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October 12, 2001

Sargassum: Weed of Life

Much remains a mystery about this magical pelagic habitat

Your blood runs hot at the first sight of the scattered yellow dots floating in the vast sea of blue. The little blobs of weed start to get thicker and thicker until you see the makings of every offshore angler's dream - a fat, well-defined weed line stretching off into the distance as far as the eye can see. Visions of patrolling game fish flash through your mind's eye as you scramble to get baits over the side, and the captain turns parallel to the line. It won't be long now, you whisper to yourself, and your thoughts are affirmed seconds later with the tantalizing pop of the rigger and the shrill screech of pulling drag. The weed lines have once again shared their bounty.

Ranging throughout the world's sub-tropical oceans, sargassum might just represent the most important offshore environment on earth. Yet even though you can run across gigantic patches or miles-long lines of the stuff almost anywhere game fish are caught, there is still much to be learned about the life-sustaining properties of this ubiquitous weed and the role it plays in the continued health of the pelagic environment. The realm of discoveries to be made regarding this habitat appear to be virtually limitless at the moment, ranging from the biology of the algae itself to the species which inhabit it. Whereas the possibilities of new discoveries about sargassum are great, our knowledge of all aspects of this important natural resource is minuscule in comparison.

What Is It?
Sargassum gets its name from the small gas-filled bladders that help keep surface varieties afloat and anchored species standing upright. Portuguese sailors cutting across the Sargasso Sea on the early voyages to the New World called the weed salgazo because the round bladders reminded them of a small variety of grape native to their homeland. Eventually salgazo turned into salgaco, and from that scientists derived the common name for the genus of brown algae now known as Sargassum.

There are at least six known species of sargassum, but the two most commonly found on the surface are Sargasso natans and Sargasso fluitans. The two species are very similar, and many clumps and weed lines contain both species.

But according to Ray Waldner, professor of biology at Palm Beach Atlantic College, "There are an awful lot of sargassum species that are anchored to the bottom. During storms they can become detached, and since these species also have bladders to help keep them upright, they will float to the surface. While doing some marine botany work, I found several species anchored in 6 to 8 feet of water off Puerto Rico. I'm sure that some of the sargassum we see on the surface comes from detached plants that originated in the Caribbean - but that's not the main source for most we see."

It is theorized that millions of years ago some of these detached clumps managed to survive and evolved into the two free-floating species that anglers seek out today. Jerry Goss, also a professor of biology at Palm Beach Atlantic, says that sargassum reproduces asexually through fragmentation and that every piece of the same species may come from a single ancestor. "It seems that no matter where you go on the planet, these two species are going to be the same genetically. In fact, they appear to be huge clones - and some people say that sargassum might just be the world's largest organism," says Goss. Once a piece breaks off, it doesn't just die; it can float on indefinitely, reproducing over and over again.

But very little is known about how long individual pieces of sargassum actually live; it isn't easy to keep alive in an aquarium. "We really aren't sure how long it can survive," says Jim Franks, a fisheries research biologist at the University of Southern Mississippi's Institute of Marine Sciences. "It is believed that mats can live for quite some time - maybe two years or more - but not terribly long." Older patches darken with age and become heavily encrusted with a myriad of life forms that depend on the weed for their survival. As more and more animals and invertebrates cling to the aging fronds, the weight begins to overcome the buoyancy provided by the sargassum's gas bladders and the pieces begin to sink. As the weed sinks deeper in the water column, the resulting increase in pressure will completely collapse the bladders, speeding the patch's - and the animals clinging to it - demise.

The Sargasso Sea
The main source of the sargassum found floating throughout the Gulf of Mexico, Atlantic Ocean and Caribbean comes from the Sargasso Sea. This "sea" is a two-million-square-mile chunk of the Atlantic Ocean stretching from the West Indies to the Azores. Although the sea is bordered on all sides by circulating currents - the Gulf Stream to the west and north, the Canary Current to the east and the North Equatorial Current to the south - in its interior lies a vast calm. It is here where huge patches of sargassum collect en masse. The weed is drawn to the center of the sea by two different methods: the earth's rotation and the process of evaporation. The rotation of the earth spins the currents to the center of the sea, which carry the weed along with them. And, since the water in the center of the sea is relatively stable, it evaporates faster, creating small surface currents that bring in water to replace that lost to evaporation.

