Nitrogen pollution from agriculture threatening health of marine ecosystems

A new study examining the effects of nutrient pollution on fragile coastal ecosystems in Central America has found that agricultural operations are having a detrimental impact.

Researchers at UC Santa Barbara’s National Center for Ecological Analysis and Synthesis (NCEAS) tracked how much nitrogen pollution was flowing into the Mesoamerican Reef Region (MAR), the second largest barrier reef in the western hemisphere.

After using several models to examine the watersheds feeding into the MAR, they found most of the pollution was coming from agricultural sources, contributing to 92% of nitrogen in the area.

This is due to fertiliser and livestock waste runoff making their way to the ocean via rivers and streams, with two rivers, the Rio Ulua in Honduras and Rio Motagua in Guatemala, making up 50% of the modelled nitrogen pollution.

‘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 lead author of the study Madeline Berger. ‘An increased influx of nutrients can therefore disrupt ecosystem functioning.

‘Our study highlights that different management strategies will need to be employed in different watersheds to help reduce nutrient input that can have detrimental impacts on coral reef and seagrass health in this area.’

The 700 mile MAR is home to hundreds of fish species, critically endangered turtles, marine mammals and shorebirds and sustains local fishing and tourism.

However, the reef is threatened by ocean warming, unsustainable fishing and pollution, with 80% of coral reefs and 68% of seagrass beds exposed to nitrogen pollution.

‘Nutrient pollution is a known threat to coral reef and seagrass,’ Berger said.

An excess of nitrogen results in oxygen-free ‘dead zones’ and acidic conditions which can severely weaken or kill fish and other animals.

It can also cause a disease, sindromé blanco, which leads to white lesions that spread over coral, leading to death in mere weeks.

Nutrient pollution in this region, according to the researchers, “comes from four sources: agricultural production, human wastewater, atmospheric deposition and wild animal scat,” with agriculture and human wastewater suspected of contributing the majority of the pollution.

Human wastewater was also found to have a negative impact on the reef, with millions of tourists visiting the coast each year.

Tourism was found to represent only a small proportion of overall pollution, but watersheds contributing the most to pollution tended to have a high number tourists nearby.

Berger said: ‘The exercise also raised the question of how tourists, or tourist demand, has larger impacts beyond just the physical presence of more people — such as increased building or expanding agriculture — that can also exacerbate pollution input.’

The researchers hope that tracking the origins and paths of nitrogen and nutrient runoff will enable coastal managers to develop and tailor action plans to mitigate pollution.

Photo by Rabih Shasha