A monster seaweed bloom is taking up the Atlantic

Once considered primarily confined to the nutrient-poor waters of the Sargasso Sea, sargassum is now acknowledged as a quickly rising and extensively distributed marine organism, whose growth throughout the Atlantic is carefully linked to each pure processes and human-induced nutrient enrichment.

The evaluation, revealed within the journal Harmful Algae, sheds new gentle on the origins and growth of the Great Atlantic Sargassum Belt, a large recurring bloom of sargassum that stretches throughout the Atlantic Ocean from the coast of West Africa to the Gulf of America.

Since its first look in 2011, this belt has shaped almost yearly – besides in 2013 – and in May, reached a brand new report biomass of 37.5 million tons. This doesn’t embody the baseline biomass of seven.3 million tons traditionally estimated within the Sargasso Sea.

By combining historic oceanographic observations, fashionable satellite tv for pc imagery, and superior biogeochemical analyses, this evaluation offers a complete framework for understanding the dramatic adjustments in sargassum distribution, productiveness and nutrient dynamics. It additionally highlights the broader implications of anthropogenic nutrient enrichment on ocean ecology and the necessity for coordinated worldwide efforts to watch and handle the impacts of those large seaweed blooms.

“Our review takes a deep dive into the changing story of sargassum – how it’s growing, what’s fueling that growth, and why we’re seeing such a dramatic increase in biomass across the North Atlantic,” stated Brian Lapointe, Ph.D., lead writer and a analysis professor at FAU Harbor Branch. “By examining shifts in its 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.”

Early within the evaluation, Lapointe and co-authors Deanna F. Webber, analysis coordinator; and Rachel Brewton, Ph.D., an assistant analysis professor, each with FAU Harbor Branch, clarify that early oceanographers charted the Sargasso Sea based mostly on floor sightings of sargassum, believing the seaweed thrived in its heat, clear, however nutrient-poor waters. However, this notion created a paradox when mid-Twentieth-century oceanographers described the area as a “biological desert.”

However, current satellite tv for pc observations, ocean circulation fashions, and subject research have resolved this paradox by tracing the seasonal transport of sargassum from nutrient-rich coastal areas, notably the western Gulf of America, to the open ocean through the Loop Current and Gulf Stream. These findings help early theories by explorers who proposed that Gulf-originating sargassum may feed populations within the Sargasso Sea.

Remote sensing expertise performed a pivotal function in these discoveries. In 2004 and 2005, satellites captured in depth sargassum windrows – lengthy, slim strains or bands of floating sargassum – within the western Gulf of America, a area experiencing elevated nutrient masses from river methods such because the Mississippi and Atchafalaya.

“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,” Lapointe stated. “A major focus of our review is the elemental composition of sargassum tissue and how it has changed over time.”

Laboratory experiments and subject analysis relationship again to the Nineteen Eighties confirmed that sargassum grows extra shortly and is extra productive in nutrient-enriched neritic waters than within the oligotrophic waters of the open ocean. Controlled research revealed that the 2 main species, sargassum natans and sargassum fluitans, can double their biomass in simply 11 days below optimum circumstances. These research additionally established that phosphorus is usually the first limiting nutrient for development, though nitrogen additionally performs a vital function.

From the Nineteen Eighties to the 2020s, the nitrogen content material of sargassum elevated by greater than 50%, whereas phosphorus content material decreased barely, resulting in a pointy rise within the nitrogen-to-phosphorus (N:P) ratio.

“These changes reflect a shift away from natural oceanic nutrient sources like upwelling and vertical mixing, and toward land-based inputs such as agricultural runoff, wastewater discharge and atmospheric deposition,” stated Lapointe. “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.”

The evaluation additionally explores how nutrient recycling inside sargassum windrows, together with excretion by related marine organisms and microbial breakdown of natural matter, can maintain development in nutrient-poor environments. This micro-scale recycling is vital in sustaining sargassum populations in elements of the ocean that may in any other case not help excessive ranges of productiveness.

Data from sargassum collected close to the Amazon River mouth help the speculation that nutrient outflows from this main river contribute considerably to the event of the GASB. Variations in sargassum biomass have been linked to flood and drought cycles within the Amazon basin, additional connecting land-based nutrient inputs to the open ocean.

The formation of the GASB seems to have been seeded by an excessive atmospheric occasion – the destructive part of the North Atlantic Oscillation in 2009 to 2010, which can have helped shift floor waters and sargassum from the Sargasso Sea southward into the tropical Atlantic.

However, the researchers warning that there isn’t any direct proof of this motion. Moreover, genetic and morphological knowledge counsel that some sargassum populations, notably the dominant S. natans var. wingei, have been already current within the tropical Atlantic previous to 2011, indicating that this area could have had an missed function within the early growth of the GASB.

“The expansion of sargassum isn’t just an ecological curiosity – it has real impacts on coastal communities. The massive blooms can clog beaches, affect fisheries and tourism, and pose health risks,” stated Lapointe. “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.”

This work was funded by the Florida Department of Emergency Management, United States Environmental Protection Agency, South Florida Program Project, and the NOAA Monitoring and Event Response for Harmful Algal Blooms program. Historical research included inside the evaluation have been funded by the NASA Ocean Biology and Biogeochemistry Program and Ecological Forecast Program, NOAA RESTORE Science Program, National Science Foundation, “Save Our Seas” Specialty License Plate and discretionary funds, granted by means of the Harbor Branch Oceanographic Institute Foundation, and a Red Wright Fellowship from the Bermuda Biological Station.