Red tide. Photo by NOAA, Public Domain
(Click on the following link to listen to an audio version of this blog .. Sargassum on the sea!
Back in 2018 (Signs of Winter 3, December 12, 2018), I wrote a series of blogs about algae growing out of control in both freshwater and marine ecosystems. These Harmful Algae Blooms (“HAB’s”) are caused by the out-of-control growth of cyanobacteria (“blue-green algae”) in almost any freshwater ecosystem (including the Great Lakes and the ponds and reservoirs of the Great Plains) or by the explosive growth of single cell algae called dinoflagellates (especially the dinoflagellate species Karina brevis) in marine shoreline ecosystems. These shoreline HAB’s include the infamous “red tides.”
A third type of HAB is seen out in the open ocean although the algae masses then drift onto shorelines depositing great piles of debris. This third type of HAB is, of course, the out-of-control replication of the algae called Sargassum.
Sargassum. Photo by B. Giusca, Wikimedia Commons
Sargassum is a brown algae genus that has over two hundred and fifty species that are 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 “Sargasso Sea” which 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 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 broad, typically calm, blue-water sea that accumulates the floating Sargassum usually in long, continuous masses. Much of the Sargassum originates in the Gulf of Mexico and floats on some bounding currents to the Sargasso Sea out through the Straits of Florida.
American Eel. Drawing by E. Edmouson and H. Chrisp. Wikimedia Commons
The floating Sargassum of the Sargasso Sea plays an important role in the life cycle of several significant types of eels. Eels, in spite of their snake-like appearance, are fish, and a number of these long, slender fish are capable of living 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! The tiny, hatchling eels use the Sargassum as protection against predators and eventually follow the respective currents on the edge of the sea to swim and float toward the shoreline of North America or Europe. There, these eels may swim even further up river where they establish themselves as bottom dwelling, nocturnal predators in their freshwater ecosystems. American eels can be found throughout the Mississippi-Ohio River watershed. A few years ago one of these eels was caught by a fisherman in the Allegheny River near Pittsburgh. This was the first time in many years that eels had been seen so far upstream. Conger eels also breed in the Sargasso Sea and follow similar dispersion patterns. Congers, though, live exclusively in marine ecosystems and do not enter the freshwater riverine systems.
Many 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 Sargassum plays in the life cycles of all of these species. It may turn out to be just as vital, though, as it is for those three species of eel.
Sargassum on beach. Photo by P. Bourjon, Wikimedia Commons
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 Sargassum. Experts debate the influences of warming ocean temperatures and the increasing abundance of land-originated nutrients (especially nitrogen and phosphates from agricultural field run-off and sewage effluents). In particular the rich waters of the Congo, Amazon and Mississippi Rivers which are all augmented by abundant human-generated nutrients, are likely causes of this massive Sargassum bloom.
The Sargassum HAB had affected over twenty islands in the Caribbean, beaches all across Mexico’s Yucatan Peninsula, and beaches in Texas, Louisiana, Mississippi and Florida. Shorelines facing to the south and to the east are most often affected because of the direction of Sargassum flow. A new “Sargasso Sea” area has even been observed in the south Atlantic between Brazil and West Africa. This new area is adding more and sargassum to the already overloaded system.
Recently, a belt of Sargassum stretching thousands of miles from the Western Coast of Africa to the Gulf of Mexico has been detected via satellite imagery. In March 2023, the mass of this Sargassum belt was estimated to be 13,5 million tons! And, it is drifting our way!
Sargassum on beach. Photo by Pixabay
In some places Sargassum can pile up in twenty-two feet thick mats smothering everything beneath it. Many fish populations have been negatively affected, and sea turtles are under significant threat both at sea (where the large turtles can get sometimes fatally entangled in the unnaturally thick Sargassum mats) and also on shore (where nesting beaches for sea turtles become impossibly buried by the accumulating masses of 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 is beginning to make the cleanup effort economically unsustainable. Piles of rotting Sargassum, as one tourist put it, smells “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 animals can 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, is just another side of the consequences of human-alteration of our ecosystems. We have red tides and blue-green algae poisoning our shorelines and lakes and ponds. We are constantly reacting to these calamities, but have yet reached consensus on how to prevent them from occurring in the first place.