Des écosystèmes naturellement stressés sous menace anthropique : réponses de la faune des plages de sable macrotidales aux marées vertes

Highly dynamic systems, often considered as resilient systems, are characterised by abiotic and biotic processes under continuous and strong changes in space and time. Because of this variability, the detection of overlapping anthropogenic stress is challenging. Coastal areas harbour dynamic ecosystems in the for of open sandy beaches, which cover the vast majority of the world’s ice-free coastline. These ecosystems are currently threatened by increasing human-induced pressure, among which mass-development of opportunistic macroalgae (mainly composed of Chlorophyta, so called green tides), resulting from the eutrophication of coastal waters. The ecological impact of opportunistic macroalgal blooms (green tides, and blooms formed by other opportunistic taxa), has long been evaluated within sheltered and non-tidal ecosystems. Little is known, however, on how more dynamic ecosystems, such as open macrotidal sandy beaches, respond to such stress. This thesis assesses the effects of anthropogenic stress on the structure and the functioning of highly dynamic ecosystems using sandy beaches impacted by green tides as a study case. The thesis is based on four field studies, which analyse natural sandy sediment benthic community dynamics over several temporal (from month to multi-year) and spatial (from local to regional) scales. In this thesis, I report long-lasting responses of sandy beach benthic invertebrate communities to green tides, across thousands of kilometres and over seven years; and highlight more pronounced responses of zoobenthos living in exposed sandy beaches compared to semi-exposed sands. Within exposed sandy sediments, and across a vertical scale (from inshore to nearshore sandy habitats), I also demonstrate that the effects of the presence of algal mats on intertidal benthic invertebrate communities is more pronounced than that on subtidal benthic invertebrate assemblages, but also than on flatfish communities. Focussing on small-scale variations in the most affected faunal group (i.e. benthic invertebrates living at low shore), this thesis reveals a decrease in overall beta-diversity along a eutrophication-gradient manifested in the form of green tides, as well as the increasing importance of biological variables in explaining ecological variability of sandy beach macrobenthic assemblages along the same gradient. To illustrate the processes associated with the structural shifts observed where green tides occurred, I investigated the effects of high biomasses of opportunistic macroalgae (Ulva spp.) on the trophic structure and functioning of sandy beaches. This work reveals a progressive simplification of sandy beach food web structure and a modification of energy pathways over time, through direct and indirect effects of Ulva mats on several trophic levels. Through this thesis I demonstrate that highly dynamic systems respond differently (e.g. shift in δ13C, not in δ15N) and more subtly (e.g. no mass-mortality in benthos was found) to anthropogenic stress compared to what has been previously shown within more sheltered and non-tidal systems. Obtaining these results would not have been possible without the approach used through this work; I thus present a framework coupling field investigations with analytical approaches to describe shifts in highly variable ecosystems under human-induced stress.Les plages de sable sont des écosystèmes dynamiques couvrant 70% des côtes mondiales. Ces systèmes abritent un cortège spécifique unique et assurent des fonctions essentielles de nourricerie, de nurserie et d’épuration des eaux. Or à proximité des côtes, la population humaine est en développement constant ce qui accroît les multiples pressions d’origine anthropique sur les écosystèmes côtiers. L’apport en excès de nutriments constitue une menace majeure qui peut se traduire par le développement d’importantes biomasses de macroalgues opportunistes (eutrophisation). La fréquence et l’intensité de ces blooms, communément formés de chlorophycées à court cycle de vie (ulves) et appelés marées vertes (MV), s'amplifient sur les côtes françaises et dans le Monde menaçant le fonctionnement de systèmes prépondérants et uniques.La plupart des études visant à déterminer les effets des MV sur la structure et le fonctionnement d’écosystèmes sédimentaires ont été conduites dans des environnements abrités, micro- ou atidaux. Cette problématique est restée presqu’inexplorée dans des écosystèmes plus dynamiques (systèmes ouverts et macrotidaux) en raison des difficultés de mise en place d’échantillonnage et de détection des effets de stress d’origine anthropique, inhérentes à la variabilité de ces systèmes. Cette thèse a donc eu pour objectif principal de combler ce manque et produire des connaissances en étudiant les réponses in situ des communautés benthiques de plages de sable fin macrotidales en présence ou non de MV. Quatre études ont été menées à différentes échelles spatio-temporelles et en considérant différents compartiments biologiques pour répondre à cette question générale.Ce travail de thèse montre qu’à l’échelle de la région Bretagne (variabilité intégrée sur 2700km de côtes et 7 ans) les communautés benthiques d’écosystèmes dynamiques sont modifiées significativement et de manière conservative par la présence de MV. Ce travail démontre aussi que les marées vertes impactent différemment la faune benthique en fonction du type d’habitat (plages semi-exposées vs. exposées), de la profondeur (mediolittoral vs. infralittoral), et du compartiment biologique (macrofaune benthique vs. juvéniles de poissons plats). Ces comparaisons ont permis d’identifier la faune benthique de médiolittoral inférieur des plages exposées comme étant le système le plus affecté par les MV. L’étude des variations à fine échelle spatio-temporelle de ce dernier montre que les caractéristiques faunistiques (uni- et multi-variées) sont modifiées le long d’un gradient de couverture d’algues vertes. Par exemple, la β-diversité décroît significativement le long de ce gradient. Afin d’explorer les processus pouvant expliquer ces modifications, et déterminer si ces changements ont des répercussions sur le fonctionnement de l’écosystème « plage de sable », les effets de l’accumulation d’ulves sur le réseau trophique à différent(e)s niveaux/échelles ont été mesurés. Les résultats de cette étude montrent qu’une importante biomasse d’ulves induit un changement de la structure entière du réseau trophique et une modification importante du fonctionnement trophique des plages. Les expérimentations menées au cours de cette thèse montrent que les changements observés sont induits par des effets directs (consommation de débris d’ulves) et indirects (modifications d’autres sources de nourriture) de la présence des MV.Cette thèse propose un cadre de travail visant à mieux détecter les effets de stress anthropiques sur la structure et le fonctionnement d’écosystèmes dynamiques. Dans un contexte de changement global forçant les écosystèmes à faire face à de multiples stress, cette approche pourrait se révéler particulièrement utile pour démêler, comprendre et prédire les effets de perturbations induites par les activités humaines sur le fonctionnement des écosystèmes et constituer une aide à la gestion de ces environnements particuliers

