Star coral cores trace up to 80 percent of a Gulf reef's nitrogen to the Mississippi basin

Nitrogen locked into a coral's calcium carbonate skeleton carries the ratio of heavy 15N to light 14N of whatever the animal ate, and that ratio holds information about where those nutrients came from ^[9]. Star corals in the northern Gulf grow slowly but continuously, live to a great age, and lay down their skeletons in layers, like tree rings ^[8]. The cores came off a NOAA expedition ^[10]. That combination is what makes a single colony usable as a 270-year archive of seawater chemistry ^[2].

For the first 97 years of the series, 1753 to about 1850, the values look like those expected from a largely natural setting, with little or no detectable direct river input ^[11]. The 173 years after that, roughly 64 percent of the record, carry a growing human contribution, rising alongside European settlement and agricultural expansion in the Mississippi region ^[21][12]. Guano-derived nitrogen appears from 1856, the year the U.S. Congress authorized guano mining on Pacific and Caribbean islands ^[13]. The signal rose again after the mid-1870s removal of the Second Great Raft, a natural log dam in the Red River ^[17]. That removal cut inland flooding, but it also sped the flow in the Mississippi and its Atchafalaya branch and moved nutrients seaward faster ^[18]. Levee construction along the river began in 1882 ^[19].

"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," said Jonathan Jung, the study's first author and a postdoctoral researcher at the Max Planck Institute for Chemistry in Mainz ^[14][16]. "We were amazed at how precisely the historical events are documented in the core samples" ^[15].

The isotope shifts came first and the history was looked up afterward, so the dated coincidences support the interpretation without independently testing it.

The headline number is a share. More than 60 percent of the reef's nitrogen in recent decades, and up to 80 percent in some periods, came from the Mississippi basin ^[4]; that describes the mixture the corals were sitting in, not the tonnage arriving each year. The damage claim rests on narrower ground still. The highest inputs fall in 2016 and in 2022 to 2023 ^[6], and those are the years of exceptional heat waves, mass bleaching and elevated disease at the same reefs ^[7]. Two intervals in which both stressors peak together will not apportion the bleaching between them, and the study, published in Science Advances, reports the co-occurrence ^[5][7].

What the work does supply is a dated record of where the nitrogen came from at a site long held up as exceptionally healthy on coral cover ^[20], and it puts a majority of that site's nitrogen supply upstream of the sanctuary itself ^[1][4].