Effect of eutrophication on air-sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years

The RIVERSTRAHLER model, an idealized biogeochemical model of the river system,has been coupled to MIRO-CO2, a complex biogeochemical model describing diatom andPhaeocystis blooms and carbon and nutrient cycles in the marine domain, to assess thedual role of changing nutrient loads and increasing atmospheric CO2 as drivers of air–seaCO2 exchanges in the Southern North Sea with a focus on the Belgian coastal zone (BCZ).The whole area, submitted to the influence of two main rivers (Seine and Scheldt), ischaracterized by variable diatom and Phaeocystis colonies blooms which impact on thetrophic status and air–sea CO2 fluxes of the coastal ecosystem. For this application, theMIRO-CO2 model is implemented in a 0D multibox frame covering the eutrophiedEastern English Channel and Southern North Sea and receiving loads from the riversSeine and Scheldt. Model simulations are performed for the period between 1951 and1998 using real forcing fields for sea surface temperature, wind speed and atmosphericCO2 and RIVERSTRAHLER simulations for river carbon and nutrient loads. Modelresults suggest that the BCZ shifted from a source of CO2 before 1970 (low eutrophication)towards a sink during the 1970–1990 period when anthropogenic DIN and P loadsincreased, stimulating C fixation by autotrophs. In agreement, a shift from net annualheterotrophy towards autotrophy in BCZ is simulated from 1980. The period after 1990 ischaracterized by a progressive decrease of P loads concomitant with a decrease ofprimary production and of the CO2 sink in the BCZ. At the end of the simulation period,the BCZ ecosystem is again net heterotroph and acts as a source of CO2 to the atmosphere.R-MIRO-CO2 scenarios testing the relative impact of temperature, wind speed, atmosphericCO2 and river loads variability on the simulated air–sea CO2 fluxes suggest thatthe trend in air–sea CO2 fluxes simulated between 1951 and 1998 in the BCZ was mainlycontrolled by the magnitude and the ratio of inorganic nutrient river loads. Quantitativenutrient changes control the level of primary production while qualitative changesmodulate the relative contribution of diatoms and Phaeocystis to this flux and hencethe sequestration of atmospheric CO2.