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Climate Change Enhancing Chesapeake Bay Algal Blooms

24 August 2016

US - Nitrogen-rich agricultural runoff into the Chesapeake Bay presents an ongoing environmental and economic concern for the bay's massive watershed. While efforts to restore the bay have been successful during the past several years, a study led by Princeton University researchers shows that weather patterns tied to climate change may nonetheless increase the severity of algal blooms by changing how soil nutrients leach into the watershed.

Extreme rainfall cycles caused by increased climate variability flush larger amounts of nitrogen-containing nutrients from fertilizer and other sources into the Susquehanna River, which carries them into the Chesapeake Bay, according to a report in the journal Geophysical Research Letters.

Moreover, a spike in rainfall can increase nitrogen levels in the bay even if the amount of fertilizer used on land remains the same.

These chemicals feed explosive algae growth that can produce toxins that harm people, fish, wildlife and drinking water. Decaying algae also suck oxygen from the surrounding water, creating a low-oxygen state known as hypoxia that results in "dead zones" that suffocate fish and other species important to the aquatic food chain.

The researchers constructed a model that they say provides the most complete picture to date of how nitrogen moves from place to place in the Chesapeake Bay watershed. It connects weather and pollution in places as far away as upstate New York to the water conditions in the bay.

The research is just one example of how climate change's effects run beyond an increase in temperature, said Amilcare Porporato, a Duke University professor of civil and environmental engineering who is familiar with the research but was not involved in the study.

"It shows how climate change often produces the most dramatic consequences when the alterations involve rainfall and the hydrologic cycle," he said. "Increasing temperatures are only the tip of the iceberg, so to speak."

"This highlights the need for different management strategies," said co-author Elena Shevliakova, a visiting research scholar in the Princeton Environmental Institute who works as a physical scientist at the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Lab located on Princeton's Forrestal Campus.

Algal blooms can be prevented by decreases in fertilizer use, but using the same low amount of fertilizer every year could have subtle and varying impacts on algal blooms in coastal waters depending on the amount of precipitation during the past several years.

The study's authors envision adaptive watershed-management plans where models are combined with data from recent years' weather and predictions for the upcoming year to understand where and how much fertilizer can be applied in the watershed before it threatens algal blooms.

"It's a balance," said Peter Jaffe, a Princeton professor of civil and environmental engineering.

 "If you tell the farmer to put in less nitrogen, maybe your crop yield goes down, but then the oyster yield goes up in the bay."

Such an environmental strategy would require understanding other sources of nitrogen -- such as automobile pollution and sewage -- and other nutrients, such as phosphorus, that contribute to algal blooms. Lee's model already accounts for all sources of nitrogen, and was developed in a manner that allows it to be expanded to a global-scale model that includes other chemicals and land-use patterns.

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