Constructional and destructional patterns - Void classification of rhodoliths from Giglio Island, Italy

Micro-CT analysis has been established as a useful, non-destructive method for assessing the inner arrangement of rhodoliths. In this study, micro-CT analyses and sectioning techniques are used for void space assessment, the reconstruction of growth histories, and their related environmental conditions in present-day rhodoliths from Giglio Island (Tyrrhenian Sea, Italy). The rhodoliths are investigated with respect to shape, taxonomy and growth forms of coralline red algae, constructing fauna and flora, degree of porosity, and types of void formation. Porosity within the nodules is calculated using image processing software based on slab surfaces and micro-CT, enabling recognition of different void types at various scales as well as their distribution throughout the rhodoliths. The studied rhodoliths range in sizes from 4 to 16 cm and are spheroidal to sub-spheroidal in shape. The nodules are dominated by coralline red algae and associated with subordinate serpulid worm tubes and bryozoans. Calculated porosity values range from 3 to 41% in volume. Two different types of porosity were distinguished within the rhodoliths: (1) constructional voids are present as primary voids present within single cells and conceptacles and as voids produced by conjoined protuberances of coralline algal thalli, and (2) destructional voids are caused by dissolution and decay of nuclei as well as soft-body organisms and a wide range of bioerosion, including Gastrochaenolites and Trypanites ichnotaxa. The degree of bioerosion (bioerosion index, BI) ranges from low (BI = 2) to moderate (BI = 3).