Effects of Eutrophication on Plankton Abundance and Composition in the Gulf of Gabès (Mediterranean Sea, Tunisia)

A total of 42 phytoplankton and 24 Copepoda taxa were identified in the coastal environment of Gabès (Tunisia). Bacillariophyta were the most abundant group (67.7–89.2% of total phytoplankton abundance). Shannon–Weaver diversity index of phytoplankton and Copepoda showed negative correlations with Eutrophication Index. Opportunistic and r strategist species (Chaetoceros costatus, Euglena acusformis, and Thalassiosira sp.) showed a positive correlation with Eutrophication Index. Among Copepoda, Oithona similis and Euterpina acutifrons were indifferent to eutrophication. Highlights: Phytoplankton and Copepoda were investigated in the industrialized Gabès coast (Mediterranean Sea, Tunisia) to understand consequences of the Eutrophication Grade on the community composition. In the past 40 years, intensive agrochemical activities have developed in Gabès, discharging wastewater along the coast. In parallel, Gabès City has experienced a rapid demographic expansion (population: 131,000) that further increased sewage discharged into the sea. The present study was carried out in the Gulf of Gabès in March 2014. The abiotic analysis of seawater showed high concentrations of nutrients and eutrophication in all the studied fifteen stations. A growing eutrophic gradient was revealed from Zarrat to Gannouche. During this study, 42 phytoplankton taxa and 24 Copepoda taxa were identified. Bacillariophyta were the most abundant group, ranging from 67.7% to 89.2% of total phytoplankton specimens. Chaetoceros costatus, Euglena acusformis, and Thalassiosira sp. showed a positive correlation with Eutrophication Index (profited of nutrient availability). Therefore, the Shannon–Weaver diversity index of phytoplankton and Copepoda showed negative correlations with Eutrophication Index. The relatively high H′ values for phytoplankton suggest that the eutrophicated waters of Gulf of Gabès are not a hostile environment for them. Among Copepoda, Oithona similis, and Euterpina acutifrons seem to be insensible species to eutrophication.