琼东水体后向散射系数与浮游植物生物量的关系模型

Particle backscattering as a function of chlorophyll a concentration off the eastern Hainan coast in the South China Sea

海洋中光后向散射系数的变化包含了浮游植物生物量的信息,可应用于卫星遥感和光学剖面观测平台获取海洋中大时空尺度-高分辨率剖面的浮游植物生物量变化特征。本文选取了琼东上升流影响下生物—光学变异性较为显著的海域,基于2013年航次实测数据,建立了颗粒物后向散射系数(b_(bp))与叶绿素a浓度(Chl a)间的区域性关系模型。模型假定颗粒物后向散射系数由不随叶绿素浓度变化的固定背景值,以及较大粒级(>2μm)和pico级(微微型,<2μm)两类浮游植物的后向散射贡献累加所得。采集的数据集进行了模型检验,结果表明,模型能很好地模拟琼东海域水体的b_(bp)与Chl a间的变化趋势,性能优于常用的幂函数关系模型,尤其在低叶绿素浓度范围,很好地解决幂函数显著低估的现象;琼东海域的b_(bp)和Chl a关系存在显著的水层变化,底层后向散射固定背景值显著高于上层水体背景值,表明底层受上升流的影响,水体中不随Chl a共变的颗粒物浓度增大,其后向散射相应增强;叶绿素最大层的后向散射固定背景值显著低于上层其他水体的固定背景值,后向散射固定背景值的贡献百分比约为21%~35%;随着叶绿素浓度增大,较大粒级的浮游植物对颗粒物后向散射系数的贡献也显著增大,可达到50%以上,pico级浮游植物贡献稳定在40%附近。本研究的结果将为琼东海域浮游植物生物量的光学遥感、生物地球化学过程研究提供更为精确的区域性模型和基础支撑数据。

Variability of light backscattering coefficient in the ocean can be used to quantify the biomass of phytoplankton,which can be applied in remote sensing and optical profile observation platforms to obtain the characteristics of phytoplankton biomass spanning various spatio-temporal scales and vertical profiles in the ocean.Significant variabilities of the bio-optical properties exist off the eastern Hainan coast,being influenced by upwelling.Based on the in-situ data collected off this area in 2013,we established a regional algorithm between the backscattering coefficient of particle[b_(bp)]and chlorophyll a concentration[Chl a].The algorithm is based on the assumption that the particle backscattering coefficient is determined by the constant background backscattering value independent of Chl a and two components by pico(<2μm)and large(>2μm)phytoplankton.Results show that the modelled b_(bp) values agree well with measurements,especially for low Chl a concentration.The relationships between b_(bp) and Chl a vary in different water layers.For example,the constant background backscattering values at the bottom layer are significantly higher than those in the upper layer,indicating that driven by upwelling,the increasing concentration of submicron particles not covariant with Chl a might produce an increasing backscattering coefficient.At the chlorophyll maximum layer,the background backscattering values are significantly lower than those in the upper water,and the contribution percentage of background backscattering value is about 21%~35%.With the increase of Chl a concentration,the contribution of large phytoplankton to b_(bp) increases significantly,accounting for 50%or higher,while the contribution of pico phytoplankton remains stable around 40%.Understanding the regional relationship of bbp and Chl a off the eastern Hainan coast can improve our understanding of marine biogeochemical processes in the region.