Impact of isopycnal transport of nitrate along the Kuroshio on the high productivity in the Kuroshio-Oyashio interfrontal zone

The Kuroshio-Oyashio interfrontal zone (K-O zone), transition area between the subtorpical and subarctic gyres in the western boundary of the North Pacific, is known as one of the most productive fishing grounds. Recent studies revealed that the Kuroshio transports considerable amount of nutrients to the downstream regions as is the case in the Nutrient Stream along the Gulf Stream. It suggests significant impacts of nutrient transport along the Kuroshio on the high productivity in K-O zone, though supplies from the nutrient-rich subarctic region have been focused in most previous studies. In order to elucidate quantitatively the nutrient supply along the Kuroshio to K-O zone, we analyzed budgets of isopycnal transport of nitrate in the region which covers K-O zone and includes parts of the Kuroshio, the Kuroshio Extension and the Oyashio, using vertical profiles of temperature, salinity, nitrate and current velocity observed at 200 stations enclosing the target region in spring 1998. Budget of nitrate transport integrated from the sea surface to 27.5σ_θ isopycnal in the enclosed region was found to be not balanced but in excess of imports, which was comprised of water transported along the Kuroshio and that along the Oyashio at the same rate. The Kuroshio water accounted for a larger rate in the nitrate excess on the lighter isopycnal shallower than nitracline, where new production averaged in the whole of the target region was estimated to correspond to a few hundred mgCm^-2d^-1, simply assuming that all the nitrate excess was consumed by phytoplankton. Comparison of the simple estimate with in situ observations of primary production suggests that the isopycnal transport of nutrient along the Kuroshio contributes significantly to the new production in K-O zone.