Carbonate Chemistry and Air-Sea CO2 Flux in a NW Mediterranean Bay Over a Four-Year Period: 2007-2011
Résumé
The Service d'Observation de la Rade de Villefranche-sur-Mer is designed to study the temporal variability of hydrological conditions as well as the abundance and composition of holo- and meroplankton at a fixed station in this bay of the northwest Mediterranean. The weekly data collected at this site, designated as ``Point B'' since 1957, represent a long-term time series of hydrological conditions in a coastal environment. Since 2007, the historical measurements of hydrological and biological conditions have been complemented by measurements of the CO2-carbonic acid system parameters. In this contribution, CO2-carbonic acid system parameters and ancillary data are presented for the period 2007-2011. The data are evaluated in the context of the physical and biogeochemical processes that contribute to variations in CO2 in the water column and exchange of this gas between the ocean and atmosphere. Seasonal cycles of the partial pressure of CO2 in seawater (pCO(2)) are controlled principally by variations in temperature, showing maxima in the summer and minima during the winter. Normalization of pCO(2) to the mean seawater temperature (18.5 degrees C), however, reveals an apparent reversal of the seasonal cycle with maxima observed in the winter and minima in the summer, consistent with a biogeochemical control of pCO(2) by primary production. Calculations of fluxes of CO2 show this area to be a weak source of CO2 to the atmosphere during the summer and a weak sink during the winter but near neutral overall (range -0.3 to +0.3 mmol CO2 m(-2) h(-1), average 0.02 mmol CO2 m(-2) h(-1)). We also provide an assessment of errors incurred from the estimation of annual fluxes of CO2 as a function of sampling frequency (3-hourly, daily, weekly), using data obtained at the Hawaii Kilo Nalu coastal time-series station, which shows similar behavior to the Point B location despite significant differences in climate and hydrological conditions and the proximity of a coral reef ecosystem.