Fertilizers and extreme heat are harming coral reefs in the Gulf of Mexico, according to a recent study using isotope analysis. The Flower Garden Banks National Marine Sanctuary, once known for its healthy coral ecosystems, is now facing serious threats from rising ocean temperatures and an overabundance of nutrients. Researchers from the Max Planck Institute for Chemistry and Louisiana State University found that high nitrogen levels in the northern Gulf come primarily from the Mississippi River, which carries runoff from agricultural areas. This runoff is largely due to the use of synthetic fertilizers. To determine the source of nitrogen in the Gulf, the team analyzed nitrogen isotopes in the calcareous skeletons of corals. These analyses allowed them to track nitrogen levels in the seawater over the past 270 years. The results show that in recent decades, more than 60% of the nitrogen in the Flower Garden Banks came from the Mississippi River basin, with some periods reaching up to 80%. Coral bleaching and disease outbreaks have been observed during these times of high nitrogen inflow. The study, published in the journal Science Advances, notes that the highest nitrogen inputs occurred in 2016 and between 2022 and 2023—years marked by unusually high seawater temperatures. The increase in nitrogen levels can be traced back to changes in the Mississippi River system. In the mid-1870s, the removal of the "Second Great Raft," a massive natural log dam in the Red River—a tributary of the Mississippi—allowed more water, sediments, and nutrients to flow directly into the Gulf. This was followed by the construction of levees along the Mississippi River starting in 1882, which further altered the river's natural flow. In the 1960s, the Green Revolution brought widespread use of synthetic fertilizers, causing a sharp rise in nitrogen levels in the region. By the end of the 20th century, 30% to 50% of the nitrogen in the corals came from the Mississippi River, and that proportion has continued to grow, reaching over 60% in the last decade and peaking at 80% in 2023. The researchers conclude that the combination of high nutrient levels and extreme heat is the main cause of the recent decline in the health of the Flower Garden Banks coral reefs. The Mississippi River basin, which covers about 41% of the continental United States, stretches from Idaho in the west through Canada in the north to New York in the east. Through natural water mixing processes, nitrogen-rich water from the river reaches the coral reefs of the Flower Garden Banks, which are located about 448 kilometers (278 miles) from the river’s mouth.