Biological control of pCO2 at station L4 in the Western English Channel over 3 years

Understanding carbon fluxes in shelf systems is an important aspect of quantifying global carbon budgets. Three years of pCO2 observations are analysed from spring to autumn during 2005, 2007 and 2008 at L4, a seasonally stratified station in the Western English Channel. A general trend from low to high seawater pCO2 during each year was observed, punctuated by episodic low seawater pCO2 events. Air–sea CO2 flux dynamics derived from seawater pCO2 showed spring and summer to be times of atmospheric CO2 drawdown during stratified water conditions while autumn saw the breakdown of stratification and CO2 outgassing. The largest CO2 instantaneous drawdown was observed during high wind events. Seawater pCO2 at L4 is controlled by metabolic processes, solubility and advection processes, although to a varying extent between years. While tidal influence, movement of water masses and rapid phytoplankton blooms contribute to large pCO2 fluctuations between adjacent samples, distinct quasi-seasonal phases are observed due to the natural physical and biological cyclic controls on seawater pCO2.