Modeling deoxygenation in fjord systems

Fjords are characterized by restricted exchange with open-ocean waters, often due to the presence of a sill that limits horizontal transport and isolates bottom water masses. Oxygen-depleted conditions can develop even in the absence of substantial freshwater discharge from land, as fjords may effectively trap and accumulate organic matter exported from adjacent open-sea surface waters (e.g., Framvaren, Hunnbunn). When a river discharges into a fjord, an additional estuarine effect comes into play, whereby river-borne organic matter fluxes further enhance bottom-water deoxygenation (e.g., Drammensfjord, Bærumsbassenget). In both scenarios, strong vertical stratification develops, severely limiting oxygen supply to deeper layers. In populated coastal regions, anthropogenic discharges often introduce pollutants whose biogeochemical impacts—such as enhanced mercury methylation—are amplified under low-oxygen conditions.

In this study, we implemented a coupled three-dimensional hydrodynamic–biogeochemical model in Julia, integrating the Oceananigans modeling framework with the OxyDep biogeochemical module. The analysis focuses on the Oslofjord and Drammensfjord, with the objectives of (i) simulating present-day conditions and (ii) quantitatively assessing the relative contributions of different drivers to the persistence of permanent bottom anoxia, seasonal anoxia, and episodic anoxia. The coupled model is further used to evaluate how changes in anthropogenic forcing—specifically variations in nutrient loading—affect the interannual variability of the fjords’ oxygen state.