The phenomenon of Sargassum blooms in the Atlantic Ocean has been clarified by researchers from the Max Planck Institute for Chemistry. Their study uncovers the mechanisms behind these extensive algae blooms, which are increasingly impacting coastal areas, particularly along the Caribbean and the Gulf of Mexico. By June 2023, approximately 38 million tons of Sargassum were reported drifting towards these coastlines, marking a troubling record.
These brown algae become a significant issue during the summer months, leading to beach closures and environmental degradation as they accumulate and decompose, releasing unpleasant odors. Tourists are often deterred, and local ecosystems face threats due to the sheer volume of Sargassum. This algae, initially from the Sargasso Sea east of Florida, forms large mats in the ocean and serves as a habitat for various marine species.
Research has long sought to identify the sources of nutrients that fuel these blooms. While it was previously thought that agricultural runoff and deforestation might be responsible, these theories could not fully account for the observed increases in Sargassum biomass over recent years.
Understanding the Mechanism of Algal Blooms
The international team led by the Max Planck Institute for Chemistry utilized coral drill cores to reveal the primary mechanisms driving these algal blooms. In their findings published in the journal Nature Geoscience, they explain that strong wind-driven upwelling near the equator transports phosphorus-rich deep water to the ocean’s surface, which then flows northward into the Caribbean.
This influx of phosphorus promotes the growth of nitrogen-fixing cyanobacteria that colonize Sargassum. These bacteria are capable of capturing atmospheric nitrogen and converting it into a usable form for the algae, enhancing their growth. This symbiotic relationship gives Sargassum a competitive edge over other algae in the region, according to the study.
The researchers undertook an analysis of coral cores from various Caribbean locations, allowing them to reconstruct historical changes in ocean chemistry. By examining the nitrogen isotopic composition of corals, they were able to track the rates of nitrogen fixation over the past 120 years. Their findings indicated significant spikes in nitrogen fixation in 2015 and 2018, coinciding with record Sargassum blooms.
PhD student Jonathan Jung, the first author of the study, noted, “In the first set of measurements, we noticed two significant increases in nitrogen fixation in 2015 and 2018, two years of record Sargassum blooms. So we compared our coral reconstruction with annual Sargassum biomass data, and the two records aligned perfectly.”
Future Implications and Predictions
The research team concluded that the excess phosphorus from upwelling water is a critical factor driving Sargassum blooms, effectively ruling out earlier hypotheses, such as the influence of iron-rich Saharan dust. Although this dust is known to travel from Africa to the Atlantic, its presence did not correlate with the observed increases in Sargassum biomass.
Temperature variations in the tropical North Atlantic contribute to this phenomenon, as cooler sea surface temperatures alongside warmer southern Atlantic waters create wind patterns that facilitate the upwelling of nutrient-rich water. Researchers believe that monitoring these climatic conditions could improve predictions regarding future Sargassum growth.
Alfredo Martínez-García, group leader at the Max Planck Institute for Chemistry, emphasized the importance of these findings, stating, “Ultimately, the future of Sargassum in the tropical Atlantic will depend upon how global warming affects the processes that drive the supply of excess phosphorus to the equatorial Atlantic.”
The research team plans to continue their work by analyzing new coral records from various Caribbean locations to gain further insights. By doing so, they hope to inform strategies aimed at mitigating the impacts of Sargassum blooms on coastal ecosystems and local communities.
