Abstract
From 2015 to 2019 we installed high-frequency (HF) sea surface temperature (SST), salinity, fluorescence, dissolved oxygen (DO) and partial pressure of CO 2 (pCO 2) 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 pCO 2 and pH. The maximum tidal variability associated to spring tides (>7 m) during phytoplankton blooms represented up to 40% of the pCO 2 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 pCO 2 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 pCO 2 dynamics. HF monitoring enabled us to accurately constrain, for the first-time, annual estimates of air-sea CO 2 exchanges in the nearshore tidally-influenced waters of the sWEC, which were a weak source to the atmosphere at 0.51 mol CO 2 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 CO 2 fluxes in contrasted coastal ecosystems. The latitudinal comparison showed a clear gradient from a weak source of CO 2 in the tidal mixing region toward sinks of CO 2 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 CO 2 in the last century, weak CO 2 sources in such tidal mixing areas could potentially become sinks of atmospheric CO 2 in coming decades.
Highlights
The dynamics of the carbonate system in the ocean are complex and simultaneously controlled by physical, chemical, and biological processes (Zeebe and Wolf-Gladrow, 2001)
We used the bimonthly discrete measurements of sea surface salinity (SSS), dissolved oxygen (DO) and Chl-a from the SOMLIToffshore station to determine whether post-calibration of the corresponding sensors of the SBE16 + deployed at the ASTAN buoy was necessary
These samples were collected during high tide slack when the ASTAN and SOMLIT-offshore surface waters had similar biogeochemical properties according to our transect data
Summary
The dynamics of the carbonate system in the ocean are complex and simultaneously controlled by physical, chemical, and biological processes (Zeebe and Wolf-Gladrow, 2001). In terms of anthropogenic CO2 sink, coastal ecosystems represent 4.5% (Bourgeois et al, 2016) of the latest estimates of 2.6 ± 0.3 Pg C yr−1 for the 1994–2007 period (Gruber et al, 2019) and 2.6 ± 0.6 Pg C yr−1 for the last decade (Friedlingstein et al, 2019). Due to their proximity with human activities, coastal ecosystems are vulnerable to anthropogenic forcing such as eutrophication and ocean acidification (OA) (Borges and Gypens, 2010; Borges et al, 2010; Cai et al, 2011, 2017; Bauer et al, 2013). Coastal ecosystems can show extremes of OA hotspots due to the intrusion of acidified water with low saturation state arag (Feely et al, 2016, 2010; Chan et al, 2017; Fennel et al, 2019) or constitute refuge with more stable pH (Chan et al, 2017)
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