南海初級生產力之估算與SEATS測站顆粒態有機質之觀測：應用於南海物理-生地化耦合模式之驗證與改進

Estimation of primary production in the South China Sea and observation of particulate organic matter at SEATS station: application in the validation and improvement of the coupled physical-biogeochemical model of South China Sea

陳英傑(Ying-Jie Chen)

指導教授 ： 劉康克

國立臺灣大學/理學院/海洋研究所/碩士(2004年)

https://doi.org/10.6342/NTU.2004.01898

摘要

目前已有之南海三維物理-生地化耦合模式（Liu et al., 2002）雖然在模擬葉綠素分佈與初級生產力方面，與衛星水色資料及實際觀測結果在定性上有相當吻合之處，然而仍有許多問題尚待解決。首先，耦合模式模擬之南海初級生產力較衛星遙測估計為低（Liu et al., 2002）。其次，除了葉綠素與溶解態無機氮以外，耦合模式其他的生地化變數如碎屑物質等則未與觀測值比較過。因此本研究主要目的在於以耦合模式模擬之初級生產力與顆粒態有機碳（POC）濃度與遙測或實測數據比較，以更瞭解模式的表現；並針對模擬結果不佳之處提出改進之道。
為了達成以上的目的，本研究進行的工作包括：（1）以衛星水色數據估計初級生產力，並與實測值比較以獲得較準確之南海初級生產力；（2）分析SEATS計畫觀測到之POC資料，建立SEATS測站POC的垂直分佈與時序變化；（3）利用以上的分析結果驗證並改進耦合模式模擬之初級生產力與顆粒態有機碳時空分佈。
以SeaWiFS水色數據結合生物光學模式（VGPM）估算之南海初級生產力與實測值比較後發現，水色數據估算之初級生產力在底深超過50 m的海域與實測值相近，但在底深50 m以內之沿岸區域則有高估的現象。因此根據比較結果將水色數據估算之沿岸初級生產力調降1/2後，估計南海初級生產力年平均值為412 mg C m-2 d-1。
此外， 在SEATS測站之POC的垂直分佈上，上層（0-200 m）可測到較高濃度之POC（1-10

關鍵字

初級生產力 ； 耦合模式 ； 南海時間序列研究 ； 南海 ； 顆粒態有機碳

並列摘要

Though the existing coupled physical-biogeochemical model of the South China Sea (Liu et al., 2002) has achieved reasonable success in predicting the temporal variation and spatial distribution of chlorophyll and primary production, there are still plenty of room for improvement. In the first place, the magnitudes of modeled primary production are considerably smaller than those derived from the ocean color data. Secondly, some biogeochemical variables of the model, such as the particulate organic carbon (POC), have not been compared with observed data yet. The major objectives of this research are to better understand the performance of the model by comparing modeled primary production and POC with data from direct observations or remote sensing, and to improve the model by modifying the algorithm or the parameters.
In order to achieve the objectives above, this study has accomplished the following items: (1) to estimate primary production from the ocean color data and to compare the results with the observed data to acquire more precise estimates of primary production in the South China Sea; (2) to collect and analyze the POC samples from the SEATS (South-East Asia Time-series Study) station and to establish the vertical distribution and seasonal variation of POC at the SEATS station; (3) to validate and improve the modeled temporal and spatial distribution of primary production and POC according to the results above.
According to the comparison between the primary production results derived from the SeaWiFS ocean color data by a biological-optical model and the observed data, the former are in reasonable agreement with the observed data in the deep zone (depth > 50 m), but significantly overestimated in the coastal zone (depth < 50 m). Therefore, we reduce the primary production values in the coastal zone by 1/2 and obtained the estimated average annual PP of 412 mg C m-2 d-1 for the South China Sea.
The vertical distribution of POC at the SEATS station reveals the following pattern. The POC concentration fluctuates between 1 and 10 uM in the top 200 m and drops to around 1 uM in the water column below. The standing stock of POC (0-100 m) shows considerable seasonal variation with high values (250-350 mmol C m-2) occurring between October and March and low values (100-200 mmol C m-2) occurring between May and July. The standing stock of chlorophyll (0-30 m) at SEATS station also reaches a peak in winter.
Comparing the modeled results with the observations described above, we have found quite a few shortfalls of the model, which need to be improved. In the case of primary production, modeled and ocean color derived primary productions in the South China Sea both show a stronger peak in winter and a weaker peak in summer, but the average annual value of modeled primary production is much smaller. Further comparison has revealed that the modeled result seriously underestimates the primary production in the coastal zone (depth < 100 m). This underestimation results from the setup of the model that the detritus is assumed to be buried and, therefore, removed from the system as it reaches the seafloor, and, hence, the model fails to simulate the high nutrient conditions in the coastal zone. Therefore, the model is modified to include the process that the detritus is decomposed to release nutrients. This improves the performance of the model in the coastal zone considerably and makes the spatial distribution of modeled primary production more reasonable (coastal zone > deep zone).
Regarding the vertical distribution and seasonal variation of POC at the SEATS station, the modeled results exhibit the basic pattern of observed distributions, but the magnitude is a little too small. Because decreasing the sinking velocity of detritus can increase the residence time of detritus, we have adopted this method to improve the simulation of the POC distribution. As a compromise to optimize both the modeled POC and the modeled chlorophyll, we have chosen 7.5 m d-1 as the sinking speed for modeling POC. This decrease of sinking speed does not affect the modeled POC flux much.
By modifying both the model setup and the sinking speed at the same time, we have found that the former primarily affects the coastal zone and the latter primarily affects the deep zones with relatively little cross effects. Therefore, we suggest that the model may include both mechanisms for better performance. In the future we may further improve the model by modifying the model structure and importing more detailed mechanism of nutrient regeneration from sediments.

並列關鍵字

coupled model ； particulate organic carbon(POC) ； South East Asia Times-series Station(SEATS) ； South China Sea ； primary production

參考文獻

Bannister, T. T. (1974) Production equations in terms of chlorophyll concentration, quantum yield, and upper limit to production. Limnology and Oceanography 19, 1-12.

Behrenfeld, M.J., P.G. Falkowski ( 1997) Photosynthetic rates derived from satellite-based chlorophyll concentration. Limnology and Oceanography 42, 1–20.

Broecker, W. S., W. C. Patzert, J. R. Toggweiler, M. Stuiver, (1986) Hydrography, chemistry and radioisotopes in the south-east Asian basins, Journal of Geophysical Research 91, 14345-14354.

Campbell, J., D. Antoine, R. Armstrong, K. Arrigo, W. Balch, R. Barber, M. Behrenfeld, R. Bidigare, J. Bishop, M-E. Carr, W. Esaias, P. Falkowski, N. Hoepffner, R. Iverson, D. Kiefer, S. Lohrenz, J. Marra, A. Morel, J. Ryan, V. Vedernikow, K. Waters, C. Yentsch, J. Yoder (2002) Comparison of algorithms for estimating ocean primary production from surface chlorophyll, temperature, and irradiance. Global Biogeochemical Cycles 16, 1-15.

Chai, F., R.C. Dugdale, T.-H. Peng, F.P. Wilkerson, R.T. Barber (2002) One-dimensional ecosystem model of the equatorial Pacific upwelling system. Part I: model development and silicon and nitrogen cycle. Deep-Sea Research II 49, 2713–2745.