Photobiogeochemical cycling of carbon monoxide in the southeastern Beaufort Sea in spring and autumn

We investigated the distribution, photoproduction, microbial uptake, and air–sea exchange of carbon monoxide (CO), a key photoproduct of chromophoric dissolved organic matter (CDOM), in open waters of the southeastern Beaufort Sea in autumn 2003 and spring 2004. Diurnal cycles of surface water CO concentration ([CO]) occurred in autumn but not in spring. In both seasons [CO] was well above air-equilibrium at most stations (maximum of 12,500% saturation) and dropped with depth to undetectable levels below 50 m. Mean surface water [CO] and CO water-column burdens (0–50 m) were 0.45 nmol L21 and 5.0 mmol m22 in autumn and 4.7 nmol L21 and 64.8 mmol m22 in spring, and the sea-to-air CO flux was 33 times higher in spring. The efficiency of CO photoproduction correlated linearly with CDOM across the Mackenzie River estuary, the Mackenzie Shelf, and the Amundsen Gulf. Modeled water-column CO photoproduction in spring was 15 times that in autumn (45.8 vs. 3.0 mmol m22 d21). Microbial CO uptake followed first-order kinetics in autumn while Hill-type, saturation, and inhibition kinetics were common in surface waters in spring. Bio-oxidation was the dominant CO loss term in autumn while gas exchange was almost equally important in spring. Higher photoproduction and slower bio-uptake in spring resulted in the wide autumn–spring differences in the [CO] distribution pattern and air–sea CO flux. CO cycling in cold northern waters differs both quantitatively and qualitatively from that in warmer seas. Carbon monoxide (CO) is the dominant sink for hydroxyl radicals in the troposphere, thus affecting the lifetimes of greenhouse gases such as methane (Thompson 1992). The ocean has long been recognized as a source of atmospheric CO (Swinnerton et al. 1970; Bates et al. 1995; Stubbins et al. 2006a), albeit with large uncertainties in its source strength. Estimates of global open-ocean CO photoproduction also span an enormous range (30–820 Tg CO-C yr21), with recent studies giving lower values (30– 90 Tg CO-C yr21; Zafiriou et al. 2003; Stubbins et al. 2006b). Photoproduction and microbial uptake are the primary source and sink for oceanic CO (Conrad et al. 1982), and the interplay between these processes results in large diurnal variations in CO concentration ([CO]) in the upper ocean (Conrad et al. 1982; Bates et al. 1995; Zafiriou et al. 2008). CO is the second most abundant inorganic carbon product (after dissolved inorganic carbon [DIC]) resulting from the photodecomposition of chromophoric dissolved organic matter (CDOM) in marine waters, and it is thereby a significant component of the oceanic carbon cycle (Mopper and Kieber 2000). CO has been suggested as a proxy for the photoproduction of DIC (Miller and 1 Corresponding author ([email protected]). Acknowledgments O. Zafiriou and M. Scranton each loaned a CO analyzer and O. Zafiriou also loaned titanium samplers and commented on the manuscript. T. Lou and M. Norroy assisted in sample collection and analysis. C. Osburn helped with collecting the Arctic RiverDelta Experiment (ARDEX) samples and J. Caveen with MATLAB programming. We thank the captains and crews of the Canadian Arctic Shelf Exchange Study (CASES) and ARDEX cruises, and all of our colleagues in these programs, for their cooperation. We are also grateful to two anonymous reviewers for their insightful comments on the manuscript. This study was supported by grants from the Natural Sciences and Engineering Research Council, Fonds Québécois pour la Recherche sur la Nature et les Technologies, and the Canada Foundation for Innovation. This is a contribution to the CASES research network and the Institut des Sciences de la Mer de Rimouski. Limnol. Oceanogr., 54(1), 2009, 234–249 E 2009, by the American Society of Limnology and Oceanography, Inc.