Seasonal Water Mass Evolution and Non-Redfield Dynamics Enhance CO 2 Uptake in the Chukchi Sea.

The Chukchi Sea is an increasing CO 2 sink driven by rapid climate changes. Understanding the seasonal variation of air-sea CO 2 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 2 ( p CO 2 ) and discrete samples of biogeochemical properties collected in five consecutive cruises in 2014 and examine the seasonal variations in air-sea CO 2 flux and net community production (NCP). We found that thermal and non-thermal effects have different impacts on sea surface p CO 2 and thus the air-sea CO 2 flux in different water masses. The Bering summer water combined with meltwater has a significantly greater atmospheric CO 2 uptake potential than that of the Alaskan Coastal Water in the southern Chukchi Sea in summer, due to stronger biological CO 2 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 p CO 2 and DIC, which reveals that, during the intensive growing season (late spring to early summer), 30%-46% CO 2 uptake in the Chukchi Sea was supported by a flexible stoichiometry of phytoplankton. These findings have important ramification for forecasting the responses of CO 2 uptake of the Chukchi ecosystem to climate change.