Vertical variation of coralligenous cliff assemblages in marine biogeographic areas

Estimating spatial patterns of variability in coastal marine priority habitats may allow to plan monitoring programs and impact evaluation studies and to optimize sampling designs in environmental investigations. In this study the spatial variability of the vertical distribution (18 m, 23 m, 28 m, 33 m, and 38 m of depth) of coralligenous cliff assemblages was estimated in three marine biogeographic areas of the Mediterranean Sea: the Sardinia Sea, the Tyrrhenian Sea and the Bonifacio Strait. Sampling sites were selected in the West, North, and East coast of Sardinia, all in pristine conditions (far from any anthropogenic sources of influence and in highly oligothrophic water). Important variations among areas in the vertical zonation of the coralligenous cliffs have emerged, especially at the shallower depths, and the dominant taxa have been highlighted. Despite these unconsistencies, a common increase in the number of taxa/groups per sample was found with depth. Thermal environment and hydrodynamics have been discussed as the most likely important drivers of such variability. Overall, these data support the hypothesis that coralligenous cliff assemblages unaffected by local anthropogenic disturbance may be relevantly different in structure and indicate that the lack of gorgonians and bryozoans (commonly used as ecological indicators) is not necessarily a sign of local human impact, as it may be the result of biogeographic patterns.