Research On Euphotic Zone Depth Derived From Remote-sensing And Its Spatial-temporal Characteristics Over China Sea

Euphotic zone depth (Zeu) is an important water property. It is not only a measure of water clarity, but also widely used in biogeochemical studies of world oceans. Because of the spatial and temporal limitations of ship measurements, Zeu of the world oceans are obtained via ocean color remote sensing. This study employs SeaWiFS data over East China Sea, Taiwan Strait and South China Sea to evaluate a semi-analytical algorithm for euphotic zone depth. This algorithm is based on water' s inherent optical properties (IOP), which can be near-analytically calculated from spectral remote-sensing reflectance (Rrs, Lee approach). In this study, Zeu calculate from Rrs was compared with in situ data to evaluates the accuracy of Lee approach.The analyses show average errors for data of summer and fall cruises of East China Sea are 13.3% and 24.9%, respectively, average error for data in Taiwan Strait is 15.0%, and average error for winter and summer cruises of South China Sea are 16.2% and 5.5%, respectively. The root mean square error for data of each cruise is 0.066,0.106,0.074,0.091,0.023, respectively. It has higher accuracy compared to chlorophyll-a approach (average error over 40%, root mean square over 0.2, except for data of summer cruise in South China Sea, where average error is 6.5% and root mean square is 0.029). These results indicate that Lee approch not only worked well with waters of the Gulf of Mexico,Monterey Bay and Arabian Sea, but also worked well with waters of complex China sea. Compared with simple chlorophyll-a approach, it has great advantages.Based on SeaWiFS monthly mean Rrs data from 1998 to 2006, this research employs Lee approach to caculate euphotic zone depth over all China Sea and discuss its temporal and spatial characters. Euphotic zone depth over all China Sea represents obvious seasonal periodical changes. On average, Zeu is deeper in summer than in winter. Also, as a result of phytoplankton growth, Zeu in East China Sea and South China Sea is depressed in spring and autumn. Spatial distribution of euphotic zone depth is very differenct. Northern waters have larger difference than southern waters, so does western waters compared with eastern. Yellow Sea and Bohai Sea's euphotic zone depth is shallowest, with greatest relative change, however. Zeu in East China Sea takes on great changes along landform. Ocean currents in Taiwan Strait are more complex, so euphotic zone depth shows wide spatial variation. South China Sea has deepest euphotic zone, with low values distributed along the shore in a strip shape. Ryukyu, Taiwan and Luzon Island obstruct Kuroshio and North Equatorial current with deep euphotic zone. Landform of the sea, seasonal wind, current and river discharges are all the factors that influence water properties and then euphotic zone depth.Yellow sea and Bohai Sea's annual mean euphotic zone depth is deepest in 1998, while shallowest in 2006. Zeu in East China Sea shores between 1998 and 2000, and then deepens, reach biggest value in 2006. Taiwan Strait's euphotic zone depth fluctuates greatly from 1998 to 2002. South China Sea's euphotic zone depth is biggest in 1998 and decreased to smallest in 2004. All these findings indicate significant environmental changes along with interactions with the climate.