UNH researchers find high levels of nitrate in Puerto Rico streams after storms

Sensors were placed in streams like this one, Quebrada Sonadora, in the Luquillo MountainsWilliam McDowell/UNH

Many people suffered from Hurricane Maria in Puerto Rico. But scientists, in a new study, say they’ve uncovered a worrisome impact on the environment, too.

University of New Hampshire researchers have measured unusually high levels of nitrate in the island territory’s streams.

The theory is that stronger and more frequent storms caused by climate change have knocked down vegetation, and the resulting decomposition elevated nitrate levels in the soil. Storms are then washing the nitrate into the streams, researchers said in a statement from the university.

The increased levels of the plant nutrient threaten ecosystems along the island’s coastline with algal blooms that could harm marine life, the researchers said.

Following Hurricanes Irma and Maria in 2017, both Category 4 storms, researchers observed unexpectedly high levels of nitrate in the streams for a prolonged period.

“Nitrate is important for plant growth but this is a case where you can have too much of a good thing,” said William McDowell, professor of environmental science at the University of New Hampshire, lead author of the study. “Over the last three decades, we’ve noticed elevated levels of nitrate right after a hurricane, but after these back-to-back major storms, the wheels came off the bus.”

The researchers’ findings were reported earlier this month at the fall meeting of the American Geophysical Union in Washington, D.C.

Researchers placed sensors in the streams in the tropical Luquillo Mountains of Puerto Rico. The sensors monitored nutrient concentrations. Researchers compared the new data to weekly stream chemistry results collected over the past 35 years, the statement said.

“We’re shining a light into the inner workings of the watershed in ways we could never see before,” McDowell said.

When hurricanes hit land, trees topple, and branches and leaves fall to the forest floor, where microbes decompose the matter and release nitrate into the soil, raising the levels, McDowell said. The amount of nitrate in the soil also increases because damaged trees are unable to absorb as much of the nutrient as they normally would.

“After Hurricanes Irma and Maria, there seems to be a ‘new normal’ for the base level of nitrate,” McDowell said. “If this continues and the mountain streams transport these higher levels of nitrate to the ocean, it could disrupt the coastal ecosystem, possibly endangering coral and other sea life.”

Increasing amounts of nitrate allow algae to grow exponentially. When the algae die and decompose, it depletes the water of oxygen, which marine organisms need to live, McDowell said. The result can be “dead zones” in the water, he said.

McDowell said the rainfall removing nitrate from the soil also causes “uncertain effects on forest productivity and regrowth” because trees, as the forest makes a comeback, need the nitrate.

“It’s hard to understand the full nitrogen cycle in the forest, so we’re not entirely sure whether this big loss in nitrogen will affect the regrowth of trees,” McDowell said.

Future research will focus on forest recovery and levels of nitrate in the soil, McDowell said.

Researchers did not see any effect on the streams themselves from the increased nitrate, he said.