Seasonal and spatial variations of in situ measured benthic fluxes of nutrients and carbon at the sediment water interface of the southern North Sea

Within coastal areas like the North Sea, nutrient and carbon cycles are driven by a close coupling of benthic-pelagic processes. Due to shallow water depths, most of the organic matter which is produced via primary production in surface waters is transferred to the seafloor. Most of the organic matter is degraded within surface sediments and nutrients such as NH₄ or PO₄are transported back into the water column, whereas only a small amount of organic carbon is buried within the sediment. Consequently, benthic carbon and nutrient fluxes have a direct impact on biological and geological processes such as the availability of nutrients in the water column, nutrient budgets or the storage of carbon within marine sediments.

By in situ as well as ex situtechniques, benthic nutrient fluxes and carbon mineralization rates were quantified within the years 2012 to 2014 at different time series sites in the southern North Sea. Benthic nitrogen and carbon fluxes are close to the Redfield ratio of 106 moles of carbon to 16 moles of nitrogen, indicating that mainly marine organic matter is settling at the seafloor.

In situ flux measurements by the benthic lander system NuSObs (Nutrient and Suspension Observatory) revealed considerable seasonal and spatial variations of benthic fluxes. For example, benthic mineralization rates of organic carbon are about three to five times higher in summer when compared to winter. Our In situ studies, investigations of pore water and macro fauna, as well as tracer studies revealed that seasonal and spatial variations are dominantly controlled by the activity and abundance of benthic macro fauna. For example, in situasphyxiation experiments during the lander deployments suggests that carbon mineralization rates can be about 20 to 30 times higher when benthic macro fauna is active.

Estimated rain rates of organic carbon and organic nitrogen to the seafloor are up to 200 g C m^-2 yr^-1 and up to 56 g N m^-2 yr^-1 respectively, while approximately 80 % of the organic carbon and 90 % of the organic nitrogen reaching the seafloor are recycled at the sediment-water interface. These datasets show that the southern North Sea is a highly dynamic system in terms of deposition and mineralisation of organic matter at the sediment-water interface and underlines the importance of the benthic macro fauna as ecosystem engineers.