Benthic communities as indicators of habitat saprobity.

We delineate the concept of saprobity as a state of an ecosystem resulting from organic matter (OM) metabolism processes. An overview is given on conceptual models describing the succession of benthic communities along a gradient of organic enrichment. Similarities between different models, e.g. between the Pearson- Rosenberg [1] and the Guelorget-Perthuisot [2] models, are highlighted. Based on a critical analysis of existing models and indices, we propose a general framework where the processes of OM metabolism are a major structuring factor the benthic communities in microtidal lagoons, and saprobity is used as a state descriptor of these process. We assume that saprobity cannot be quantified by considering only the amount of OM. In fact, saprobity is the result of both input of OM and other processes, such as mineralization, sinking, dilution and export of OM. The same organic input can, therefore, generate different degrees of saprobity in different systems/ areas. Saprobity acts on benthic communities together with other components of the transitional gradient, such as salinity but, in coastal lagoons, the effect of the latter is primarily in oligohaline and hyperhaline areas. Due to difficulties to quantify saprobity itself, we are developing a numerical index of saprobity which takes into account the memberships of benthic species to different saprobic groups. Main characteristics of this index are described in this paper and a zonal approach is also foreseen.