As of early June this yr, an estimated 38 million tons headed for the Caribbean islands, Gulf of Mexico and northern South American coastlines, setting an unlucky recent record. During the summer, these floating mats of brown algae accumulate on the beach in large quantities where they decay and emit an unpleasant odor. This construction discourages visitors and stresses the coastal ecosystem. Far from shore, surface currents provide food and shelter for a lot of marine animals.
The algae originated within the Sargasso Sea east of Florida. Since 2011, scientists have tracked the recurring appearance of the Great Atlantic Sargassum Belt, a widespread band of gulfweed that moves from the equator toward the Caribbean in periods of strong winds. Until recently, the source of phosphorus (P) and nitrogen (N) fueled its rapid growth. Some had suggested that agricultural runoff or nutrients released by deforestation of rainforests were responsible. However, these explanations don’t match the regular increase in biomass lately.
Identifying the forces behind flowers
An international team of researchers led by the Max Planck Institute for Chemistry has now determined the underlying process behind the huge bloom. They have also identified climate patterns that set the stage for this growth, allowing them to start developing a system that may predict future arrivals.
In a recent publication, the researchers describe how high wind speeds near the equator bring phosphorus-rich deep water to the surface and transport it northward into the Caribbean. An increased supply of phosphorus advantages the cyanobacteria that continue to exist the surface of the brown algae. These microorganisms capture atmospheric nitrogen gas (N2) and convert it right into a form that might be used, a process often known as nitrogen fixation. Cyanobacteria often colonize and establish a partnership that gives the algae with an extra nitrogen source. According to the study, this trait offers a competitive advantage over other algae within the equatorial Atlantic and helps explain changes in abundance recorded over time.
Coral cores reveal a century of nitrogen fixation
The team linked the invention of cooler, nutrient-rich waters by studying algae growth, enhanced nitrogen fixation, and coral cores deposited within the Caribbean. Corals act as long-term environmental archives because their skeletons incorporate chemical cues from the encompassing water as they grow. By examining their annual growth layers, like tree rings, scientists can reconstruct changes in ocean chemistry over the centuries.
In this study, researchers measured the nitrogen isotopic composition of corals to estimate how much nitrogen has been fixed by microorganisms over the past 120 years. During nitrogen fixation, bacteria reduce the stable nitrogen isotopes in seawater from 15N to 14N. When corals show low 15N to 14N ratios, this means periods of increased nitrogen fixation. To confirm the meaning of those chemical signatures, seawater samples collected by the research vessel Eugen Seibold were used to calibrate nitrogen isotopes in modern corals, showing that they reliably record nitrogen fixation.
Couple trends since 2011
Jonathan Jung, a PhD student on the Max Planck Institute for Chemistry and lead writer of the study, explains, “In the first set of measurements, we saw two significant increases in nitrogen fixation in 2015 and 2018, two years of records open. So we compared with annual biomass data, and precisely from the two records,! link.”
A deeper comparison shows that algal biomass and nitrogen fixation have been consistently correlated since 2011, including each high and low values. The timing is notable because strong winds in 2010 transported brown algae from the Sargasso Sea into the tropical Atlantic for the primary time.
Rejection of other sources of nutrients
After eliminating other ideas, the team concluded that phosphorus is the most important factor behind numerous critical events. Earlier theories that Saharan dust picked up iron that would stimulate algae growth didn’t match the biomass record. Similarly, nutrient inputs from the Amazon or Orinoco rivers showed no correlation with the timing or intensity of blooms.
A technique that improves future predictions
The researchers describe a process through which phosphorus from deep water and nitrogen provided by nitrogen-fixing bacteria combined to fuel blooms that occurred over the past a long time. Geochemist Jung notes, “Our mechanism explains growth variability better than any previous approach. However, there is still uncertainty as to whether and to what extent other factors also play a role.”
The influx of phosphorus-rich water depends upon cooler sea surface temperatures within the tropical North Atlantic and warmer conditions within the South Atlantic. These temperature differences change wind pressure patterns, causing changes in wind strength and direction that push surface water aside and permit deeper phosphorus-rich water to rise.
Monitoring wind conditions, sea surface temperature, and associated upwelling patterns within the equatorial Atlantic Ocean could help improve predictions of future growth, in response to researchers in Mainz. “Ultimately, the future of the tropical Atlantic will depend on how global warming affects the processes that supply excess phosphorus to the equatorial Atlantic,” explains Alfredo MartÃnez-Garcia, group leader on the Max Planck Institute for Chemistry and senior writer of the study. The team plans to expand their evaluation by examining recent coral records from multiple locations throughout the Caribbean. They expect these insights will support efforts to guard coral reefs and help coastal communities manage the growing environmental and economic impacts of blooms.