Aqueous Carbon Monoxide Cycling in a Fjord-Like Estuary

Dissolved carbon monoxide, [CO], was measured in oxic surface waters and in the anoxic layer of the Pettaquamscutt River, Rhode Island, from March to August 2008. Samples were collected at near-shore locations to examine spatial and seasonal changes at solar noon. Each month, a set of diel samples was collected at the surface stations to evaluate photoproduction and biological processing. In July and August, anoxic samples from depths >6 m were collected to examine the presence of an active anaerobic CO metabolism. The surface [CO] decreased from 65 to 5 nmol kg−1 from spring to summer, which was attributed to a decline in dissolved organic matter. Diel [CO] showed a strong mid-afternoon maximum with a late evening and early morning minimum. Inferred first-order loss rates, attributed to biological processing, ranged from 0.1–0.6 h−1 without a clear seasonal pattern. [CO] saturation ratios were typically >200 at mid-day, >7 in morning/late evening, and never below 1, implying the river is always a net source of CO to the atmosphere. [CO] in the anoxic layer averaged 2 nmol kg−1.