However, the use of hyperspectral imagery on drones is not developed even though it allows species identification even in heterogeneous environments such as intertidal rocky shores. Based on hyperspectral drone imagery acquired in summer 2021, this study aims to identify and validate an algorithm suitable for easy integration into an operational framework for monitoring macroalgal dominated shore. The study focuses on two sites along the Brittany coast (Western France). Species identification and abundance were determined in the field. Six algorithms were tested: Mahalanobis, Minimum Distance, Maximum Likelihood, Random Forest, Spectral Angle Mapper and Support Vector Machine. Classifications showed overall accuracies ranging from 70% to 90% depending on the algorithm. The Maximum Likelihood is retained as it provides good accuracies and valuable information about the species distributions. Our analyses based on a combination of field and remote sensing data reveals globally consistent results when considering the main Phaeophyceae species but a divergence was highlighted for Rhodophyta. Despite environmental differences, the two studied sites were faithfully characterized in terms of intertidal species and habitat distribution, highlighting the potential of hyperspectral drone imagery to better understand seaweed-dominated ecosystem dynamics.

Background and Aims Maerl-associated communities have received considerable attention due to their uniqueness, biodiversity and functional importance. Although the impacts of human activities are well documented for maerl-associated macrofauna, the spatio-temporal variations of macroalgae have comparatively been neglected, and the drivers that influence their dynamics are poorly known. We investigate the links between maerl-associated macroalgal communities, anthropogenic pressures and environmental conditions, and hypothesize that sites under human pressure would exhibit different dynamics when compared to reference sites.Methods To better understand community variation through space and time, four subtidal maerl beds under different pressures were consistently monitored over one year in the bay of Brest, Brittany, France. Both macroalgae community monitoring and environmental data were acquired through field sampling and available models.Key Results Higher macroalgal biomass was observed within eutrophic sites, especially in summer (more than ten times higher than in the Unimpacted site), caused by free-living forms of opportunistic red macroalgae. The Dredged site also exhibited distinct macroalgal communities during summer from the Unimpacted site. Nutrient concentrations and seasonality proved to be key factors affecting the macroalgal community composition, although dredging and its effects on granulometry also had a strong influence. Over the long term, fewer than half of the species identified during historical surveys were found, indicating major temporal changes.Conclusions Human pressures have strong impacts on maerl-associated macroalgal communities. Nutrient concentrations and dredging pressure appear as the main anthropogenic factors shaping maerl-associated macroalgal communities. Additionally, our results suggest historical changes in maerl-associated macroalgal communities over 25 years in response to changes in local human pressure management. This study suggests that maerl-associated macroalgal communities could be used as indicators of anthropogenically driven changes in this habitat.

Abstract Non-indigenous seaweed species have been introduced to the coasts of Brittany, France for decades, with an increasing arrival rate since the 1970s due to both the introduction of the Pacific oyster and increased maritime traffic. In this study, seven species of red macroalgae originating from the Pacific Ocean were found in new locations around the coasts of Brittany between 2018 and 2022. The seaweed species belong to four different orders: Ceramiales (Antithamnion hubbsii, Polysiphonia morrowii, and Symphyocladiella dendroidea), Halymeniales (including Pachymeniopsis lanceolata and Polyopes lancifolius), Rhodymeniales (Botryocladia wrightii), and Gigartinales (Solieria sp.). The dispersal mode and putative invasive potential of each species were examined, and the species were described in terms of their macroscopic and microscopic appearance and habitat. These finds result from a combination of citizen science and long-term monitoring. Used together, these two approaches can aid in the detection of these species on other European coasts and in understanding their dispersion.

Biological invasions are one of the main global threats to biodiversity in terrestrial, freshwater and marine ecosystems worldwide, requiring effective inventorying and monitoring programs. Here, we present an updated list of non-indigenous species in French marine and transitional waters. Focused on eukaryote pluricellular species found throughout the three metropolitan French marine regions (Western Mediterranean Sea, Bay of Biscay and the Northern Seas), a total of 342 non-indigenous, including 42 cryptogenic, species are listed as having been introduced since the 13th century. The majority of the species originated from the temperate Northern Pacific. They mainly arrived through both ballast and hull fouling and also are associated with shellfish farming activities. Most of them have been introduced since the 1970s, a time when maritime and aquaculture trade intensified. Despite important human-aided opportunities for species transfer between the three marine regions (for instance, via recreational boating or aquaculture transfers), only a third of these NIS are common to all regions, as expected due to their environmental specificities.

Maerl beds are unique marine habitats hosting a great diversity of organisms while macroalgae are a major component of this diversity. The bay of Brest is one of the most studied coastal ecosystems in the world; in addition, it has a significant background concerning historical seaweeds check-lists associated to maerl beds. However, no recent work aimed at compiling and completing these data. In this study, a total of 7 subtidal and 3 intertidal maerl beds have been surveyed between 2020 and 2022. These data complete 4 previous inventories, giving the most accurate description of seaweeds colonizing maerl beds in the bay of Brest. The total number of macroalgal species reaches 170 among which 127 are Rhodophyta, 22 Phaeophyceae, and 21 Ulvophyceae, with 51 additions to the French and 19 additions to the European maerl beds check-list. A comparison with other maerl beds of the northeastern Atlantic coasts is compiled. It appears that the maerl beds from the bay of Brest are the most diverse in Atlantic France and among the most diverse in Europe. A description of the stratification of maerl beds and the associated macroalgal communities is proposed. It includes maerl species, encrusting species, maerl-entangling species, erect isolated species, and free-living species. In addition, specimens of the rarely recorded Rytiphlaea tinctoria, reaching its northernmost population in the bay, a possible introduced species/relict population, are described.

