季風與大河主導之東海表層海水二氧化碳分壓時空變化與其控制因子量化分析

Abstract

目前對沿岸海洋二氧化碳吸收之季節轉換及控制過程，尤其是位在季風-大河系統之邊緣海域，還未完全量化明白，因此本研究利用2003 ~ 2011年東海8個不同月份航次之實測二氧化碳分壓數據，進行兩端點混合機制解析，並定量影響表水二氧化碳分壓時空變化的控制因子及評估其貢獻量。首先，東海表水二氧化碳分壓結果顯示，在秋季時有最高值（381.1 ± 10.6 μatm, n=2)，其次為夏(350.7 ± 31.8, n=3)、冬（342.8 ± 19.3, n=1)，春季最低（317.5± 39.2, n=2)。從兩端點混合機制量化發現，在夏季時有最強的生物作用消耗量(ΔpCO2_Bio，平均-31.0 μatm)，在冬季時則有最強的溫度效應影響(ΔpCO2_Temp，-144.3 μatm)與次表層垂直補充作用(∆pCO2_Vmix，100.8 μatm)；此三種作用分別與溫度變化量、長江流量以及混合層深度有良好的相關性 (R2分別為0.99、0.73、0.88, n=8)。東海在不同時期，表水二氧化碳分壓分布及其主控因子不一樣，暖季時，主要受控於溫度效應，其次為生物及垂直混合作用，溫度效應分別為兩者的1.8與1.7倍；而在冷季時，主要受控於溫度及垂直補充作用，生物作用甚弱，溫度效應分別是兩者的15與1.3倍。東海整體的二氧化碳海氣交換通量，強匯出現在春（-3.0 ± 0.6 mol C m-2 yr -1）冬冷季(-3.0 ± 0.7），弱匯在夏季（-0.5 ± 1.2, n=3)，而秋季則呈現弱源（0.2 ± 0.4，n=2)，整年平均為-1.6 ± 1.6。評估東海若無生物作用機制，整年通量會從原本的-1.6 mole C m-2 yr -1減弱至-1.0 mole C m-2 yr -1，使得東海對大氣二氧化碳的吸收能力降低約30%，此結果也與關掉溫暖季節時旺盛生物作用的長江沖淡水後計算之整年通量約-1.2 mol C m-2 yr -1近似，代表東海表水二氧化碳分壓變化最主要是受控於季風季節物理差異，而長江營養鹽物質輸入造成強烈生物作用的影響量僅為20%。

The quantification of controls on the seasonal alternation and dynamics of sea surface CO2 uptake in coastal oceans, especially for the monsoon-large river dominated marginal seas, has not yet been revealed. The study is to de-convolute quantitatively controlling processes with factor contribution on the spatiotemporal CO2 dynamics in the East China Sea (ECS) by use of the two-endmembers mixing method for analyzing the field partial pressure data of CO2 (pCO2) collected from 2003 to 2011 during the 8 different-month OR-1 cruises. First, the sea surface pCO2 in the ECS showed the highest values in autumn (381.1 ± 10.6 μatm, n=2), followed by summer (350.7 ± 31.8, n=3) and winter (342.8 ± 19.3, n=1). ), and the lowest in spring (317.5 ± 39.2, n=2). Quantification from the two-endmembers mixing analysis found that in the summer, there was the strongest bio-uptake (ΔpCO2_Bio, mean -31.0 μatm), and in the winter the strongest temperature effect (ΔpCO2_Temp, -144.3 μatm) and subsurface vertical mixing effect (∆pCO2_Vmix, 100.8 μatm); these three effects have significant correlations with the temperature change, Changjiang discharge, and mixed layer depth (R2 = 0.99, 0.73, 0.88, n = 8), respectively. In different periods of the ECS, the pCO2 distribution and main controlling factors were different. In the warm season, the main controlling factor was temperature effect which is 1.8 and 1.7 times larger than biological uptake and vertical replenishment ; while in the cold season, the entire ECS was mainly controlled by temperature effect and vertical mixing, and a little by biological uptake, the temperature effect is 15 and 1.3 times larger than them. In the ECS, as a whole, the annual CO2 uptake averaged -1.6 ± 1.6 mol C m-2 yr-1 with a strong sink in spring (-3.0 ± 0.6) and winter of cold season (-3.0 ± 0.7), a weak sink in summer (-0.5 ± 1.2), instead of a weak source in autumn (0.2 ± 0.4). If the bio-uptake mechanism in the ECS was shut down, the annual flux will be reduced from the normal -1.6 mol C m-2 yr -1 to -1.0, the ability of the ECS absorbing atmosphere CO2 will hence decrease ~30%. This result was also consistent with the annual -1.2 mol C m-2 yr -1 calculated after the CDW was replaced with the KW during the algal-bloom warm season. This indicates that the change in sea surface pCO2 in the ECS is mainly governed by the seasonally physical effects related to monsoon alternation, while the biological effect on the pCO2 uptake contributed from riverine nutrients by the Changjiang is only 20%.