Nutrient flux and transport in the upper ocean of the Kuroshio east of Taiwan

The Kuroshio is the North Pacific western boundary current, originating from the east coast of the Philippines to the southeast coast of Japan. It has significant impact on fisheries and ecosystem dynamics in coastal regions of Northeast Asia. To better understand its nutrient flux and transport in the upper 250 m water column, six ship-board surveys were conducted along a transect across the Kuroshio at 23.75^oN east of Taiwan between September 2012 to July 2014. Result shows the Kuroshio was mostly northward flow with maximum normal velocity (v) varied between 0.7 to 1.4 m s^-1 located within 100 m water depth. The water volume transport in the upper 250 m ranged from 13.43 to 17.51 Sv (1 Sv = 10⁶ m³ s^-1), and it accounted for 51 – 67% of transport down to 1000 m water depth. Similar to temperature distribution pattern, there was a westward uplifting trend for nutrient concentration along the transect. It suggests that plankton communities along the coastal ecosystem might be significantly benefited from this nutrient uplifting. The averaged nitrate and phosphate concentrations over the 250 m water column were in the range of 1.74±1.35 – 2.27±1.17 μM and 0.10±0.07 – 0.32±0.18 μM, respectively. The ‘nutrient stream’ almost located within 300 – 600 m water depth, and the maximum flux ranged from 4.15 to 7.94 mmol m^-2 s^-1 for nitrate and 0.38 to 0.72 mmol m^-2 s^-1 for phosphate. As for nitrate and phosphate transport above the 250 m water depth, there were in the range of 16.77 to 31.09 kmol s^-1 for nitrate and 0.78 to 3.86 kmol s^-1 for phosphate. It accounted for 12.0 – 16.8% of nitrate and 4.7 – 20.2% of phosphate transport above the 1000 m water depth. Compared to previous studies, result also suggests the amount of phosphate transport was reduced during the Kuroshio downstream transport.