Physical constraints, and particularly hydrodynamics, are major drivers of macroalgal canopy-dominated rocky shore communities. The abundance of habitat-forming seaweeds decreases with increasing wave exposure, triggering cascading effects on their associated communities. This study aims at describing the structure of benthic communities along a hydrodynamic gradient and understanding the role of wave height in structuring the ecological transition from macroflora to macrofauna dominance. In situ wave height was measured together with biological communities (macroalgae, macrograzers and sessile fauna) along an exposure gradient on 3 rocky shores of western Brittany. Results showed that Fucales cover and the abundance of several understory organisms are negatively correlated to in situ wave height, except for barnacles and limpets which were positively correlated. Results revealed the occurrence of a wave height threshold at approximately 80 cm from high to mid-shore levels. Beyond this threshold, we observed a large dominance of barnacles, while below this threshold, Fucales and the associated organisms abounded. A similar threshold was observed on low shore habitats, for barnacle cover only. The results of this study challenge the commonly accepted paradigm of a gradual shift from seaweed-dominated to sessile fauna-dominated zones with increasing wave exposure on rocky shores. Finally, this study sheds new light onto interspecific interactions which drive the structure of rocky shore-associated communities.

Intertidal macroalgal habitats are major components of temperate coastal ecosystems. Their distribution was studied using field sampling and hyperspectral remote mapping on a rocky shore of Porspoder (western Brittany, France). Covers of both dominating macroalgae and the sessile fauna were characterized in situ at low tide in 24 sampling spots, according to four bathymetric levels. A zone of ca. 17,000 m(2) was characterized using a drone equipped with a hyperspectral camera. Macroalgae were identified by image processing using two classification methods to assess the representativeness of spectral classes. Finally, a comparison of the remote imaging data to the field sampling data was conducted. Seven seaweed classes were distinguished by hyperspectral pictures, including five different species of Fucales. The maximum likelihood (MLC) and spectral angle mapper (SAM) were both trained using image-derived spectra. MLC was more accurate to classify the main dominating species (Overall Accuracy (OA) 95.1%) than SAM (OA 87.9%) at a site scale. However, at sampling points scale, the results depend on the bathymetric level. This study evidenced the efficiency and accuracy of hyperspectral remote sensing to evaluate the distribution of dominating intertidal seaweed species and the potential for a combined field/remote approach to assess the ecological state of macroalgal communities.

The present study gives an updated list of macroalgal species from the Channel and Atlantic coasts of France. It includes a total of 707 macroalgal species with their taxonomic treatment

Wave height was used as a proxy to assess the effect of hydrodynamics on the development and structure of intertidal North-East Atlantic rocky macroalgal communities (Brittany). The characterization of hydrodynamics at small-scale (about 10 m) was performed through wave height in situ monitoring using pressure sensors. Both the diversity and the cover of the macroalgal communities were sampled in parallel. Wave heights exhibit large variations with values ranging from ca. 5 cm to 1.60 m. We show that wave height directly controls the cover of macroalgal canopies (Pearson's r between -0.62 and -0.39). In all communities, most of the fucoid covers regressed with increasing wave height values. By contrast, positive correlations were found at low shore levels between wave heights and the cover of the kelp Laminaria digitata and also of several species of Rhodophyta. Redundancy analysis points out the significant effect of wave height on the inner variability of macroalgal communities, explaining up to 19% of their upright structure and more than 15% of the distribution of canopy-forming groups. In assemblages dominated by either Pelvetia canaliculata or Fucus serrates, a significant negative correlation was also evidenced with the index of community structure (Ics), which gives an appraisal of both the structure and the development of macroalgal communities. In conclusion, this study provides a novel experimental approach helping to quantify the effects of hydrodynamics on the structure of macroalgal communities using a small-scale in situ quantification of wave heights, revealing hydrodynamics as the main environmental driver of inner structural variations in seaweed assemblages.

Sheltered intertidal environments are characterized by highly diversified macroalgal communities which vary over short spatial scales depending on various environmental pressures. In this study, relations between wave heights and vertical elevation were analyzed in order to identify the respective roles of these factors on the structure of macroalgal communities. We measured both wave heights and elevation on six macroalgal communities of three Northeast Atlantic rocky shores. The effects of these structuring factors on seaweed communities were seasonally evaluated in early summer and late autumn using pressure recorders and digital terrain model data. Results reveal that waves were higher during late autumn than in early summer. At shore-scale, elevation was the main factor determining both the structure and the species composition of the macroalgal communities (13.7% of total variance explained), while hydrodynamics played a lower, although significant role (< 1% of the total variance). Based on the cover of canopy-forming species, intertidal macroalgal communities can be grouped into three clusters, corresponding to three height levels on the shore (higher, middle, and lower shore). Canopy composition and structure were mainly explained by the effect of elevation (45%) in the higher level of the shore and by both elevation (42%) and wave height (2.5%) in the middle shore. On the contrary, low shore communities were affected by a more balanced effect of elevation (explaining 14% of the total variance and wave height associated with site effect explaining 15% of such effect). This study gave new insights of how hydrodynamics and elevation condition the extension of macroalgal communities.