Carbon dynamics and CO2 air-sea exchanges in the eutrophicated coastal waters of the Southern Bight of the North Sea: a modelling study

A description of the carbonate system has been
incorporated in the MIRO biogeochemical model to investigate
the contribution of diatom and Phaeocystis blooms to
the seasonal dynamics of air-sea CO2 exchanges in the Eastern
Channel and Southern Bight of the North Sea, with focus
on the eutrophied Belgian coastal waters. For this application,
the model was implemented in a simplified three-box
representation of the hydrodynamics with the open ocean
boundary box ‘Western English Channel’ (WCH) and the
‘French Coastal Zone’ (FCZ) and ‘Belgian Coastal Zone’
(BCZ) boxes receiving carbon and nutrients from the rivers
Seine and Scheldt, respectively. Results were obtained by
running the model for the 1996–1999 period. The simulated
partial pressures of CO2 (pCO2) were successfully compared
with data recorded over the same period in the central
BCZ at station 330 (51 26.050 N; 002 48.500 E). Budget
calculations based on model simulations of carbon flow
rates indicated for BCZ a low annual sink of atmospheric
CO2 (−0.17 mol C m−2 y−1). On the opposite, surface water
pCO2 in WCH was estimated to be at annual equilibrium
with respect to atmospheric CO2. The relative contribution of
biological, chemical and physical processes to the modelled
seasonal variability of pCO2 in BCZ was further explored
by running model scenarios with separate closures of biological
activities and/or river inputs of carbon. The suppression
of biological processes reversed direction of the CO2
flux in BCZ that became, on an annual scale, a significant
source for atmospheric CO2 (+0.53mol C m−2 y−1). Overall
biological activity had a stronger influence on the modelled
seasonal cycle of pCO2 than temperature. Especially
Phaeocystis colonies which growth in spring were associated
with an important sink of atmospheric CO2 that counteracted
the temperature-driven increase of pCO2 at this period of the
year. However, river inputs of organic and inorganic carbon
were shown to increase the surface water pCO2 and hence the
emission of CO2 to the atmosphere. Same calculations conducted
in WCH, showed that temperature was the main factor
controlling the seasonal pCO2 cycle in these open ocean
waters. The effect of interannual variations of fresh water
discharge (and related nutrient and carbon inputs), temperature
and wind speed was further explored by running scenarios
with forcing typical of two contrasted years (1996 and
1999). Based on these simulations, the model predicts significant
variations in the intensity and direction of the annual
air-sea CO2 flux.