The nitrogen, coupled with warmer oceans, has boosted the seaweed's growth. Environmentalists warn that the algae is a disaster for the area's biodiversity.
While it serves as a habitat for marine life out at sea, once it hits the shore it dies and produces toxic gases and acid which leak back into the sea, damaging coral reefs and marine ecosystems.
Perhaps worse for tourists is the rotting egg smell the algae releases as it decays.
The floating island, estimated to weigh more than 200 fully loaded aircraft carriers, has been captured by Nasa satellite images and is more than 547km in diameter and is about 995km east from the coast of Mexico's Quintana Roo state.
Dr Chuanmin Hu, a marine scientist at the University of South Florida, has studied the algae's growth. He warned that based on the last 20 years of data, the belt of algae "is very likely to be a new normal".
"The ocean's chemistry must have changed in order for the blooms to get so out of hand," he added.
Dr Woody Turner, manager of the Ecological Forecasting Program at NASA Headquarters in Washington, said: "The scale of these blooms is truly enormous, making global satellite imagery a good tool for detecting and tracking their dynamics through time."
Mexico's president, Andrés Manuel López Obrador, has deployed the country's marine corps to tackle the algae but has also enraged local businesses by downplaying the issue.
The issue has come centre stage after Mr Obrador said last week that the seaweed's impact on the region was "not very serious", while some countries, such as Barbados, have previously declared national emergencies because of the toll on tourism.
Mr Obrador elaborated on his comments on Thursday, now claiming that people opposed to his controversial Tren Maya railroad project are also behind claims that the sargassum is at emergency levels.
"I have to tell the people that we are attending to the [algae] problem but that is's not how some are presenting it."
However according to industry data, hotel resorts on Mexico's Riviera Maya, which stretches from Puerto Morelos to Punta Allen, have lost an estimated $12m this year from a sargasso-related downturn in visitors.
Hotels have placed nets in the sea to try to keep the sargassum away from the beaches while staff remove up to a ton day using shovels and barrows.
However the removal is time-consuming and largely ineffective against such large amounts.
Obrador said he has budgeted 52 million pesos ($3.9 million) for measures such as seaweed-catching ships and barriers - but this falls short of what the president's own tourism secretary, Marisol Vanegas Pérez, said was necessary to tackle the issue.
Mexico seaweed: The Great Atlantic Sargassum Belt
• The algae, known as the Great Atlantic Sargassum Belt, forms in response to ocean currents and emerges seasonally.
• In spring and summer waste from the Amazon pumps nitrogen and nutrients into the ocean. The levels have increased in recent years due to more intensive deforestation and fertiliser use.
• In the winter cold, deep water rises off the West African coast and delivers nutrients to the surface where the sargassum grows.
• In small quantities sargassum is critical for marine life to feed, hide and spawn, but in such a large number it hinders some species' ability to move and breathe.
• When it dies at sea, it sinks to the ocean floor and can smother corals and seagrasses.
• On the beach, rotten sargassum releases hydrogen sulfide gas and smells like rotten eggs and can trigger asthma attacks.