Dynamic ecosystems under anthropogenic stress : how does macrotidal sandy beach fauna respond to green tides ?

Les plages de sable sont des écosystèmes dynamiques couvrant 70% des côtes mondiales. Ces systèmes abritent un cortège spécifique unique et assurent des fonctions essentielles de nourricerie, de nurserie et d’épuration des eaux. Or à proximité des côtes, la population humaine est en développement constant ce qui accroît les multiples pressions d’origine anthropique sur les écosystèmes côtiers. L’apport en excès de nutriments constitue une menace majeure qui peut se traduire par le développement d’importantes biomasses de macroalgues opportunistes (eutrophisation). La fréquence et l’intensité de ces blooms, communément formés de chlorophycées à court cycle de vie (ulves) et appelés marées vertes (MV), s'amplifient sur les côtes françaises et dans le Monde menaçant le fonctionnement de systèmes prépondérants et uniques.La plupart des études visant à déterminer les effets des MV sur la structure et le fonctionnement d’écosystèmes sédimentaires ont été conduites dans des environnements abrités, micro- ou atidaux. Cette problématique est restée presqu’inexplorée dans des écosystèmes plus dynamiques (systèmes ouverts et macrotidaux) en raison des difficultés de mise en place d’échantillonnage et de détection des effets de stress d’origine anthropique, inhérentes à la variabilité de ces systèmes. Cette thèse a donc eu pour objectif principal de combler ce manque et produire des connaissances en étudiant les réponses in situ des communautés benthiques de plages de sable fin macrotidales en présence ou non de MV. Quatre études ont été menées à différentes échelles spatio-temporelles et en considérant différents compartiments biologiques pour répondre à cette question générale.Ce travail de thèse montre qu’à l’échelle de la région Bretagne (variabilité intégrée sur 2700km de côtes et 7 ans) les communautés benthiques d’écosystèmes dynamiques sont modifiées significativement et de manière conservative par la présence de MV. Ce travail démontre aussi que les marées vertes impactent différemment la faune benthique en fonction du type d’habitat (plages semi-exposées vs. exposées), de la profondeur (mediolittoral vs. infralittoral), et du compartiment biologique (macrofaune benthique vs. juvéniles de poissons plats). Ces comparaisons ont permis d’identifier la faune benthique de médiolittoral inférieur des plages exposées comme étant le système le plus affecté par les MV. L’étude des variations à fine échelle spatio-temporelle de ce dernier montre que les caractéristiques faunistiques (uni- et multi-variées) sont modifiées le long d’un gradient de couverture d’algues vertes. Par exemple, la β-diversité décroît significativement le long de ce gradient. Afin d’explorer les processus pouvant expliquer ces modifications, et déterminer si ces changements ont des répercussions sur le fonctionnement de l’écosystème « plage de sable », les effets de l’accumulation d’ulves sur le réseau trophique à différent(e)s niveaux/échelles ont été mesurés. Les résultats de cette étude montrent qu’une importante biomasse d’ulves induit un changement de la structure entière du réseau trophique et une modification importante du fonctionnement trophique des plages. Les expérimentations menées au cours de cette thèse montrent que les changements observés sont induits par des effets directs (consommation de débris d’ulves) et indirects (modifications d’autres sources de nourriture) de la présence des MV.Cette thèse propose un cadre de travail visant à mieux détecter les effets de stress anthropiques sur la structure et le fonctionnement d’écosystèmes dynamiques. Dans un contexte de changement global forçant les écosystèmes à faire