Estuaries face higher nutrient loads in the future -- particularly on the Atlantic coast

Nutrient loadings are of interest in large part because they are key contributors to algal blooms, which pose risks for both human health and the environment.

Depending on the type of algae, blooms can produce toxins that harm both human and animal life. Algal blooms also contribute to "dead zones," where there is little oxygen in the water to support a healthy ecosystem. In addition, algal blooms can also increase costs for drinking water treatment and for industrial sectors that rely on clean water. And the risk of algal blooms is increasing due to warmer temperatures, related to global climate change.

"We know the importance of nutrient loadings to ecosystem health, so we wanted to assess estuaries across the 48 contiguous states to determine how vulnerable these estuaries are to increased nutrient loads," says Lise Montefiore, co-corresponding author of a paper on the work and a postdoctoral researcher at North Carolina State University. "Essentially, our goal was to use predictive modeling to estimate average annual nutrient loads for estuaries between the years 2035 and 2065. We were able to conduct assessments of all major estuaries in the lower 48 states, with the exception of the Mississippi River estuary."

"One of the drivers for this study is that once an estuary has elevated nutrient loads, it is exceptionally difficult to restore that estuary," says Natalie Nelson, co-corresponding author of the study and an assistant professor of biological and agricultural engineering at NC State. "It's more effective to prevent high nutrient loads in the first place than it is to address nutrient loading problems once they're established. We wanted to help policymakers identify which systems are most at risk and could benefit substantially from conservation efforts to prevent increased nutrient loads."

For the study, researchers drew on both historical data and existing research that predicted land use and climate conditions for the continental U.S. between 2035 and 2065. This data was then incorporated into a model that predicted the annual average nitrogen and phosphorus loads for 112 estuaries in the lower 48 states.

The researchers applied the model to the time periods spanning the years 1990-2020 and 2035-2065.

"Our goal with this modeling was not to identify what the specific nutrient loads would be for any given estuary -- this model is not designed to provide that level of detail," Nelson says. "Rather, our goal was to identify which estuaries are most likely to see the biggest increases in nutrient loadings. That's something this model can do well."

"We found that almost all estuaries will see increases in nutrient loadings," Montefiore says. "But the highest increases appear to be in the North Atlantic region -- which stretches from New England to Virginia -- with the greatest increases occurring in coastal regions north of the Chesapeake Bay."

To help place the modeling results in context, the researchers also assessed the sensitivity of each estuary to increased nutrient loads. The researchers drew on existing research to classify estuaries based on the extent to which their conditions are likely to decline if their nutrient loads went up.

The researchers also evaluated each estuary's "adaptive capacity," essentially accounting for the resources available in each estuary that authorities could use to address nutrient loadings in a meaningful way. These factors included things such as state laws and regulations, availability of monitoring data, and the amount of wetlands in the estuary. The more resources available, the higher the estuary's adaptive capacity score.

"Taking into account sensitivity, adaptive capacity and predicted increases in nutrient loadings offers a more complex -- and more complete -- assessment of which estuaries are most vulnerable," Nelson says.

"For example, the North Atlantic region will likely see the greatest increase in nutrient loads," Montefiore says. "However, estuaries in the North Atlantic region also have a great deal of adaptive capacity. As a result, the region has more resources than many other regions that could be used to reduce the predicted increase in nutrient loading, if authorities in the region choose to take action."

"In addition, many of the estuaries in the North Atlantic already have high nutrient loads -- so it's not clear whether a further increase would have a significant impact," Nelson says.

However, this overarching assessment also highlights the vulnerability of estuaries on the South Atlantic Coast and along the eastern coast of the Gulf of Mexico.

"Our modeling suggests that these regions will see significant increases in nutrient loadings, and several states in these regions have relatively low adaptive capacity," says Montefiore.

"That being said, all of the states in the South Atlantic and eastern Gulf have substantial wetlands, which can be a valuable natural resource for reducing nutrient loadings," says Nelson. "If these states choose to act and conserve wetlands, they may be able to mitigate the predicted increase in nutrient loads."

"One of the key takeaways from this work is that land use plays a major role in nutrient loads -- perhaps larger than many people would anticipate, when compared to climate change," says Montefiore. "State and regional officials have limited ability to influence climate change, but they do have authority to control land-use decisions. That means they are in a position to help limit future nutrient loads and protect their water resources."

The work was done with support from the U.S. Geological Survey Southeast Climate Adaptation Science Center; a National Climate Adaptation Science Center Science to Action Fellowship under grant number G18AC00336; an Early-Career Research Fellowship from the Gulf Research Program of the National Academies of Science, Engineering, and Medicine; and the U.S. Department of Agriculture's National Institute of Food and Agriculture Hatch project 1016068.