Monitoring a sentinel species from satellites: Detecting emiliania huxleyi in 25 years of AVHRR imagery

Abstract

Blooms of the coccolithophore Emiliania huxleyi were detected around the world from 1982 through 2006 using Advanced Very High Resolution Radiometer (AVHRR) remote sensing reflectances. The annually averaged surface area of these blooms has decreased over the past 25 years in regions where E. huxleyi blooms are most prevalent - the Bering Sea, the North Atlantic south of Iceland, the Norwegian and Barents Sea, and the Patagonian Shelf. Though less sensitive than satellite ocean color sensors, AVHRR offers the longest continuous global dataset of visible reflectances and has been used previously to identify regional E. huxleyi blooms. This declining long-term trend in bloom surface area was correlated to warm sea-surface temperature anomalies. The trend was weakly correlated to increased mixed-layer depths. There were mixed results when comparing bloom surface area to climate indices. Previous studies associated individual E. huxleyi blooms to warmer temperatures and increased stratification. This apparently contrary result may indicate that the dynamics of large-scale changes are different from those of individual, local blooms. The decreased extent of blooms could also mean that E. huxleyi respond to additional factors over the long term, such as ocean chemistry.