face à de multiples stress, cette approche pourrait se révéler particulièrement utile pour démêler, comprendre et prédire les effets de perturbations induites par les activités humaines sur le fonctionnement des écosystèmes et constituer une aide à la gestion de ces environnements particuliers.Highly dynamic systems, often considered as resilient systems, are characterised by abiotic and biotic processes under continuous and strong changes in space and time. Because of this variability, the detection of overlapping anthropogenic stress is challenging. Coastal areas harbour dynamic ecosystems in the for of open sandy beaches, which cover the vast majority of the world’s ice-free coastline. These ecosystems are currently threatened by increasing human-induced pressure, among which mass-development of opportunistic macroalgae (mainly composed of Chlorophyta, so called green tides), resulting from the eutrophication of coastal waters. The ecological impact of opportunistic macroalgal blooms (green tides, and blooms formed by other opportunistic taxa), has long been evaluated within sheltered and non-tidal ecosystems. Little is known, however, on how more dynamic ecosystems, such as open macrotidal sandy beaches, respond to such stress. This thesis assesses the effects of anthropogenic stress on the structure and the functioning of highly dynamic ecosystems using sandy beaches impacted by green tides as a study case. The thesis is based on four field studies, which analyse natural sandy sediment benthic community dynamics over several temporal (from month to multi-year) and spatial (from local to regional) scales. In this thesis, I report long-lasting responses of sandy beach benthic invertebrate communities to green tides, across thousands of kilometres and over seven years; and highlight more pronounced responses of zoobenthos living in exposed sandy beaches compared to semi-exposed sands. Within exposed sandy sediments, and across a vertical scale (from inshore to nearshore sandy habitats), I also demonstrate that the effects of the presence of algal mats on intertidal benthic invertebrate communities is more pronounced than that on subtidal benthic invertebrate assemblages, but also than on flatfish communities. Focussing on small-scale variations in the most affected faunal group (i.e. benthic invertebrates living at low shore), this thesis reveals a decrease in overall beta-diversity along a eutrophication-gradient manifested in the form of green tides, as well as the increasing importance of biological variables in explaining ecological variability of sandy beach macrobenthic assemblages along the same gradient. To illustrate the processes associated with the structural shifts observed where green tides occurred, I investigated the effects of high biomasses of opportunistic macroalgae (Ulva spp.) on the trophic structure and functioning of sandy beaches. This work reveals a progressive simplification of sandy beach food web structure and a modification of energy pathways over time, through direct and indirect effects of Ulva mats on several trophic levels. Through this thesis I demonstrate that highly dynamic systems respond differently (e.g. shift in δ13C, not in δ15N) and more subtly (e.g. no mass-mortality in benthos was found) to anthropogenic stress compared to what has been previously shown within more sheltered and non-tidal systems. Obtaining these results would not have been possible without the approach used through this work; I thus present a framework coupling field investigations with analytical approaches to describe shifts in highly variable ecosystems under human-induced stress

Dynamic ecosystems under anthropogenic stress : how does macrotidal sandy fauna respond to green tides?

