Interannual variability of coccolithophore Emiliania huxleyi blooms in response to changes in water column stability in the eastern Bering Sea

During the late summer of 1997, most of the continental shelf in the southeastern Bering Sea was covered by aquamarine-colored waters as a result of massive blooms of the coccolithophore, Emiliania huxleyi (E. huxleyi). Since then, E. huxleyi blooms in the eastern Bering Sea have become common. This study was conducted to examine spatio-temporal variability in E. huxleyi blooms in the eastern Bering Sea and to determine what factors are responsible for the blooms. In this study, we used datasets from the satellite ocean color sensor, SeaWiFS (Sea-viewing Wide Field-of-view Sensor) and MODIS (MODerate resolution Imaging Spectroradiometer) to detect the E. huxleyi blooms. The E. Huxley bloom was only found in the middle of the continental shelf domain (MSD), where the water depth is 20 to 100 m. We found large-scale interannual and seasonal variability in the area of the E. huxleyi blooms, with massive blooms observed in the spring of 1998 and 2000, and in the fall of 1997 and 2000. The blooms now typically peak in September, and only small bloom areas have also been observed since 2001. hat we proposed is that the key parameter for the E. huxleyi blooms is the strength of the density stratification resulted from two water masses formed in different season, surface warm layer and cold bottom water (CBW). Warming of the CBW since 2001 in the middle shelf have induced weakening the aforementioned stratification during summer. With less stratification, the water column is much more easily mixed, resulting in an increased nutrient (nitrate and silicate) supply from subsurface layers by storms in the summer. This is likely to be responsible for the recent decrease in E. huxleyi blooms in the eastern Bering Sea shelf. Conversely, the strong stratification induced by an eruptive warming of surface layer and intensified cold bottom water may result in the development of the massive E. huxleyi blooms like in 1997.