In what's termed the North Atlantic Bloom, an immense number of microscopic phytoplankton burst into color, first "greening" then "whitening" the sea as one species follows another.
Longer hours of sunlight were assumed behind the annual blooms, but oceanographers report they've studied eddies swirling across the surface of the ocean that sustain phytoplankton in shallower waters where sunlight fuels their growth by keeping them from being pushed downward by the ocean's rough surface.
"Our results show that the bloom starts through eddies, even before the sun begins to warm the ocean," said Amala Mahadevan, an oceanographer at the Woods Hole Oceanographic Institution in Massachusetts and lead author of the study published in Science.
The finding helps explain the timing of the spring and summer bloom known to mariners and fishers for centuries and clearly visible in satellite images, researchers said.
It also accounts for the patchy appearance of the blooms, shaped by the eddies responsible for their formation, they said.
The scientists focused on phytoplankton known as diatoms.
"When conditions are right, diatom blooms spread across hundreds of miles of ocean," researcher Craig Lee of the University of Washington said, "bringing life-sustaining food to sometimes barren waters."