Coral reefs are considered one of the most threatened ecosystems on the planet and are dying at alarming rates around the world. Scientists attribute coral bleaching and ultimately massive coral death to a number of environmental stressors, in particular, warming water temperatures due to climate change.

A study published in the international journal Marine Biology, reveals what’s really killing coral reefs. With 30 years of unique data from Looe Key Reef in the lower Florida Keys, researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute and collaborators have discovered that the problem of coral bleaching is not just due to a warming planet, but also a planet that is simultaneously being enriched with reactive nitrogen from multiple sources.

Improperly treated sewage, fertilizers and top soil are elevating nitrogen levels, which are causing phosphorus starvation in the corals, reducing their temperature threshold for “bleaching.” These coral reefs were dying off long before they were impacted by rising water temperatures. This study represents the longest record of reactive nutrients and algae concentrations for coral reefs anywhere in the world.