Unfortunately, these same currents can concentrate all the debris and pollution introduced by man, and no one is sure how these pollutants might affect the sargassum. "Drifting in the ocean, sargassum has the opportunity to encounter many different things - debris and pollutants - and we don't know what effect they have on the sargassum or the animals living on and around it," says Franks.

One thing we do know: There's an awful lot of it out there. In a Fishery Management Plan submitted by the South Atlantic Fishery Management Council to the U.S. Department of Commerce, there is an estimated standing crop of sargassum in the North Atlantic that may be 4 to 11 million metric tons (roughly 9 to 24 billion pounds). These estimates are very rough, however. "There have been a few studies that try to estimate the coverage, but they come up with such a wide variability that no one really knows for sure just how much there is," says Read Hendon, a fisheries biologist and tagging coordinator at the Institute of Marine Sciences at the University of Southern Mississippi.

Spreading Out
Even though the prevailing currents work to keep the sargassum centrally located in the Sargasso Sea, prevailing winds, storms and spiraling currents help disperse the weed throughout the world's oceans. Franks says that it is much more prevalent in some areas like the Gulf of Mexico and around the islands of the Pacific, Japan, and the Indian Ocean.

"Pieces and parts break away from the Sargasso Sea, and then currents and winds carry it all over the place. It drifts down through the Caribbean and is pushed down through the Yucatan Straits. Once there, it can join the loop current in the southern Gulf of Mexico, which eventually pushes it up into the northern Gulf. Gyres spinning off the currents bring the offshore waters in close to shore, carrying with it the rafts of weed," says Franks. It also gets trapped in the Gulf Stream and works its way up the northeast coast of the United Sates, where it hitches a ride on giant eddies and back currents along the stream's edge that bring it into the nutrient-rich waters over the continental shelf.

What Makes It So Important?
Aside from the fact that sargassum seems to attract and hold just about every species of big-game fish that anglers pursue, the weed plays an important role in the life cycles of hundreds if not thousands of marine animals. A multitude of marine invertebrates spends part or all of their lives associated with floating patches of the weed. According to Hendon, as a marine habitat, sargassum is one of the most dynamic and significant features in the ocean utilized by pelagic organisms, particularly young fish. Since young fish are not as mobile as their adult forms, they become easy prey for predators, and sargassum provides a source of refuge from this predation. Paradoxically, and luckily so for us anglers, this large presence of juvenile fish attracts predatory game fish. Tiny billfish, as well as juvenile dolphin, swordfish and even some species of snapper, grouper and permit, can be found seeking refuge in the protective fronds.

"Fish are drawn to sargassum for many reasons," says Franks. "It provides both shelter and food, making it a good place for fish to congregate; schooling behavior may even start here. We liken the habitat to that of the inshore estuary - estuaries provide a nursery area inshore, and the sargassum provides one offshore."

Mats and weed lines may also provide a point of orientation for some species of pelagic fish, a landmark that they can return to throughout the day to gather or feed. Young sea turtles of all apecies also seek refuge among the weed, finding both food and shelter from hungry predators.

In short, there is no other habitat like it in the open ocean. It provides young fish shelter from predators, and it harbors a constant supply of small prey to feed upon.

I would especially like to thank Read Hendon and Jim Franks of the Gulf Coast Research Laboratory and the Institute of Marine Sciences at the University of Southern Mississippi. Several passages from their joint paper, "How Well Do We Know the Sargassum Community?" were used with their permission in the writing of this article. Additional information was gleaned from the article, "The Sargasso Sea," Smithsonian Magazine, November 1998 by Henry Genthe.