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

• Published in: Frontiers in Marine Science
• Main Authors: Jean-Philippe Gac, Pierre Marrec, Thierry Cariou, Christophe Guillerm, Éric Macé, Marc Vernet, Yann Bozec
• Format: Article in Journal/Newspaper
• Language: English
• Published: Frontiers Media S.A. 2020
• Subjects: buoy of opportunity; high-frequency; tidal cycle; multi-annual; air-sea CO2 exchanges; ocean acidification; Science; Q; General. Including nature conservation; geographical distribution; QH1-199.5; Ocean acidification
• Online Access: https://doi.org/10.3389/fmars.2020.00712; https://doaj.org/article/fa6ae1ea12c34599b217b98cc6330070

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 ...