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Excess nitrogen has made sargassum the world's largest harmful algal bloom

Excess nitrogen has made sargassum the world's largest harmful algal bloom
Local governments, resorts and beach communities have spent millions of dollars cleaning up the mess caused by sargassum blooms, such as the buildup pictured in Palm Beach County, Fla. Photo by Brian Lapointe/FAU

May 24 (UPI) -- Sargassum provides vital nursery habitat for crabs, fish, sea turtles and other marine species in the North Atlantic.

But new research -- published Monday in the journal Nature Communications -- suggests the proliferation of nitrogen over the last three decades has helped transform the brown seaweed into the planet's largest harmful algal bloom.

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Typically, sargassum blooms are relegated to low-nutrient waters off the coast of the North Atlantic, but tides, winds and excess nutrients from natural and human sources can fuel massive blooms that encroach on coastal waters and deplete oxygen reserves in local ecosystems.

In recent years, sargassum blooms have gotten so big that governments have spent millions of dollars picking up seaweed from sands and shallows of popular beaches.

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To better understand how the relationship between nutrient levels and sargassum blooms has evolved over the last few decades, researchers collected and analyzed hundreds of sargassum seaweed samples over the course of several years.

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For reference, they compared the samples to a historic baseline from 1980s.

The data showed macroalgae's elemental composition has varied significantly over the last 30 years, but revealed increases in nitrogen to be responsible for the biggest changes.

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Most notably, the average nitrogen to phosphorus ratio in sargassum samples increased 111 percent between 1980 and 2010.

Seaweed blooms across the Great Atlantic Sargassum Belt -- which stretches from the Gulf of Mexico to the Sargassum Sea in the middle of the North Atlantic -- are taking advantage of the excess nitrogen in ocean waters.

But phosphorous, researchers said, remains the most essential element for macroalgae growth.

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"Data from our study supports not only a primary role for phosphorus limitation of productivity," senior author Brian Lapointe said in a press release.

"[The data] also suggests that the role of phosphorus as a limiting nutrient is being strengthened by the relatively large increases in environmental nitrogen supply from terrestrial runoff, atmospheric inputs, and possibly other natural sources such as nitrogen fixation," said Lapointe, an expert on sargassum and a research professor at Florida Atlantic University.

Though phosphorous fertilizers have been around for decades, nitrogen-based fertilizers have become increasingly popular over the last 30 years.

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Energy production and biomass burning has also led to increased nitrogen inputs global waterways.

"Over its broad distribution, the newly-formed Great Atlantic Sargassum Belt can be supported by nitrogen and phosphorus inputs from a variety of sources," Lapointe said.

These sources include discharges from the Congo, Amazon and Mississippi rivers, upwelling off the coast of Africa, vertical mixing, equatorial upwelling, atmospheric deposition from Saharan dust and biomass burning of vegetation in central and South Africa.

In addition to harming economies that rely on tourist dollars, growing sargassum blooms can disrupt other types of vital marine habitat, including seagrasses and coral reefs.

Sargassum blooms can also cause human health problems, as the decaying process can trigger to respiratory problems.

"Human activities have greatly altered global carbon, nitrogen, and phosphorus cycles, and nitrogen inputs are considered now 'high risk' and above a safe planetary boundary," Lapointe said.

"Based on scientific research, population growth and land-use changes have increased nitrogen pollution and degradation of estuaries and coastal waters since at least the 1950s ... The trend toward higher [nitrogen:phosphorus] ratios in the major rivers in the Atlantic basin parallel the increased N:P ratios we now see in sargassum."

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