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

Abstract

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 pralines (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. A predictive 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).