Carbon dynamics and CO<sub>2</sub> air-sea exchanges in the eutrophied coastal waters of the Southern Bight of the North Sea: a modelling study

Abstract

Abstract.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 CO 2 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 CO 2 (pCO 2 ) were successfully compared with data recorded over the same period in the central BCZ at station 330 (51 • 26.05 N; 002 • 48.50 E). Budget calculations based on model simulations of carbon flow rates indicated for BCZ a low annual sink of atmospheric CO 2 (−0.17 mol C m −2 y −1 ). On the opposite, surface water pCO 2 in WCH was estimated to be at annual equilibrium with respect to atmospheric CO 2 . The relative contribution of biological, chemical and physical processes to the modelled seasonal variability of pCO 2 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 CO 2 flux in BCZ that became, on an annual scale, a significant source for atmospheric CO 2 (+0.53 mol C m −2 y −1 ). Overall biological activity had a stronger influence on the modelled seasonal cycle of pCO 2 than temperature. Especially Phaeocystis colonies which growth in spring were associated with an important sink of atmospheric CO 2 that counteracted the temperature-driven increase of pCO 2 at this period of the year. However, river inputs of organic and inorganic carbonCorrespondence to: N. Gypens (ngypens@ulb.ac.be) were shown to increase the surface water pCO 2 and hence the emission of CO 2 to the atmosphere. Same calculations conducted in WCH, showed that temperature was the main factor controlling the seasonal pCO 2 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 CO 2 flux.

The stoichiometry of inorganic carbon and nutrient removal in the Mississippi River plume and adjacent continental shelf

“…These studies have led to an improved understanding of the relationships between nutrient availability and biogeochemical processes such as photosynthesis and remineralization in all aquatic systems. Redfield stoichiometric ratios have been applied in coastal ocean biogeochemical models in the northern Gulf of Mexico (Fennel et al, 2011;Green et al, 2006Green et al, , 2008, and used in linking river nutrient loading with sea surface partial pressure of carbon dioxide (Gypens et al, 2004) and ocean acidification (Borges and Gypens, 2010). These studies have widely assumed that the dissolved inorganic carbon (DIC) uptake and nitrogen removal ratio in the water column follows the Redfield value (i.e.…”

How phosphorus limitation can control climate-active gas sources and sinks

“…The description of the carbonate system is based on the evolution of dissolved inorganic carbon (DIC) and total alkalinity (TA). It allows the calculation of partial pressure of CO 2 (pCO 2 ), pH and airsea CO 2 fluxes (Gypens et al, 2004). The DMS(P) cycle describes the DMSP and DMS dynamics; those consist of biological transformations by phytoplankton and bacteria as well as physico-chemical processes (i.e., photodegradation and DMS air-sea exchange) .…”

“…The model also includes the description of the carbonate system (Gypens et al, 2004) and the DMS(P) cycle . The description of the carbonate system is based on the evolution of dissolved inorganic carbon (DIC) and total alkalinity (TA).…”

“…Phaeocystis, as other haptophytes in general, is an important producer of dimethylsulfoniopropionate (DMSP) (Keller et al, 1989), the precursor of DMS. The impact of organic P use on (i) ecosystem primary production and (ii) CO 2 and DMS fluxes is studied using an extended version of the R-MIRO-BIOGAS model (previously validated in the SBNS; Gypens et al, 2004 that includes a description of the ability of phytoplankton organisms to use DOP as a source of P (Ghyoot et al, 2015).…”

Kinetic bottlenecks to respiratory exchange rates in the deep-sea – Part 1: Oxygen

“…K, however, is equivalent to the "gas transfer velocity" or "piston velocity" commonly used to describe air-sea gas exchange, both for the open ocean (Wanninkhof, 1992) and coastal seas (e.g. Gypens et al, 2004) and estuaries (Hofmann et al, 2008), so the underlying simplified physical formalism is the same. The limit case of u 100 → 0 would yield L → ∞.…”