SPATIAL AND SEASONAL VARIABILITY IN THE RELATIONSHIPS BETWEEN BENTHICCOMMUNITIES AND PHYSICAL-ENVIRONMENT IN A LAGOON ECOSYSTEM

Hydrodynamic processes within a shallow Mediterranean lagoon, the Etan g du Prevost in southwestern France, are essentially controlled by the combined effects of tide and wind, which induce both horizontal advec tion and vertical turbulent diffusion. The spatial distribution of mac robenthic organisms within this lagoon varies seasonally, according to variability in the hierarchy of the forcing environmental conditions. A 2-dimensional numerical model is used to compute spatial distributi ons in the lagoon of the tide- and wind-induced hydrodynamic kinetic e nergy under typical environmental conditions. An Alternating Condition al Expectation (ACE) algorithm is used to demonstrate non-linear spati al and seasonal relationships in multiple regression between benthic c ommunities and physical environment. Mollusc and crustacean biomasses at 8 sampling stations are considered as dependent variables in the AC E analyses, while the distance of each sampling station from the sea i nlet, the granulometry of the upper sediment (fraction of fine particl es below 40 mum), and the computed tide- or wind-induced kinetic energ y are considered successively as predictors. Results provide insight i nto the relationships between benthic macrofauna and sediment or hydro dynamic features, and especially into the spatial and seasonal variabi lity of these relationships. Spatially, results emphasize the distinct ion between the optimal development of molluscs, associated with the e nergetic physical environment prevailing at seaward locations, and tha t of crustaceans associated with the more confined environment prevail ing landwards. In addition, a distance of 1.6 km from the sea inlet is computed and proposed as the maximum spatial extension in the inner l agoon of a specific marine influence. Temporally, the major contributi ons of distance from the sea inlet, granulometry and tide-induced hydr odynamics, in regressions from January to May, confirm the central rol e of sediment features and tidal impact in explaining the seasonal var iability of benthic macrofauna from winter to spring, related to the r ecruitment dynamics of marine larvae. Also seasonally, the increasing efficiency as a predictor of tide-induced kinetic energy from May to O ctober supports a beneficial impact of marine water circulation on ben thic macrofauna in preventing, at seaward locations, the anoxia which prevails in the inner lagoon in summer.