Highly dynamic systems, often considered as resilient systems, are characterised by abiotic and biotic processes under continuous and strong changes in space and time. Because of this variability, the detection of overlapping anthropogenic stress is challenging. Coastal areas harbour dynamic ecosystems in the form of open sandy beaches, which cover the vast majority of the world’s ice-free coastline. These ecosystems are currently threatened by increasing human-induced pressure, among which mass-development of opportunistic macroalgae (mainly composed of Chlorophyta, so called green tides), resulting from the eutrophication of coastal waters. The ecological impact of opportunistic macroalgal blooms (green tides, and blooms formed by other opportunistic taxa), has long been evaluated within sheltered and non-tidal ecosystems. Little is known, however, on how more dynamic ecosystems, such as open macrotidal sandy beaches, respond to such stress. This thesis assesses the effects of anthropogenic stress on the structure and the functioning of highly dynamic ecosystems using sandy beaches impacted by green tides as a study case. The thesis is based on four field studies, which analyse natural sandy sediment benthic community dynamics over several temporal (from month to multi-year) and spatial (from local to regional) scales. In this thesis, I report long-lasting responses of sandy beach benthic invertebrate communities to green tides, across thousands of kilometres and over seven years; and highlight more pronounced responses of zoobenthos living in exposed sandy beaches compared to semi-exposed sands. Within exposed sandy sediments, and across a vertical scale (from inshore to nearshore sandy habitats), I also demonstrate that the effects of the presence of algal mats on intertidal benthic invertebrate communities is more pronounced than that on subtidal benthic invertebrate assemblages, but also than on flatfish communities. Focussing on small-scale variations in the most affected faunal group (i.e. benthic invertebrates living at low shore), this thesis reveals a decrease in overall beta-diversity along a eutrophication-gradient manifested in the form of green tides, as well as the increasing importance of biological variables in explaining ecological variability of sandy beach macrobenthic assemblages along the same gradient. To illustrate the processes associated with the structural shifts observed where green tides occurred, I investigated the effects of high biomasses of opportunistic macroalgae (Ulva spp.) on the trophic structure and functioning of sandy beaches. This work reveals a progressive simplification of sandy beach food web structure and a modification of energy pathways over time, through direct and indirect effects of Ulva mats on several trophic levels. Through this thesis I demonstrate that highly dynamic systems respond differently (e.g. shift in δ13C, not in δ15N) and more subtly (e.g. no mass-mortality in benthos was found) to anthropogenic stress compared to what has been previously shown within more sheltered and non-tidal systems. Obtaining these results would not have been possible without the approach used through this work; I thus present a framework coupling field investigations with analytical approaches to describe shifts in highly variable ecosystems under human-induced stress

Characterization of marine eukaryotic biofilms at offshore wind farm sites: assessment of DNA extraction methods and marker gene used for metabarcoding approaches

Among marine lifestyles, biofilms are considered as diversified communities embedded in complex exopolymers whose development depends on several factors, related to both environmental conditions and physical-chemical characteristics of substrates (Antunes et al. 2019, Bellou et al. 2012). For the maritime industry, bio-colonization and its impact on human activities were well-described (Schultz et al. 2011). However, this phenomenon represents a new challenge in Renewable Marine Energies (RME) due to their specificities (materials, structures, localization…). In particular, macro-organism assemblages appeared to include a wide variety of eukaryotic groups but the literature is sparse considering the sequencing of eukaryotic diversity in comparison to those of bacterial communities (Briand et al. 2018, Dang and Lovell 2000, Salta et al. 2013). As a matter of fact, the very small size of some of the eukaryotes and/or their insufficient morphological discernible features appear to considerably limit their detection and identification, leading to underestimate their diversity (Carugati et al. 2015). When talking about molecular approaches, analysis of eukaryotes also represents a challenge because such organisms possess resilient cellular structures which can give poor DNA extraction yield (Hermans et al., 2018Hermans et al. 2018). In addition, SSU rRNA in eukaryotes fails to be as universal as for prokaryotes (Bik et al. 2012, Medinger et al. 2010). However, the use of marker genes from environmental DNA, when focused on the targeted eukaryotic community, remains critical to decoding the complexity of marine biofilms diversity.In this study, four extraction methods, including a preliminary mechanic cell lysis, both soil and biofilm kits, and global approaches, have been compared. We also examined the coverage and the identification capability of several primers to characterize eukaryotic communities colonizing three plastic surface types (polyvinyl chloride, HD polyethylene, and polyamide) which have been immersed in several locations along the French Mediterranean and Atlantic coasts. Sequence quality and number remain the same whatever the extraction method. However, the richness and community structure were clearly affected regardless of the sample type (Figure 1). Finally, two kits (PowerMaxSoil, and PowerBiofilm kits) evaluated in this study were considered as the most powerful overall.Secondly, we amplified and sequenced short fragments of two genes: one region of the mitochondrial Cytochrome Oxidase subunit I (COI) and five variable regions of the 18S small subunit ribosomal DNA (rDNA) gene (V1V2, V4TAR, V4UNI, V7, and V9). The Chao1 index was considerably lower for the CO1 gene compared to those of the 18S rDNA regions. The V4TAR and V7 regions showed a significant highest richness, followed closely by the V1V2 and V9 regions. The 18S rDNA gene sequences were dominated by microeukaryotes whereas the COI sequences were dominated by macro-organisms. Each of the 18rDNA primer pairs also exhibited dissimilar community structures although the dominant taxa seemed to be common.To conclude, our results provided a global assessment of tools dedicated to the description of the diversity of marine eukaryotes biofilms from three surfaces used in the design of RME. Among the four extraction methods described here, PowerMaxSoil and PowerBiofilm kits allowed recovering the highest diversity. COI and 18S rDNA gene sequencing covered different groups including at high taxonomic levels. Despite limitations, metabarcoding will help in the characterization of marine biofilms diversity on RME. Especially, it may be relevant to use primers targeting these two genes to better cover the eukaryotic diversity

