Every year, the north Atlantic ocean turns green with plankton, and for more than fifty years, scientists thought they knew why. Now, a decade’s worth of satellite measurements suggest they were wrong.

The common-sense idea is that in the spring, the sun warms up the water column until it hits a key threshold and suddenly comes alive. But the true beginning of the plankton blooms probably begins in the dark of winter.

Phytoplankton harness the energy of the sun and draw on nutrients in the ocean like nitrogen, iron, and other elements that land plants get from soil. So scientists focused on solar and nutrient availability as the key to their growth. But they found that the phytoplankton's growth rate started to accelerate in mid-winter, when the conditions for their growth were presumably the worst.

Scientists devised an alternative thesis that squares better with the new data. During the winter months, cold winds blowing across the water cool the top surface layers. Cold water sinks, pushing up some warmer water, which gets cooled itself and drops. The process creates convection and carries the tiny plankton through a much larger volume of the ocean, diluting them.

When the phytoplankton are spread out, it’s harder for the zooplankton that eat them to find them. Suddenly, the phytoplankton can breed like crazy without as much interference by predators. As spring arrives, the temperature of the surface water and the layers underneath it equalize. The convection stops. The water stops mixing. At that moment — when the phytoplankton get stuck at the top of the ocean — we notice the blooms.

If the new thesis is right, the new model would have dramatic implications for ocean health in a warming world. If a warmer ocean is all that’s needed to spark plankton blooms, then global warming would lead to larger and larger blooms. With the new model, a warming ocean would hurt the blooms. Considering that the blooms are the base of the oceanic food chain, that would hurt species ranging from the tiniest fish to the largest whales.

Original article here.