The seasonal blooming characteristics of phytoplankton and POC export flux in the waters around South Georgia Island: Based on BGC-Argo and satellite remote sensing observations

doi:10.3969/j.issn.1001-909X.2023.04.001

Abstract

Abstract:

The waters surrounding South Georgia Island are one of the highest primary productivity regions in the Southern Ocean with enormous carbon sequestration potential. However, the strength of the biological pump efficiency in this area is still uncertain due to the lack of continuous upper ocean observation data.In this study, the hydrological and biogeochemical parameters obtained from the Biogeochemical Argo (BGC-Argo) floats deployed in the South Georgia Island vicinity during the period of 2017-2020 were utilized to investigate the impacts of physical processes on biogeochemical processes and to estimate the carbon export flux in the Antarctic summer. Results indicated that both upstream (northeast of the Antarctic Peninsula) and downstream (Georgia Basin) regions of South Georgia Island exhibited strong seasonal characteristics in Chl-a, with the latter area having a 4-month sustained period of phytoplankton bloom, suggesting a stable and continuous supply of iron. Using the temporal variability of the seasonal particulate organic carbon (POC) export, the summer POC export fluxes of the upstream and downstream regions were estimated to be 7.12±3.90 mmol·m^-2·d^-1 and 45.29±5.40 mmol·m^-2·d^-1, respectively, indicating that the difference might be due to enhanced downward export of organic carbon after the deepening of the mixed layer. The study found that the region maintained a high biological pump efficiency, contrary to the previous conclusion that the Georgia Basin had “high productivity low export efficiency”, which might have been caused by the limited “real-time” representation of the entire seasonal characteristics during ship-based surveys. BGC-Argo provides high spatiotemporal resolution of multi-parameter observation data, and this study demonstrates that it can more accurately quantify and evaluate marine biogeochemical processes and carbon sequestration potential.

Key words: Southern Ocean, South Georgia Island, BGC-Argo, POC export production, Chl-a

CLC Number:

P734

ZHAO Yueran, FAN Gaojing, WU Jiaqi, SUN Weiping, PAN Jianming, HAN Zhengbing. The seasonal blooming characteristics of phytoplankton and POC export flux in the waters around South Georgia Island: Based on BGC-Argo and satellite remote sensing observations[J]. Journal of Marine Sciences, 2023, 41(4): 1-11.

Figures/Tables 7

Fig.1 Study area and the movement trajectory of BGC-Argo floats (Contour lines represent water depth, unit: m. Blue dots represent the profiling float stations. The thick black line and red line indicate the southern Antarctic circumpolar current front (SACCF) and polar front (PF), respectively. Color bar uses log transformation, and the color represents the climatological distribution of Chl-a from December to February in the southern hemisphere from 1997 to 2017. The data are from the multi-sensor merged product of the European Space Agency’s GlobColour project with a spatial resolution of 25 km × 25 km and a temporal resolution of 1 month.)

Tab.1 Summer mean values of MLD, POC storage, NCP, and POC export flux based on BGC-Argo

年份	位置	混合层深度/m	POC储量/(mmol·m^-2)	NCP/(mmol·m^-2)	POC输出通量/(mmol·m^-2·d^-1)
2017/2018	南极半岛东北部海域	38.49±18.93	610.32±124.83	3 976.14±124.53	7.12±3.90
2018/2019	乔治亚海盆	62.40±24.55	691.62±158.92	4 591.86±316.12	45.29±5.40

Fig.2 Profiles of biogeochemical parameters of BGC-Argo floats (The white dashed line represents the depth of the mixed layer.)

Fig.3 Comparison of time series of Chl-a mass concentration observed by remote sensing and BGC-Argo

Fig.4 Floating bar chart of the difference in Chl-a mass concentration between 5 m and 50 m observed by BGC-Argo (The top and bottom of the bars represent Chl-a5 m or Chl-a50 m respectively. Green bars indicate that Chl-a5 m is higher than Chl-a50 m, while orange bars indicate the opposite, and the length of the bars represents the difference.)

Fig.5 Time series of NCP and POC storage in the upper 100 m of the waters surrounding South Georgia Island

Fig.6 Time series of POC export in the waters near South Georgia Island (The shaded area represents the summer seasons of 2017/2018 and 2018/2019; the trend line shows the temporal variation of POC export)

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