Researchers trace source of nitrogen pollution affecting world's second largest barrier reef

Tropical coastal ecosystems are among the most biodiverse areas on the planet. And they are also on the front line of effects caused by human activity. This is why it becomes increasingly crucial, especially as the human population grows, to manage the impacts of runoff and sewage flowing into the sea.

“Tropical coastal ecosystems, such as coral reefs, are oligotrophic, meaning they are located in nutrient-poor waters and have therefore adapted to these conditions,” said Madeline Berger, researcher at UC Santa Barbara’s Nationwide Middle for Ecological Assessment & Synthesis (NCEAS). ). “An increased supply of nutrients can therefore disrupt the functioning of the ecosystem.”

In a write-up published in the journal Ocean and Coastal Administration, lead author Berger and his colleagues address the issue of air nutrient pollution through a case study on the Central American coast. The result? Farms are responsible for the vast majority of the nitrogenous air pollution that pours into the Mesoamerican reef region. Knowing where air pollution comes from, researchers say, will help managers tailor mitigation options.

“Our study highlights that different management strategies will need to be used in different watersheds to help reduce the input of nutrients that can adversely affect the health of coral reefs and seagrass beds in this area,” Berger said.

The Mesoamerican Reef (MAR) is the largest barrier reef in the Western Hemisphere, second only to the Australian Great Barrier Reef in size. Shared by Mexico, Belize, Guatemala, and Honduras, it stretches nearly 700 miles and is home to a variety of creatures, including hundreds of species of fish, critically endangered sea turtles, as well as molluscs, marine mammals and shorebirds. These habitats and communities, in turn, support local fishing as well as tourism. Millions of people flock to the resorts and ports of Cancun, Cozumel, Belize Metropolis and other points along the coast each year on their Caribbean vacations.

But the reef is also in trouble. More than half of it is in disrepair due to various threats, including ocean warming, non-resilient fishing and air pollution.

“Air pollution by nutrients is a known threat to coral reefs and seagrasses,” Berger said. Too much nitrogen, a common pollutant, leads to a chain of events that results in oxygen-free “dead zones” and acidic conditions that can severely weaken or kill fish and other animals. Water quality issues are also among the main reasons for sindroma blanco, a pathogenic disease that results in white lesions that spread on coral, leading to death in just a few weeks. Air nutrient pollution in this region, according to the researchers, “comes from four sources: agricultural production, human sewage, atmospheric deposition, and wildlife feces,” with agriculture and human sewage being believed to contribute to the majority of air pollution.

To say how much of the air pollution came from upstream agriculture or human sewage, the researchers dove into several models looking at the watersheds that feed into the MAR. One was a global wastewater model that estimates the amount of nitrogen pollution based on population maps, protein consumption and known rates of nitrogen excretion by humans, which has been adapted to this specific region.

“Another NCEAS group was also creating a spatially explicit world-wide model quantifying the ecological footprint of food production, so we saw a opportunity to synthesize the two models to compare nutrient pollution from wastewater to air nutrient pollution from crop and animal production,” Berger said.

The researchers also modeled the impacts of the millions of tourists traveling the coast using hotel location data, cruise ship location data and monthly statistics reported by each country’s tourism bureaus. “We were basically trying to figure out where the tourists were pooping, which is proving quite tricky,” she said.

Researchers found that the agriculture was by far the largest (80%) contributor of nitrogen to MAR, due to runoff from fertilizers and livestock waste that goes to the ocean through rivers and streams. Two rivers, the Rio Ulua in Honduras and the Rio Motagua in Guatemala, contributed more than 50% of the modeled nitrogen pollution, collecting the waters runoff from several tributaries upstream and flowing into the ocean. Meanwhile, 80% of the modeled nitrogen air pollution has been attributed to 20 (out of 430) watersheds, of which are located in Guatemala or Honduras . It is estimated that 80% of coral reefs and % of seagrass beds have been exposed to nitrogen pollution from watershed plumes.

The results are indicative of the complexity of the air pollution problem – even land use changes and agricultural expansion very far inland can have impacts on marine ecosystems, Berger said. Additionally, the researchers found that habitats with high biodiversity were more exposed to plumes from smaller coastal watersheds. And while tourism accounted for a very small proportion of overall pollution, the watersheds that contributed the most to pollution also tended to have high numbers of tourists or be close to those that did.

“The exercise also raised the question of how tourists, or tourism demand, have larger impacts beyond the basic physical presence of more people — such as increased construction or the expansion of agriculture — which can also exacerbate pollution input,” Berger said. Researchers hope that ultimately a granular approach that can be used to track the origins and trajectories of nitrogen and other nutrient runoff and their impacts will allow coastal managers to develop the sophisticated designs of action needed. to mitigate air pollution of some of the most biodiverse coastal areas in the world. Regions.

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