Sediment Type and Their Relation to the Presence of Seagrass Posidonia oceanica in the Mediterranean Lagoon

This study aims to determine the sediment pattern changes and seagrass response to natural and anthropogenic factors in urbanized coastal lagoons in Ghar El Melah, Tunisia, in the central Mediterranean. We explore seagrass metrics over 37 radials and 106 spatial lagoon stations and sediment characteristics in 21 stations in December 2020 and April 2021. Floristical assessment is based on structural and functional metrics and seagrass distribution. The result reveals the presence of Cymodocea nodosa, Zostera marina, Z. nolti, Ruppia maritima, and R. cirrhosa, while P. oceanica is reported for the first time in lagoon systems of northern Tunisia, which cover ~70 ha, showing a density ranging from 56 shots/m2 to 2111 shots/m2. Grain-size analysis of marine lagoon sediment revealed a predominance of very fine sand (D50 goes from 0.063 to 0.120 mm) and mud distribution (D50 ranges from 0.001 to 0.063 mm) inside the lagoon except in the lagoon channel when it dropped by coarser sand (Median equal to 0.2 mm). The sediment and seagrass distribution variability is controlled by the historical deltaic deposits inside the lagoon and the neighboring marine processes. The lagoon’s center and urban sides are marked by the absence of seagrass meadows where sediment is composed of clay fractions. The ecological quality of the lagoon is strongly affected by urban activities and coastal management.