Impacts of phytoplankton blooms on the benthic phosphorus cycle in a macro-tidal coastal ecosystem (Bay of Vilaine, France)

Temporal and spatial variations in benthic phosphorus (P) cycle were studied in the eutrophic temperate macro-tidal Vilaine Bay (VB) using field observations, laboratory measurements and an early diagenetic model. Time series of sedimentary P speciation, benthic fluxes measurements and dissolved P pore water profiles at a monitoring station, together with a spatial survey, were complemented by an extensive set of physical, chemical, and biological descriptors of the water column. Benthic dissolved inorganic P (DIP) fluxes ranged from 0.5 to 7.0 μmol m -2 h -1 , while benthic dissolved organic P (DOP) fluxes remained weak and lacked a clear pattern, likely due to rapid microbial turnover of a labile DOP pool, contributing indirectly to DIP regeneration. Sedimentary P was dominated by Organic P (Org-P) and iron oxides-bound P (Fe-P). The model correctly reproduced observed variations in benthic DIP fluxes and identified phytoplankton-derived Org-P mineralisation and the Fe-P formation/dissolution balance as the main controls on benthic P dynamics. In spring, a fraction of DIP produced by bloom mineralisation was temporarily retained in sediments through Fe-P formation. This P was subsequently released during summer as bottom-water oxygen saturation decreased, promoting Fe-P dissolution. In summer, increased organic matter degradation further elevated sediment oxygen demand, enhancing DIP release through Fe-P dissolution and ongoing Org-P mineralisation. Benthic DIP release therefore constitutes a sustained internal P source that can prolong summer phytoplankton biomass and potentially exacerbate the effects of riverine nutrient inputs. These results highlight that reducing nitrogen inputs is necessary to mitigate eutrophication in coastal waters.