RELATIONSHIPS BETWEEN BENTHIC COMMUNITIES AND PHYSICAL-ENVIRONMENT IN A LAGOON ECOSYSTEM

The Thau lagoon differs from other mediterranean lagoon sites in the apparent complexity of its benthic Community structures, which seem to result from the relative complexity of its physical environment. A statistical analysis of the relationship between the abundances and biomasses of benthic taxa and the lagoonal hydrodynamic characteristics was carried out. An ACE-type statistical algorithm is used to expose the non-linear relationships in multiple regressions. Polychaete and crustacean biomasses along with mollusc densities at 15 sampling stations are considered as dependent variables. The granulometric fraction of the sediment below 40 mu as well as hydrodynamic descriptors such as kinetic energy and the exchange term H2V2, resulting from a numerical modeling of circulation forced by stochastic winds, are considered as predictors. The results demonstrate that the exchanges resulting from the hydrodynamic conditions within the watermass have a dominant beneficial influence on the development of each of the zoological groups considered. In addition, the importance of the sedimentary substrate is confirmed, but gives an interesting non-linear result, with a threshold between 3 and 6 % of the fraction of fine particles below 40 mu. Apart from the quantification of this sedimentological indicator it appears that the dominant influence of the hydrodynamic exchange processes throughout the lagoon on the development of the benthic biological structures, quite independently of any bathymetric considerations, could give a new insight into the spatial organization of benthic communities in lagoonal ecosystems.