By Jake Spring

(Reuters) – Scientists were baffled when a band of seaweed longer than the entire Brazilian coastline sprouted in 2011 in the tropical Atlantic – an area typically lacking nutrients that would feed such growth.

The prime suspect is human sewage, as well as agricultural runoff that was carried to the ocean by rivers. This has been identified by a group of U.S. scientists.

However, the science is still not conclusive. It is only one possible cause of an explosion in seaweeds in America’s warm water. Six researchers told Reuters that they believe a combination of Amazon (NASDAQ) rainforest destruction, climate change and dust blowing westward from Africa’s Sahara Desert could be the cause of massive blooms of dark brown seaweed called sargassum.

Scientists recorded 20,000,000 metric tons seaweed in June 2018. This is a 1000% increase over the previous month’s bloom.

According to Ajit Subramaniam, a Columbia University oceanographer, “There may be multiple reasons” for the seaweed’s growth. It would surprise me if there was one villain.

A recent study that examined the chemical composition of seaweed between the 1980s and 2019 provides strong evidence to the contrary. It shows water from cities and farms has played a significant role in the expansion of the Great Atlantic Sargassum belt, now nearly 9,000 km long.

NITROGEN RUNOFF

That study, co-authored by biologist Brian Lapointe at Florida Atlantic University, found that sargassum collected recently in coastal waters from Brazil to the southern United States, and including several Caribbean nations, contained levels of nitrogen that were 35% higher on average than in samples taken more than three decades earlier. These findings were published by Nature Communications in May.

The compound nitrogen can be found in both human and animal wastes and fertilizers. According to these results, sewage from farms and other runoff is flowing into the Americas’ rivers and eventually the ocean. This feeds the offshore growth of sargassum. This seaweed is carried by currents to the Caribbean Sea where it can be devastating the coastal economy dependent on tourism.

The same samples showed a 111% increase in the nitrogen/phosphorus ratio. This ratio is almost constant over the oceans of the globe, going back many decades. It suggests that water chemistry is being radically altered.

For particular attention, the researchers focused on the Amazon River.

CLIMATE CHANGE

As global temperatures rise, scientists believe that rainstorms are intensifying in certain areas of the globe, including over the Amazon. Lapointe explained to Reuters that extreme flooding has been increasing due to these storms, likely pushing more of the nitrogen-rich runoff into the sea. He calls it a “double whammy.”

Experts say that in April and March, peak Amazon River flooding sends hundreds of kilometers of nutrients out to sea. This coincides with significant sargassum blossoms. The currents then push seaweed along the Venezuelan coast into the Caribbean Sea, sometimes further north to the Gulf of Mexico.

The climate change has also led to stronger hurricanes that at sea draw more nutrients up from seabeds in order to fertilize Sargassum.

AFRICAN DUST AND ASH

Scientists have also theorized that dust from the Sahara Desert, along with smoke and ash, could be contributing to the seaweed boom. These particles blow westward from the Atlantic Ocean. Once they reach clouds, the rains become fertilizing iron or phosphorus.

Research and funding will be required to prove how each factor contributes to sargassum’s growth. This research may take many years. However, scientists argue that this does not mean that governments should wait to act in order to reverse the trend.

Carlos Noriega from Brazil’s Federal University of Pernambuco, said that this phenomenon would continue until there are changes in government policy. Brazil is one example of a country that could slow the deforestation. Brazil’s cattle ranching boom has allowed loose soil, manure, fertilizer and other nutrients to seep into rivers.

The burgeoning population of the Amazon region in Brazil was also mentioned by him. Nearly 900,000. people have moved to the area’s five major cities since 2010. The region also lacks adequate sewage treatment.

Noriega explained that treating sewage and stopping the deforestation is the only way of controlling it.