潮汐作用對東海生地化循環及初級生產力之影響; The effect of tidal action on the biogeochemical process and the primary production in the East China Sea

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Title:	潮汐作用對東海生地化循環及初級生產力之影響;The effect of tidal action on the biogeochemical process and the primary production in the East China Sea
Authors:	葉子嫈;Tzu-ying Yeh
Contributors:	水文與海洋科學研究所
Keywords:	數值模擬;東海;生地化;潮汐;East China Sea;Model;Biogeochemical;tide
Date:	2011-07-04
Issue Date:	2012-01-05 14:11:31 (UTC+8)
Abstract:	本研究利用東海的三維物理-生地化耦合模式，模擬東海之營養鹽(溶解性無機氮，簡稱DIN)、葉綠素-a濃度分布與初級生產力。原先之模式結果(Liu et al., 2010)與Gong et al. (2003)在東海觀測所得不同季節之平均初級生產力相比較，在時間變化的趨勢上大致相符。但必須藉著微推(nudge)的方式趨向長期平均DIN，才能正確的摸擬營養鹽分布；若任由模式自由運算，則會導致表層營養濃度偏低，進而使總初級生產力也過低，模擬之年平均生產力僅及觀測值的67.5%。而在長江口附近因長江排出大量DIN，所以硝酸鹽的模擬結果較高，使得所推算出之葉綠素濃度和初級生產力也較實際觀測結果要高；而對外部陸棚及開放海域的模擬，則硝酸鹽濃度偏低而使得推算出來的葉綠素濃度及初級生產力值也偏低。因為長江口是一個中等強度的潮汐河口，潮汐對長江水舌分布有一定程度之影響；故本研究的目的在於將原本的物理模式加入潮汐作用，探討其對生地化參數的分布影響，並與實際觀測結果比較。模式中的潮汐作用是在開放邊界上，將潮汐所造成的水位變化加在邊界條件上，令其作用傳入模式範圍之中。潮汐作用讓長江水舌擺盪及擴散情形更接近實際觀測現象，也使得營養鹽在水平混合及垂直混合均較原來強烈，也更接近真實情形。由DIN積分(0?60m)來看，加入潮汐作用並不會使得原本的營養鹽含量有太大的差別，而是使得上層水體之營養鹽儲量增加，有利光合作用。再由DIN對深度的變化考慮，潮汐作用明顯地加強垂直混合作用，因而使次表層營養鹽因混合作用進入表層。表層營養鹽的濃度增加使得全年的初級生產力估算較原本提高了40%，更接近實測結果。所估算的初級生產力年平均值可達觀測值之93%。由0?30m之營養鹽深度來看，加入潮流作用之後，平均濃度約由1?M增加為2?M，經由米-曼酶動力學(Michaelis–Menten kinetics)來看，藻類生長率可達最高生長率之91%，接近模式結果與實際觀測值之比較。 Abstract A coupled physical-biogeochemical model has been used to simulate the distributions of nutrient(i.e. DIN, dissolved inorganic nitrogen) and chlorophyll(Chl-a) in the East China Sea and to predict primary productivity(Liu et al.,2010). Concentrations of DIN, Chl-a and the integrated primary production(IPP) are similar to observations of Gong et al.(2003). However, the model relies on nudging towards observed climatological distribution DIN. Without nudging, the modeled DIN is much higher than observations near the Changjiang river mouth and much lower than observations in most of the far field. The mostly underestimated DIN causes underestimating Chl-a and PP. The high values in the near field result from the high DIN concentration of the ChangJiang run off prescribed for the model. Since Changjiang estuary is an estuary of medium tidal range, its tidal action can influence the dispersal of Changjiang plume. In this study, we add the tidal action in the ECSmodel to explore how tidal action influences the biochemistry of the East China Sea and compare the output with observations. The tidal action is incorporated in the model by prescribing the sea level changes caused by tides on the open boundaries, so that the tidal actions may propogate into the model domain. The tidal action makes the Changjiang plume more dispersive and better resembling observations. In addition, it induces stronger horizontal and vertical mixing so that the distribution of DIN concentration in the surface layer close to the observations. On the other hand, the total DIN in the upper 60m remains nearby the same, indicating the tidal action merely moves. DIN from the subsurface layer to the surface layer, which enhances photosynthesis. The additional concentration of DIN in the surface layer increases the IPP by 40% on an annual basis. The modeled annual mean IPP reaches 93% of the observated mean value. For the water depth of 0 ~ 30m, the mean concentration of modeled DIN increases from 1 ?m to 2 ?m in the ECS shelf region by tidal action. From Michaelis-Menten kinetics, the phytoplankton growth rate can reach 91% of maximam phytoplankton growth rate, which may explain the enhanced primary productivity.
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