Using BROM (Bottom RedOx Model) model to quantify the contribution of sediments in the eutrophication of a shallow Mediterranean coastal lagoon

Due to their semi-closed hydrological nature, lagoonal ecosystems are the location of important biogeochemical transformations. Over the past century, these systems have accumulated organic matter (OM), pollutants, and nutrients, that was predominantly stored in sediments. Today, sediments are thus largely involved in elements cycles in shallow coastal areas through their role in oxygen consumption, OM and nutrient recycling and chemical contaminant release, and are further involved in the ecological degradation. Thus, sediment reactions and associated benthic fluxes of oxygen, nutrients and chemicals elements are essential processes that act as a critical driver of the biogeochemical coastal cycles and important to better understand the environmental and chemical degradation. This recycling may also limit the depuration role that sediment may play on carbon, nutrients and other pollutants storage by burial. Quantifying sedimentary contributions to ecosystem degradation and epuration is challenging. Indeed, mass balance of oxygen, nutrient, and pollutant exchanges at the sediment-water interface (SWI) has strong temporal variability in relation to temperature, quality of the OM and availability of oxidants that impact the efficiency of OM mineralization and burial and nutrients recycling over time. To address these gaps, the Bottom RedOx Model (BROM), a 1D diagenetic coupled benthic-pelagic modelling tool with O-N-P-Si-C-Fe-Mn-S biogeochemical module is particularly relevant as it is able to simulate organic matter production and mineralization, as well as major elemental cycles and trace elements fluxes at the sediment-water interface. It was applied in the Bolmon lagoon, a shallow Mediterranean lagoon impacted by eutrophication, deoxygenation and chemical pollution. In this study, the model was firstly calibrated using vertical profiles of diagenetic variables (O₂, nutrients, trace elements, etc.) collected during seasonal field campaigns. It was then employed to better understand the diagenetic response to change in the physicochemistry of the water column and to reconstruct continuous fluxes over time and finally to estimate the net mass budget at the sediment-water interface. Results revealed substantial temporal variability in fluxes, predominantly driven by water column oxygen concentration and benthic macrofaunal activity that evolved seasonally in response to hydroclimatic and ecological conditions. The net mass balance highlighted that the sediment acted as a significant oxygen sink and nutrient source. Resulting OM degradation and burial was then discussed with respect to the prevalent physicochemical conditions in the lagoon. Comparison with riverine inputs underscored the sediment compartment as a critical factor influencing the ecological state of the Bolmon lagoon, necessitating its integration into future management strategies.