Seasonal and latitudinal variability of the CO2 system in the western English Channel based on Voluntary Observing Ship (VOS) measurements

We investigated the dynamics of the CO2 system across the Western English Channel (WEC) between Roscoff (France) and Plymouth (UK) using a Voluntary Observing Ship (VOS). From December 2010 to December 2011, 20 return crossings were carried out to collect a comprehensive dataset of CO2 system parameters and ancillary data. The hydrographical structure of the water column across the latitudinal transect was investigated at 3 fixed stations: ASTAN (southern WEC, offshore Roscoff), E1 and L4 (northern WEC, offshore Plymouth). Based on these profiles, we defined two provinces, the stratified northern WEC (> 49.5°N) and the well-mixed southern WEC (< 49.5°N), which were periodically separated by a thermal front. These contrasted hydrographical properties strongly influenced the ecosystem dynamics. Biological production/respiration processes were the main driver of pCO2 variability during the year except for winter cooling in the northern WEC. The seasonally stratified northern WEC showed enhanced biological activities characterized by an extensive autotrophic phase, which maintained the pCO2 below the atmospheric equilibrium until early fall and acted as sink for atmospheric CO2 at a rate of 1.1 mol C m– 2 y– 1. The permanently well mixed southern WEC was characterized by a shorter autotrophic phase due to a delayed spring phytoplankton growth and an early start of the fall heterotrophic phase, resulting in an annual air-sea CO2 flux close to equilibrium at a rate of − 0.4 mol C m– 2 y– 1. On annual scale, calculation of Net Ecosystem Production (NEP) revealed that surface waters at E1 and ASTAN were both autotrophic at rates of 1.5 mol C m– 2 y– 1 and 1.0 mol C m– 2 y– 1, respectively. Our latitudinal approach resolved the discrepancy between the directions of the fluxes in the WEC observed in previous studies by differentiating between the hydrological regions. The combined approach of using data from VOS tracks and fixed coastal observatories stations provided new insights into the control of air-sea CO2 fluxes in the different provinces of the WEC. This combined approach could be applied in other continental shelf systems where data on the CO2 system are sparse.

Marrec P., Cariou T., Collin E., Durand A., Latimier M., Macé E., Morin P., Raimund S., Vernet M. & Bozec Y., in press. Seasonal and latitudinal variability of the CO2 system in the western English Channel based on Voluntary Observing Ship (VOS) measurements. Marine Chemistry. Article (subscripton required).

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