Phosphorus diagenesis in sediment of the Thau Lagoon

We describe the depth distribution of phosphorus in relation to the distribution of major redox species (dissolved O2, NO3−, NH4+, Mn, and Fe, and particulate S, organic C, reactive Mn- and Fe-oxides) in modern sediments of two stations in the Thau Lagoon. Sediments close to the oyster bank zone are enriched in organic carbon and are highly bioturbated, while those outside the bank are less bioturbated and organic carbon levels are lower. In all sites, early diagenesis follows the well-accepted depth sequence of redox reactions of organic matter mineralization. The upper sediments of the station enriched in organic carbon contain high amounts of reactive particulate organic phosphorus that arrives at the sediment surface through biodeposition. Only a part of this phosphorus is released to the bottom water after mineralization, since more than 50% of total P is buried as an authigenic phosphate mineral. In the middle of the lagoon, outside the oyster bank zone, organic matter seems to be much more refractory, but the distribution of the major redox compounds indicates that this organic matter is partially mineralized. A portion of the phosphate and ammonium released during mineralization does not escape the sediment, since the concentration gradient is close to zero between 1 and 25 cm depth below the sediment–water interface. Pore water N and P are likely fixed by biological uptake. The characterisation and magnitude of this process require further study.