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From the Gulf of Guinea to the mouth of the Senegal River, a sprawling shroud of seaweed is choking coastal ecosystems along West Africa and the fishing communities that depend on them.

Offshore, it snags lines, breaks nets and clogs up outboard motors. On shore, it accumulates in vast brown heaps, tide after tide, obstructing both boatmen and nesting sea turtles. As the seaweed rots, it releases foul-smelling hydrogen sulfide gas, causing respiratory illness in local populations. And in nearshore habitats, the same processes of decay depletes the oxygen in the water, killing marine animals in gruesome mass die-offs.

“It came as a shock to people,” said Paul Lamin of Sierra Leone’s Environment Protection Agency, describing the first inundation of June 2011. “We had never experienced such a large quantity of seaweed washing on our beaches.”

Scientists at the University of Sierra Leone subsequently analysed the seaweed, identifying two species of non-native brown macroalga of the genus SargassumS. fluitans and S. natans. Both are abundant in the Sargasso Sea, a vast, shoreless, gyre-bound body of water encompassing nearly two-thirds of the North Atlantic Ocean.

Pelagic Sargassum is bushy, rootless and buoyant. It reproduces asexually by propagating from detached fragments. Kept afloat by gas-filled vesicles that resemble berries, Sargassum aggregates into floating mats that provide nourishment and shelter to a host of rare fauna. At least 10 endemic species inhabit the Sargasso Sea, including the Sargassum anglerfish (Histrio histrio), a cannibal frogfish with flawless camouflage and grasping, prehensile fins that enable it to clamber around the tangle of seaweed.

Flying fish, marlin, and migratory eels all spawn in Sargassum, and several species of shark and tuna migrate through the region, too. Thirty species of whales and dolphins have been observed in the Sargasso Sea, including humpback and sperm whales. And four species of endangered turtle use the Sargassum as a nursery.

Once described as “the golden rainforest of the ocean” by U.S. oceanographer Sylvia Earle, the Sargasso Sea is a haven for life. However, large-scale Sargassum beaching events are singularly destructive to local biodiversity.

Small, occasional and harmless quantities of Sargassum have always washed up on Atlantic beaches, but the enormous quantities observed since 2011 are a startling new occurrence that almost certainly heralds a long-term and irreversible shift in the ecology of the ocean.

The source of this seaweed is not the Sargasso Sea, but a massive, novel, transcontinental algal bloom, stretching 8,850 kilometres (5,500 miles) from West Africa to South America, north to the Caribbean and beyond to Mexico, called the Great Atlantic Sargassum Belt (GASB). it burgeons to life from April to October every year, though it can vary in mass, timing and distribution. In 2013, the GASB did not bloom at all. In 2018, it grew to more than 20 million tons. Scientists and policymakers around the world are now scrambling to understand the phenomenon.

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According to Lamin, in Sierra Leone, funding for further studies and on-the-ground responses has been particularly stretched this year due to the COVID-19 pandemic, which has shifted government focus away from Sargassum. It has been almost impossible to mobilize youth volunteers for beach cleanups, an activity usually undertaken with the help of community groups, NGOs and local businesses, as well as the national tourist board.

To combat this scourge of seaweed, researchers in Ghana are participating in an innovative interdisciplinary project involving partners in Barbados, Jamaica and the United Kingdom, called “Teleconnected SARgassum risks across the Atlantic: Building capacity for TRansformational Adaptation in the Caribbean and West Africa (SARTRAC).”

Led by the University of Southampton, U.K., SARTRAC is a three-year project and the only one in the world to investigate Sargassum on both sides of the Atlantic.

The researchers’ work includes developing a Sargassum early-warning system using satellite imagery, modeling and drones. Their main goal is to identify adaptation opportunities and build resilience among the poorest and worst-impacted communities, including those that derive their primary income from artisanal fishing.

At the University of South Florida, Chuanmin Hu heads an optical oceanography lab that tracks the GASB and publishes a monthly outlook bulletin for the Caribbean Sea. He says he thinks the complexity and connectivity of unbounded ocean systems pose a major challenge to understanding what drives Sargassum.

“We are not looking at a local problem like your backyard pond,” he said. “The vast ocean connects the western and eastern planet, the North and the South Atlantic, the surface and the deep Atlantic, the air and the water. So it’s a connected problem and there are several variables that can drive Sargassum transport and growth.”

