Carbon cycling in the German Bight: An estimate of transformation processes and transport

Carbon cycling in the shallow, near-coastal environment of the German Bight, south-eastern North Sea was studied during different seasons (spring, summer, winter). Special emphasis was placed on the investigation of processes: biologically induced internal transformation within the water column as well as external influences (rivers, Wadden Sea). Detailed analysis of dissolved inorganic carbon, total alkalinity, particulate organic carbon, dissolved oxygen and calculated CO2 fluxes revealed carbon cycling in the German Bight to be highly variable between seasons, with either dominance of physical processes during winter or pronounced biological control during productive times in spring and summer. During winter, enhanced input of particulate carbon was gained from the Wadden Sea by ice transport while biological processes stayed low. In spring, high primary production decreased the inorganic carbon pool and pronounced CO2 fluxes from the atmosphere into the surface water amounted to 40 mmol m-2 d-1. About sixty per cent of the produced biomass was rapidly remineralised within the surface layer, while in the bottom waters remineralisation stayed low. Degassing of CO2 to the atmosphere could only be observed during times of enhanced river discharge. During summer, high regenerated production accounted for CO2 fluxes of up to 25 mmol m-2 d-1 into the surface water. Replenishment of DIC in the surface layer through vertical mixing was hindered by strong stratification. Ninety-five per cent of the primary produced biomass was remineralised. Our investigations clearly show the German Bight to be a net carbon sink in spring and summer. Apart from degassing of CO2 as a result of late autumn vertical mixing or short term events like enhanced freshwater input, CO2 concentrations close to equilibrium during winter suggest that the German Bight constitutes a net annual sink for atmospheric CO2.