Première signalisation du nudibranche aeolidien Spurilla neapolitana en mer d’Iroise (Bretagne ouest)

National audienceSpurilla neapolitanaest un mollusque gastéropode de la famille des Aeo-lidiidae. La distribution de ce nudibranche de grande taille s’étend des eauxchaudes de la Méditerranée à l’Atlantique Nord-Est. Plus précisément, on notela présence deS. neapolitanadu golfe de Gascogne (au nord), juqu’au Séné-gal (limite sud) et aux Açores (à l’ouest). En France métropolitaine,Spurillaneapolitanaest commune dans le bassin d’Arcachon principalement, mais a étérégulièrement recensée au Croisic. Sa récente découverte en presqu’île de Cro-zon (mer d’Iroise), étend son aire de distribution d’une centaine de kilomètresvers le nord, à proximité immédiate de l’entrée de la Manche

Eukaryotic diversity of marine biofouling from coastal to offshore areas

International audienceMarine biofouling communities, including biofilms, are composed of many eukaryotes with high taxonomic and functional diversities. However, molecular characterization of eukaryotic diversity of marine biofouling has been barely developed due to the only recent interest in research areas such as marine renewable energies, antifouling technologies, or plastic pollution. The aim of this study was to compare the diversity and taxonomic composition of biofouling through different metabarcoding approaches used to detect the widest range of taxa from samples collected in several contrasted marine environments (French Atlantic and Mediterranean coasts). Thus, we assessed four DNA extraction methods and six primers pairs targeting the 18S rDNA gene (including the V1-V2, V4TAR, V4UNI, V7 and V9 regions) and the COI gene, the latter with two databases (BOLD and MIDORI). In addition the influence of primers selection was analyzed at three sites to compare geographic variations in eukaryotic diversity. Although none of the extraction methods greatly altered the community diversity or composition. we have observed that eukaryotic biofouling community diversity and structure varied depending on primers pairs, reference databases and sites. 18S rDNA regions allowed the detection of more taxa at the species level, including microeukaryotes, while the COI recovered more ASVs, but with a large proportion that remained taxonomically unassigned probably because BOLD and MIDORI specifically targeted metazoans. Interestingly, the spatial pattern obtained with both COI and 18S rDNA markers were similar showing that spatial selection occurred throughout a wide diversity of eukaryotic taxa. These results encouraged the use of these two complementary markers for future metabarcoding investigations but also highlighted the relevance of completing databases to enhance the identification of biofouling eukaryotes

Opportunistic basal resource simplifies food web structure and functioning of a highly dynamic marine environment

