<p>Three sediment cores from dredged and undredged areas of the Northern Lake of Tunis, a Mediterranean lagoon in northern Tunisia, were used to investigate the behavior of nutrients related to diagenetic reactions in the sediment and to assess the release of reduced nitrogen and phosphorus from the surface sediment to the water, 30 years after dredging. The degradation of organic matter and the resulting N and P fluxes are greater in the undredged area than the dredged area, mainly due to the lability of fresh organic matter in the undredged site compared to the older refractory organic matter at the bottom of the dredged sites. The biological stability of microbial communities in sediment at the undredged site may play a significant role in the microbial mineralization of organic matter. A comparison with the pre-dredging sediments shows that the influence of dredging is very remarkable over time. It has effectively reduced organic matter content (TOC levels after dredging were 2 to 9.5 times lower) and, consequently, the fluxes of reduced nitrogen and phosphorus species across the sediment-water interface. Fluxes of ammonium and phosphorus decreased approximately 150 times after dredging. The improvement in the water quality of the lagoon following dredging and the resulting oxygenation of the water promote the nitrification of ammonium ions into nitrites and nitrates in the surface layer of the sediment, thereby trapping the reducing species resulting from the anaerobic mineralization of organic nitrogen at depth.</p>