Such variables might include sea temperatures, upwellings, fertiliser runoff, untreated effluence, nutrient discharges from the Congo and Amazon rivers, deposits of iron-rich Saharan dust, and wind and current anomalies associated with negative phases of the North Atlantic Oscillation. Understanding how these variables interact will require a huge, coordinated, international effort.

“We have some hypotheses,” Hu said. “My take is perhaps we have just touched the sides of a giant elephant — but not all sides.”

Featured image by: Flickr 

This article was originally published on Mongabay, written by Richard Arghiris, and is republished here as part of an editorial partnership with Earth.Org.

According to researchers at the University of Hawaii, a newly discovered seaweed has been wreaking havoc on the pristine coral reefs throughout the Northern Hawaiian islands. Individual mats of this seaweed are as big as football fields, have the ability to break off and form tumbleweed-like structures, and- most dangerously- compete with corals for nutrients and light. They have described this seaweed as ‘highly destructive with the potential to outgrow entire reef systems’. 

The seaweed was discovered in the Papahanaumokuakea Marine National Monument, a pristine conservation area with high ecological value considered as a World Heritage Site. The area is home to over 7 000 marine species, 14 million seabirds, and is home to the threatened green sea turtle and endangered Hawaiian monk seal. Each of these species relies on the existing reef systems for shelter, food and structural protection (erosion protection from waves). 

The red seaweed, known as Chondria tumulosa, was initially discovered through surveys conducted by the National Oceanic and Atmospheric Administration (NOAA) back in 2016. The first appearance was in the Pearl and Hermes Atoll. A follow-up survey was conducted in 2019 and it was then that researchers discovered their alarming growth rates that had covered entire reef systems. From DNA testing, there is no existing match of known algae, hence it is considered a new type of seaweed in the genus Chondria. 

According to the US National Invasive Species Information Center, the new seaweed is not considered invasive, as it is unknown whether it was introduced to the native islands by humans or if it originated from there in very small numbers and only recently exploded. Normally, marine invasive species are introduced into new areas via human activities, through fish being accidentally transported on ships, for example. 

Researchers have instead characterised the seaweed as a ‘nuisance species’ due to the sudden ecological impact caused by its expansive and explosive growth. There is conclusive evidence of their abilities to destroy coral reefs; according to the published article, the seaweed was found to outgrow native reef species and replace keystone species, fundamentally, changing the ecological structure of reefs. This has caused irreversible damage to coral reefs in Hawaii because the seaweed is outcompeting the corals for light and nutrients, causing a collapse in the ecosystem. 

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The researchers are also concerned at the seaweed’s tendency to break off into tumbleweed-like structures and travel far distances, posing risks for nearby coral reef ecosystems. 

Another example where an invasive species has decimated the native ecosystem is that of the lionfish. Originating from the South Pacific and Indian Ocean, the lionfish has invaded the Atlantic ocean, with frequent sightings along the southeastern coast of the US and the Bermuda Islands. The Lionfish hunts and outcompetes native fish for food, causing an imbalanced food chain.

In areas where the seaweed had taken over, Heather Spalding, one of the researchers and an algae specialist from the University of Hawaii says that “everything was dead underneath.” The large mats of seaweed essentially block light that corals need to live, making it difficult for these and other marine life to flourish. Known species of fish that graze on algae do not touch the new seaweed, and such areas are void of marine life. 

Determining the causes of a systemic change in the ecosystem is rather difficult but one important factor favouring seaweed growth is increasing ocean temperatures. Opportunistic seaweed can adapt to fluctuating warmer waters and completely overtake coral-dominated systems. An example of this is in the Gulf of Maine where there used to be an abundance of kelp beds but these are currently being overtaken by turf seaweed. Growth of kelp beds favour lower temperatures and they provide large areas of cover for the native fish. Without this cover, native fish are being predated on by migratory fish species.

Seasonal changes can contribute to seaweed blooms throughout the year, but with over 20 years of NOAA observations and surveys in the area, the researchers have concluded that it is not an ordinary seaweed seasonal growth, but rather a symptom of an existing problem, such as increasing water temperature from climate change. 

To combat this nuisance seaweed and save coral reefs in Hawaii and beyond, it is vital to find out the origins and causes of growth, both of which are still unknown at this stage. The researchers are returning to the area to study for more information.

Featured image by: Mike Cialowicz

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