Abstract
<title>Abstract</title> <p>Photochemical transformations of dissolved organic matter play a key role in regulating elemental cycles, ecosystems and trace gas production in the oceans. To assess the impact of photochemistry on these processes, global photochemical rates are often estimated from inshore or coastal data, or by using one measured rate to make global estimates. However, this approach is fraught with uncertainties and untested assumptions. To test the validity of these approaches, carbon monoxide (CO) photoproduction rates were determined in estuarine, coastal, and open-ocean waters of the Northwest Atlantic. Here we show that a significant nonlinear relationship was observed between chromophoric dissolved organic matter absorption coefficients and CO photoproduction rates. However, this relationship does not capture open-ocean variability, and it cannot be used to estimate open-ocean water rates which varied by nearly an order of magnitude demonstrating that extrapolation is unreliable even within waters of similar optical classification. This study emphasizes the need for photoproduction measurements across diverse marine environments to constrain global photochemical carbon fluxes and assess the impact of photochemistry on biogeochemical processes and organic carbon turnover in the oceans.</p>