Sargassum natans, unlike the many benthic Sargassum species (i.e., those that live attached to the sea bottom), is a pelagic (free-floating) brown seaweed that occurs mainly far out in the North Central Atlantic Ocean, but washes ashore regularly. It is often associated with the Sargasso Sea, a very large region situated offshore from the southeastern United States, approximately south and east of Bermuda and seaward from the Gulf Stream. The Sargasso Sea, which accumulates large masses of Sargassum, results from a ring of prevailing ocean currents that enclose an enormous eddy (about 5.2 million square kilometers) which rotates clockwise as a result of the Earth's eastward rotation. Sargassum natans apparently reproduces only asexually, by fragmentation. (Gosner 1978; Kaplan 1988) Gower and King (2008) used satellite imagery to track the origin, distribution, and fate of floating Sargassum. Using this approach, they analyzed data from 2002 to 2008 and were able to present the first mapping of the full distribution and movement of pelagic Sargassum in the Gulf of Mexico and western Atlantic. Their results revealed a seasonal pattern in which Sargassum typically shows strong growth in the northwest Gulf of Mexico in the spring of each year, is transported into the Atlantic in about July, appearing east of Cape Hatteras as a “Sargassum jet", and ends up northeast of the Bahamas in February of the following year. Gower and King note that the idea that pelagic Sargassum originates in the Gulf of Mexico, as they propose, was fairly common in the 19th century (based on ship observations), but became less widespread in the following century. Based on their findings, Gower and King conclude that that most pelagic Sargassum has a life span of one year or less, with the major “nursery area” being in the northwest Gulf of Mexico. Given their estimated average flow of about one million tons of Sargassum out of the Gulf of Mexico each year, the authors suggest the implied carbon flux should be accounted for in productivity and carbon models.
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