Sedimentary geochemistry in P-limited freshwater drained marshes (Charente-Maritime, France): Original drivers for phosphorus mobilization

Phosphorus bioavailability is a major issue in aquatic environments, where it generally limits primary production. In this work, the analysis of the pore water and the solid phase of the sediment was carried out over a 9-month monitoring period between February 2020 and April 2021 in two drained marshes (Marans and Genouillé, France) distinct by their uses and management tools. Soluble reactive phosphorus (SRP) enrichment in the sediment was intimately controlled by iron oxide dissolution. The latter seemed highly controlled by seasonal nitrate inputs (winter and early spring) that favoured denitrification as a major benthic mineralization process promoting iron curtain development and stability. Following benthic mitigation of nitrate other anaerobic metabolisms developed such as iron dissolutive reduction promoting P recycling and planktic bioavailability. Surprisingly, sulphur cycle seemed to affect P dynamics, especially in the absence of nitrate. The absence of NO₃⁻ triggered high sulphate reduction rates in the two first centimeters depth, reaching −8.9 E⁻⁰³ ± 0.5 E⁻⁰³ and -5.0 E⁻⁰³ ± 0.2 E⁻⁰³ nmol cm⁻³ s⁻¹ in August and July at Marans and Genouillé respectively. These values placed this process at higher rates than the denitrification (maximum in May at Marans with −5.0 E⁻⁰³ ± 1.1 E⁻⁰³ nmol cm⁻³ s⁻¹) and reduced iron production (maximum in July at Genouillé with 0.5 E⁻⁰³ ± 0.1 E⁻⁰³ nmol cm⁻³ s⁻¹). The rapidity with which process changes occur (monthly scale) testified to the dynamism of these systems. The similarity in geochemical patterns regarding NO₃⁻ pressure at both sites underlines the importance of diffuse pollution in coastal systems for nitrogen mitigation and phosphorus trapping. The results obtained in this study could lead to the development of a generalized diagenetic operating model for temperate systems with high agricultural pressure. This would enable to target management efforts to both optimize the purification function and limit eutrophication risks in these systems.