Distributions And Variations And Cross Shelf Exchange Of Nutrients In The East China Sea

The nutrients are the most important factor affecting the marine primary productivity, and they are the most critical element of the study of marine ecosystems. Relate-d research of nutrients is an important aspect for marine scientific research, and it is the major priority of international cooperation projects.Understanding the linkages between cross shelf transport of terrestrial material in the marginal seas and oceanic processes is crucial to establishing the global fluxes of biologically utilizable elements and thus has an important impact on global carbon cycle. The functions of continental pump for cross-shelf expo-rt of terrestrial materials in the marginal areas are highlighted recently.The East China Sea extends from the Cheju Island, to the north to the northern coast of the island of Taiwan to the south.It is connectedto the Japan Sea at the northeastern portion of its northern boundary through the Tsushima Strait. The Tsushima Warm Current flows from the East China Sea through this Strait into the southern Jap-an Sea and it eventually reaches the northwestern Pacific through the Tsugaru Strait. At its southern boundary, the East China Sea is connected to the South China Sea through the Taiwan Strait where the Taiwan Strait Warm Water enters the East China Sea. The Kuroshio, the western boundary current in the western north Pacific, affects the complicated circulation in the East China Sea. Many valuable studies have been carried out to understand the Kuroshio variability and the interplay between the ECS shetf water and the water brought by the Kuroshio. As the Kuroshio travels along the northwestern slope of the Okinawa Trough, it exchanges extensively with the East China Sea Shelf Water, especially through the actions of eddies atong the shelf break. The Kuroshio Subsurface Water is a major source of nutrients that support primary production in the East China Sea She If. However, it is not clear yet how and where the nutrients in the East China Sea Shelf Water is exported to the Kuroshio across the 200 m isobath. The nutrient concentrations and distribution in the East China Sea are discussed in this study, and the nutrient exchange fluxes between the Shelf Water and the Kuroshio are evaluated, as well.Based on the results of observations, the nutrient concentrations and distribution in the East China Sea show obvious temporal and spatial changes. In the East China Sea, the distribution of nutrients is significantly influenced by the comprehensive current regime, i.e. the invasion of Kuroshio Subsurface Water, the Changjiang Diluted Water and Taiwan Current Warm Water. The distribution of nutrient shows the impact of riverine input along the coastal area and also the intrusion of Kuroshio waters along the conti-nental shelf.Two investigations in spring and autumn show similar distributions with high values in surface layer in the inner shelf of the East China Sea, and the contour is dense and parallel to the coast. From the inner shelf to the outer shelf, the concentrations of nutrients decrease quickly that indicates the contributions from continent terrestrial material. In the near-bottom layer, the distribution of nutrients is similar to that of surface layer, except that there is another high-value area affected by the invasion of Kuroshio Subsurface Water. The vertical distribution of temperature, salinity and nutrients is well-mixed due to strong mixing of seawater in the inner shelf with depth less than 50m, and the concentrations of nutrients decreased from the inner shelf to outer shelf. But the shelf water with depth more than 50m begins stratify in spring. In sum-mer, the Changjiang Diluted Water extends more far than spring and water stratificat-ion is more significant in the ECS, the nutrient concentration is higher in lower layer than that in the upper layer.Because of the thermocline and pycnocline, nutrients in the bottom layer can’t move to the upper layers.The nutrients concentrations decrease from the coastal area where receives huge amount of riverine inputs to the middle and out shelf region of ECS in the surface layer, this significant onshore-offshore gradient of nutrients indicates the huge potential of crossshelf transport to the open ocean during the dynamic water exchanges between shelf waters and Kuroshio waters.An nutrient plume originates from the near bottom layer along coast and transports southeastward to the shelf edge along the isopycnal surface is detected in spring, strongly suggesting cross shelf transport in the surface water over the continental shelf. Plots of nutrient along different isopycnal surfaces suggest that this exceptionally nutrient-rich near bottom layer along coast flows eastward and then northeastward to the shelf edge through isopycnal layer with σt less than 23.4.Onshore and offshore fluxes of water and nutrient transport at different isopycnal surfaces across 200m isobath are calculated, assuming 200m isobath as the boundary of water exchange between shelf and Kuroshio waters. Due to the meandering of Kuroshio when it flows northward along the ECS shelf, sequential changing of conveying directions between onshore and offshore along the isobath occurs dramatically. Fluxes of water and nutrient transport across 200 m isobath are used to understand the function of continental pump in the ECS. Nutrients in the coastal water transport out shelf across 200m isobath through the surface layer with σt less than 23.4, which highlight the importance of surface cross shelf transport. But the nutrients intrud onto the shelf under the surface layer. There were two areas where positive onshore flux was concentrated. One was northeast of Taiwan where the Kuroshio bumps against the shelf break and induces a large onshore flux of volume and nutrients. The other was located southwest of Kyushu where the Kuroshio veers toward Tokara Strait. Along the shelf break between them, the Kuroshio onshore flux of volume and nutrients was generally negative, indicating an offshore transport of water and nutrients. The Kuroshio onshore flux in spring was 0.44 Sv for volume,8.93 kmol·s-1 for DIN,0.46 kmol·s-1 for DIP, and 8.22 kmol·s-1 for silicate in spring. The Taiwan Warm Current onshore flux in spring was 2.40 Sv for volume,6.65 kmol·s-1 for DIN,0.41 kmol·s-1 for DIP, and 6.52 kmol·s-1 for silicate. The values of the nutrient flux were on the same order as The Kuroshio.The Kuroshio onshore flux in summer was 1.07 Sv for volume,5.13 kmol·s-1 for DIN,0.41 kmol·s-1 for DIP, and 6.42 kmol·s-1 for silicate.