Increasing nitrogen loading in the Gulf of Mexico undermines coral reef resilience

The ongoing global decline of coral reefs underscores the need to identify key mechanisms driving reef deterioration. The Flower Garden Banks (FGB) reefs in the northern Gulf are widely regarded as some of the healthiest in the continental United States, but recent disease outbreaks in the FGB have raised concerns about their long-term resilience. As human and agricultural expansion intensified across the Mississippi River basin, the river has become the dominant source of nutrients to the northern Gulf; however, the evolution of this influence is poorly understood. Here, we reconstruct nitrogen inputs to the FGB using annually resolved coral-bound nitrogen isotopes (CB-δ¹⁵N) from two coral cores that collectively span the period from 1753 to 2023. Our results show that anthropogenic nitrogen began to rise around 1850, coinciding with the early use of fertilizers, and increased sharply after the 1960s with the onset of the Green Revolution and the use of synthetic fertilizers, suggesting the Mississippi discharge as the source. Fractional endmember modeling further reveals that the accumulation and mobilization of legacy nitrogen stored in Midwestern soils and groundwater have amplified modern riverine fluxes, driving Mississippi-derived nitrate to more than 60 to 80% of the nitrogen reaching the FGB in recent decades. The highest riverine contributions of the past 270 years occurred in 2016 and 2022-2023, coinciding with disease outbreaks and thermal stress events. These findings suggest that the combined effects of unprecedented nutrient loading and extreme heat are key synergetic drivers of the recent deterioration in coral health observed for this historically resilient reef system.