Foraminiferal-based biotic indices to assess the ecological quality status of the Gulf of Gabes (Tunisia): Present limitations and future perspectives

Abstract

[eng] Biotic indices are tools to assess the ecological status of marine systems, and can be based on different metrics(such as ecological groups, specific diversity). The present study applies five biotic indices based on living(stained) benthic foraminiferal assemblages to assess ecological conditions in a wide area of the MediterraneanSea in the heavily polluted Gulf of Gabes and along the western and eastern coasts of the Djerba Island, with theeastern coast considered as "pristine". The applied indices are based on epiphytic foraminifera e.g., the "longversus short life span" index (ILS) and the modified FoRAM-Index (FI'), benthic foraminiferal assemblage com-position and diversity, e.g., the Tolerant Species index (TSstd), the Foram Stress Index (FSI), the diversity Index(exp(H'bc)) and the newly developed Foram-AZTI Marine Biotic Index (Foram-AMBI). This latter index wastested using species of benthic foraminifera presently assigned to ecological groups in the Mediterranean Sea. Allindices confirm good ecological conditions eastern of Djerba. The FI' and ILS, % TSstd, FSI, indicate a severeanthropogenic impact especially in the central part of the Gulf of Gabes, next to the phosphate industry of Skhiraand along the western coast of Djerba Island and confirm direct observations and geochemical data. The indicesbased on epiphytic foraminifera seem to better reflect actual environmental conditions in the investigated regionbecause of the extendedPosidoniaoceanicameadow in the area. The exp(H'bc) reflects lower degrees of pollution.The application of the Foram-AMBI index presents limitations that prevent its current use. It strongly under-estimates the pollution impact along the Tunisian coast, and additional work is needed to increase the number ofspecies attributed to ecological groups and to possibly modify the coefficients to be used in its equation.