Winter Storms Lead to Spring Bloom

New hypothesis supported by satellites and waterborne sensors

Highest chlorophyll concentrations show as red in this spring 2003 image. The East Coast — Long Island, the Gulf of Maine and Nova Scotia — is to the left. (Image courtesy of the SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE)

If you separate predators from their prey, you get more prey. Now that simple relationship has been used to explain one of the most important annual events in the ocean: the North Atlantic spring phytoplankton bloom.

Since the 19th century, oceanographers have sought to explain its origins and have settled on the wintertime mixing of ocean waters followed by increasing light and temperature in the spring, a process known as Sverdrup’s hypothesis.

However, using NASA satellite data, Michael Behrenfeld, OSU professor in the Department of Botany and Plant Pathology, reported in 2010 that phytoplankton abundance begins to increase in the depths of winter, well before light and warmth return. He offered another explanation: As winter storms stir the water, predators of phytoplankton get separated from their prey, allowing more of the tiny plants to survive and initiating a bloom that lasts until the end of spring.

Critics who took issue with Behrenfeld’s use of satellite data noted that space-borne sensors capture light from only the ocean surface. However, in a second 2010 paper, Emmanual Boss of the University of Maine and Behrenfeld used additional data from a waterborne “profiling float” that sampled from deep in the ocean to the surface. They reported in the journal Geophysical Research Letters that float and satellite data are consistent. Phytoplankton begin to rebound in the short, dark days of winter. Move over Dr. Sverdrup.

Behrenfeld’s 2010 report in the journal Ecology is available online: http://bit.ly/aTUM3V.

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