Heavy rains deliver largest amounts of fertilizer-derived nitrogen pollution to the Gulf of Mexico, new study finds

Chestnut Hill, Mass (12/10/2024) – As opposed to a constant flow from sources above and below ground, periods of heavy rain and runoff deliver the greatest amounts of fertilizer-derived nitrogen through creeks, rivers, and storm drains into the northern Gulf of Mexico, a team of scientists led by Boston College researchers reports today in the journal Communications Earth and Environment.

The findings clarify how nitrogen is delivered to the gulf and can be used to develop policies to protect the northern Gulf of Mexico, according to Boston College Assistant Professor of Earth and Environmental Sciences Xingchen (Tony) Wang, a lead author of the report.

“The northern Gulf of Mexico faces significant environmental challenges, including nutrient overloading, harmful algal blooms, and oxygen-depleted ‘dead zones’,”  Wang said. “These issues are primarily driven by nitrogen pollution transported via rivers, with the Mississippi-Atchafalaya River Basin—a major agricultural hub in North America—being a key contributor.”

With funding from the Army Corps of Engineers, Wang's lab teamed up with researchers from Caltech and the University of California, Santa Barbara, to determine how different nitrogen sources, particularly fertilizers, are mobilized and transported into the Gulf under different hydrological conditions. The answers, Wang said, can improve the understanding of these pollution dynamics and guide effective mitigation strategies.

The Gulf of Mexico is fed by 33 major rivers that drain water from 31 states and 2 Canadian provinces. In 2021, the researchers collected water and particle samples from the Wax Lake Delta, a key distributary of the Mississippi-Atchafalaya River Basin. At BC, the nitrogen isotopic composition of the samples was analyzed in the Wang Stable Isotope Biogeochemistry Lab.

Nitrogen contains two stable isotopes: nitrogen-14 and nitrogen-15, Wang said. Different nitrogen sources often exhibit distinct nitrogen-15 to nitrogen-14 ratios, allowing the scientists to effectively trace and characterize the sources of nitrogen in the Wax Lake Delta.

Lower nitrogen isotope ratios characterize nitrogen derived from fertilizers. The team found that the nitrogen-15 to nitrogen-14 ratios in samples collected during the wet season were consistently lower than those from the dry season, according to Wang and co-authors including U.C. Santa Barbara’s Gen Li, Caltech’s Michael Lamb, and Boston College professors Noah Snyder and Hanqin Tian.

In contrast, samples collected during the dry season showed that groundwater nitrogen becomes the dominant nitrogen source in those months, Wang said. 

“We were somewhat surprised to observe such a significant shift in anthropogenic nitrogen sources between the dry and wet seasons, which had not been identified in previous studies,” said co-author Jian-Jhih (Kenji) Chen, a postdoc researcher in the Wang Lab during the project, who is now an assistant professor at National Kaohsiung University of Science and Technology in Taiwan.

The findings underscore the critical role of hydrology in driving nitrogen pollution in the northern Gulf of Mexico, Wang said. 

“With climate change predicted to increase the frequency of extreme precipitation events, the transport of fertilizer-derived nitrogen into the Gulf is likely to intensify, further exacerbating hypoxic conditions,” Wang said.

The study suggests the impacts can be mitigated by optimizing the timing of fertilizer application to crops to reduce nitrogen loading and its environmental consequences in the Gulf of Mexico.

The next steps involve exploring the fate of anthropogenic nitrogen in the Gulf of Mexico, Wang said. Specifically, the researchers want to determine whether this nitrogen remains confined to coastal zones or is transported into the open waters of the Gulf.