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New Bloom Theory for Phytoplankton in North Atlantic Ocean – August 29th, 2009

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August 29th, 2009 Category: Climate Change, Image of the day

Phytoplankton in Atlantic Ocean - August 16th, 2009

Phytoplankton in Atlantic Ocean - August 16th, 2009

Here, ocean currents pull a phytoplankton bloom in the North Atlantic Ocean into an elongated swirl. For over fifty years, scientists explained the north Atlantic phytoplankton blooms with a theory, formalized by Harald Sverdrup, that the sun’s warming of the water in springtime gave phytoplankton abundant nutrients and sunlight, thus allowing them to reproduce at a great rate.

However, new research and the information provided by a decade of satellite data suggests that Sverdrup’s theory is incorrect; the true beginning of the plankton blooms probably begins in the dark of winter.

“We found that the north Atlantic bloom was starting much earlier than we thought and it didn’t coincide with an improvement in the growth conditions from the phytoplankton,” Michael Behrenfeld, an phytoplankton ecologist at Oregon State University. “It started in January.”

Behrenfeld found that the phytoplankton’s growth rate started to accelerate in mid-winter, when the conditions for their growth were presumably the worst, reports Wired Science.

His new theory is that 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 very quickly without as much interference by predators. As spring arrives, the temperature of the surface water and the layers underneath it equalize. The convection stops, and the water stops mixing, causing the phytoplankton to get stuck at the top of the ocean and allowing us to notice the blooms.

If Behrenfeld 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 Behrenfeld’s 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.

These winter plankton blooms probably did not show up in previous data because very few research vessels would explore the North Atlantic in mid-winter. Behrenfeld thinks that introduced a bias in the data that seemed to show that the blooms began suddenly in the spring, when more researchers were out looking for them. Satellite imagery, however, makes it possible to observe the blooms year-round.

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