Scales of coastal heterogeneity and benthic intertidal species richness, diversity and abundance

Species richness, diversity, total biomass of the benthic macrofauna and macroflora, and the biomass of the 2 dominant taxa (Fucus spp. and Mytilus edulis) were examined in relation to coastal heterogeneity at different scales in the intertidal zone. The sampling design included randomness at all scales and replication of treatments. A 103 km portion of the south shore of the St. Lawrence Estuary, Canada, was divided into 1 km stretches of shore (stations)-the large scale-which were classified using a shore heterogeneity index (SHI) into 3 categories (low, medium and high). Species richness was qualitatively evaluated for each station and substratum heterogeneity on a 100 m (medium scale) was measured as covariate. At the smallest scale 4 types of surface (smooth, crevices of 1, 10 and 20 cm) were quantitatively sampled. Species richness tended to increase with Sill category but this tendency was not statistically significant. A multiple regression analysis was carried out to find which scale of heterogeneity was the most significant for defining species richness. Diversity in types of surface did not vary significantly among SHI categories. Our results show that large-scale heterogeneity explained a higher proportion of the variance in species richness than substratum heterogeneity on a 100 m scale. No statistically significant difference was found in total biomass, M. edulis and Fucus spp, biomass or percent cover among the SHI categories. At the small scale (types of surface), the abundance increased significantly from smooth surfaces to 20 cm crevices except for mussels, where abundance was higher in 10 cm crevices. The types of surface explained 42% of the variation in total biomass and 21% of that in Fucus spp. biomass. Variation in percent cover was explained by the types of surface (40%) and to a lesser extent by the SHI (7%). The present study showed that the scales which influenced abundance were smaller than 20 cm in the intertidal zone. Thus, our results indicated that 2 distinct spatial scales explained the variability within the same marine intertidal community, i.e. variability in species richness (scale of 1 km) and in abundance (types of surface; scale of less than or equal to 20 cm).