Researchers say Chesapeake Bay cleanup initiatives have triggered a major resurgence of the underwater grasses that are at the center of the estuary's fragile food web.
Scientists from across Maryland and Virginia say that from 1984 to 2014, concentrations of nitrogen in the bay fell by 23 percent while the acreage of areas covered with submerged vegetation more than tripled, to nearly 100 square miles.
They wrote in the Proceedings of the National Academy of Sciences, one of science's premier journals, that the finding “speaks directly to the value” of coordinated cleanup efforts like a so-called pollution diet that was imposed on the Chesapeake watershed in 2010.
“The diet is working,” said Bill Dennison, vice president for science applications at the University of Maryland Center for Environmental Science in Cambridge, and one of the study’s authors.
“We can definitely link this reduction of nutrients to the surge of grasses, and hopefully as a harbinger of ecosystem improvement overall,” Dennison said.
The research analyzed data including aerial surveys of grass coverage, monitoring of land uses, agricultural land nutrient loads and water quality.
Jonathan Lefcheck, the study’s lead author and a postdoctoral researcher at the Virginia Institute of Marine Science, said the longevity and detail of that data allowed researchers to make firm statements about the improvement of bay health — assertions that had previously been noted only in isolated studies and anecdotal evidence.
“We now have such a critical mass of information we can start pulling all the threads together and seeing what’s the general status,” Lefcheck said.
The data collection used in the study started in earnest in 1983 with the creation of the Chesapeake Bay Program, a unit of the federal Environmental Protection Agency.
Efforts to improve bay health by reducing the flow of nitrogen, phosphorus and sediment into the bay stepped up in 2010 when the EPA established the pollution diet, which seeks to limit bay pollution by reducing the amount of nutrients being spread on farms and lawns and cleaning up the discharges from sewage treatment plants and septic systems.
This is how nutrients harm underwater grass growth: They fertilize growth of algae blooms that can block sunlight, or that can grow directly on blades of grass, smothering and killing them. The grasses are considered a key indicator in the bay’s health because they remove and store carbon dioxide from the water and air, and create vital habitat for young fish, crabs and other creatures.
“We often call the grass beds the secret garden,” Dennison said. “They’re absolutely teeming with life.”
When algae growth causes grasses to die off, that causes direct harm to iconic species such as blue crabs and rockfish. Blue crabs spend some of their earliest days in the grass beds of the lower Chesapeake, and without that refuge, the crustaceans are vulnerable to predators.
“When these grasses disappear, there’s really no alternative,” Lefcheck said. “These blue crabs would be out in the open, ripe for the picking.”
The study also emphasizes the importance of biodiversity among vegetation. The researchers found that while one set of conditions could cause some types of grasses to suffer, other species might be better adapted to survive, and their resilience helps protect grass beds from becoming barren.
In the paper published Monday, the researchers wrote that “conservation or restoration of species diversity could further enhance recovery of underwater grasses.”
The researchers said their findings stress the importance of maintaining and further improving programs and initiatives that have come about as part of the pollution diet.
President Donald Trump’s administration has proposed significant budget cuts, if not total elimination, of the Chesapeake Bay Program, alarming state leaders who have stressed the importance of protecting the program and the gains in bay health.
“We don’t want to backslide,” Dennison said. “We want to keep that going.”