Influence of cross-shelf water transport on nutrients and phytoplankton in the East China Sea: a model study

A three dimensional coupled biophysical model was used to examine the supply of oceanic nutrients to the shelf of the East China Sea (ECS) and its role in primary production over the shelf. The model consisted of two modules: the hydrodynamic module was based on a nested model with a horizontal resolution of 1/18 degree, whereas the biological module was a low trophic level ecosystem model including two types of phytoplankton, three elements of nutrients, and biogenic organic material. Model results suggested that seasonal variation in chlorophyll-a had a strong regional dependence over the shelf of the ECS. The area with high chlorophyll-a appears firstly at the outer shelf in winter, and gradually migrates toward the inner shelf (offshore region of Changjiang estuary) from spring to summer. Vertically, chlorophyll-a was generally homogenous from the coastal zone to the inner shelf. In the middle and outer shelves, high chlorophyll-a appeared in the surface in spring but moved to the subsurface from summer to early autumn. The annual averaged onshore flux across the shelf break was estimated to be 1.53 Sv for volume, 9.4 kmol s−1 for DIN, 0.7 kmol s−1 for DIP, and 18.2 kmol s−1 for silicate, which are supplied mainly from the northeast of Taiwan and southwest of Kyushu. From calculations that artificially increased the concentration of nutrients in the Kuroshio water, the additional oceanic nutrients were distributed in the bottom layer from the shelf break to the region offshore of Changjiang estuary from spring to summer, and appeared in the surface layer from autumn to winter. The contribution of oceanic nutrients to primary production over the shelf was found not only in the surface layer (mainly at the outer shelf and shelf break in winter and in the region offshore of Changjiang estuary in summer) but also in the subsurface layer over the shelf from spring to autumn.