Assessment of old eutrophication in the sediments of Marchica Lagoon (a post-restored lagoon, Mediterranean): The role of geochemistry and granulometry of the sediments

The aim of this study is to present the seasonal variation of organic matter (OM), total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) in the bottom sediments of Marchica Lagoon, a post-restorated lagoon located in the Moroccan Mediterranean, and to estimate the potential role of the geochemistry and granulometry of bottom sediments in the retention of old eutrophication. The organic index of bottom sediment was found generally higher in old-eutrophic and enclosed areas. The surface sediments show variable levels of OM, TOC, TP, and TN; high in some cases (12.3%, 6.71%, 0.70%, and 0.30% respectively). These high levels were recorded in the central part of the lagoon and in areas subjected to high anthropogenic pressure, including wastewater discharges. Our results indicated that sediments are an excellent immobilizer of organic matter (C, N, and P), although, there was no correlation between sediment grain size and organic matter content. The absence of correlation might be explained by currentology and hydrodynamics, but also by the bathymetry of the lagoon. Therefore, sediments must be implicated in any lagoon restoration strategy and the site development plan.