Temporal and spatial variability of coccolithophore blooms in the eastern Bering Sea, 1998-2001

In the late summer of 1997, aquamarine waters, resulting from a massive bloom of coccolithophore algae, covered most of the continental shelf of the eastern Bering Sea. This was the first such event recorded in the area, but since then, coccolithophore blooms have been common. The objectives of this study were first to determine the threshold value of the Sea Wide Field-of-view Sensor (SeaWiFS) coccolithophore algorithm using a comparison of satellite and ship-based optical observations, and second to analyze temporal and spatial variability using SeaWiFS images of the coccolithophore blooms in the Bering Sea from 1998 to 2001. In late July 1998 and 2000, we made bio-optical measurements of the coccolithophore blooms, which were composed of Emiliania huxleyi. In situ measurements of water leaving radiance (nLw) at characteristic wavelengths yielded values that differed from the standard NASA coccolithophore values. We therefore defined new threshold values for the mask and applied it to a time series of SeaWiFS images from the Bering Sea. Generally, the coccolithophore bloom began in February each year, as melting began along the edge of the sea ice, and then expanded northward. The bloom reached its peak in April, and then from May through August the area covered by the bloom dwindled in size. However, its area increased again in September. Throughout the study period, the blooms were located at depths of 20 to 100 m. There was annual and seasonal variability in the area affected by the coccolithophore bloom. There were large blooms in 1998 and 2000, whereas in 1999 and 2001 the area affected was smaller. These differences might be related to sea surface temperature (SST), since the largest blooms occurred in the warmest years.