Abstract
Abstract. 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 parts: the hydrodynamic module was based on a nested model with a horizontal resolution of 1/18 degree, whereas the biological module was a lower trophic level ecosystem model including two types of phytoplankton, three elements of nutrients, and biogenic organic material. The model results suggested that seasonal variations occurred in the distribution of nutrients and chlorophyll a over the shelf of the ECS. After comparison with available observed nutrients and chlorophyll a data, the model results were used to calculate volume and nutrients fluxes across the shelf break. The annual mean total fluxes were 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. Two areas, northeast of Taiwan and southwest of Kyushu, were found to be major source regions of oceanic nutrients to the shelf. Although the onshore fluxes of nutrients and volume both had apparent seasonal variations, the seasonal variation of the onshore nutrient flux did not exactly follow that of the onshore volume flux. Additional calculations in which the concentration of nutrients in Kuroshio water was artificially increased suggested that the oceanic nutrients were distributed in the bottom layer from the shelf break to the region offshore of the Changjiang estuary from spring to summer and appeared in the surface layer from autumn to winter. The calculations also implied that the supply of oceanic nutrients to the shelf can change the consumption of pre-existing nutrients from rivers. The response of primary production over the shelf to the oceanic nutrients was confirmed not only in the surface layer (mainly at the outer shelf and shelf break in winter and in the region offshore of the Changjiang estuary in summer) but also in the subsurface layer over the shelf from spring to autumn.
Highlights
In addition to receiving the terrestrial input of nutrients, marginal seas receive nutrients from the open ocean through cross-shelf water exchange
Our objective is to examine the onshore nutrient flux and its effects on primary production over the shelf
The area with high chlorophyll a concentration first appeared in the central Yellow Sea in spring, and moved toward coastal zones in summer; in the East China Sea (ECS), the area with high chlorophyll aconcentration first appeared northeast of www.ocean-sci.net/7/27/2011/
Summary
In addition to receiving the terrestrial input of nutrients, marginal seas receive nutrients from the open ocean (hereafter referred to oceanic nutrients) through cross-shelf water exchange. The East China Sea (ECS) is one of the major marginal seas of the northwestern Pacific (Fig. 1). The ECS receives water and associated nutrients from the South China Sea through the Taiwan Strait, for which the volume transport is greater than river discharge into the ECS by as much as two orders of magnitude (Isobe, 2008). In addition to the Taiwan Strait, Chen (2008) suggested that the upwelling of the South China Sea water on the western side of the Kuroshio in the ECS is a possible pathway for the nutrients transported from the South China Sea to the ECS. The Kuroshio in the ECS flows along the shelf break (see 200-m isobath in Fig. 1), and the onshore volume transport of the Kuroshio
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