The low water-leaving radiances phenomena around the Yangtze River Estuary

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1 The low water-leaving radiances phenomena around the Yangtze River Estuary He Xianqiang* a, Bai Yan a, Mao Zhihua a, Chen Jianyu a a State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration, Hangzhou, China ABSTRACT Based on the in-situ data and ocean color remote sensing data of SeaWiFS, we found there was a black water region with the normalized water-leaving radiances less than. mw/(cm 2 µm sr) at the visible light wavelength. Yangtze River Estuary locates in the East China Sea shelf with shallow water. Affected by the tide mixing and the runoff of the Yangtze River and Qiantang River, the turbidity is very high. Generally, the water-leaving radiance is high in the turbid water because of the large particle scattering. The reason of the occurrence of this black water was analyzed by the inherent optical properties and the ocean color components. The results showed that black water was caused by the relative low values of the suspended particle matter concentration and the back scattering ratio. Key words: Yangtze River Estuary, water-leaving radiance, ocean color remote sensing, inherent optical properties.. INTRODUCTION Yangtze River is the third largest river around the world with annual flux of 9.2 m. Because of the shallow water and affection by the tide mixing and the runoff of the Yangtze River, the turbidity is very high []. Generally, the waterleaving radiance is high in the turbid water with the particles scattering. However, in some specific conditions, the waterleaving radiance may very low, which is called as the black water phenomena. Hu found the black water in the Florida Bight between the March and May in 22 using the SeaWiFS data, and suggested that the decline in the benthic communities was related to the passage of the black water over the reef sites [2]. In this paper, we found there was a black water region around the Yangtze River Estuary in spring of 2. The research area is between 9 E~2 E and N~6 N, covered the Southern Yellow Sea and Northern East China Sea, Yangtze River Estuary and Subei Shallow with the extremely high turbid water. The water depth is all less than m, and most of them are less than m. The insitu dataset measured in spring of 2(Mar. 22-Apr. 2) in East China Sea was used to analyze the distribution of the black water and its optical principle [, ]. There were total 8 stations, and the dataset included the apparent optical properties, inherent optical properties and the ocean color components. Both the above- and in-water methods were used to measure the apparent optical properties. For the above-water method, two independent optical radiometers named Satlantic SAS-II and ASD FieldSpec Dual were used to measure the water-leaving radiance or remote sensing reflectance. Also, two independent in-water profile radiometers named Satlantic SPMR and BioSPherical PRR-8 were used to measure the profiles of the downwelling irradiance and upwelling radiance, and the water-leaving radiance or remote sensing reflectance was deduced from these profiles. The relative difference of the two methods was less than % for the clear water, and the difference of the two above-water optical radiometers was less than %. According to the NASA ocean optical protocol, the in-water method dataset was used in this research. Two methods were used to measure the inherent optical properties, one was the in-situ method, and the other was the laboratory analysis method. For the in-situ measurement, AC-9 was used to measure the total absorption coefficient and attenuation coefficient, and HS-6 was used to measure the back scattering coefficient. Two independent spectrophotometers were used to measure the CDOM, total particle, non-pigment particle and pigment absorption coefficients in the laboratory []. Two independent methods were used to measure the chlorophyll concentration, one was the fluorescence method by Turner-2 fluorescence spectrometer, and the other was HPLC method. In this research, the fluorescence method data was used. Suspended particle matter concentration was determined from weight measurements on duplicate filters. *hexianqiang@sina.com; phone Remote Sensing of the Ocean, Sea Ice, and Large Water Regions 29, edited by Charles R. Bostater Jr., Stelios P. Mertikas, Xavier Neyt, Miguel Velez-Reyes, Proc. of SPIE Vol. 77, 77R 29 SPIE CCC code: X/9/$8 doi:.7/2.82 Proc. of SPIE Vol R-

2 2. THE LOW WATER-LEAVING RADIANCES PHENOMENA Figure was the normalized water-leaving radiances distribution in the research area. We could see there was a black water region at the Southeast of the Yangtze River Estuary, with the normalized water-leaving radiances less than.mw/(cm 2 µm sr) at all wavelengths. Also, the existence of this region was proved by the remote sensing data from SeaWiFS on Apr. 2, as shown in figure 2. There was even larger black water region along the Yangtze River Estuary and costal of the Zhejiang province between 22 E~2 E and 28 N~ N. It was worth to note a high chlorophyll concentration region existed at the Eastern of the Yangtze River Estuary with about km distance from the mouth of the Yangtze River, as shown in figure. Although the high chlorophyll concentration, the normalized waterleaving radiances at green light wavelength (nm) and red light wavelength (67nm) were not low enough to becoming a black water. (a) (c) (e) (d) (f) Figure the normalized water-leaving radiances by the in-situ measurement with the unit of mw/(cm 2 µm sr), and the red circle showed the black water region. (a)2nm, (b)nm, (c)9nm, (d)2nm, (e)6nm, (f)67nm. (b) Proc. of SPIE Vol R-2

