Decadal changes of phytoplankton chlorophyll-a in the East Sea/Sea of Japan

The East Sea has very dynamic environmental conditions with respect to its physical phenomena and biological characteristics. In order to detect any longterm change in patterns of phytoplankton biomass and a longterm trend in phytoplankton spring bloom characteristics in the East Sea, recent decade chlorophyll-a data were obtained for the East Sea from NASA SeaWiFS and MODIS. We found that the average Chl-a concentration from 2008-2011 was rather higher than that from 1998-2001 in the East Sea, especially along the coastal areas and the sub-polar fronts. Distinctively, the patterns of the Chl-a concentration such as duration and intensity of the phytoplankton spring bloom were significantly different between the periods from 2008-2011 and 1998-2001. During the observation period, there were no distinct trends in environmental conditions in the East Sea. Several potential mechanisms for these changes are suggested for further intensive field studies. First, a recent increase in anticyclonic eddies could seed an early spring bloom of the phytoplankton. An increase of sea surface temperature in winter could also cause an earlier stratification leading to an earlier start of the bloom. Furthermore, a change of light regime might take place in the euphotic zone in the East Sea.