Possible contribution of typhoon- and internal tide-induced offshore nutrient supply to the occurrence of a Chattonella bloom in the Yatsushiro Sea, Japan

The bloom dynamics of the raphidophycean flagellate, Chattonella antiqua during summer 2016 in the Yatsushiro Sea, Japan were studied using a hydrodynamic model and monitoring data after typhoon Namtheun passed. The bloom formed in the southern area of the Yatsushiro Sea. The bloom dynamics differed from the dynamics of the 2008-2010 Chattonella blooms, which initially formed in the northern area and subsequently expanded to the southern area by effects of tidal currents. In 2016, high DIN levels appeared in the period preceding the bloom after the passage of the typhoon and the bloom formed in the southern area. The high DIN levels did not originate from the deeper water within the Yatsushiro Sea by vertical mixing because of the low DIN in the water column prior to the typhoon. A hydrodynamic model was used in order to examine how the high DIN was supplied. Model results suggested that deep offshore water intruded onshore and through the Ushibuka Strait into the Yatsushiro Sea in the period preceding the bloom. This deep offshore water intrusion was controlled by an internal tide, typhoon-induced current and vertical mixing induced by tidal stirring. The internal tide and typhoon-induced current transported offshore water to the lower layer in the Ushibuka Strait, and subsequently tidal mixing transported nutrients to the euphotic zone in the Ushibuka Strait by vertical diffusion.