Massive sargassum seaweed blooms in the past few years, which were among the largest ever recorded, have fouled beaches in the tropical Atlantic with their huge, stinky clumps. Now, scientists have found an explanation for these puzzlingly large growths. Reporting in Communications Earth & Environment, scientists have determined that sargassum seaweed has thrived incredibly in the tropics because of strong winds and ocean currents. A shift in atmospheric pressure over the Atlantic Ocean led to two straight years of a strong negative North Atlantic Oscillation (NAO), which pushed sargassum into tropical regions around 2009. After that, it had ideal growing conditions with abundant nutrients and sunlight all year; the blooms started in 2011 and have persisted.
"At first, we saw just a few patches of sargassum being pushed south by the NAO," said study co-author Frank Muller-Karger, Distinguished University Professor and biological oceanographer at the University of South Florida College of Marine Science. "But these algae patches were met with the right conditions to grow and perpetuate blooms."
Sargassum has not only negatively affected the beauty of beaches and tourism to these areas, it has also disrupted marine and human health. It's been costly to clean up sargassum blooms as well.
In this study, the researchers used computational tools to model the impact of the NAO on sargassum. These models indicated that sargassum was taken up by wind and ocean currents, and moved from the Sargasso Sea eastward, then southward, and then into the tropics.
Additional modeling suggested that wind and currents could also move nutrients to feed the blooms. Vertical mixing in the ocean, in which winds mix masses of water on a seasonal cycle, can move deep water to the surface. This movement fueled blooms, as nutrients moved towards areas that were sunlit, and the sargassum could bloom extensively.
The time lapse video above shows how seaweed moved and bloomed in the Atlantic Ocean.
"This was a surprising result," Muller-Karger said. "We had posed the hypothesis before that it is not the rivers that feed the formation of the sargassum blooms in the tropical Atlantic. This model supports the idea that nutrients from slightly deeper layers in the ocean feed the blooms."
Sources: University of South Florida, Communications Earth & Environment