Simulating North Sea biogeochemical dynamics using a high-resolution unstructured-grid model

The distribution, transformation, and impacts of marine pollutants are strongly modulated by background biogeochemical conditions, including nutrient availability, organic matter cycling, and oxygen dynamics. Accurately resolving these processes in shelf seats therefore requires modelling approaches that combine advanced ecosystem representation with high spatial resolution. In this study, we present a high-resolution coupled hydrodynamical–biogeochemical modelling framework developed for the North Sea to simulate seasonal variability in nutrients, oxygen, and organic matter.

The system is based on the unstructured-grid Finite Volume Community Ocean Model (FVCOM) coupled to the Oxygen Depletion biogeochemical model (OxyDep) via the Framework for Aquatic Biogeochemical Models (FABM). The horizontal grid resolution varies from approximately 1–2 km at the open boundaries to about 200 m or finer in targeted regions, with 42 sigma layers in the vertical. The model is forced by atmospheric, tidal, and open-boundary conditions from operational products, while biogeochemical boundary conditions are derived from Copernicus Marine Environment Monitoring Service datasets.

Simulations for a full annual cycle reproduce the major seasonal phases of North Sea biogeochemistry. Winter conditions are characterized by light limitation of phytoplankton growth, elevated surface nutrient concentrations, and high dissolved oxygen associated with low temperatures. In spring, the development of the phytoplankton bloom leads to rapid nutrient consumption and pronounced oxygen gradients that closely follow phytoplankton biomass. Enhanced zooplankton activity and increased production of dissolved and particulate organic matter occur during late spring and early summer, followed by reduced primary production and lower oxygen concentrations during summer stratification driven by intensified organic matter mineralization. In autumn and early winter, the system gradually returns to well-mixed, winter-like conditions. The simulated spatial patterns and seasonal evolution of nutrients, chlorophyll, and dissolved oxygen are consistent with available observational products.

By resolving the natural seasonal baseline of nutrient and oxygen dynamics at high spatial resolution, the model provides a robust reference state against which additional anthropogenic nutrient inputs can be assessed. External nutrient sources associated with activities such as aquaculture, coastal discharges, and other human pressures may substantially alter local biogeochemical conditions and oxygen regimes. The presented unstructured-grid framework offers the spatial detail required to investigate how such inputs interact with physical transport and ecosystem processes, influencing nutrient dispersion, pollutant transformation, and potential environmental impacts. As such, it provides a suitable high-resolution tool for studying nutrient enrichment and pollution-related processes in the North Sea and comparable shelf environments.