[en] The RIVERSTRAHLER model, an idealized biogeochemical model of the river system,
has been coupled to MIRO-CO2, a complex biogeochemical model describing diatom and
Phaeocystis blooms and carbon and nutrient cycles in the marine domain, to assess the
dual role of changing nutrient loads and increasing atmospheric CO2 as drivers of air–sea
CO2 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), is
characterized by variable diatom and Phaeocystis colonies blooms which impact on the
trophic status and air–sea CO2 fluxes of the coastal ecosystem. For this application, the
MIRO-CO2 model is implemented in a 0D multibox frame covering the eutrophied
Eastern English Channel and Southern North Sea and receiving loads from the rivers
Seine and Scheldt. Model simulations are performed for the period between 1951 and
1998 using real forcing fields for sea surface temperature, wind speed and atmospheric
CO2 and RIVERSTRAHLER simulations for river carbon and nutrient loads. Model
results 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 loads
increased, stimulating C fixation by autotrophs. In agreement, a shift from net annual
heterotrophy towards autotrophy in BCZ is simulated from 1980. The period after 1990 is
characterized by a progressive decrease of P loads concomitant with a decrease of
primary 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, atmospheric
CO2 and river loads variability on the simulated air–sea CO2 fluxes suggest that
the trend in air–sea CO2 fluxes simulated between 1951 and 1998 in the BCZ was mainly
controlled by the magnitude and the ratio of inorganic nutrient river loads. Quantitative
nutrient changes control the level of primary production while qualitative changes
modulate the relative contribution of diatoms and Phaeocystis to this flux and hence
the sequestration of atmospheric CO2.
Disciplines :
Aquatic sciences & oceanology
Author, co-author :
Gypens, N.
Borges, Alberto ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Océanographie chimique
Lancelot, C.
Language :
English
Title :
Effect of eutrophication on air-sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years
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