Seasonal Water Mass Evolution and Non-Redfield Dynamics Enhance CO Uptake in the Chukchi Sea.

The Chukchi Sea is an increasing CO sink driven by rapid climate changes. Understanding the seasonal variation of air-sea CO exchange and the underlying mechanisms of biogeochemical dynamics is important for predicting impacts of climate change on and feedbacks by the ocean. Here, we present a unique data set of underway sea surface partial pressure of CO (CO) and discrete samples of biogeochemical properties collected in five consecutive cruises in 2014 and examine the seasonal variations in air-sea CO flux and net community production (NCP). We found that thermal and non-thermal effects have different impacts on sea surface CO and thus the air-sea CO flux in different water masses. The Bering summer water combined with meltwater has a significantly greater atmospheric CO uptake potential than that of the Alaskan Coastal Water in the southern Chukchi Sea in summer, due to stronger biological CO removal and a weaker thermal effect. By analyzing the seasonal drawdown of dissolved inorganic carbon (DIC) and nutrients, we found that DIC-based NCP was higher than nitrate-based NCP by 66%-84% and attributable to partially decoupled C and N uptake because of a variable phytoplankton stoichiometry. A box model with a non-Redfield C:N uptake ratio can adequately reproduce observed CO and DIC, which reveals that, during the intensive growing season (late spring to early summer), 30%-46% CO uptake in the Chukchi Sea was supported by a flexible stoichiometry of phytoplankton. These findings have important ramification for forecasting the responses of CO uptake of the Chukchi ecosystem to climate change.