The macrofauna in the north Marmara Sea were identified and evaluated with univariate measures and biotic indices (BENTIX, AMBI). As expected, the diversity and species' variety decreased as the sediments became muddier and as the stress increased. The AMBI index failed to detect the spatial differentiation of the EcoQ between the sites whereas, the BENTIX index succeeded (in 70% of the cases) in producing an ecologically relevant classification reflecting the environmental pressures. Results suggested that risks of reduced species' richness and diversity from organic loading and other associated stressors in sediments should be relatively low at TOC concentrations < about 6 mg/g, high at concentrations > about 22 mg/g, and intermediate at concentrations in-between. It is suggested that these TOC critical points may be used as a general screening-level indicator for assessing the EcoQ in association with reduced biodiversity over coastal areas receiving organic wastes and other pollutants from human activities. It is further suggested that both assignment of species to ecological groups and classification scales for biotic indices (AMBI, BENTIX) for sites in the Mediterranean Sea should be readjusted so as to be closer related to environmental variables. (c) 2005 Elsevier Ltd. All rights reserved.