太湖浮游藻类的后向散射分离及其叶绿素a浓度反演

Phytoplankton Backscattering Coefficients Partitioning and Its Applications in Retrieving Chlorophyll-a Concentrations in Taihu Lake

浮游藻类的后向散射是水体光谱构成的重要组成部分,作为水体辐射传输模型中的重要参数,高精度的藻类后向散射系数对水体叶绿素a浓度的遥感反演精度至关重要。本文以简化的辐射传输模型-生物光学模型为基础,尝试性分离了太湖浮游藻类的后向散射系数。通过藻类后向散射规律分析,建立了浮游藻类吸收、后向散射特征的叶绿素a反演模型,改善了叶绿素a浓度的遥感反演精度。分析表明:藻类颗粒物的后向散射系数与吸收系数之间存在反比关系,且在560 nm、700 nm附近存在明显的散射峰,与叶绿素a浓度之间相关性显著;低密度藻类水体总悬浮颗粒的后向散射以非色素颗粒为主导,适合采用经典的指数模型模拟后向散射系数,而藻类密度较高的富营养化水体,水体总悬浮颗粒的后向散射以藻类颗粒为主导,传统的指数模型已不适用;采用分离藻类后向散射系数的方法,使得叶绿素a浓度的反演值与真实值相关系数从0.66提高到0.98,相对误差从55%降低到38%,均方根误差(RMSE)也由60.95μg/L降低至13.98μg/L。其真实性检验表明,与经典指数模型方法相比,利用藻类颗粒后向散射分离方法反演叶绿素a浓度,能够显著改善反演精度。

As key parameters in bio-optical model, the backscattering coefficients of phytoplankton plays an im-portant role in modelling the reflectance spectra and retrieving chlorophyll-a （CHL-a） concentrations from eutro-phic water. An exponential model is usually used to simulate the total backscattering coefficients by omitting the phytoplankton backscattering in inland water characterized by lower concentrations of CHL-a. However, the ex-ponential model is not valid for inland water with high CHL-a concentrations, and high relative errors and resi-dues may exist in retrieving the CHL-a concentrations in the algae blooming area, due to the errors made by omitting or introducing inaccurate backscattering coefficients of phytoplankton. Therefore, a precise determina-tion of the phytoplankton backscattering coefficients is of great importance in retrieving chlorophyll-a concentra-tions. Based on the classical bio-optical model, we proposed a method to partition the phytoplankton backscatter-ing coefficients. The variations of the backscattering coefficients of phytoplankton particles with wavelengths in 400~700 nm and the chlorophyll-a concentrations are illustrated and discussed in details. According to the results mentioned above, following conclusions are drawn：（a） it is appropriate to model the total backscattering coeffi-cients by using exponential function in most Case 2 waters with lower concentrations of Chlorophyll-a, where the non-algal suspended sediments dominated the optical properties. However, it is not applicable in eutrophic waters with higher concentrations of chlorophyll-a, where the algal particles dominated the optical properties;（b） phytoplankton backscattering coefficients vary inversely to their absorption coefficients, and two backscatter-ing peaks emerge in the wavelengths of 560nm and 700nm, which are significantly correlated with CHL-a con-centrations; （c） compare to the exponential model, the accuracy of the bio-optical model using the partitioned phytoplankton backscattering coefficients has improved greatly：the correlation coefficient between the retrieved and the measured CHL-a is increased from 0.66 to 0.98, the average relative error decreases from 55%to 38%, and the RMSE decreases from 60.95 to 13.98 in estimating CHL-a concentrations.