Rhodolith beds in the Mediterranean

Mediterranean rhodolith beds could be found between 20-150 m depth in normal marine conditions. The most common species is Lithothamnion corallioides and Phymatolithon calcareum is recorded less frequently. Different dominant species characterize the Mediterranean rhodolith beds, probably on the basis of biogeography and local environmental conditions.Characteristic associations include: Association with rhodoliths in coarse sands and fine gravels under the influence of bottom currents; and Association with rhodoliths on coastal detritic bottoms.

Rhodolith beds are defined by those sedimentary bottoms characterised by any morphology and species of unattached nongeniculate calcareous red algae (incompletely-coated grains excluded) with >10% of live cover. They occur in coarse clean sediments of gravels, clean sands and coastal detritic areas under the influence of bottom currents, which occur either on the open coast or in tide-swept channels of marine inlets (the latter often stony).

In general, fluent unidirectional hydrodynamism and laminar currents affect the seafloor. In the Mediterranean, the most favourable environment for this habitat to occur in biotopes with laminar bottom currents with a regular course. Changes in hydrodynamic conditions and intensity of light differentiate the presence of different associations, particularly in the community of the coastal detritic bottoms (circalittoral). The most frequent species of epiflora are Arthrocladia villosa and Sporochnus pedunculatus. Differences in intensity and typology of hydrodynamism and in species composition are able to determine differences in the growth form, structure and shape of the rhodoliths that can be summarized into three main morphologies: small and compact pralines, unattached branches, and large, irregular, boxwork rhodoliths. In the lower infralittoral zone this assemblage forms patches of organogenous pebbles of branched rhodoliths in various development stages. In the circalittoral zone the free calcareous algae formation can cover up to several square kilometres. Rhodolith beds can be considered as authentic carbonate factories, since they are among the highest producers of biogenic particles in European seas. The living part of the association is limited only to the surface of some centimetres of thickness. Subhabitat scomposed of non-nucleated, unattached growths of branching, twig-like coralline can be distinguished as maerl beds.

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