Over recent weeks, scientists and resource managers in such diverse places as Trinidad, the Dominican Republic and Sierra Leone have been reporting unusual incidents of seaweed washing ashore in massive quantities. The seaweed is a type of brown algae known as Sargassum, which commonly drifts on ocean currents of the North Atlantic and Gulf of Mexico. This algae is well-known for washing up en masse on beaches in places such as Florida, Texas and Bermuda. What’s different this year is not so much the quantities, but the geography. The islands of the Lesser Antilles report never having seen this much of the seaweed before (although it is normally present in their waters), while the country of Sierra Leone has never known it under any quantity.
Meanwhile, in the Sargasso Sea, a region named for the seaweed and where you’d expect to find it in abundance, scientists are reporting less than usual biomass for the algae. Ken Smith and Alana Sherman of MBARI recently completed a survey of pelagic Sargassum on an oceanographic cruise through the Sargasso between the Bahamas and Bermuda. They expected to find large mats of the seaweed. They did not.
In the North Atlantic Ocean, Sargassum algae has evolved a completely pelagic existence, free-floating on ocean currents of the Gulf of Mexico, Caribbean and the Sargasso Sea. Mariners have documented large drifting islands of this seaweed for hundreds of years, mistakenly believing the appearance of the plant to be a sign that land was nearby. Their false assumption was that the seaweed had been torn from reefs and shoals during heavy storms. With other species of Sargassum, found throughout the oceans of the world, the seaweed indeed is usually affixed to rocks, reef and such. But two Atlantic species have made the evolutionary leap to become “holopelagic”, meaning they permanently live and reproduce adrift on the sea.
Check out this video about life in the Sargassum:
These two species – Sargassum natans and S. fluitans – support a globally unique and important ecological community. Much like seagrass beds and mangroves in coastal areas, pelagic Sargassum provides a nursery habitat for juvenile fish. It is also a critical nursery for newly hatched sea turtles, a grazing area for adult fish and seabirds, and refuge to countless invertebrates and a few specialized fish species that make it home throughout their life histories. The habitat is economically important for its role in nurturing both coastal and offshore fisheries, and provides a mode of transportation for juvenile species that later settle out and inhabit coral reefs. Without it, the ocean would be a less dynamic place.
But it can be a nuisance when it washes ashore, particularly on beaches not prepared for it.
In a year marked by unusual weather in the Northern Hemisphere – from snow storms to tornados to heat waves – it should come as no surprise that things are amiss in the ocean as well. But what exactly is going on remains to be determined. Participants on popular scientific discussion groups still seem more perplexed than conclusive. And even the climatic cause for the inundations is under debate. Some prefer a hypothesis of abnormally high blooms of the seaweed this season – perhaps influenced by increased ocean productivity. To me, this later explanation would not account for its sparsity in the Sargasso Sea, nor it’s appearance in places it has never been documented before. But whatever the cause, one thing is certain. This is not a normal event in the places that are seeing seaweed-covered beaches this year. The ocean, in one way or the other, is behaving in ways not remembered in recent decades.
Note: Images of Sierra Leone courtesy Andrew Huckbody.