Testing foraminiferal environmental quality indices along a well-defined organic matter gradient in the Eastern Mediterranean

Coastal environments are affected by multiple pressures resulting from anthropogenic activities, among which organic enrichment (C_org) is particularly important. Foraminiferal communities react to organic enrichment by changes in community structure (density, diversity, species composition) and behaviour (metabolism, feeding strategy, reproduction, mortality, etc.). A number of biotic indices based on foraminiferal faunas have been developed to measure the impact of anthropogenic organic enrichment on the ecological quality of marine soft bottom environments. However, the multiple origins of organic matter and its diffuse dispersion make it difficult to clearly identify enrichment gradients and the community changes along them. In this context, marine disposals of activated sewage sludge, which represent localised point sources of organic matter, offer the possibility to study faunal successions along a well-developed stress gradient. The main objectives of this study are to test 1) the ecological assignments of Mediterranean foraminifera species as recently proposed by Jorissen et al. (2018), and 2) two biotic indices, based on groups of indicator species, developed in the Mediterranean Sea, TSI-Med and Foram-AMBI, along such an organic matter gradient, resulting from a sewage outlet off the Israeli coast, south of Tel-Aviv. Along the studied organic enrichment gradient, Caronia silvestrii dominated the foraminiferal faunas in the most severely impacted zone. Further away from the sewage outlet, faunas were characterised by the dominance of Ammonia tepida. Finally, in the least affected area, farthest away from the sewage outlet, Leptohalysis scottii was the dominant taxon. Our observations strongly contrast with earlier assignments of C. silvestrii and L. scottii to ecological groups III (3rd order opportunists) and V (1st order opportunists), respectively (Jorissen et al., 2018). This discrepancy strongly suggests that additional studies are urgently needed to better characterise the ecological strategies of major ecological index species. When, as we propose here, L. scottii is not included in the group of stress-tolerant taxa for TSI-Med, and C. silvestrii and L. scottii are reassigned respectively to ecological groups V and III for Foram-AMBI, both tested indices clearly show the decreasing impact of organic enrichment with greater distance from the sewage outlet. These species reassignments underline the need to distinguish between stress-tolerant and opportunistic taxa, which do not give the same information about environmental quality.