Sedimentary Phosphorus Dynamics in a Tropical Mangrove Lagoon : Baie des Milliardaires, Ébrié Lagoon, Côte d’Ivoire

Abstract

Sedimentary phosphorus can sustain eutrophication through internal loading, yet its mobility in the Baie des Milliardaires of the Ébrié Lagoon remains poorly documented. This study characterised sedimentary phosphorus fractions and assessed their mobility in relation to physicochemical conditions during the flood season. Surface sediments were collected monthly from nine stations between September and November 2024, producing 27 samples. Phosphorus fractions were determined using an optimised sequential extraction protocol, while pH, redox potential, conductivity, and salinity were measured to examine environmental controls. The sediments were generally acidic and oxidising, with relatively high conductivity and salinity. Total phosphorus concentrations varied spatially, and the extracted phosphorus pool was dominated by authigenic phosphorus, iron-bound phosphorus, and humic-associated phosphorus, whereas phosphorus associated with (oxy)hydroxides and detrital phosphorus was least represented. Total organic phosphorus accounted for more than 60% of sedimentary phosphorus, while mobilisable organic phosphorus represented more than 55% of the total mobilisable fraction. Spatial variability in phosphorus forms was evident among stations, and multivariate analyses indicated associations between selected phosphorus fractions and sediment conductivity and salinity. Overall, the results show that organic phosphorus constitutes a substantial component of the sedimentary phosphorus pool and may contribute importantly to phosphorus mobility within this tropical mangrove lagoon. These findings provide a site-specific basis for understanding the potential contribution of sedimentary phosphorus to eutrophication processes in the Baie des Milliardaires.