Study on the Seasonal and Horizontal Distributions of Phytoplankton Communities in Yatsushiro Sea, Using a Cluster Analysis

Abstract

The seasonal and horizontal distributions of phytoplankton communities in Yatsushiro Sea, Kumamoto and Kagoshima Prefectures, were investigated by means of a cluster analysis.
Neritic diatoms and dinoflagellates dominate the phytoplankton communities. The structure and the distribution are largely different between the northern and the southern regions. In the northern region, a single, wide-spread community with higher cell density and lower diversity is observed, while in the southern region a few, smaller communites with lower cell density and higher diversity are distributed. Dinoflagellate-dominated communities are found in the southern region only in May and August.
The distribution of communities, more or less, correlates with depth, degree of inshoreness and salinity. The higher values of degree of inshoreness are obtained in the northern region. There the salinity decreases to about 30‰ due to the effect of the inflowing river water. In the southern region, on the other hand, degree of inshoreness is less than 10 and salinity is more than 32.5‰, due to the effect of the inflowing offshore water through Nagashima Straight. These dif-ferences of the hydrographic condition between the northern and the southern regions are con-sidered to affect remarkably the structure and the distribution of phytoplankton communities in Yatsushiro Sea.