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Este artículo contiene 10 páginas, 2 figuras, 3 tablas., [EN]Photosynthetic features of different canopy-forming macroalgae of the order Fucales (Phaeophyceae) living in shallow and sheltered environments show a high homogeneity when compared with other morphologically similar species living across a depth gradient. Photosynthesis at saturation (situated around 5 mg O2 gAFDM 1 h1) and photosynthetic efficiency [around 0.4 mg O2·m2 s (μmol photon·gAFDM·h)-1] are relatively low, while dark respiration (around 1 mg O2 gAFDM 1 h1) and light at compensation (around 24 μmol photon·m-2·s-1) are relatively high, as it corresponds to the characteristics of “sun” plants. C:N and C:P ratios suggest a strong nutrient limitation for growth and photosynthesis, in agreement with the low dissolved nutrient levels usually found in shallow Mediterranean waters. Homogeneity in photosynthetic features points to a good local adaptation of the different species to the prevailing light conditions but opens the question of which are the factors allowing the coexistence of different species of Fucales in sheltered and shallow Mediterranean environments. [FR] Homogénéité des caractéristiques photosynthétiques des macroalgues de l’ordre des Fucales formant une canopée dans les environnements peu profonds et protégés. Les caractéristiques photosynthétiques de différentes algues de l’ordre Fucales (Phaeophyceae) qui croissent dans des environnements peu profonds et protégés montrent une grande homogénéité si on les compare avec d’autres espèces de morphologie similaire présentes dans d’autres environnements le long du gradient bathymétrique. La photosynthèse en saturation (située aux environs de 5 mg O2 g masse sèche sans cendres1 h1) et l’efficacité photosynthétique [environ 0.4 mg O2·m2 s (μmol photon·g masse sèche sans cendres1·h)-1] sont relativement basses, alors que la respiration sombre (environ 1 mg O2 g poids sec sans cendres1 h1) et la lumière à compensation (environ 24 μmol photon·m2·s-1) sont relativement élevées, comme c’est le cas avec les caractéristiques des plantes «de soleil». Les rapports C:N et C:P suggèrent une forte limitation par N et P, ce qui est congruent avec les niveaux de nutriments très bas qu’on observe régulièrement dans les eaux peu profondes méditerranéennes. L’homogénéité des caractéristiques photosynthétiques indique une adaptation locale des différentes espèces aux conditions de lumière présentes mais pose la question des facteurs permettant la coexistence des différentes espèces de Fucales dans les environnements méditerranéens peu profonds et protégés.

International audience; Dispersal is a central process that affects population growth, gene flow, and ultimately species persistence. Here we investigate the extent to which gene flow occurs between fragmented populations of the deep-water brown algae Ericaria zosteroides (Turner) Greville (Sargassaceae, Fucales). These investigations were performed at different spatial scales from the bay of Marseille (western Provence) to Corsica. As dispersal of zygotes is shown to be limited over distances beyond a few meters, we used a multidisciplinary approach, based on Lagrangian modeling and population genomics to test the hypothesis that drifting of fertile parts of thallus (eggs on fertile branches), mediated by ocean currents, enable occasional gene flow between populations. Therefore we assessed the respective contribution of oceanographic connectivity, geographical isolation, and seawater temperatures to the genetic structure of this species. The genetic structure was assessed using 10,755 neutral SNPs and 12 outlier SNPs genotyped by dd-RAD sequencing in 261 individuals of E. zosteroides. We find that oceanographic connectivity is the best predictor of genetic structure, while differentiation in outlier SNPs can be explained by the depth of populations, as emphasized by the minimum seawater temperature predictor. However, further investigations will be necessary for clarifying how depth drives adaptive genetic differentiation in E. zosteroides. Our analyses revealed that local hydrodynamic conditions are correlated with the very high divergence of one population in the Bay of Marseille. Overall, the levels of gene flow mediated by drifting were certainly not sufficient to counteract differentiation by local genetic drift, but enough to allow colonization several kilometers away. This study stresses the need to consider secondary dispersal mechanisms of presumed low dispersal marine species to improve inference of population connectivity.