The subtropical Zhujiang (Pearl River) Estuary: Nutrient, trace species and their relationship to photosynthesis

Two cruises were undertaken to study the nutrients and trace species and their relation to marine production in the Zhujiang (Pearl River) Estuary, which cover both wet (August-September, 1996) and dry (January-February, 1997) seasons. Concentrations of nutrients (NO₃^-, NH₄⁺, PO₄^3-, and dissolved SiO₂) and other major and trace species (alkalinity, DOC and Al) in the Zhujiang are generally within the range of Chinese rivers and comparable to other large systems, like the Huanghe (Yellow River) and Changjiang (Yangtze River). In the estuary, some species (e.g. SiO₂) illustrate a fairly defined relationship with chlorinity, whereas others show considerable remobilization and/or changes in freshwater end-members, owing to the fact that fresh water enters the South China Sea from eight channels with various water flow and different compositions. The incubation experiments using in situ water samples reveal that while phosphate is the limiting element in riverine effluent plume, the photosynthesis turns to be nitrogen and/or silicon limited further offshore on the shelf region. Trace species (e.g. Cu²⁺ and Fe²⁺) have important impact on the photosynthesis in the Zhujiang Estuary, with Fe²⁺ to be a more limiting element than Cu²⁺. Atmospheric wet deposition may well stimulate photosynthesis and hence demonstrate a potential link with algal blooms in this area. Atmospheric impact on the photosynthesis increases from estuary toward the interior of South China Sea. The experimental data further suggest that in the turbid region of the Zhujiang Estuary, radiation reduction by suspended materials could be an important issue limiting photosynthesis, although the eutrophication is quite common in coastal environments of China.