Spatial patterns of macrophyte composition and landscape along the rocky shores of the Mediterranean–Atlantic transition region (northern Alboran Sea)

The Alboran Sea is the westernmost ecoregion of the Mediterranean Sea. It is located in the vicinity of Strait of Gibraltar, the only natural connection of the Mediterranean Sea with global circulation. This ecoregion presents steep and highly variable environmental gradients, thus acting as a natural filter for Mediterranean and Atlantic species. This study aimed to analyse spatial patterns of littoral and upper sublittoral communities and their relationship with oceanographic conditions and coastal geomorphology, and to quantify littoral and upper sublittoral rocky shore communities at landscape scale. The results suggest that oceanographic conditions are the main factor to explain landscape patterns along the studied area, while geomorphological features should be related with local-scale variability. In this sense, three biogeographic subregions, matching with oceanographic patterns, were identified: western, central and eastern. These subregions showed significant differences in the structure and the composition of the littoral and upper sublittoral community, which can be explained by regional oceanographic dynamics. Posidonia oceanica, Cystoseira ericaefolia group and Mytilus spp. were the species that most contributed to landscape dissimilarity between the three subregions identified. The central oceanographic region, where the environmental conditions were more variable, showed the poorer and less differentiated flora, suggesting the existence of a divergent boundary between the Atlantic and the Mediterranean Sea.