Summer near-surface seawater sampling in the Canadian Arctic revealed potential for significant errors (nearly 0.1 μmol·(m -2 s -1 )) in CO 2 fluxes calculated from measured air-sea CO 2 gradients. River runoff and sea ice melt strongly stratify these waters, often resulting in surface mixed layers only a few meters thick and isolated from waters sampled by shipboard underway systems. Samples collected with the underway system, rosette, and small boats exposed substantial near-surface gradients

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... in CO 2 partial pressure (pCO 2 ) over the top 7 m at many stations. Distributions of temperature, salinity, and fluorescence indicated that the sources of the CO 2 system gradients varied between stations, precluding simple corrections to align subsurface data with shallower conditions. Overall, the strong summertime sink of atmospheric CO 2 implied by the underway data was not supported by shallower data.