Small-scale effects of hydrodynamics on the structure of intertidal macroalgal communities: A novel approach

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.