Ostracods as indicators of organic enrichment in marine sediments (Adriatic Sea, Mediterranean): ecological groups and applications in palaeoenvironmental studies

Ostracods represent valuable proxies in palaeoenvironmental reconstructions and environmental monitoring, yet their ecological response to organic enrichment remains poorly quantified compared to other benthic groups such as macrofauna and benthic foraminifera. Here, we propose the first large-scale assessment of ostracod species' tolerance to total organic carbon (TOC) in the Adriatic Sea (western Mediterranean), based on a previously published, extensive dataset of 298 samples analyzed using a weighted-averaging approach. Ninety marine species are assigned into five canonical Ecological Groups (EGs): twenty-six sensitive (EG I), thirty-one indifferent (EG II), twenty third-order opportunists (EG III), ten second-order (EG IV), and three first-order opportunists (EG V). This classification allowed the development of a novel Ostra-AMBI index, a modified version of the AZTI Marine Biotic Index (AMBI), based on species-specific tolerance to organic matter enrichment. The reliability of species assignment is supported by significant positive correlations between Ostra-AMBI and both TOC (R 2 = 0.59) and Foram-AMBI (R 2 = 0.75), calculated on an independent testing dataset. Besides promoting ostracods as reliable marine (palaeo)ecological indicators, results reveal a clear link between ostracod EGs distribution and the Adriatic sediment routing system, reflecting the interplay between river plumes, current-driven sediment dispersal, and organic matter accumulation. Sensitive and indifferent species dominate the coastal belt and north-central shelf, whereas opportunistic taxa prevail in TOC-enriched deposits (i.e., Adriatic Mud Wedge and deep basins). Although further studies are needed to refine the Ostra-AMBI index, this work highlights the importance of integrating biological and sedimentological processes for both environmental and stratigraphic studies.