Spatial variability of sessile benthos in a semi-submerged marine cave of a remote Aegean Island (eastern Mediterranean Sea)

The spatial heterogeneity of sessile benthos was investigated, for the first time in the eastern Mediterranean, in a semi-submerged cave of a NATURA 2000 Special Protection Area in the North Aegean Sea. The use of a non-destructive photographic method and advanced image analysis revealed the presence of 46 taxa of Chlorophyta, Ochrophyta, Rhodophyta, Foraminifera, Porifera, Anthozoa, Polychaeta, Bryozoa, and Ascidiacea, including new records of rare species. Sponges and macroalgae were the dominant groups, in terms of substrate coverage and number of taxa. Sponges were found in all cave sectors covering considerable part of the substrate, whereas macroalgae dominated the entrances and were not recorded further than the middle part of the cave. Different patterns were observed between the walls and the floor with regard to both the biotic coverage, which decreased from the entrances towards the inner part of the cave, and diversity, possibly due to the higher sedimentation rate on the floor. Both the distance from the entrances and the position within the cave, as well as the combination of these factors, had a statistically significant effect on the observed patterns. Resemblance analysis separated the floors at the luminous entrances from the rest of the tunnel, revealing groups that roughly corresponded to the sciaphilic algal-dominated entrance zone and the intermediate semi-dark cave sectors, where sessile invertebrates dominated. The present study, which is unique in the eastern Mediterranean basin, can be utilized for comparative studies and can serve as a solid basis for future monitoring.