Deep rhodolith distribution in the Pontian Islands, Italy: a model for the paleoecology of a temperate sea

Within the bathymetric range (27–98 m) of the soft bottoms investigated off the Pontian Islands, rhodoliths have been classified into three morphological groups: (1) the unattached, monospecific branches, mainly composed of the non-geniculate coralline red algae Phymatolithon calcareum, Lithothamnion corallioides and Lithothamnion valens: (2) the prâlines (sensu stricto), monospecific little nodules with protuberances or short branches and a compact internal structure, normally due to the concentric growth of Lithothamnion minervae or Lithophyllum racemus over a pebble or a skeletal remain; (3) the large, multispecific rhodoliths with a ‘boxwork’ internal structure, due to long periods of growth under calm conditions interrupted by episodes of overturning and/or partial covering by sediments, with evident sedimentary fillings and a first nucleus often difficult to identify. ictive model of distribution of species and growth forms of corallines and peyssonnelians in the framework of the Mediterranean circalittoral benthic zonation has been developed along two opposite gradients: (1) the frequency of overturning of the rhodoliths and (2) environmental stability of the bottom, conditioned by: sedimentation rate; absolute hydrodynamics of the environment; shape and density of the single rhodolith (depending on its nucleus).