Sargassum Seaweed Blooms Transform from Aquatic Flora to Coastal Nuisance

Seaweed is no longer just an aquatic flora, it has now transformed into a nuisance, especially for scientists. Sargassum, a genus of brown macroalgae that was once largely confined to the Sargasso Sea has evolved into the Great Atlantic Sargassum Belt that stretches across 5,000 miles. In May alone, scientists detected 37.5 million tons of this seaweed drifting between the western coast of Africa and the Gulf of Mexico. According to a study published in the journal Science titled 'Harmful Algae', these algae have been developing at varying levels over the past four decades, with many of the nutrients that fueled their early growth originating from the Amazon River. Researchers at the Harbour Branch Oceanographic Institute at Florida Atlantic University explained that they have spent four decades analysing satellite data, field observations and chemical studies to understand how a massive bloom first detected in 2011 has evolved into an almost annual event. They also believe that Portuguese explorers and adventurers may have already described the phenomenon in accounts from their voyages to the Americas during the 15th century.

What is Sargassum and why does it matter?

Sargassum is a genus of brown macroalgae that drifts freely across the ocean.

Sargassum is a genus of brown macroalgae that drifts freely across the ocean. In the open sea, it provides a habitat for a wide variety of marine species. What functions as a valuable ecosystem offshore transforms into a nuisance on land. Along the coastlines, it becomes a hazard, posing an environmental, economic and public health threat. It also affects tourism as visitors expecting a clean beach arrive at foul-smelling shores covered with seaweed that are difficult to access. As the seaweed decomposes, it produces an unpleasant odour and can release hydrogen sulfide, a toxic gas associated with the decay of organic matter. Large accumulations when washed ashore can clog beaches, disrupt coastal ecosystems, create low-oxygen zones and damage coral reefs. Local authorities have to hire labour and resources for continuous clean-up efforts, with little lasting success. For economies that depend on tourism, the impact can be particularly severe. According to Brian Lapointe, PhD, lead author of the study and research professor at FAU Harbour Branch, the team has deep dived into the changing story of sargassum- how it's growing, what's fueling the growth and why we're seeing such a dramatic increase in biomass across the North Atlantic. By examining the shifts in the nutrient composition, particularly nitrogen, phosphorus and carbon and how those elements vary over time and space. "We're beginning to understand the larger environmental forces at play," he said.

Environment at risk

According to satellite observations, ocean circulation models and field studies, the sargassum has a seasonal movement from the nutrient-rich coastal waters, especially in the western Gulf of Mexico into the open ocean through the Loop Current and the Gulf Stream. These findings also substantiate the early theories suggesting that sargassum originating in the Gulf could help sustain populations in the Sargasso Sea. “These nutrient-rich waters fueled high biomass events along the Gulf Coast, resulting in mass strandings, costly beach cleanups and even the emergency shutdown of a Florida nuclear power plant in 1991. A major focus of our review is the elemental composition of sargassum tissue and how it has changed over time,” Lapointe added. Researchers identified a major shift "away from natural oceanic nutrient sources like upwelling and vertical mixing towards land-based inputs such as agricultural runoff, wastewater discharge and atmospheric deposition. "Carbon levels in sargassum also rose, contributing to changes in overall stoichiometry and further highlighting the impact of external nutrient loading on marine primary producers,” Lapointe explained. According to Lapointe, the expansion of sargassum is not just a matter of ecological curiosity, it has real impacts on coastal communities. “The massive blooms can clog beaches, affect fisheries and tourism, and pose health risks. Understanding why sargassum is growing so much is crucial for managing these impacts. Our review helps to connect the dots between land-based nutrient pollution, ocean circulation, and the unprecedented expansion of sargassum across an entire ocean basin. ”