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Surface Ocean and Lower Atmosphere Study—Air-Sea interactions and their climatic and environmental impacts
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Short-term dynamics of sea surface pCO2 in a large subtropical estuary system Wednesday 9th @ 1530-1550, Conference Room 7 Yangyang Zhao* , State Key Laboratory of Marine Environmental Science, Xiamen University Yi Xu, State Key Laboratory of Marine Environmental Science, Xiamen University Xianghui Guo, State Key Laboratory of Marine Environmental Science, Xiamen University Minhan Dai, State Key Laboratory of Marine Environmental Science, Xiamen University Presenter Email: yyzhao@stu.xmu.edu.cn |
The constraint of air-sea CO2 fluxes and their variability in coastal oceans at different timescales and spatial scales is at the base of the controversy concerning their role in the global carbon cycle. The lack of high-resolution sampling of the partial pressure of CO2 (pCO2) would largely underestimate monthly air-sea CO2 fluxes, even reverse the direction of the fluxes at annual level. However, mechanistic understanding towards the diurnal to weekly dynamics of pCO2 remains challenging due to the complicated interaction of biogeochemical and physical forcings in coastal waters. Here we examined the short-term variability of sea surface pCO2 recorded by a Battelle Seaology pCO2 Monitor amounted on a buoy located in the outer Pearl River estuary, a large subtropical estuary system in the northern South China Sea, from July 2 to August 11, 2015. Surface seawater pCO2 was highly variable, ranging from 78 to 640 μatm with an average of 279 μatm, mostly ~100 μatm lower than the atmospheric pCO2. On diel timescales, pCO2 changes of ~40-100 μatm occurred in response to diurnal primary production and respiration; temperature and tidal effects played a minor role. Over longer timescales, pCO2 was influenced by atmospheric forcing, physical advection and biological metabolism. For example, a surface cooling of ~3 ˚C and increase in pCO2 of ~110 μatm occurred after Typhoon Linfa passed near Hong Kong on July 9, 2015. The increase in pCO2 probably resulted from enhanced upward mixing of high-salinity, rich-inorganic carbon subsurface water. Subsequently, intense plume due to typhoon-induced strong freshwater discharge, coupled with elevated biological productivity, reduced pCO2 back to the pre-typhoon level. Overall, freshwater inputs, wind-driven upwelling and biological processes were the main drivers of short-term variability of pCO2 in the subtropical estuary, in contrast to temperate coastal ecosystems mostly subject to the thermal and tidal effects. Net fluxes of CO2 for the observation period (-4.8 mmol CO2 m-2 d-1) were directed from atmosphere to ocean, but with a wide range from ~-30 to +20 mmol CO2 m-2 d-1 depending largely on wind speed. We also found that gas exchange direction between the air-sea interface was closely related to wind direction when strong wind events occurred. Therefore, careful consideration of the short-term variability of surface seawater pCO2 and wind regime was needed to accurately estimate air-sea CO2 fluxes in the subtropical estuary system. |
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