Intrusions of excess nitrate in the Kuroshio subsurface layer

Abstract

Vertical profiles of nutrient concentration were investigated in the southeastern East China Sea and the Kuroshio (30°N–31°N, 128°E–130°E) in June 2013, in order to understand the effects of less-saline water intrusions on nutrient composition. The ratio of nitrate to phosphate is commonly <16; however, values >16 were observed in the northern edge of the Kuroshio at a depth of 50–100m at the beginning of June, associated with a less-saline water intrusion with a density (σt) of 23.5–24.8. This less-saline water was not present at the end of June. A positive excess nitrate concentration was found in water with a salinity of <34.17, revealing a significant negative correlation between salinity and excess nitrate concentrations. This less-saline excess-nitrate water originated from winter mixing of the East China Sea coastal waters, which were affected by the Changjiang River according to a back-trajectory analysis, and was transported to the east by the Kuroshio >500km away from the source. The excess-nitrate water existed below the euphotic layer, and it prevented the utilization of nutrients by the phytoplankton during the transportation. The intrusion of this less-saline water increased the nitrate to phosphate ratio and supplied excess nitrate to the Kuroshio subsurface layer.