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The study of extant organisms is the base of the environmental micropaleontology, since the knowledge of natural mechanisms, which regulate their distribution in present environments, is the necessary base for the reconstruction of fossil environments. Since most shallow-water taxa well tolerate a wide range of environmental conditions, the organic flux, which determines trophic levels, appears the most important factor that regulates the distribution of benthic foraminifera. On the contrary, other variables like substrate, temperature and salinity are less important for their occurrence.

Since the end of the 90s, many studies have been performed on recent benthic foraminifera from a small marine basin located along the southern Tuscan shelf, characterized by the sedimentary contribution of the Ombrone River. The geographical features of this basin, named Ombrone River Basin (ORB), are unfavourable to the organic matter dispersion, determining a particularly suitable environment for the study of eutrophicated sea-bottoms. Researches evidenced the correlation between a typical assemblage, dominated by the opportunistic Valvulineria bradyana and the organic flux of fluvial origin. Particularly, the statistical multivariate analysis performed on 127 samples from the ORB helped to determine the distribution of such assemblage. In this work, the quantitative parameters describing the structure of the V. bradyana assemblage (species richness, heterogeneity, dominance) are evidenced and discussed, defining an actualistic model for assemblages that could be used to evidence the organic paleo-flux in coastal marine areas influenced by river …