报告题目：Primary production in the South China Sea: Recent observations and modeling approaches

报告人：Kon-Kee Liu，台湾中央大学水文科学研究所所长

时间：2008年8月29日，早上9：30－11：00

地点：曾呈奎楼B－206

摘要：

The South China Sea (SCS) is the largest marginal sea in the world aside from the ice-covered Arctic Ocean.  Driven by the alternating monsoons, the biogeochemistry of SCS is very dynamic and potentially sensitive to climate change.  Such a premise has inspired new biogeochemical observations and modeling work.  A better understanding of the SCS biogeochemistry is essential for the interpretation of paleo records in the SCS obtained during paleoceanographic expeditions, which have flourished in the last decade.  The primary production in the South China Sea (SCS) has been assessed by a coupled physical-biogeochemical model with a simple NPZD ecosystem.  In recent years there have been an increasing number of observations in the SCS that may be used to check the validity of the previous approach.  The coupled model of the SCS mentioned above employs a photo-adaptation scheme for the phytoplankton growth and uses the simplest bottom boundary condition of an inert benthic layer.  These adopted schemes are checked against observations at the South-East Asia Time-series Study (SEATS) Station in the northern SCS and in the Gulf of Thailand.  Numerical experiments with or without photo-adaptation or active benthic processes are carried out in this study.  Additional experiments are performed with different parameters used for these processes.  The observations at the SEATS Station provide direct evidence for the variable chlorophyll-to-nitrogen ratio in phytoplankton as required by photo-adaptation.  It is concluded that the photo-adaptation scheme is critical to the phytoplankton growth, especially for the development of the subsurface chlorophyll maximum.  However, the modeled subsurface chlorophyll maximum occurs at depths shallower than observations.  Increasing of the upper limit of the chlorophyll-to-nitrogen ratio, as suggested by observations, enhances chlorophyll level in the lower part of the euphotic zone and raises primary productivity in areas with rich nutrient supply.  The observed values of the vertically integrated primary production (IPP) in the Gulf of Thailand clearly demonstrate the importance of the benthic-pelagic coupling to the nutrient cycle.  Without benthic nutrient regeneration the model grossly underestimates primary production due to failure to build up the nutrient reserve in the Gulf.  On the other hand, a fully regenerated flux of particulate organic nitrogen at the seafloor without denitrification produces too strong a primary productivity.  The improved model uses a higher upper limit for the chlorophyll-to-phytoplankton ratio of 3.5 g Chl/mol N and adopts benthic processes of a coupled nitrification-denitrification scheme with denitrification consuming 14% of the detritus flux at the bottom.  The model predicts a mean annual IPP value of 406 mgC mmgC m^-2d^-1 for the SCS, which may be broken down as 390 mgC m^-2d^-1 for the basin region (>200 m) and 429 mgC m^-2d^-1 for the shelf region (<200 m).  The modeled monthly mean IPP values for the shelf and basin regions compare favorably with observed mean values in different seasons.  The model also predicts a mean nitrogen removal flux of 0.16 mmol N m^-2d^-1 during denitrification for the shelf region.  The contribution of internal waves to the IPP has recently assessed with a tide-driven POM model, which produces an estimate of 3-5%.