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- Title
Seasonal Water Mass Evolution and Non‐Redfield Dynamics Enhance CO<sub>2</sub> Uptake in the Chukchi Sea.
- Authors
Ouyang, Zhangxian; Collins, Andrew; Li, Yun; Qi, Di; Arrigo, Kevin R.; Zhuang, Yanpei; Nishino, Shigeto; Humphreys, Matthew P.; Kosugi, Naohiro; Murata, Akihiko; Kirchman, David L.; Chen, Liqi; Chen, Jianfang; Cai, Wei‐Jun
- 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. Plain Language Summary: The Chukchi Sea has been suggested to take more CO2 from the atmosphere as a result of decreased sea ice coverage and increased inflow of nutrient‐rich Pacific Water. In order to better understand the seasonal variations in CO2 uptake and net community production (NCP) on the Chukchi shelf, we examined the data of sea surface partial pressure of CO2 and biogeochemical properties collected in five consecutive cruises from spring to fall in 2014. We found that the nutrient‐rich Bering Summer Water combined with meltwater has a larger CO2 uptake potential than that of the nutrient‐poor Alaska Coastal Water. In addition, we estimated NCP based on the seasonal drawdown of dissolved inorganic carbon and nutrients, and found that NCP derived from carbon deficit was consistently higher than NCP derived from NO3− ${\text{NO}}_{3}^{-}$. We attributed this inconsistency to a high C:N uptake ratio because phytoplankton growth may not always follow the canonical Redfield ratio. With a model simulation, we further quantified that this non‐ Redfield C:N uptake in phytoplankton enables more efficient carbon‐fixation and contributes 30%–46% CO2 uptake from atmosphere during the intensive growing season in the Chukchi Sea. Key Points: Atmospheric CO2 uptake potential is larger in the nutrient‐rich non‐Alaska Coastal Water than the nutrient‐poor Alaska Coastal WaterFor the most intensive growing period (spring to early summer), net community production estimation was 66%–84% higher based on dissolved inorganic carbon (DIC) than NO3− ${\text{NO}}_{3}^{-}$A non‐Redfield C:N uptake ratio by phytoplankton enables more efficient DIC‐fixation and contributes 30%–46% of CO2 uptake
- Subjects
WATER masses; CLIMATE feedbacks; SEA ice; CARBON fixation; SEASONS; SPRING; CLIMATE change; ATMOSPHERIC carbon dioxide
- Publication
Journal of Geophysical Research. Oceans, 2022, Vol 127, Issue 8, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2021JC018326