Saprobity in Coastal Transitional Ecosystems: An overlooked aspect of ecosystem functioning?

In the context of environmental quality assessment of coastal transitional ecosystems, we delineate the concept of habitat saprobity as a state of an ecosystem resulting from numerous processes of organic matter (OM) metabolism (Tagliapietra et al., submit-ted). We review and expand upon classic conceptual models describing the succes-sion of benthic communities along a gradient of organic enrichment (e.g. Pearson & Rosenberg, 1978; P-R) or confinement (e.g. Guélorget & Perthuisot, 1983; G-P). Simi-larities between different approaches and models are highlighted, whereby the P-R and the G-P models are unified under a single conceptual framework. 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 coastal lagoons, and saprobity is used as a state descriptor of these processes. We assume that saprobity cannot be quantified by considering only the amount of OM per se. In fact, saprobity is the result of both input of OM and other processes, such as mineralization, burial, dilution and export of OM. The same or-ganic input can, therefore, generate different degrees of saprobity in different systems and in different areas within a system. In addition, saprobity acts on benthic commu-nities together with other components of the transitional gradient, such as salinity and sediment type. Due to difficulties to quantify saprobity itself, we foresee the use of benthic communities and the memberships of dominant species to different sapro-bic groups, as indicators of habitat saprobity.