RELATIONSHIP BETWEEN PHYSICAL ENVIRONMENT AND 3 PHYTOPLANKTON SPECIES, SKELETONEMA SPP., EUCAMPIA ZODIACUS, AND ASTEROPLANUS KARIANUS CAUSING COLOR BLEACHING OF CULTURED NORI IN THE ARIAKE SEA DURING WINTER

Abstract

 Color bleaching of Nori by HAB (harmful algal blooms) due to 3 phytoplankton species, Skeletonema spp., Eucampia zodiacus, Asteroplanus karianus, occurs in the inner part of Ariake Sea. This study evaluated the relationship between the growth dynamics of these red tide causing species and physical environments by analysis of weekly field observation data and conducting numerical simulations using unstructured triangular mesh. Red tide index (Number of phytoplankton cells × duration period) was high in the western area of the inner Ariake Sea and has high correlative relationship with the water residence time, which means that the strength of water exchange strongly controls the phytoplankton dynamics. Horizontal distribution of the strength of seawater exchange was determined by the horizontal difference in the strength of density-driven residual current which is strengthened in the eastern part of the inner Ariake Sea by fresh water inflow from 4 large 1st class rivers. Nori aquaculture increased the water residence time, so that the aquaculture formed more favorable environments for phytoplankton growth in terms of physical environment.