Statistical Analysis of High-Frequency pCO2 Data Acquired with the ASTAN buoy (South-Western English Channel, Off Roscoff)

Since 2007, in the context of the SOMLIT network, we installed a CTD (SeaBird SBE19+) below the ASTAN buoy located in the Western English Channel (WEC) off Roscoff (48° 46′ 40 N 3° 56′ 15 W; depth 5 m) between the SOMLIT-Coast and SOMLIT-Offshore sampling sites. These sensors provided high-frequency (HF) measurements (hourly) of sea surface salinity (SSS), sea surface temperature (SST), fluorescence and dissolved oxygen (DO) all year. Since 2014, we added a SAMI-CO2 sensor, which measured the sea surface partial pressure of CO2 (pCO2) to investigate the seasonal dynamic of the pCO2 and associated air-sea CO2 exchange at the ASTAN site. From this high-frequency database, we were able to separate the signal according to the tide to determine the physicochemical and biological characteristics of two water masses: the coastal water mass (CWM) and the offshore water mass (OWM). These two water masses were strongly influenced by the biological productivity of the environment, more importantly in the CWM, and limited by the available light. Surface waters at the ASTAN site were near equilibrium with the atmosphere with an annual air-sea flux of 0.12 mmol C m−2 d−1 during 2016. The year can be divided into three distinct regimes: at the beginning of the year (winter period), the ASTAN site was a source of CO2, controlled by thermodynamic at 1.23 mmol C m−2 d−1, from April to September (spring–summer), it became a sink of CO2 with an important impact of biological activity (−0.64 mmol C m−2 d−1), and at the end of the year (fall), it reversed to a weaker source of CO2 at 0.26 mmol C m−2 d−1. In order to further assess the signal, we used signal processing tools such as wavelet analysis. From the frequency/time representation, it was possible to follow how the signal varied over time, and thus deduce the presence of the two water masses due to the tidal effect, but also observes a significant impact of the day/night cycle on the biological parameters.