NASA data shows hurricanes help plants bloom in 'ocean deserts'

06/16/04

Whenever a hurricane races across the Atlantic Ocean, chances are phytoplankton will bloom behind it. According to a new study using NASA satellite data, these phytoplankton blooms may also affect the Earth's climate and carbon cycle.

Dr. Steven Babin, a researcher at the Johns Hopkins University in Applied Physics Lab in Laurel, Maryland studied 13 North Atlantic hurricanes between 1998 and 2001. Ocean color data from the SeaWiFS instrument on the SeaStar satellite were used to analyze levels of chlorophyll, the green pigment in plants. The satellite images showed tiny microscopic ocean plants, called phytoplankton, bloomed following the storms.

"Some parts of the ocean are like deserts, because there isn't enough food for many plants to grow. A hurricane's high winds stir up the ocean waters and help bring nutrients and phytoplankton to the surface, where they get more sunlight, allowing the plants to bloom," Babin said.

Previous research has relied largely on sporadic, incomplete data from ships to understand how and when near-surface phytoplankton bloom. "This effect of hurricanes in ocean deserts has not been seen before. We believe it is the first documented satellite observation of this phenomenon in the wake of hurricanes," Babin noted. "Because 1998 was the first complete Atlantic hurricane season observed by this instrument, we first noticed this effect in late 1998 after looking at hurricane Bonnie," Babin said.

The study found the physical make-up of a storm, including its size, strength and forward speed, is directly related to the amount of phytoplankton that blooms. Bigger storms appear to cause larger phytoplankton blooms. An increased amount of phytoplankton should have more chlorophyll, which satellite sensors can see.

Hurricane-induced upwelling, the rising of cooler nutrient-rich water to the ocean surface, is also critical in phytoplankton growth. For two to three weeks following almost every storm, the satellite data showed phytoplankton growth. Babin and his colleagues believe it was stimulated by the addition of nutrients brought up to the surface.

Whenever the quantity of plants increases or decreases, it affects the amount of carbon dioxide in the atmosphere. As phytoplankton grow, they absorb carbon dioxide, a heat-trapping greenhouse gas. The gas is carried to the ocean floor as a carbon form when the tiny plants die. This enables atmospheric carbon to get into the deep ocean. It is one of several natural processes that contribute to Earth's carbon cycle.

By stimulating these phytoplankton blooms, hurricanes can affect the ecology of the upper ocean. Phytoplankton is at the bottom of the food chain. The factors that influence their growth also directly affect the animals and organisms that feed on them. In addition, since climate-related phenomena like El NiƱo may change the frequency and intensity of hurricanes, storm-induced biological activity may have even greater contributions to future climate change.

Scientists are still trying to determine how much carbon dioxide might be removed from such a process. "Better knowledge of the carbon cycle will improve our understanding of global ecology and how climate change might affect us," Babin said.

The research appeared as a paper in a recent issue of the Journal of Geophysical Research-Oceans. Study co-authors include J.A. Carton, University of Maryland, College Park, Md.; T.D. Dickey, Ocean Physics Laboratory, University of California, Santa Barbara, Calif.; and J.D. Wiggert, Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, Va.

Source: Eurekalert & others

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