Seasonal and annual dynamics of phytoplankton off Tajima, southwestern Sea of Japan

In the Sea of Japan, changes in the oceanographic conditions affect fisheries, and new phenomena such as blooms of the harmful dinoflagellate Margalefidinium polykrikoides and mass occurrences of the giant jellyfish Nemopilema nomurai have been observed since the 2000s. In order to elucidate and formulate countermeasures to such new issues, it is essential to enhance the oceanographic/biological data. However, data especially on the phytoplankton assemblage are limited. In the present study, we investigated the population dynamics of the phytoplankton community together with environmental factors at two stations off Tajima, southwestern Sea of Japan for seven fiscal years from September 2009 to March 2016. The oceanographic observations revealed the general pattern in the seasonal and annual abundance of phytoplankton off Tajima. The major component of size-fractionated chlorophyll a concentrations were micro-size (filtered pore size: >10 mu m). The results indicated that there was usually a low abundance of phytoplankton in this area. Relatively high abundances (cell densities over 100 cells mL(-1)) of micro-sized phytoplankton were observed only in spring, but total cell densities were less than 10 cells mL(-1) in most months of the other three seasons. The major phytoplankton component was diatoms. Based on the abundance and frequency, the dominant seven diatom taxa were classified into three groups: Skeletonema spp., Thalassiosira spp. and Eucampia zodiacus were dominant in spring, Chaetoceros spp. and Pseudo-nitzschia spp. were dominant in spring and autumn, and Rhizosolenia spp. and Leptocylindrus danicus were dominant in autumn.