Carbon monoxide and chromophoric dissolved organic matter cycles in the shelf waters of the northern California upwelling system

Abstract

An annual program of approximately bimonthly seawater sampling was conducted off the coast of the Bodega Marine Laboratory in the California Current upwelling system (CCUS) to investigate controls on dissolved carbon monoxide (CO) and its photochemical parent, chromophoric dissolved organic matter (CDOM). A diel steady state model was developed to quantify CO sources and sinks throughout the day. Observed midday CO spanned 2.7–17 nmol L−1, and correlated strongly with coincident CDOM absorption; however, rapid microbial consumption rates (0.05–0.80 h−1) were such that ∼95% of the CO generated was metabolized within the ocean surface. Further indication of the dominant influence of the microbial ecosystem was the persistent observation of a strong “dark production,” which contributed ∼25% of the diurnal source strength within the top 17 m of the ocean. The dark source correlated strongly with biological oxidation rates and CDOM, suggesting that it is the result of incomplete respiration of biologically labile DOM. CDOM was lowest in recently upwelled waters, implying that its origin is from the induced primary production and not from shelf sediment resuspension. The estimated CO air‐sea flux was highly variable (average of 5 μmol m−2 d−1), and most likely inconsequential to the atmospheric budget in coastal regions. The photolytic source strength of CO2 from CDOM (2.7 mmol m−2 d−1) was estimated by scaling to modeled CO photoproduction rates. This photolytic remineralization rate represents ∼2.5% of the net primary production in the CCUS, implying that CDOM photobleaching may play a significant role in the carbon cycling of upwelling systems.