Above average river flows impact underwater grasses

Underwater grasses—also known as submerged aquatic vegetation or SAV—are critical to the Chesapeake Bay, providing food and habitat to wildlife and improving water quality by absorbing excess nutrients and trapping sediment. Grass abundance has fluctuated over the years, but updated research recently released from the Chesapeake Bay Program reveals a 38% decline from the previous year, when the Bay may have supported up to 108,078 acres of underwater grasses.

In 2019, an estimated 66,387 acres of underwater grasses were mapped in the Chesapeake Bay and its tidal tributaries. Although this is a 70% increase from the 38,958 acres observed during the first survey in 1984, it is a 17% decrease from the preceding 10-year average of 79,738 acres.

Experts attribute the losses largely to a decline in widgeon grass, a species whose abundance can rise and fall from year to year and responds easily to weather events or changes in water quality. The decline in widgeon grass observed in 2019 mirrors a similar situation that occurred in 2001-2003, when a rapid increase in widgeon grass in 2001 and 2002 was followed by a 50% decline in 2003. Though the precise cause for the fall in widgeon grass in 2019 is unknown, a contributing factor may have been higher than average river flows entering the Bay, which reduces water clarity and blocks sunlight from reaching the grasses.

“The past two years have tested the resilience that our SAV beds have been building since the Chesapeake Bay pollution diet was implemented,” said Brooke Landry, chair of the Chesapeake Bay Program’s Submerged Aquatic Vegetation Workgroup, in a media release. “The good news is that many of the Bay's tributaries stood up well against the high flows and associated sediment and nutrient runoff from 2018 and 2019, and SAV acreage even continued to expand in some. But there's still much more work to do in order to mitigate unpredictable impacts from climate change.”

In 2019, the largest decline in terms of total area—an estimated 34,986 acres—was observed in moderately salty waters, particularly in the Tangier Sound area, which experienced an 18,452-acre decline. Overall, the lower central Chesapeake Bay saw fewer grasses, both in the Maryland and Virginia portions. However, certain areas did see a growth in grass abundance. The Sassafras River in northeast Maryland, the Severn River in central Maryland and the middle and upper portions of the Rappahannock River in Virginia noted modest increases.

Elle Bassett, the Miles-Wye riverkeeper with ShoreRivers notes that “while it is disappointing to see a decline in SAV acreage this past year, I also see this as an opportunity to refocus our efforts on the restoration and protection of SAV beds. SAV plays a critical role in our local waterways and in the fight for swimmable, fishable rivers. We have the capability to reach our SAV acreage by improving water quality and being better stewards of our land.”

The Chesapeake Bay Program and its partners continue to work toward improving water quality, in hope of reversing these recent losses. Among various efforts, reducing pollution and storm water runoff, installing natural buffers that filter out sediments and planting additional grasses will be instrumental moving forward.

To learn more about our progress toward achieving and sustaining 185,000 acres of underwater grasses Bay-wide, visit ChesapeakeProgress.