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The circulation systems and ecological environment of the East China Sea (ECS) are greatly influenced by the Kuroshio. The nutrient-rich Kuroshio Subsurface Water (KSSW) has been subject to considerable research attention, but the intrusion pathway, extent and variation of the KSSW over the entire southern ECS shelf are still debated. In June 2015, the dissolved inorganic iodine species (DIIS) were determined, and hydrographic data were collected and analyzed in the southern ECS. Analysis of the collected data suggests that the KSSW mainly intrudes onto the ECS shelf from northeast of Taiwan and bifurcates into a nearshore branch and an offshore branch. The nearshore branch ultimately appears in the subsurface layer within the 50-m isobath off the Zhejiang coast, north of 30 °N, and the offshore branch flows along the 100-m isobath and covers the area outside the 80-m isobath. Another KSSW intrusion along the shelf edge is located at ∼28 °N. This additional KSSW intrusion is weaker, and the offshore area in the southern ECS is mainly influenced by the KSSW intrusion northeast of Taiwan. Compared with the KSSW intrusion on the ECS continental shelf (depth < 100 m) in May 2014, the KSSW intrusion northeast of Taiwan in June 2015 was shifted seaward, had a greater geographical reach and intruded farther north off the Zhejiang coast. The KSSW intrusion in the nearshore area off the Zhejiang coast may promote a decrease in dissolved oxygen (DO) in the bottom water as well as the growth of phytoplankton in the upper water. The expansion of the range of the KSSW intrusion northward in 2015 led to the movement of the low-DO, high-chlorophyll area farther north in 2015 relative to its position in 2014 and may have contributed to the variations in hypoxia and algal blooms. [ABSTRACT FROM AUTHOR]

The Kuroshio has an important influence on the circulation systems and ecological environment of the East China Sea (ECS), but the intrusion pattern of the Kuroshio on the ECS continental shelf and its seasonal variability are still debated. Our study shows that evidence from both hydrographic data and dissolved inorganic iodine species (DIIS) suggest that a cold (< 19 °C), saline (> 34.4 PSU), dense (> 24.5 kg/m 3 ), R-iodate-rich (> 0.35 μM) and R-iodide-poor (< 0.10 μM) water (R-iodate and R-iodide are represented by iodate and iodide concentrations normalized to a salinity of 35) occurs at the bottom of the ECS continental shelf (depth < 100 m). The characteristics of this water are similar to those of the Kuroshio Subsurface Water (KSSW), and the intrusion pattern of the KSSW onto the ECS shelf consists of an intrusion from the northeast of Taiwan and another intrusion at ∼ 28.5°N along the 100-m isobaths. In the spring of 2014, the KSSW intrusion northeast of Taiwan flowed northwestward then bifurcated into a nearshore branch and an offshore branch at ∼ 27.5°N. The nearshore branch flowed approximately northward and finally appeared in the subsurface layer within the 50-m isobath off the Zhejiang coast (∼ 29.5°N). In addition, the offshore branch flowed along the 100-m isobath and reached ∼ 29.5°N. The KSSW intrusion northeast of Taiwan was obviously weaker in autumn than in spring. In autumn, the nearshore branch was not observed, and the offshore branch reached only ∼ 28°N. In addition to the main intrusion center northeast of Taiwan, the DIIS concentrations indicate that another weaker intrusion from the KSSW occurs at ∼ 28.5°N along the 100-m isobaths. However, this phenomenon could not be observed via hydrographic data alone. Moreover, seasonal fluctuation in this additional KSSW intrusion was observed. In spring, this water flowed northwestward and reached 29.5°N, whereas this water was confined along the 100-m isobath in autumn. Overall, DIIS can be used as complementary tracers of the KSSW intrusion, and more details of the intrusion pattern of the KSSW were well revealed. [ABSTRACT FROM AUTHOR]