Fertilizers and Extreme heat are damaging coral reefs in the Gulf of Mexico

Isotope analyses of corals show that 60 to 80 percent of the nitrogen comes from the Mississippi River basin.

In a nutshell:

• Coral as an environmental archive: Core samples reveal the settlement history of the Mississippi over the past 270 years.
• Stress despite high resilience: One of the most resilient U.S. reefs is increasingly suffering from nutrient pollution and warming.
• The Mississippi as the source: The majority of the nitrogen in the Gulf comes from the Mississippi River basin.
• Deteriorating conditions: Nutrient overload and heat stress are leading to the first instances of coral bleaching and disease outbreaks on the reef.

Who isn’t familiar with the colorful coral reefs from movies like “Finding Nemo” or “The Little Mermaid”? Unfortunately, the reality is often quite different, as these vital ecosystems are under pressure worldwide. Climate change and pollution are causing corals to bleach and die. Even the most resilient coral reefs of the United States are now threatened. The reefs in the Flower Garden Banks National Marine Sanctuary in the northern Gulf of Mexico were long considered exceptionally healthy due to their high coral coverage. But rising water temperatures and an excess of nutrients are causing even these reefs to lose their resilience.

An international research team led by the Max Planck Institute for Chemistry and Louisiana State University has now discovered that the high levels of nitrogen in the northern Gulf originate from the Mississippi River and are attributable to the use of fertilizers.

The team analyzed nitrogen isotopes in the calcareous skeletons of corals and used the data to reconstruct the source of nitrogen in seawater over the past 270 years. The results show that, in recent decades, more than 60 percent, and during certain periods, up to 80 percent, of the nitrogen in the Flower Garden Banks originated from the Mississippi River basin.

Coral bleaching and disease outbreaks coincide with high nitrogen inflows

According to the study, now published in the journal Science Advances, the highest nitrogen inputs occurred in 2016 and between 2022 and 2023. It was precisely during these years that the reefs suffered from exceptional heat waves, massive coral bleaching events, and increased disease outbreaks.

The Mainz researchers analyzed core samples from star corals in the northern Gulf of Mexico. These stony corals grow slowly but continuously and live to a very old age. Similar to the annual rings of trees, the corals form their calcareous skeletons in layers, making them an excellent archive of environmental conditions. As a result, the scientists were able to determine the nitrogen isotopes present in the corals from 1753 to 2023. The isotopic fingerprint of nitrogen (N) more specifically, the ratio of heavy ¹⁵N to light ¹⁴N, carries information about the origin of the nutrients consumed by the coral. Therefore, using this technique, the team could determine the origin of the water in which the corals grew. The coral cores were collected during an expedition by the U.S. National Oceanic and Atmospheric Administration (NOAA).

Corals provide a record of the recent settlement history of the Mississippi

From 1753 to around 1850, the nitrogen isotope values recorded in the coral cores resembled those expected from a largely natural environment. This suggests that, during this period, there was little to no detectable influence of direct nitrogen inputs from rivers. After about 1850, however, the coral records show a growing contribution of nitrogen from human activities. This shift coincides with European settlement and agricultural expansion in the Mississippi River region, including the increasing use of organic fertilizers. One signal that stands out is the rise in guano-derived nitrogen, which appears in the coral record from 1856 onward — the same year the U.S. Congress authorized guano mining on Pacific and Caribbean islands.

“In the areas where we detected significant changes in the corals’ nitrogen signal, we investigated what was happening around the Mississippi River during those periods,” reports Jonathan Jung, first author of the study and postdoctoral researcher at the Max Planck Institute for Chemistry (MPIC) in Mainz. “We were amazed at how precisely the historical events are documented in the core samples.”

The nitrogen signal intensified once again after the removal of the “Second Great Raft”, a massive, naturally formed log dam in the Red River, a tributary of the Mississippi, in the mid 1870s. Although its removal led to fewer inland floods, it also increased the flow velocity in the Mississippi River and its Atchafalaya branch, thereby accelerating the transport of nutrients into coastal waters. In addition, the construction of levees along the Mississippi began in 1882 to contain floodwaters. As a result, river water, together with sediments and nutrients, flowed directly into the Gulf.

The Mississippi is the primary source of nutrients in the northern Gulf of Mexico

With the Green Revolution and the availability of synthetic fertilizer beginning in the 1960s, the concentration of anthropogenic nitrogen in the corals studied rose sharply and rapidly. This trend has continued, so that the proportion of nitrogen washed in from the Mississippi stood at 30 to 50 percent by the end of the 20th century and exceeded 60 percent in the last decade. In 2023, the figure even reached 80 percent. The researchers conclude that nowadays the Mississippi is the primary source of nutrients in the Gulf of Mexico.

More disease outbreaks and coral bleaching expected

“Our study shows that the combination of unprecedentedly high nutrient loads and extreme heat are the decisive factors behind the recent significant deterioration in the health of the coral reefs at the Flower Garden Banks National Marine Sanctuary”, concludes Alfredo Martínez-García, group leader at the MPIC and senior/last author of the study. In 2016, 2022, and 2023, years marked by particularly high seawater temperatures, corals became diseased or bleached for the first time. The researchers expect disease outbreaks and coral bleaching to increase as long as nitrogen pollution from the Mississippi River watershed remains at its current high levels along with increasing temperatures.

“The corals of the Flower Garden Banks are valuable archives of past ocean and environmental conditions. We have much to learn from “reading” the chemical records preserved in the coral skeletons about the conditions of the oceans before industrialization. These coral records provide resource managers with natural baselines of what the Gulf was like before industrialization, to help them make management decisions to maintain and protect the reefs and the Gulf,” states Kristine DeLong, Professor at Louisiana State University and second author of the study.

The Mississippi River basin today covers about 41 percent of the land area of the continental United States and extends from Idaho in the west, through Canada in the north, to New York in the east. Through mixing, nitrogen-rich water reaches the coral reef ecosystems of the Flower Garden Banks, 448 kilometers (278 miles) from the river mouth.