The enormous “dead zone” in the Gulf of Mexico will take decades to recover even if the flow of farming chemicals that is causing the damage is completely halted, new research has warned.

Intensive agriculture near the Mississippi has led to fertilizers leeching into the river, and ultimately the Gulf of Mexico, via soils and waterways. This has resulted in a huge oxygen-deprived dead zone in the Gulf that is now at its largest ever extent, covering an area greater than the state of New Jersey.

A new study has found that even if runoff of nitrogen, a fertilizer chemical, was fully stemmed, the Gulf would take about 30 years to recover. Even this scenario is “not only considered unrealistic, but also inherently unsustainable”, researchers stated in the work, published in Science.

“We have been building up nitrogen for the past 50 years and it will take time to go through the system,” said Nandita Basu, associate professor of environmental sciences at Canada’s University of Waterloo and the study co-author.

“Money is being spent on the landscape in an ad hoc way. We need to focus better. If we make the right changes it will have an impact, it’s just that it’ll take a few decades. It’s like when you go on a diet – you can’t expect results right away.”

The ailing Gulf of Mexico is emblematic of a global suffocation of the oceans caused by modern agriculture, sewage and climate change, which is causing waters to warm and hold less oxygen. At least 500 sites experiencing hypoxia, or oxygen deprivation, have been reported near coasts worldwide, up from just 50 in 1950. The true number may, in fact, be much higher, experts believe.

Fertilizers spilling into the oceans promote the growth of algae, which can trigger toxic blooms harmful to fish, shellfish, marine mammals and birds. These outbreaks can discolour water and befoul beaches. It also depletes oxygen in the water, leading to further damage to marine creatures and dwindling supplies for the people who rely upon them for food.

In the US, a federally led taskforce set a goal of shrinking the Gulf’s dead zone to less than 5,000 sq km by 2015. However, the hypoxic area was three times that size by the target year, prompting the deadline to be pushed back to 2035.

There is also an interim goal of cutting nitrogen flow to the Gulf by 20% by 2025, but that too looks in peril.

“That short term 2025 goal, based on the course we’re on now, isn’t really possible,” said Kim Van Meter, a colleague of Basu’s and a fellow co-author. “It would take an immediate change and it takes time for that to happen. The legacy of nitrogen in the system means that it will take decades.”

Nitrogen pollution can be curbed with a more careful application of fertilizers, the planting of certain grasses, trees and shrubs that can stop the chemicals getting into waterways and reducing the amount of tilling of the soil, to prevent soil erosion and runoff.

Some US farmers are given governmental support in these efforts, although environmentalists argue that funds have been wasted on projects that merely help farms to increase production of crops and meat, rather than tackle pollution.

“This study shows we need a scientific strategy and can’t expect instant results, but we know what needs to be done to improve things,” said Denise Breitburg, a marine scientist at the Smithsonian Environmental Research Center who was not involved in the report.