Abundance and Diversity of Molluscan Fauna Related to the Sediment Grain Size Trend in the Foreshore of the Gulf of Gabes, Mediterranean Sea

This study aims to provide ecological statement based on molluscan fauna distribution and related coastal sediment facies in urbanized regions, in the southern Gulf of Gabes, Mediterranean Sea. Foreshore survey of surface marine sediment sample and Malacological fauna are carried out in April 2017 and April 2018 in harbor and river coastal outlets area, respectively. Samples were collected from the intertidal area in high tide level (~105 tidal coefficient) along 72 stations extended in the southern coast of the Gulf of Gabes. Grain size particle analysis reveals unimodal (Mo = 0.1 to 0.125 mm), fine to very fine (2Ф < Mz < 3Ф), well-sorted (0.35 < σ < 0.5) sand distribution in 95% of cases. Harbor sediment facies is characterized by fine sand distribution rich in dead seagrass leaves. The sediment of the river mouth is characterized by fine-grained muddy distribution. The longshore spatial evolution of the particle size shows a decreasing pattern under the North to the southern coastal drift direction. The cross-shore grain size pathways is mainly influenced by the tidal current with an increasing pattern from the supratidal to the infratidal areas. The inventory of molluscan fauna identified 27 species of bivalves where the Veneridae family is the most represented (by 25%) with 7 species and 23 species of Gastropods. The specific richness (SR) is ranging from only one species to 22 species. The abundance rate (A) was highly variable and was fluctuating between 6 individuals/m2 and 22 near the harbor zone, and the maximum values were recorded in fine-sandy bottom zones which are closer to river mouth areas. The dominance (D) was highest for the Bivalve Ruditapes decussatus with a range of 19%, due to the favorable nearshore biotope conditions such as the sandy bottom and the coupled tidal waves physical action. Secondary, the Gastropods Cerithuim scabridium (D) record ranges 14% and Pirenella conica (D) rate is about 12%. According to diversity and molluscan fauna composition, the nearby harbor area (i.e., Zarrat) are characterized by a low richness and abundance. However, the southward coastal areas adjacent to river outlets which are under the active wave-tide hydrodynamic processes, reveals a higher molluscan fauna diversity mainly dominated by Ruditapes decussatus.