3 (a) (b) (c) (d) (e) (f) Figure 2 the distribution of the normalized water-leaving radiances by SeaWiFS data on Apr., 2. The red circle in the image was the black water region. (a)2nm, (b)nm, (c)9nm, (d)nm, (e)nm, (f)67nm Proc. of SPIE Vol R-

4 Figure The distribution of the chlorophyll concentration by SeaWiFS data on Apr.,2.. REASONS OF LOW WATER-LEAVING RADIANCES AROUND THE YANGTZE RIVER ESTUARY There could be three reasons caused the black water, one was large gelbstoff absorption coefficient, second was large total absorption coefficient, and the third was small total backscattering coefficient. For () Large gelbstoff absorption coefficient The percentage of gelbstoff absorption to the total absorption coefficient (excluding the pure water absorption) at nm was less than 6 %( figure ). In the costal water, because of the high turbidity, absorption by suspended particle matter was the dominator, and the percentage of the gelbstoff absorption was less than 2%. With the increase of the distance to the coastal, the percentage of the gelbstoff absorption increased. In the black water region, the percentage of the gelbstoff absorption was the largest in the research area. However, the gelbstoff absorption was not the reason of the black water in the research area because of the low value of the absolute gelbstoff absorption coefficient as described in the next. (2) Large total absorption coefficient Figure was the spatial distribution of the total absorption coefficient (excluding the pure water absorption) at nm. Costal water has very high total absorption coefficient, and in the Subei Shallow, the largest total absorption coefficient was as high as 8 m-. However, total absorption coefficient decreased rapidly as the increasing of the off shore distance. In the black water region, the total absorption coefficient was relatively low. Therefore, Total absorption coefficient was not the reason of the black water. () Small total backscattering coefficient Figure 6 was the spatial distribution of the total backscattering coefficients (excluding the pure water backscattering coefficient) at 2nm, 88nm,2nm and 676nm measured by HS-6. Costal water has very high backscattering coefficient, and in the Subei Shallow, the largest total backscattering coefficient was as high as m-. Total backscattering coefficient decreased rapidly as the increasing of the off shore distance. In the black water region, the total backscattering coefficient was the lowest in the research area. Therefore, low value of the total backscattering coefficient was the reason of the black water. Proc. of SPIE Vol R-

5 In the black water region, the concentration of the suspended particle matter was the lowest in the research area, and the spatial distribution consisted with the total backscattering coefficient (figure 7). Therefore, low concentration of the suspended particle matter was one of the reasons causing the low value of the total backscattering coefficient. Particle backscattering ratio was another reason which could cause the low value of the total backscattering coefficient. Figure 8 showed the distribution of the particle backscattering ratios at 2nm, 88nm, 2nm and 676nm. The range of the particle backscattering ratio at 2nm was from. to.78 with the average value of.2, which consisted with the other researches [6-9]. In the black water region, the particle backscattering ratio was the lowest in the research area with the value less than.7 at 2nm. Thus, low value of the particle backscattering ratio was the reason of the black water. Backscattering ratio depended on the components and size of the particle. Generally, organic particle (phytoplankton) had large size and small refractive index, but inorganic particle (sediment) had the contrary properties. Figure 9 showed the distribution of the percentage of the phytoplankton absorption to the total absorption (excluding the pure water absorption) at nm. In the black water region, this percentage was as high as 6%, and the specific absorption coefficient of the chlorophyll was less than. m2/mg with the minimum of.2 m2/mg (figure ), which was greatly less than the value in the open sea [-2]. According to the principle of the package effect of the phytoplankton particle, low value of the specific absorption coefficient of the chlorophyll corresponded to the large size of the phytoplankton particle. Therefore, large size of the phytoplankton particle was the reason causing the low value of the particle backscattering ratio in the black water region Figure the percentage of the gelbstoff absorption to the total absorption (excluding the pure water absorption) at nm. Figure the total absorption coefficient (excluding the pure water absorption) at nm with the unit of m Proc. of SPIE Vol R-

6 (a) (b) (d) Figure 6 the total backscattering coefficients (excluding the pure water backscattering coefficient) measured by HS-6 with the unit of m -, (a)2nm, (b)88nm, (c)2nm, (d)676nm. (c) Figure 7 the concentration of the suspended particle matter with the unit of mg/l. Proc. of SPIE Vol R-6

