While the Chesapeake Bay saw a 38% decline in underwater grasses in 2019, the damage was largely in the saltier waters of the southern bay, according to a new report from the Chesapeake Bay Program.
In Anne Arundel County, rivers such as the Severn and Magothy both saw an increase in the grass.
The annual report found 66,387 acres were mapped in Maryland and Virginia in 2019, down from the 10-year average of 79,738 acres and from 2018′s estimate of 108,078 acres.
Chris Patrick of the Virginia Institute of Marine Science, which does an aerial survey of bay grasses each year, said the story of bay grass isn’t uniform, but rather many small stories. In the West, Rhode and South rivers there usually isn’t much grass, and that was the case in 2019. The Severn went from 224 acres of grasses in 2018 to 404 in 2019, and the Magothy went from 36 acres to 183.
“The upper bay, in general, is doing better,” he said. “The last 15 years or so are a really good success story for the fresher water parts of the bay.”
Things are worse south of the Honga River in Dorchester County, where the water is saltier. Losses in Tangier Sound, right on the border between Maryland and Virginia, accounted for 40% of the overall decline, Patrick said.
There are dozens of underwater plants growing in the bay, but this decline is tied to a specific species: widgeon grass, which responds quickly to water quality and temperature, according to the program. The bay program said the growth of widgeon grass in recent years followed by a sharp decline mimics a rapid increase in 2001 and 2002 that was followed by a 50% decline in 2003.
The grass can grow in salty water and fresher water, but is temperamental, Patrick said. His team plans on researching that specific species more to try to identify the causes of that variability.
Bay grasses fill a number of essential roles in the Chesapeake. The grasses help to prevent erosion, stabilize sediment, break up the turbidity in water, absorb and store carbon, and they play an important role as a hideaway for baby fish, crabs and other critters.
While fresher waters in Anne Arundel are faring better, the decline in bay grasses farther south could hit Marylanders where it hurts: at the crab shack. According to the Maryland Department of Natural Resources female crabs migrate to the mouth of the Chesapeake each fall, where they fertilize their eggs. The eggs need a salinity level of 18 parts per million to survive, and are swept into the Atlantic Ocean after hatching.
About 31-49 days later those larvae reach their next stage of growth, drop to the bottom to feed and start to return to the bay, the department says on its website.
Grasses are a refuge for those juvenile crabs, according to Brooke Landry, who works for the Maryland Department of Natural Resources and chairs the Bay Program’s submerged aquatic vegetation workgroup.
“We need these southern bay (submerged aquatic vegetation) beds to provide nursery habitat for blue crabs,” she said. “The less habitat they have, the more vulnerable they are to predation.”
As scientists track the amount of grass in the bay from year to year, they are also thinking about how climate change will factor in.
Landry said given the amount of rain that fell last year the decline in the grass isn’t alarming. Rain can flush sediment and nutrients from the land into the water, making it cloudier and fueling algae blooms, both of which hurt grasses.
The rain also reduced the salinity in the water, which affects what type of plants can grow. The concern is what will happen to the system if the weather experienced in 2019 and 2018 become the normal pattern.
“I think the last couple of years are what climate scientists see as becoming more of a normal pattern,” Landry said.
More rainwater could shift the salinity farther south, which would allow for an expansion of plants that thrive in freshwater.
Patrick said to help bay grasses, people can build rain gardens or rain barrels on their property to reduce the amount of nutrients washed from land into the water. They can also drive less: what goes up as aerosol nitrogen pollution can end up in the water.