Seasonal Water Mass Evolution and Non-Redfield Dynamics Enhance CO2 Uptake in the Chukchi Sea

Zhangxian Ouyang; Andrew Collins; Yun Li; Di Qi; Kevin R Arrigo; Yanpei Zhuang; Shigeto Nishino; Matthew P Humphreys; Naohiro Kosugi; Akihiko Murata; David L Kirchman; Liqi Chen; Jianfang Chen; Wei-Jun Cai

doi:10.1029/2021JC018326

Seasonal Water Mass Evolution and Non-Redfield Dynamics Enhance CO₂ Uptake in the Chukchi Sea

J Geophys Res Oceans. 2022 Aug;127(8):e2021JC018326. doi: 10.1029/2021JC018326. Epub 2022 Aug 4.

Authors

Zhangxian Ouyang¹, Andrew Collins^{1

2}, Yun Li¹, Di Qi^{3

4}, Kevin R Arrigo⁵, Yanpei Zhuang^{3

6}, Shigeto Nishino⁷, Matthew P Humphreys⁸, Naohiro Kosugi⁹, Akihiko Murata¹⁰, David L Kirchman¹, Liqi Chen⁴, Jianfang Chen⁶, Wei-Jun Cai¹

Affiliations

¹ School of Marine Science and Policy University of Delaware Newark DE USA.
² NOAA Pacific Marine Environmental Laboratory Seattle WA USA.
³ Polar and Marine Research Institute Jimei University Xiamen China.
⁴ Key Laboratory of Global Change and Marine-Atmospheric Chemistry of Ministry of Natural Resources Third Institute of Oceanography MNR Xiamen China.
⁵ Department of Earth System Science Stanford University Stanford CA USA.
⁶ Key Laboratory of Marine Ecosystem Dynamics Second Institute of Oceanography Ministry of Natural Resources Hangzhou China.
⁷ Institute of Arctic Climate and Environment Research Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Yokosuka Japan.
⁸ Department of Ocean Systems (OCS) NIOZ Royal Netherlands Institute for Sea Research Texel The Netherlands.
⁹ Meteorological Research Institute Tsukuba Japan.
¹⁰ Global Ocean Observation Research Center Research Institute for Global Change Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Yokosuka Japan.

Abstract

The Chukchi Sea is an increasing CO₂ sink driven by rapid climate changes. Understanding the seasonal variation of air-sea CO₂ 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 CO₂ (pCO₂) and discrete samples of biogeochemical properties collected in five consecutive cruises in 2014 and examine the seasonal variations in air-sea CO₂ flux and net community production (NCP). We found that thermal and non-thermal effects have different impacts on sea surface pCO₂ and thus the air-sea CO₂ flux in different water masses. The Bering summer water combined with meltwater has a significantly greater atmospheric CO₂ uptake potential than that of the Alaskan Coastal Water in the southern Chukchi Sea in summer, due to stronger biological CO₂ 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 pCO₂ and DIC, which reveals that, during the intensive growing season (late spring to early summer), 30%-46% CO₂ uptake in the Chukchi Sea was supported by a flexible stoichiometry of phytoplankton. These findings have important ramification for forecasting the responses of CO₂ uptake of the Chukchi ecosystem to climate change.