Michel Frankignoulle

European continental shelf as a significant sink for atmospheric carbon dioxide

The concentration of carbon dioxide was measured during 18 cruises in the surface waters of the North Atlantic European Shelf (Galician sea, Gulf of Biscay, Armonican Sea, Celtic Sea, English Channel, North Sea), covering all four seasons (9 out of 12 months) at interannual scale. This is the very first intensive field study of continental shelves, in terms of source/sink for atmospheric CO2, which allows to integrate fluxes on an annual basis and over a large surface area. Here we show that European continental shelves are a sink of 90–170 million tons of carbon per year, which is an additional appreciable fraction to the presently proposed flux for the open North Atlantic Ocean (∼ 45%). The air-sea fluxes of CO2 we obtained are similar to those recently reported in the East China Sea, allowing us to conclude that the coastal ocean plays a considerable role in the global oceanic carbon cycle.

The Portugal coastal counter current off NW Spain: new insights on its biogeochemical variability

Abstract Time series of wind-stress data, AVHRR and SeaWiFS satellite images, and in situ data from seven cruises are used to assemble a coherent picture of the hydrographic variability of the seas off the Northwest Iberian Peninsula from the onset (September–October) to the cessation (February–May) of the Portugal coastal counter current (PCCC). During this period the chemistry and the biology of the shelf, slope and ocean waters between 40° and 43°N have previously been undersampled. Novel information extracted from these observations relate to: 1. The most frequent modes of variability of the alongshore coastal winds, covering event, seasonal and long-term scales; 2. The conspicuous cycling between stratification and homogenisation observed in PCCC waters, which has key implications for the chemistry and biology of these waters; 3. The seasonal evolution of nitrite profiles in PCCC waters in relation to the stratification cycle; 4. The Redfield stoichiometry of the remineralisation of organic matter in Eastern North Atlantic Central Water (ENACW)—the water mass being transported by the PCCC; 5. The separation of coastal (mesotrophic) from PCCC (oligotrophic) planktonic populations by a downwelling front along the shelf, which oscillates to and fro across the shelf as a function of coastal wind intensity and continental runoff; and 6. The photosynthetic responses of the PCCC and coastal plankton populations to the changing stratification and light conditions from the onset to the cessation of the PCCC.

Atmospheric CO2 flux from mangrove surrounding waters

[1] The partial pressure of CO2 (pCO2) was measured at daily and weekly time scales in the waters surrounding mangrove forests in Papua New Guinea, the Bahamas and India. The pCO2 values range from 380 to 4800 matm. These data, together with previously published data, suggest that overall oversaturation of CO2 with respect to atmospheric equilibriuminsurfacewatersisageneralfeatureofmangrove forests, though the entire ecosystems (sediment, water and vegetation) are probably sinks for atmospheric CO2. The computed CO2 fluxes converge to about +50 mmolC m � 2 day � 1 . If this conservative value is extrapolated for worldwide mangrove ecosystems, the global emission of CO2 to the atmosphere is about 50 10 6 tC year � 1 . Based on this tentative estimate, mangrove waters appear to be regionally a significant source of CO2 to the atmosphere and should be more thoroughly investigated, especially at seasonal time scale. INDEX TERMS: 4806 Oceanography: Biological and Chemical: Carbon cycling; 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4820 Oceanography: Biological and Chemical: Gases; 4231 Oceanography: General: Equatorial oceanography; 4235 Oceanography: General: Estuarine processes. Citation: Borges, A. V., S. Djenidi,

Inorganic and organic carbon biogeochemistry in the Gautami Godavari estuary (Andhra Pradesh, India) during pre‐monsoon: The local impact of extensive mangrove forests

tidal mangrove creeks in the delta. Here, variations in the concentration and d 13 Co f the DIC pool were shown to result largely from the mineralization of organic matter. The present study clearly identifies the mangrove creeks as an active site of mineralization and CO2 efflux to the atmosphere, but shows that these changes in the aquatic biogeochemistry are a localized feature, rapidly fading in the adjacent Kakinada Bay. Our data indicate that mineralization of dissolved organic carbon (DOC) of mangrove origin, and its subsequent efflux as CO2 to the atmosphere may represent an important fate for mangrove carbon. Although further quantification of this process in a variety of systems is required, we suggest that some of the current ideas on the role of mangroves in the carbon budget of the coastal zone may need to be reconsidered. INDEX TERMS: 4235 Oceanography: General: Estuarine processes; 4504 Oceanography: Physical: Air/sea interactions (0312); 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4815 Oceanography: Biological and Chemical: Ecosystems, structure and dynamics; KEYWORDS: mangroves, mineralization, stable isotopes

Daily and seasonal variations of the partial pressure of CO2 in surface seawater along Belgian and southern Dutch coastal areas

Abstract The variations of the partial pressure of CO2 (pCO2) and related parameters were determined in surface seawater along the Belgian coast, from January 1995 to June 1996, at both daily and seasonal time scales. The distribution of pCO2 in this area is regulated by river input from the Scheldt, biological activity and hydrodynamics. The contribution of each of these processes varies as a function of the considered time scale: (i) the daily variation of pCO2 depends on the tide although modulated by the biological diel cycle; (ii) the seasonal variation of pCO2 depends on the input from the Scheldt and the seasonal variations of phytoplanktonic biomass. During winter, the plume of the river Scheldt is oversaturated in pCO2 with respect to the atmosphere. During spring and summer, phytoplankton blooms occur both in the lower Scheldt estuary and in the river plume and may lead to undersaturation of pCO2 in the easternmost area of the river plume. However, the degradation of phytoplankton induces oversaturation of pCO2, in the westernmost area of the plume. Furthermore, the inter-annual variation of pCO2 depends partly on the fluctuations of the discharge of the Scheldt. Our preliminary results strongly suggest that, on an annual basis, the Scheldt plume behaves as a net source of CO2 to the atmosphere.

