Hydrography, suspended sediments, chlorophyll A, chromophoric dissolved organic matter and optical characteristics of the Pearl River (Zhujiang) Estuary during July and November, 1998 (China)

The Pearl River (Zhujiang) is the second largest river in China and the largest in southern China, with a strongly seasonal discharge driven by Asian monsoon rainfall patterns. Optical characteristics of the plume region are expected to strongly affect light limitation of primary production and the interpretation of satellite remote sensing data. The river is enriched with nutrients from agriculture, aquaculture, and human waste disposal, and experiences frequent phytoplankton blooms. An understanding of factors controlling phytoplankton primary production is currently lacking, but is essential for effective water quality management.; This study reports on the hydrography (temperature, salinity, dissolved oxygen, pH, turbidity, chlorophyll fluorescence) and several bio-geo-optical properties of waters in the Pearl River (Zhujiang) Estuary between Hong Kong and Macau during July and November 1998. Spatial distributions of nitrate, phosphate and three optically active constituents are reported: suspended particulate matter (SPM), chromophoric dissolved organic matter (CDOM), and chlorophyll a (Chl). Spatial and temporal changes in light availability for primary producers was then investigated by modeling the light field using the Hydrolight radiance transfer model.; In July, maximum surface water Chl concentrations were found in coastal waters south of Lantau Island, while in November higher concentrations were achieved, and the spatial pattern was reversed, with higher values in the estuary and lower values in adjacent coastal waters. CDOM absorption coefficient ay400 varied conservatively with salinity in July, indicative of a riverine CDOM source. Sites sampled in November were all representative of high salinity coastal waters, with low ay400.; Modeling using the Hydrolight radiance transfer model with AER absorption and scattering subroutine was used to parameterize the inherent optical properties. Spatial patterns of modeled photosynthetically available radiation (PAR) during July suggested light limitation of primary production in low salinity turbid western waters due to high light attenuation. During November, co-limitation by light and nutrients existed in high-salinity, coastal waters southeast of Hong Kong, with light availability most influenced by hydrography and bathymetry (deep vertical mixing) rather than optical clarity.