Nutrient reductions over the last 30 years are the primary factor behind the resurgence of underwater grasses in the Chesapeake — something that scientists cite in a new study as tangible evidence that efforts to improve Bay water quality are paying off.
Seagrass beds are in decline globally, but the Chesapeake Bay is one of the few places — and the largest example — where that trend has been successfully reversed, according to an article that published in March in the Proceedings of the National Academy of Sciences.
That’s good news for the Bay as underwater grasses provide important habitat for fish, crabs and waterfowl. The scientists who led the study also said that the recovery likely foreshadows a broader comeback in the estuary’s health.
“We are thinking of the resurgence of the grasses as being the harbinger of things to come,” said Bill Dennison, vice president for science applications at the University of Maryland Center for Environmental Science and a co-author of the study. “We are using them now as an early signal for the restoration of the Bay.”
The study, built upon an analysis of a wide variety of data collected over three decades, found that a 23 percent decline in nitrogen concentrations in the Bay and an 8 percent decline in phosphorus were the primary factors behind a nearly threefold increase in underwater grasses since 1984.
Like all plants, underwater grasses require sunlight to survive, and scientists have long known that algae blooms and sediment in the water can block light from reaching plants, causing them to die.
But the study found that nutrients play a “dominant role” in causing the loss of grass beds because they not only spur algae blooms, but also promote epiphytic algae growth directly on the plants. That epiphytic growth, the study found, was three times more harmful to plants than the indirect effects of phytoplankton blooms in the water column.
“We show that nutrients are actually the primary control over these underwater grasses,” said Jonathan Lefcheck, the lead author of the report, who conducted this work while a post-doctoral student at the Virginia Institute of Marine Science, but now works at the Bigelow Institute of Marine Science in Maine.
The amount of underwater grasses still fluctuates from year to year, in large part because of weather — rainy years drive more water-fouling nutrients into the water than dry ones. Nonetheless, while the amount of grasses has varied, their overall acreage has increased over time, from a low of 38,229 acres in 1984 to a high of 97,400 acres mapped in 2016.
“Beyond the noise of inter-annual variability, we’ve got the right trajectory, and we can link it to specifically the nutrient reductions,” Dennison said.
While nutrients are the driving force, other factors still play a role. Areas with several underwater grass species do better over time than those with a single species, the study found.
The importance of diversity may explain, in part, why grass bed recovery in high-salinity areas, which has always been dominated by a single species — eelgrass — has been more muted than in other parts of the Bay.
It also offers a clue as to how to maintain comebacks in mid-salinity parts of the Bay, where widgeon grass dominates but its abundance often fluctuates greatly from year to year. The researchers said that Bay restoration efforts — which now focus only on water quality — should put more focus on restoring a mix of species in mid-salinity areas.
“When we look at those beds historically, we know diversity was important,” said Bob Orth, also of the Virginia Institute of Marine Science, who co-authored the study and has overseen the Bay’s annual underwater grass survey since its inception. “When you add one species, it has a significant effect on the stability of the meadow.”
The study has 14 co-authors representing universities and agencies from around the Bay region and the country. This team met five times over the course of two years in Annapolis digging deep into the data. They compiled extensive datasets about land use, manure and fertilizer applications, wastewater treatment plant discharges and water quality, as well as the abundance, diversity and density of grass beds.
Using sophisticated new analytical techniques unavailable just a few years ago to analyze that data, the scientists were able to draw conclusions that sometimes challenged their assumptions about factors affecting the grasses.
For instance, while wastewater treatment discharges may be locally important for grass beds, actions on the landscape — such as changes in land use or fertilizer applications on farms — were more important to larger trends in grass bed acreage.
Similarly, while sediment in the water column may be locally important, it was less important than nutrients in Baywide underwater grass abundance.
“With this multi-author, multi-partner synthesis type of science, you can bring in different types of expertise,” said Jennifer Keisman, a scientist with the U.S. Geological Survey and co-author of the study. “It is really important.”
Further analyzing that data, the authors said, could provide new insights for managers and promote an additional comeback of grass beds. “This is not the end, but the end of the beginning for all of this work,” Orth said.
The Chesapeake is still far short of the goal to restore 185,000 acres of underwater grasses, but it is doing better than any other place on the planet, the article said.
Seagrasses have declined globally by 29 percent, largely because of nutrient and sediment runoff. While they have come back in places such as Tampa Bay and the Wadden Sea in the Netherlands, researchers found that the Chesapeake has seen a “greater total and proportional recovery.”
A continued comeback would be good news for the Bay. Grass beds are a critical component of its ecosystem. They pump oxygen into the water, trap sediments, buffer shorelines from wave action, provide food for waterfowl and shelter for fish and blue crabs.
That trajectory is likely to continue, at least for now. Orth said a preliminary review of data from last year suggests that the Bay’s underwater grasses will likely set yet another record.