長期モニタリングデータからみた大阪湾における環境変化と有毒渦鞭毛藻Alexandrium tamarenseブルーム規模の拡大

抄録

The expansion of blooming in the toxic dinoflagellate Alexandrium tamarense in Osaka Bay, eastern Seto Inland Sea, Japan, is examined from investigations at 13 or 20 sampling stations from 2002 to 2016, as well as long-term environmental monitoring from 1975 to 2016. Alexandrium tamarense blooms were mainly observed from March to May in Osaka Bay, at approximately 15˚C and 30 PSU. High cell densities were observed at low nutrient conditions, especially low phosphate, and low chlorophyll a. Temperatures in April and May gradually rose over the study period, but salinity from February to May showed no trend. Dissolved inorganic nitrogen (DIN) declined from the 1980s onward, and the downward trend became conspicuous after the 2000s. Phosphate drastically declined until the 1990s and then remained at a low level. Alexandrium tamarense blooms in Osaka Bay increased in scale approximately from the 2000s onward. Long-term trends in the dynamics of A. tamarense populations were considered to relate to the concentration of DIN and the cell density of the dominant diatom in Osaka Bay, Skeletonema spp. Short-term blooms of A. tamarense in Osaka Bay may have been induced by declines in phosphate concentration. However, the long-term expansion of A. tamarense appears to be related to a decline in DIN concentrations and subsequent decreased spring blooms, especially of Skeletonema spp.