Credit: J. SCHMALTZ/MODIS RAPID RESPONSE TEAM/NASA GSFC

Ecology doi:10.1890/09-1207.1 (2010)

An analysis of satellite data suggests that the vast phytoplankton blooms that grace oceans around the globe in springtime actually start to develop in the depths of winter, challenging long-held theories about what drives them.

With almost 10 years' worth of data from NASA's SeaStar spacecraft, Michael Behrenfeld of Oregon State University in Corvallis found that phytoplankton populations begin to increase during winter, when deeper, nutrient-rich waters mix with the ocean surface layer and disperse zooplankton grazers that keep phytoplankton in check.

The visible bloom (pictured above) occurs in spring, months after the active surface layer stops deepening. Phytoplankton, multiplying quickly, are concentrated in the layer and nourished by spring sunshine — previously believed to be a major factor in initiating the bloom.

The bloom ends when grazing zooplankton or dwindling surface nutrients bring the phytoplankton population back under control.