2006年秋季東海表水二氧化碳之空間分佈與控制機制探討

Abstract

本研究主要是探討2006年秋季東海表水二氧化碳之空間分佈、控制機制與海氣交換通量之變化。本研究於2006年11月6日至16日利用海研一號815航次，在東海進行現場偵測海水與大氣中之二氧化碳分壓 (fCO2) 和水文參數。表水fCO2 範圍介於322.9 ~ 519.4 μatm，平均值為359.8 ± 20.3 μatm (n = 1425)。秋季 fCO2 在空間上具有獨特分佈，主要分四區 (長江口區、陸棚沿岸區、陸棚區與台灣東北海域)，最高值出現在較低溫、低鹽之長江口區並有一個明顯向外海遞減的空間分佈，平均值為 422.1 ± 39.6 μatm，此區域同時也是營養鹽及葉綠素a峰值區；其他區域(陸棚沿岸區、陸棚區與台灣東北海域) 呈現較均值之空間分佈，平均值分別為367.1 ± 9.1 μatm、356.1 ± 11.7 μatm 與376.4 ± 11.7 μatm。依據過往溫度與葉綠素a所建立的經驗公式，將溫度、生物和垂直混合補充之作用對以上區域 fCO2之貢獻度進行分離，顯示溫度與生物的作用在各個區域變化差異少於20 μatm，而垂直混合之補充對各區域貢獻差異可大於100 μatm，這意味著秋季東海的空間分佈主要受控於垂直混合的補充。在夏季時沖淡水 (River plume) 內高的基礎生產力雖然增強了二氧化碳的吸收能力，但累積在次表層的有機碳並沒有完全移除至深海，至秋季時次表層高fCO2的海水帶至表層，提高表水fCO2之分佈，並抑制秋季東海對fCO2的吸收能力，甚至在長江口區驅使fCO2釋放至大氣。秋季東海陸棚區之二氧化碳海氣交換通量為 -0.7 ± 0.9 mol/m2/yr，是大氣二氧化碳的弱「匯」 (weak sink)。觀測結果與過往文獻中提到秋季東海是大氣二氧化碳的「源」 (source) 並不相符，主要因為過往文獻只侷限於長江河口和北東海域等小區域的研究，並不能代表整體東海的值。東海海氣交換通量在空間與時間分佈上有明顯的差異，並且受到人為與自然環境之因子的干擾，如何把每個機制分離出來，值得進一步研究。

The spatial distribution of sea surface fCO2 (fCO2w) with its controlling mechanisms and related air-sea exchange fluxes in the East China Sea (ECS) in fall 2006 was investigated. We performed the underway measurements of the air and sea surface fCO2, and hydrographic variables from November 6th to 16th 2006 during the field survey (OR1-815). The results showed that fCO2w ranged from 322.9 to 519.4μatm with an average of 359.8±20.3μatm (n=1425). The fCO2w in fall spatially exhibited a unique distribution pattern with four distinctive areas (e.g., Changjiang estuary (averaged: 422.1±39.6μatm), China coastal (367.1±9.1μatm), shelf (356.1±11.7μatm) and upwelling waters in northeastern Taiwan (376.4±11.7μatm). The highest fCO2w (>400μatm) was observed within the outer Changjiang estuary, which waters were cold and less saline with high chlorophyll-a and nutrient concentrations. Additionally, fCO2w levels apparently decreased from near-shore river mouth to offshore regions with contents which were near the whole averaged values. Based on empirical algorithms developed from SST and chlorophyll-a data in summer, the contributions of temperature, biological and vertical mixing effects on spatial fCO2 variations could be respectively estimated in four aforementioned areas of the ECS shelf. Consequently, variations on contributions of temperature and biological effects were less than 20μatm respectively among them, but that due to vertical mixing effect was greater than 100μatm. It suggests vertical mixing as the main factor controls the spatial fCO2 variation in the ECS in fall. In summer, a high CO2 uptake via biological activity takes place within the Changjiang plume in which high nutrients induce high biomass growth, and finally the CO2 was stored in subsurface and bottom layers of inner ECS shelf instead of being exported to the deep ocean. Further, the enriched-CO2 waters in subsurface layer would be brought to the surface in fall via vertical mixing. This effect would enhance the fCO2w contents in the surface, further hinder the CO2 uptake, and finally result in a net release of CO2 to the atmosphere from Changjiang plume area. However, the whole ECS shelf in fall acted as a weak sink of atmospheric CO2 with an averaged air-to-sea flux of -0.7 ± 0.9 molm-2yr-1. The results we obtained are not consistent with those reported by previous studies, which show the ECS as a CO2 source to the atmosphere in fall. It indicates small-scale spatial surveys which were previously performed in Changjiang estuary and northern ECS can’t be a representative of the whole ECS shelf.