The dynamic observation of dissolved organic matter in the Zhujiang (Pearl River) Estuary in China from space

The distributions of estuarine colored dissolved organic matter (CDOM) are the combined results of physicalbiogeochemical processes. Remote sensing is needed to monitor highly dynamically estuarine CDOM. Using in situ data from four seasonal cruises, an algorithm is developed to estimate CDOM absorption coefficient at 400 nm (a (CDOM)(400)) in the Zhujiang (Pearl River) Estuary (ZJE). The algorithm uses band ratios of R (rs)(667)/R (rs)(443) and R (rs)(748)/R (rs)(412). By applying it to moderate resolution imaging spectroradiometer onboard Aqua satellite (MODIS/Aqua) data from 2002 to 2014, seasonal climatology a (CDOM)(400) in the ZJE is calculated. CDOM distributions are majorly influenced by water discharge from the Zhujiang River and underwater topography. Along the section vertical to a water depth gradient, the seasonal a (CDOM)(400) exponentially decreased (y=ae (bx) , b < 0), but with great differences among seasons. Riverine fresh water is the primary source of CDOM in the ZJE. Fulvic acid fraction decreases with increasing salinity. Using developed algorithms, conservative CDOM mixing equation, and river discharge, effective riverine end-member concentration and flux of dissolved organic carbon (DOC) in summer and winter from 2002 to 2014 are first estimated from the MODIS/Aqua data. Both effective riverine end-member DOC concentration and flux are positively related to the river discharge, significantly in summer with R (2) of 0.698 for concentration and 0.965 7 for flux.