Sedimentary Phosphorus Dynamics in the Mediterranean Rhône River Prodelta: Data-Modeling Approach

The Mediterranean Sea is oligotrophic due to its paucity in nutrients especially phosphorus. As a consequence, riverine fluxes at its boundaries and the controlling processes at the river–sea connection are of prime importance to understand P cycling in this oligotrophic sea. The Rhône River delta is characterized by intense loads of terrestrial and marine particulate organic carbon and nutrients (N, P), leading to important burial and mineralization. The organic matter mineralization in sediments and induced fluxes to the water column allows internal recycling of phosphorus by biogeochemical processes. A reactive transport model was constrained by carbon, oxygen, and nutrient measurements, to quantify the role of sediments in organic matter degradation in benthic P retention and regeneration. At the selected station located in the Rhône River prodelta near the river outlet, pore water (dissolved oxygen, nitrate, sulfate, Dissolved Inorganic Carbon, Oxygen Demand Units and Dissolved Inorganic Phosphorus (DIP)) and solid data (organic carbon, Fe-bound P, Ca-bound P and organic phosphorus) were in good agreement with the model simulations. The organic C flux to sediments (43 mmol m⁻² d⁻¹) was characteristic of organic-rich ecosystems. Organic P mineralization (348 µmol m⁻² d⁻¹) represented 99% of internally produced DIP, compared to net Fe-bound P release (1%). The calculated P budgets showed that 92% of DIP released in sediments was recycled to the overlying waters, while Ca-bound P precipitation was a minor sink of reactive P (8%). The modeling approach highlighted the sediment role in P cycle as a source of regenerated DIP.