Spring phytoplankton blooms in the Northern Satsunan region, Japan, stimulated by the intrusion of Kuroshio Branch water

The Kuroshio current greatly affects biological processes in coastal areas. The Kagoshima Bay has good spawning ground for major migratory fish, serves as an important feeding area for fish larvae, and is affected by the Kuroshio current; however, there is little information concerning the biological response to the Kuroshio intrusion in this location. During the mixing period, the Kuroshio current approaches the Kagoshima Bay and a density flow, consisting of surface inflow and bottom outflow, is generated. We hypothesize that the bottom water in the bay, having a high nutrient concentration, mixes with the surface layer when the water flows out of the bay and that this contributes to the high biological production at the bay mouth. In this study, we conducted a field survey of the water quality in March 2016 and, using a three-end member mixing model, examined whether the bottom water of the bay could be a nutrient source supporting the phytoplankton bloom in the bay mouth. We discuss the fate of the organic matter derived from the primary production promoted by a nutrient supply event. The results indicate that the N-based phytoplankton biomass represented less than 80% of the N uptake by the phytoplankton, as estimated by subtracting the measured nitrate concentration from the supplied nitrate concentration based on the mixing model, indicating that the phytoplankton bloom at the bay mouth was supported by the supply of bottom water from the bay. Based on the C/N molar ratio (6.4) calculated by C/chlorophyll a (Chl-a) and N/Chl-a in this study, the highest value of nitrogen uptake was 12 mg C m- 3 d(-1) in terms of the carbon assimilation. The feeding rate of the mesozooplankton was estimated to be 13.6 mg C m(-3) d(-1) using a physiological model. An analysis of the food web structure is required to assess the effects of feeding by mesozooplankton; however, most of the phytoplankton bloom caused by mixing could be efficiently transferred to mesozooplankton in this study area.