Distribution and air-water exchange of carbon dioxide in the Scheldt plume off the Belgian coast

In the present paper we report partial pressureof CO2 (pCO2) data obtained off theBelgian coast during 24 cruises. The temporaland spatial resolution of this data set allowsus to discuss satisfactorily seasonal andinter-annual variability of pCO2 in thestudy area. The dynamics of pCO2 aredescribed using two approaches: fixed referencestations and area survey cruises. The air-waterfluxes of CO2 in the Scheldt estuarineplume and in the outer-plume region areestimated quantitatively, showing that theseareas correspond respectively to a net annualsource and sink of atmospheric CO2. Theannually integrated air-water fluxes for theScheldt estuarine plume range between +1.1 and+1.9 mol m−2 year−1 as a function ofthe formulation of the exchange coefficient ofCO2. The annual net emission of CO2from the estuarine plume to the atmosphere isestimated to be between +2.3 to +4.0 Gmolyear−1 which represents 17 to 29% of theestimate reported in the literature for the Scheldtinner estuary.

Planktonic and Whole System Metabolism in a Nutrient-rich Estuary (the Scheldt Estuary)

Planktonic gross primary production (GPP), community respiration (CR), and nitrification (NIT) were measured monthly in the Scheldt estuary by the oxygen incubation method in 2003. No significant evolution of planktonic GPP was observed since the 1990s with high rates in the freshwater area (salinity 0; 97±65 mmol C m−2 d−1) decreasing seaward (22–37 mmol C m−2 d−1). A significant decrease of NIT was observed with regard to previous investigations although this process still represents up to 20% of total organic matter production in the inner estuary. Planktonic CR was highest in the inner estuary and seemed to be mainly controlled by external organic matter inputs. Planktonic net community production was negative most of the time in the estuary with values ranging from −300 to 165 mmol C m−2 d−1. Whole estuary net ecosystem production (NEP) was investigated on an annual scale using the results mentioned above and published benthic metabolic rates. A NEP of −39±8 mmol C m−2 d−1 was estimated, which confirms the strong heterotrophic status of this highly nutrified estuary. NEP rates were computed from June to December 2003 to compare with results derived from the Land-Ocean Interaction in the Coastal Zone budgeting procedure applied to dissolved inorganic phosphorus and carbon (DIP and DIC). DIP budgets failed to provide realistic estimates in the inner estuary where abiotic processes account for more than 50% of the nonconservative DIP flux. DIC budgets predicted a much lower NEP than in situ incubations (−109±31 versus −42±9 mmol C m−2 d−1) although, as each approach is associated with several critical assumptions, the source of this discrepancy remains unclear.

A new design of equilibrator to monitor carbon dioxide in highly dynamic and turbid environments

A new design of equilibrator for carbon dioxide monitoring in natural waters is described. It consists in a vertical tube filled with marbles through which water is flowing while equilibrating with a closed air circuit. It offers several advantages compared with classical equilibrators, among which is a fast response time (half-life constant approximately 30 s) and the potential to work in very turbid water. The proposed equilibrator is of particular interest to monitor carbon dioxide in coastal ecosystems, such as estuaries, which are known to be turbid and highly dynamic. Two performance tests and some field results are presented to illustrate the efficiency of the proposed system.

Off-shelf fluxes of labile materials by an upwelling filament in the NW Iberian Upwelling System

Daily changes in the concentrations of carbon and nitrogen species were monitored during the course of a Lagrangian drifter experiment in a recurrent upwelling filament south of Cape Finisterre (NW Iberian Upwelling System). A drifting buoy released at the southern edge of the upwelling centre generated by the Cape moved 60km southwestwards from 3 to 7 August 1998. Organic matter in the 50m deep study water mass (average 77±2 µM C) consisted of: 57µ MC of dissolved organic matter (DOM) with a C/N molar ratio of 19±2; 6µM C of DOM with a C/N ratio of 9±2, and 14µM C of 50% DOM and 50% suspended organic matter (POM susp) with a C/N ratio of 6.0±0.4. Net conversion of consumed inorganic salts into accumulated TOM=POMsusp+DOM was 40% for nitrogen and 30% for carbon. Since the parcel of water crossed the shelf-edge, these conversion efficiencies are equivalent to net horizontal export-ratio of 0.4 and 0.3 respectively. A second drifter was deployed in the offshore-end of the filament, and was displaced 20km west between 14 and 17 August 1998. Nitrate was exhausted in the surface water and no significant changes were observed in the variables measured during the course of the second experiment. Low C/N ratios (6.5±0.4) and rapid POMsusp/DOM inter-conversion in the 20 µM C excess observed in the study volume points to the persistence of the labile materials formed on the shelf during transport to the ocean. Our data demonstrate

Direct and Indirect pCO2 Measurements in a Wide Range of pCO2 and Salinity Values (The Scheldt Estuary)

Recent improvements in both Infra-red spectroscopy and equilibrator techniqueshave allowed to determine, for the first time, pCO2using simultaneously and continuously both the direct and indirect methods in an estuary where pCO2 values range from 500 to 8500 μatm and salinity from 0 to 30. Our results show that both methods are in excellent agreement in the wholeestuary (r2 = 0.999, n = 1075, p < 0.0001). Thus, the NBS (US National Bureau of Standards) scale, although inadequate for seawater samples, is appropriate for estuarine waters and can be applied with confidence to calculate pCO2.