In summer, vast floating islands of algae can cover parts of the tropical Atlantic Ocean. Known as Sargassum, this algae reached record levels last year. The giant brown belt it formed in June 2018 extended from the west coast of Africa into the Gulf of Mexico — a span of 8,850 kilometers (5,500 miles)! The largest bloom of seaweed ever detected, it weighed at least 20 million metric tons, a new report finds. For perspective, that is about four times the weight of the Great Pyramid of Giza in Egypt.
Mengqiu Wang is an optical oceanographer at the University of South Florida in Tampa. She uses imagery to study the seas. Wang and her colleagues tracked the Sargassum mats over the last 19 years using satellite data. These revealed those mats suddenly expanded dramatically in the summer of 2011. And those massive blooms recurred almost every year since then.
Atlantic’s July Sargassum algae bands, 2011 to 2018
Researchers have dubbed these massive mats of seaweed the great Atlantic Sargassum belt. These floating algae islands have long provided an important shelter for turtles, fish, crabs, eels and other marine species. But there can be too much of a good thing. When they crowd coastlines, these mats can smother corals and seagrass. They also can litter and wreak coastal beaches along the Caribbean and Gulf of Mexico with layers of heavy, rotting seaweed that is meters (yards) thick.
The yearly Sargassum bloom has been fueled partly by nutrients pouring into the ocean from the Amazon River, the new study suggests. Forests can both regulate the flow of water from land to ocean and filter out some of the materials in it. But farms around the Amazon River are expected to increase their use of fertilizers (nutrients) in the decades ahead. People along the river’s tributaries also have been cutting down huge swaths of forests. That, too, is expected to continue. Such trends could mean that colossal Sargassum blooms may become the new normal, Wang’s team says in the July 5 Science.
Eyed from the sky
The researchers analyzed data from satellite instruments that scan the ocean in visible and infrared light. Like photosynthetic plants, Sargassum algae contain lots of green. It comes from a pigment known as chlorophyll-a. This pigment shows up brightly at infrared wavelengths. That makes it contrast sharply with the darker water below.
In the 10 years leading up to 2010, there was little of the algae in the central Atlantic Ocean. An occasional patch would emerge near the mouth of the Amazon River in the summer and fall. But things changed in 2011. A band of seaweed suddenly developed all the way across the ocean.
This change in 2011 “was really surprising,” says James Gower. Also an optical oceanographer, he works at Fisheries and Oceans Canada in North Saanich, British Columbia. “It really stood out in the satellite data,” he noted. He coauthored a commentary on the finding in the same issue of Science.
Each year since — except for 2013 — the algae have formed a similar, vast belt. Last year’s was the largest and densest yet, Wang’s team notes.
Two sources of nutrients appear to be feeding the blooms. One is the release of nutrients from the Amazon River. The other is an upwelling of water along the coast of West Africa. This is where strong winds push aside surface waters for long periods. This allows deeper, nutrient-rich waters to rise toward the surface. Such upwellings occur off Africa naturally, due to interaction caused by winds, the ocean and Earth’s rotation.
How those nutrients might change in the future is unclear. There’s much that scientists still don’t know about the sources of those nutrients. They’ve also much to learn about how climate change will affect the seaweed’s life cycle. Nutrients carried on dust may have a role. They get blown into the ocean from the Sahara Desert as well as from places along Africa’s Congo River.
For instance, the team notes, particularly warm temperatures at the sea surface appear to suppress the algae’s growth. That’s what happened in 2013. That could cast some doubt on the size the blooms to be expected in a warming world, the researchers note.
Being able to eye seaweed from the sky has been key to witnessing its widescale patterns in growth. Even higher-resolution satellite data of the world’s oceans could help researchers better track the movement of such algae, Gower says. And that might help figure out what’s really causing the blooms.
algae Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.
Atlantic One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.
bloom (in microbiology) The rapid and largely uncontrolled growth of a species, such as algae in waterways enriched with nutrients.
Caribbean The name of a sea that runs from the Atlantic Ocean in the East to Mexico and Central American nations in the West, and from the southern coasts of Cuba, the Dominican Republic and Puerto Rico down to the northern coasts of Venezuela and Brazil. The term is also used to refer to the culture of nations that border on or are islands in the sea.
chlorophyll Any of several green pigments found in plants that perform photosynthesis — creating sugars (foods) from carbon dioxide and water.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
colleague Someone who works with another; a co-worker or team member.
commentary (in science) An opinion piece, often written to accompany — and add perspective to — a paper by others, which describes new research findings.
coral Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).
data Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
eel A fish with a snake-like body and no scales. Many migrate from freshwater to salt water when it’s time to spawn.
fertilizer Nitrogen, phosphorus and other plant nutrients added to soil, water or foliage to boost crop growth or to replenish nutrients that were lost earlier as they were used by plant roots or leaves.
filter (in chemistry and environmental science) A device or system that allows some materials to pass through but not others, based on their size or some other feature.
fishery A place where fish are raised commercially (for sale) or a site where large numbers of a fish species naturally congregate, making it a place where commercial fleets come to harvest that species.
forest An area of land covered mostly with trees and other woody plants.
infrared A type of electromagnetic radiation invisible to the human eye. The name incorporates a Latin term and means “below red.” Infrared light has wavelengths longer than those visible to humans. Other invisible wavelengths include X-rays, radio waves and microwaves. Infrared light tends to record the heat signature of an object or environment.
life cycle The succession of stages that occur as an organism grows, develops, reproduces — and then eventually ages and dies.
litter Material that lies around in the open, having been discarded or left to fall where it may. (in biology) Decaying leaves and other plant matter on some surface.
marine Having to do with the ocean world or environment.
nutrient A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.
optical An adjective that refers to light or vision.
pigment A material, like the natural colorings in skin, that alter the light reflected off of an object or transmitted through it. The overall color of a pigment typically depends on which wavelengths of visible light it absorbs and which ones it reflects. For example, a red pigment tends to reflect red wavelengths of light very well and typically absorbs other colors. Pigment also is the term for chemicals that manufacturers use to tint paint.
pyramid A monumental structure with a square or triangular base and sloping sides that meet in a point at the top. The best known are those made from stone as royal tombs in ancient Egypt.
regulate (n. regulation) To control with actions.
resolution (in optics) A term having to do with the degree of clarity or detail with which some object can be seen. (v. resolve)
Sargassum A genus of large brown marine algae, a type of seaweed. It floats and can develop into huge surface mats.
satellite A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
seagrass The name is a misnomer because these are not grasses, but flowering underwater plants. Like land plants, seagrasses use photosynthesis to power the production of food and the release of oxygen. Some 60 different species can be found around the world. How deeply they can grow tends to depend on how clear the water is, and therefore how far down the sunlight can penetrate.
seaweed Large algae growing in the sea or on rocks below the high-water mark.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
upwelling (in oceanography) A rise to the warm surface of cold — usually nutrient-rich — waters from the darks depths of the ocean. It usually takes place along the western coastlines of continents in the subtropics. It develops when prevailing winds push the surface water away from the shore. This pulls deeper water up to take its place.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.
Journal: M. Wang et al. The great Atlantic Sargassum belt. Science. Vol. 365, July 5, 2019, p. 83. doi: 10.1126/science.aaw7912.
Journal: J. Gower and S. King. Seaweed, seaweed, everywhere. Science. Vol. 365, July 5, 2019, p. 27. doi: 10.1126/science.aay0989.
Website: National Oceanic and Atmospheric Administration. What is sargassum?