The sources and sinks of carbon monoxide in the St. Lawrence estuarine system

The rates of photoproduction, dark production, microbial consumption, and air–water exchange of dissolved carbon monoxide (CO) were independently assessed for the first time for a large sub-arctic estuarine water body, the Canadian St. Lawrence estuary system. Areal CO photoproduction in all four seasons increases from the Upper Estuary to the Lower Estuary to the northwestern (NW) Gulf of St. Lawrence. Areal microbial CO consumption and water-to-air flux follow the same order as photoproduction except in spring when the sequence of the Lower Estuary and the NW Gulf is reversed. Areal CO dark production tracks the ascending order of Lower Estuary<NW Gulf<Upper Estuary. Seasonally, the areal CO photoproduction and microbial CO consumption are highest in summer followed sequentially by spring, autumn, and winter. The areal CO dark production and water-to-air flux are in the ascending sequence of winter<spring<autumn<summer. Annual CO photoproduction, dark production, microbial consumption, and water-to-air flux in the St. Lawrence estuary and NW Gulf are estimated to be 4.91, 0.78, 5.45, and 0.44–0.86 Gg (gigagram) (Gg, 1 Gg=109 g) CO-C, respectively. These independently estimated source (5.69 Gg CO-C yr−1) and sink (5.89–6.31 Gg CO-C yr−1) strengths are approximately in balance. Photoproduction accounts for 86% of the total CO source and microbial consumption takes up >86% of the total CO loss term, leaving dark production and outgassing to be the minor source and sink terms, respectively. However, the dark source is similar in magnitude to the photochemical source in the organic-rich Upper Estuary. The photochemical dissolved organic carbon (DOC) loss in the study area was elaborated using CO as a proxy of photoproduction of CO2 and biolabile DOC.