Bridging the gap: Oxygen and nutrient budgets in the Loire estuary using a benthic-pelagic box model

In hypoxic estuaries, sediment feedback is clearly a key factor in nutrient and oxygen cycling, favouring nutrient enrichment and oxygen depletion in the water column. It is crucial to understand these mechanisms to implement effective management strategies for these coastal systems. In this study, a coupled benthic-pelagic box model was developed to assess the impact of sediment and water column processes on nutrient and dissolved oxygen stocks in a hypoxic estuary. The model integrates physical and biogeochemical processes, focusing on the sediment-water column interaction during two contrasting hydrological contexts sampled in 2021: a winter flood and a summer low discharge period. The model reproduces the main processes well, despite some discrepancies with the observed data due to the underlying assumptions, which are discussed. The winter flood caused substantial erosion of upstream sediments, resulting in a strong redox gradient and high sediment-water exchange. The short residence time of the water prevented solutes accumulation, minimising their influence on oxygen and nutrient stocks. In summer, under low flow conditions, benthic recycling to the water column is intensified, resulting in an increase in ammonium concentration of up to 91 %, compared to 67 % from pelagic processes. In contrast, phosphate release remains modest (around 4 %), particularly from the sediments underneath the Turbidity Maximum Zone (TMZ), where benthic exchange is limited by the presence of an oxide-rich sediment layer that acts as a barrier. However, phosphate availability in summer is primarily driven by pelagic desorption from suspended particles, notably in the TMZ. The results clearly highlight a decoupling between nitrogen and phosphorus recycling, which is promoting nitrogen enrichment and potentially favouring harmful algal blooms and hypoxia. The proposed simplified modelling approach is a crucial step in identifying the estuarine processes controlling oxygen and nutrient dynamics and can be easily applied to any estuary.