Fertilizer Nitrogen From the Mississippi Is Damaging Gulf of Mexico Corals

An international research team led by the Max Planck Institute for Chemistry in Mainz, Germany, and Louisiana State University in the United States has found that high levels of nitrogen pollution in the northern Gulf of Mexico come from the Mississippi River and are linked to fertilizer use. The study, published in the journal Science Advances, focuses on the Flower Garden Banks National Marine Sanctuary, reefs long considered among the healthiest in the United States because of their high coral coverage.

The researchers analyzed nitrogen isotopes preserved in the calcareous skeletons of star corals, which grow in layers much like tree rings, to reconstruct the source of nitrogen in the seawater from 1753 to 2023. The coral cores were collected during an expedition by the U.S. National Oceanic and Atmospheric Administration (NOAA). The results show that in recent decades, more than 60% of the nitrogen at Flower Garden Banks came from the Mississippi River basin, rising to as much as 80% during certain periods, including 2023.

From 1753 to around 1850, the nitrogen signals in the coral cores resembled a largely natural environment. After 1850, the records show a growing contribution of nitrogen from human activity, coinciding with European settlement and the spread of agriculture in the Mississippi River region. The anthropogenic share rose sharply again after the 1960s, when synthetic fertilizer became widely available during the Green Revolution.

The researchers found that the highest nitrogen inputs occurred in 2016 and between 2022 and 2023. These were the same years the reefs suffered exceptional heat waves, massive coral bleaching and increased disease outbreaks. "We were amazed at how precisely the historical events are documented in the core samples," said Jonathan Jung, the study's first author and a postdoctoral researcher at the Max Planck Institute for Chemistry. Senior author Alfredo Martinez-Garcia said the combination of unprecedented nutrient loads and extreme heat is the decisive factor behind the recent deterioration of the reefs. The researchers expect coral disease and bleaching to increase as long as nitrogen pollution and rising temperatures continue.