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Last week I wrote about two kinds of algae (or two organisms, at least, who are often described using the word “algae”): blue green algae (the “cyanobacteria”) and dinoflagellate protists (especially Karina brevis). Blue green algae grow explosively in warm, freshwater lakes and ponds and puddles after their waters have been polluted with nitrogen and phosphate from sewage or agricultural field runoff. Likewise, Karina brevis explodes in numbers in marine ecosystems when similar nutrients from similar sources are combined with warm water and favorable winds and currents. This ocean shoreline phenomenon is called a “red tide.” These two out-of-equilibrium “algae” population events are collectively called “Harmful Algae Blooms” (HABs), and each represents a significant environmental threat to both wildlife and to human beings.
There is another kind of algae, though, that may be properly included in these HAB events. These algae are large, multicellular, marine, brown algae that are both commonly and scientifically called “sargassum.”
There are over two hundred and fifty species of sargassum distributed all around the world. The presence of the air bladders on these algae allow them to float on the surface of ocean often in thick, extensive masses. One area known for an abundance of sargassum is aptly named the “Sargasso Sea.” This sea is located in the Atlantic Ocean off of the coast of North America. The Sargasso Sea is only named sea in the world that is not bordered by any land masses. Instead it is surrounded by four, large, powerful ocean currents (the Gulf Stream in the west, the North Atlantic Current to the north, the Canary Current to the east, and the North Atlantic Equatorial Current to the south). It is a calm, typically blue-water sea where floating sargassum accumulates in long, continuous mats. Much of this sargassum originated in the Gulf of Mexico and was carried out in the current flow through the Straights of Florida into the Sargasso Sea.
The floating sargassum of the Sargasso Sea plays an important role in the life cycle of several types of eels. Eels, in spite of their snake-like appearance, are fish, and some of these long, slender fish spend parts of their lives in seawater and parts of their lives in the freshwater of rivers. The American eel and the European eel begin their lives as fertilized eggs out in the floating seaweed of the Sargasso Sea! After they hatch, the tiny eels use the sargassum as protection against predators and eventually follow the respective currents on the edges of the sea to swim and float to the shorelines of North America or Europe. There, these eels swim up into rivers where they establish themselves as bottom dwelling, nocturnal predators. This past summer a fisherman in the nearby Allegheny River caught an American eel. This was notable because it was the first time in many years that an American eel had been seen in the Allegheny River. That eel had begun its life in the Sargasso Sea and had followed the ocean currents to the mouth of the Mississippi River. It had then laboriously worked its way up the Mississippi, to the Ohio River, past Pittsburgh and into the Allegheny. Quite a trip! The fisherman, thankfully, released the eel back into the river! I hope that that eel is eventually able to return to the Sargasso Sea to lay its eggs!
Conger eels also breed in the Sargasso Sea and follow similar dispersion patterns. These eels, though, live exclusively in marine ecosystems and do not enter the freshwater riverine systems.
Many other organisms live in the entangled strands of sargassum. Crabs, shrimp, polychaete worms, flat worms, squid, snails, many kinds of fish and even sea turtles use the floating algae as a refuge and a source of food. Research is just beginning to explore the role that the sargassum plays in the life cycles of all of these species. It may turn out, though, to be just as vital as it is for those three species of eel.
Sargassum has been present in the Gulf of Mexico, the Caribbean, and the Atlantic Ocean for a long time. It has phases of boom and bust growth like any species, but has existed in a fairly predictable equilibrium for as long as people have been watching it.
That predictable equilibrium, though, ended in 2011 when a massive (and still on-going) HAB occurred for the sargassum. Experts debate whether rising ocean temperatures or the increasing abundance of land-originated nutrients caused the sargassum HAB, although most agree that it was probably a synergy between the two. They also note that iron-rich dust from the Sahara Desert has been blowing into the Atlantic Ocean and the Gulf of Mexico in greater and greater amounts, and that this dust is know to stimulate the growth of algae.
The sargassum HAB has affected beaches on twenty islands in the Caribbean and all across Mexico’s Yucatan Peninsula. Beaches in Texas, Louisiana, Mississippi and Florida have also been buried under masses of sargassum. Shorelines facing to the south and to the east are most often affected because of the direction of sargassum flow. Further, a new “Sargasso Sea” area has been observed in the south Atlantic between Brazil and West Africa. This new area is contributing more and sargassum to the already overloaded system.
On some beaches the sargassum can pile up in mats up to twenty-two feet thick smothering everything beneath it. Abnormally dense floating masses of sargassum at sea negatively affect many fish populations. Sea turtles are under significant threat both at sea (where the large turtles can get fatally entangled in the unnaturally dense sargassum) and also on shore (where their nesting beaches become impossibly buried by the accumulating seaweed).
Many tourist resorts are working hard to remove the sargassum from their beaches but are finding that the cost of the heavy machinery and the difficulties in disposing of the tons and tons gathered sargassum are beginning to make the cleanup effort economically unsustainable. Piles of rotting sargassum, as one tourist put it, smell “like sewage.” It is rich in sulfur compounds and also arsenic and destroys plant life with which it comes in contact. Composting it down to plant fertilizer has not been successful, and few land animals will eat it. There is a research is underway to see if it can be used for biofuel generation, but that project is many years away from practical application.
This sargassum problem, then, like the red tides and the blue-green algae HAB’s, is just another consequence of the human-alteration of our aquatic ecosystems. We constantly react to these calamities, but have not yet reached consensus on how to prevent them from occurring in the first place.