Effect of substrate size and depth on macroalgal communities in unstable marine rocky bottoms

Abstract

Subtidal unstable bottoms are sensitive to the environmental disturbances that determine the structure of macroalgal communities. Here, three variables (substrate size, depth and site) are tested together to explore their effect on the community structure on cobble and gravel bottoms. Diversity metrics (richness per sample, richness per area, diversity (H '), total cover), cover of life forms (hypnophyceae, ephemerophyceae, phanerophyceae, hemiphanerophyceae, chamaephyceae) and cover of the predominant species (Harveylithon samoënse, Jania pedunculata var. adhaerens, Lobophora spp., Canistrocarpus cervicornis, Halopteris scoparia) are used as descriptors of communities. Descriptors response was modeled using multiple regression analyses, in which each descriptor was considered separately as the dependent variable, while substrate size, depth and site was used as independent variables. Results show the substrate size was the most important variable with depth only being important for some perennial species although there was often site-site variability in these patterns. Richness per sample of macroalgal communities increased with substrate size; however, the highest values of richness per area and total cover increased towards the smallest substrates. Substrate size also positively influenced the cover of the perennial life forms hemiphanerophyceae and phanerophyceae, but do not on the cover of chamaephyceae. The perennial crustose Harveylithon samoënse showed a significant relationship with all variables increasing its cover towards small substrates in shallow, while the perennial erect Lobophora spp. are more abundant on large substrates at greater depths. Together these results showed that the substrate size was consistently the most important factor for algal diversity in unstable-bottom habitats.