Image_1_Cardinal Buoys: An Opportunity for the Study of Air-Sea CO2 Fluxes in Coastal Ecosystems.pdf

From 2015 to 2019 we installed high-frequency (HF) sea surface temperature (SST), salinity, fluorescence, dissolved oxygen (DO) and partial pressure of CO₂ (pCO₂) sensors on a cardinal buoy of opportunity (ASTAN) at a coastal site in the southern Western English Channel (sWEC) highly influenced by tidal cycles. The sensors were calibrated against bimonthly discrete measurements performed at two long-term time series stations near the buoy, thus providing a robust multi-annual HF dataset. The tidal transport of a previously unidentified coastal water mass and an offshore water mass strongly impacted the daily and seasonal variability of pCO₂ and pH. The maximum tidal variability associated to spring tides (>7 m) during phytoplankton blooms represented up to 40% of the pCO₂ annual signal at ASTAN. At the same time, the daily variability of 0.12 pH units associated to this tidal transport was 6 times larger than the annual acidification trend observed in the area. A frequency/time analysis of the HF signal revealed the presence of a day/night cycle in the tidal signal. The diel biological cycle accounted for 9% of the annual pCO₂ amplitude during spring phytoplankton blooms. The duration and intensity of the biologically productive periods, characterized by large inter-annual variability, were the main drivers of pCO₂ dynamics. HF monitoring enabled us to accurately constrain, for the first-time, annual estimates of air-sea CO₂ exchanges in the nearshore tidally-influenced waters of the sWEC, which were a weak source to the atmosphere at 0.51 mol CO₂ m^–2 yr^–1. This estimate, combined with previous studies, provided a full latitudinal representation of the WEC (from 48°75′ N to 50°25′ N) over multiple years for air-sea CO₂ fluxes in contrasted coastal ecosystems. The latitudinal comparison showed a clear gradient from a weak source of CO₂ in the tidal mixing region toward sinks of CO₂ in the stratified region with a seasonal thermal front separating these hydrographical provinces. In view of the fact that several continental shelf regions have been reported to have switched from sources to sinks of CO₂ in the last century, weak CO₂ sources in such tidal mixing areas could potentially become sinks of atmospheric CO₂ in coming decades.