00000International audienceCarbon flow through ecosystems is determined by the nature, availability and incorporation of basal resources (BRs) to higher trophic levels of food webs. The occurrence of abundant supplementary BR often diversifies trophic niches within food webs. Blooms of opportunistic macroalgae, which may act as additional BRs, have increased in intensity, and are expected to amplify further because of global change. Understanding the effects of high biomasses of opportunistic BRs on the functioning of naturally unvegetated coastal ecosystems is thus crucial. We set out to assess whether and how green algae (opportunistic BR) modifies macrotidal sandy beach trophic structure and functioning. We also investigated whether these effects propagate up through the web, looking at different scales and trophic levels (i.e. primary producers, specific primary consumers, and the general higher-consumer assemblage). To achieve these objectives, we used a high-resolution field sampling (over 1200 individual stable isotope-, and 180 biodiversity samples) at pristine and macroalgal-bloom-affected macrotidal sandy beaches, together with recent and novel analytical approaches (biomass-weighted isotope signatures, Isotopic Functional Indices). We found that the opportunistic BR, both directly and indirectly affects the entire beach food web across several trophic levels, by inducing shifts in trophic interactions. Overall, we found that the dominance of a BR in the form of opportunistic algae simplifies the entire food web over time. Our findings challenge the paradigm that addition of BR inherently diversifies trophic interactions within an ecosystem, and have implications for the interpretation of shifts in dynamic system food webs and eutrophicated environments

Meiofauna communities' response to an anthropogenic pressure: The case study of green macroalgal bloom on sandy beach in Brittany

International audienceOpen sandy beaches support key ecological functions and a distinctive biodiversity, but are threatened by an increasing anthropogenic pressure. Among these threats is the occurrence of green tides of Ulva spp., expression of the eutrophication phenomenon. Our study aimed at determining the impact of green macroalgal accumulations on benthic meiofauna inhabiting two macrotidal sandy beaches of Brittany: Saint Nic (impacted) and Anse de Dinan (unimpacted). The presence of Ulva enhanced total meiofaunal abundance and nematode functional diversity when compared to a state free from macroalgal mats. The stranded Ulva supported high values of meiofaunal beta-diversity as well. This unexpected response was likely due to the highly dynamic nature of open sandy beaches that prevents O-2 depletion within sediments. Natural seasonal changes in meiofaunal biomass and composition were also evident at both beaches and changes in environmental features, apart from eutrophication alone, do drive the variability in meiofaunal biomass and nematodes diversity

DataSheet_1_Eukaryotic diversity of marine biofouling from coastal to offshore areas.docx

Marine biofouling communities, including biofilms, are composed of many eukaryotes with high taxonomic and functional diversities. However, molecular characterization of eukaryotic diversity of marine biofouling has been barely developed due to the only recent interest in research areas such as marine renewable energies, antifouling technologies, or plastic pollution. The aim of this study was to compare the diversity and taxonomic composition of biofouling through different metabarcoding approaches used to detect the widest range of taxa from samples collected in several contrasted marine environments (French Atlantic and Mediterranean coasts). Thus, we assessed four DNA extraction methods and six primers pairs targeting the 18S rDNA gene (including the V1-V2, V4TAR, V4UNI, V7 and V9 regions) and the COI gene, the latter with two databases (BOLD and MIDORI). In addition the influence of primers selection was analyzed at three sites to compare geographic variations in eukaryotic diversity. Although none of the extraction methods greatly altered the community diversity or composition. we have observed that eukaryotic biofouling community diversity and structure varied depending on primers pairs, reference databases and sites. 18S rDNA regions allowed the detection of more taxa at the species level, including microeukaryotes, while the COI recovered more ASVs, but with a large proportion that remained taxonomically unassigned probably because BOLD and MIDORI specifically targeted metazoans. Interestingly, the spatial pattern obtained with both COI and 18S rDNA markers were similar showing that spatial selection occurred throughout a wide diversity of eukaryotic taxa. These results encouraged the use of these two complementary markers for future metabarcoding investigations but also highlighted the relevance of completing databases to enhance the identification of biofouling eukaryotes.</p

Green macroalgae blooms (Ulva spp.) influence trophic ecology of juvenile flatfish differently in sandy beach nurseries

International audienceOpportunistic green macroalgae blooms increasingly affect coastal areas worldwide. Understanding their impacts on organisms that use this zone, such as juvenile flatfish, is critical. By combining stable isotope data, digestive tract contents and community analyses of flatfish and their potential prey (benthic macroinvertebrates) from two North-East Atlantic sandy beaches (one impacted by blooms and one not), we detected similar and species-specific trophic changes among three co-occurring species (sand sole, plaice and turbot). Across flatfish species, juveniles displayed more opportunistic foraging behavior at the impacted site. Differently, plaice and sand sole relied more on the additional basal resource (Ulva spp.) than turbot. Finally, sand sole and turbot presented a stronger diet shift at the impacted site than plaice. We hypothesize that the species-specific response to the blooms are mostly driven by how the flatfish detect their prey (using visual and/or chemical cues) and when they forage (diurnal or nocturnal foraging)