7 (a) (c) Figure 8 the distribution of the particle backscattering ratio. (a) 2nm, (b)88nm, (c)2nm, (d)676nm. (b) (d) Figure 9 Percentage of phytoplankton absorption to total absorption (excluding pure water absorption) at nm. Figure the specific absorption coefficient of the chlorophyll at nm with the unit of m 2 /mg Proc. of SPIE Vol R-7

8 . CONCLUSION Based on the in-situ dataset measured in spring of 2 in East China Sea and the ocean color remote sensing data of SeaWiFS on Apr.,2, a black water region was found around the Yangtze River Estuary. In the black water region, the normalized water-leaving radiances were less than.mw/(cm 2 µm sr) at all wavelengths. Generally, there were three cases which could cause the black water, the first was the large gelbstoff absorption, the second was the large total absorption coefficient, and the third was the small total backscattering coefficient. All of these three cases were analyzed to determine the possibilities in the research area by the in-situ inherent optical properties and ocean color components. The results showed that small total backscattering coefficient was the main reason causing the black water. In the black water region, the concentration of suspended particle matter and the particle backscattering ratio were relative low, which caused the small total backscattering coefficient. Also, the percentage of the phytoplankton absorption to the total absorption was high in the black water region which meant that the large size organic particle might cause the small backscattering ratio. ACKNOWLEDGEMENTS This work was supported by National Basic Research and Development Program (97 Program, Grant No. 29CB222), National Natural Science Foundation of China (Grant No. 766) and National High Technology Development Program (86 Program, Grant No. 27AA2Z7 and 28AA9Z). The authors thank all the participants in the Case II water optical measurement in spring of 2 in East China Sea for providing the in-situ dataset, and the Hangzhou satellite ground station of SOED for providing the SeaWiFS data. REFERENCES [] [2] [] [] [] [6] [7] [8] [9] [] [] [2] BEARDSL EY R C,L IMEBURNER R, YU H, et al., Discharge of the Changjiang (Yangtze River) into the East China Sea, Continental Shelf Research,, 7-76(98). Chuanmin Hu, Keith E. Hackett, Michael K. Callahan, et al., The 22 ocean color anomaly in the Florida Bight: A cause of local coral reef decline?, GEOPHYSICAL RESEARCH LETTERS, (),-~-(2). Tang Junwu, Wang Xiaomei, SongQingjun, et al., The statistic inversion algorithms ofwater constituents for the Huanghai Sea and the East China Sea, Acta Oceanologica Sinica,2(), (2). Tang Junwu, Wang Xiaomei, Songqingjun, et al., Statistical inversion models for Case II water elements in theyellow Sea and East China Sea, Advances in marine science, 22(Z),-7(2). Zhu Jianhua, Li Tongji, Spectral model research about absorption coefficient of the de-pigment particles and yellow substance in Yellow Sea and East China Sea, Ocean technology, 2(2), 7-(2). Twardowski M. S., Boss E., Macdonald J. B., et al., A model for estimating bulk refractive index from optical backscattering ratio and the implications for understanding particle composition in case I and case II waters, J. Geophys. Res., 6,29 2(2). Boss E., Pegau W. S., Lee M., et al., Particulate backscattering ratio at LEO and its use to study particle composition and distribution, J. Geophys. Res.,9, C, doi:.29/22jc(2). Chang G. C., Barnard A., Zaneveld R., et al., Bio-optical relationships in the Santa Barbara Channel: Implications for remote sensing, Proceedings of the Ocean Optics XVII Conference, 2(2). Loisel H., Meriaux X., Berthon J.F., et al., Investigation of the optical backscattering to scattering ratio of marine particles in relation to their biogeochemical composition in the eastern English Channel and southern North Sea, Limnol. Oceanogr., 2(2), 79-72(27). Babin M., Therriault J. C., Legendre L., et al., Variations in the specific absorption coefficient for natural phytoplankton assemblages: Impact on estimates of primary production, Limnol. Oceanogr., 8(l), -77(99). Bricaud A., Babin M., Morel A., et al., Variability in the chlorophyll-specific absorption coefficients of natural phytoplankton: Analysis and parameterization, J. Geophys. Res.,(C7), 2-2(99). Bricaud A., Morel A., Babin M., et al., Variations of light absorption by suspended particles with chlorophyll a concentration in oceanic (case ) waters: Analysis and implications for bio-optical models, J. Geophys. Res.,(C), -(998). Proc. of SPIE Vol R-8