| Summary: | 碩士 === 國立臺灣海洋大學 === 海洋環境化學與生態研究所 === 101 === Previous studies in the 1990s have shown that the entire East China Sea (ECS) shelf was a sink for atmospheric CO2 all year round. However, more recent studies suggested that the inner shelf near the Changjiang Estuary may act as a CO2 source in autumn in response to the increased eutrophication over recent decades. In order to explore the ECS shelf currently acting as a sink or source of atmospheric CO2 in autumn, comprehensive carbon chemistry data (total alkalinity (TA), dissolved inorganic carbon (DIC), pH , and partial pressure of CO2 (pCO2) and other pertinent data (temperature, salinity, nitrate, and dissolved oxygen) were measured in the entire ECS shelf in October 2011.
Results show that the spatial variations of DIC, pH, and TA range from 1913 to 2025 μmol kg-1, 7.906 to 8.043, and 2216 to 2267 μmol kg-1, respectively, corresponding well to the seasonal circulation pattern. In addition, the inner shelf area (water depth ≤ 60m) was a source for atmospheric CO2 with an average sea-air exchange flux of 7.0±6.1 mmolC m-2 day-1, while the middle and outer shelf areas (water depth > 60 m) were a sink with an average flux of -2.3 ± 1.5 mmolC m-2 day-1. The average ΔpCO2 (ΔpCO2 = surface seawater pCO2 – atmospheric pCO2) and sea-air CO2 exchange flux in the entire study area were about 5 matm and 0.6 mmolC m-2 day-1, respectively, indicating that the ECS shelf acted as a source of atmospheric CO2 in autumn as a whole. Overall, the entire study area may release 10240 tons CO2 per day into the atmosphere in autumn. Moreover, the inner shelf near the Changjiang Estuary in autumn has changed from CO2 sink area in the early 1990s to source area in the late 2000s. This decadal change may be associated with the increased eutrophication over the past decades. This would increase both the photosynthetic removal of CO2 in surface waters and the respiratory release of CO2 in bottom waters during summertime, thereby returning more CO2 to the surface during the subsequent mixing season in autumn.
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