Long-distance nutrient-transport process in the Changjiang river plume on the East China Sea shelf in summer

Idealized physical and ecosystem numerical models are coupled to investigate the phytoplankton bloom in Changjiang Diluted Water (CDW) on the East China Sea shelf in summer. Special attention is given to the phytoplankton not exhausting nutrients in CDW over a period of a few months in spite of high biological productivity. In the present study, CDW is expressed by a bowl-shaped three-layered plume moving on an infinitely deep layer due to ambient eastward currents. This model permits layers outcropping to the sea surface. The simplest ecosystem model composed of the nutrient, phytoplankton, and detritus is coupled with the above physical model. When river plumes contain a large amount of suspended sediment, it is likely that turbid river water prevents sunlight from reaching the lower layer of the plumes. Hence in the ecosystem model for turbid CDW, it is assumed that primary production is completely restricted when a layer is shaded by upper layers. As three-layered CDW is carried eastward by the ambient currents, lower layers south of the plume outcrop to the sea surface because of the layer-to-layer interaction. The nutrient released into the lower layers is used for primary production when it reaches the outcrop area south of the plume, and causes the phytoplankton bloom there. The modeled phytoplankton concentration in the uppermost layer exhibits a similar pattern to that seen in satellite-derived chlorophyll a images in which a high concentration is observed south of CDW extending eastward.