Biogeochemistry of sedimentary phosphorus in an enriched, sandy coastal area (Seine Bay, France)

Abstract

The eutrophic Seine Bay (area: 4000 km²) with its sandy sediments provided an ideal setting to study the forms of phosphorus (P), with about 7000 tons brought by the Seine River each year. Since P biogeochemistry is little documented in sandy sediments, we investigated it through sediment profiles at various locations in the bay. General characteristics (granulometry, permeability, organic carbon, pH), sedimentary P forms bound to calcium and iron, organic P (Orga-P) and interstitial nutrients were studied. The work sought to highlight the effect of continental and marine inputs, while regarding the currents in the bay with respect to the dispersion of particles from the estuary, especially the finer ones rich in iron minerals. Total P (TP; 9–12 µmol g⁻¹) varied by less than 10% within the 10 cm surface layer. Detrital (mainly continental) and authigenic (mainly marine) calcium-bound P forms together accounted for almost 60% of TP. The other major fraction, Orga-P (23–31% of TP), originated mainly from endogenous planktonic production. Iron-P forms (crystallized or adsorbed) were less abundant (10–24% of TP) and showed a distinct redox status depending on location. At all sites, surface phosphate concentrations indicated mineralization of Orga-P of planktonic origin. In the center of the bay, the marine influence was strong, the sediment was oxic and Iron-P precipitation occurred. Weak currents in the southwest concentrated fine particles, producing anoxic sediment in which phosphate was trapped beneath unreactive iron compounds. This study describes phosphorus biogeochemistry after a stable long term input situation.