Monday, December 11, 2023

Chesapeake Bay Program Reports Decline in Underwater Grasses. Here’s Why That’s a Problem

Widgeon grass, a type of submerged aquatic vegetation (SAV), responds rapidly to impacts from extreme weather and changes in water quality. Underwater grasses can be a good indicator as to how healthy the Bay is. (Photo by Will Parson/Courtesy of the Chesapeake Bay Foundation)

REGIONAL — The Chesapeake Bay Program (CBP) is reporting another year of underwater grasses declining in the Chesapeake Bay. 

Why is This Important?

Underwater grasses, also known as submerged aquatic vegetation if you want to be fancy, are critical to the Chesapeake Bay’s overall ecosystem. They keep waters clean by absorbing excess nutrients, trapping suspended sediment and slowing wave action that helps to stabilize shorelines, protect wetlands and reduce erosion. 

Underwater grasses help keep structures built on the coast from being swept out to sea. 

Underwater grasses contribute to providing delicious crabs each summer.  Bay grasses create food for small invertebrates and migratory waterfowl, as well as a habitat for fish and blue crabs. 

In fact, bay grass abundance is one of several factors that can impact the health and stability of the blue crab population: the loss of these grasses is a loss of nursery habitat which can increase their vulnerability by pushing young crabs to gather in the limited nurseries that remain. 

In the 2021 Blue Crab Advisory Report, the population of juvenile blue crabs decreased from 185 million in 2020 to 86 million in 2021. Researchers that study underwater grasses believe that a contributing factor to the decline of the juvenile blue crab population is the decrease of grass beds from 2018—2020. 


According to a July 27 news release, the preliminary figure of 62,169 acres of underwater grasses was mapped in the Chesapeake Bay and its tidal tributaries during 2020, achieving 48% of the Chesapeake Bay Program’s 2025 restoration target of 130,000 acres and 34% of the ultimate restoration goal of 185,000 acres.

Although the 62,169 acres of underwater grasses observed in 2020 have increased from the 38,958 acres noted during the first survey in 1984, it is a 20% decrease from the current 10-year average of 78,168 acres. 

But this preliminary acreage observed in 2020 is a 42% decline from 2018 when it was estimated that the Bay may have supported up to 108,078 acres of underwater grasses. 

While this is a staggering decrease, the loss from 2019 to 2020 was only 7%.

“While the 7% decline is disappointing, the silver lining is that the 2020 survey shows that underwater grasses are stabilizing following the losses experienced in the middle Bay in 2019. We are hopeful that this is a sign that we’re poised to start regaining that lost ground in coming years,” Chris Patrick, Head of the Chesapeake Bay SAV Monitoring and Restoration Program and Assistant Professor of Biology for the Virginia Institute of Marine Science at William and Mary, said in a released statement.

One bragging right Virginia has is that the largest declines were observed in Maryland. The decline included estimated 5,684 acres  located in the Tangier Sound, the Eastern Bay, the mouth of the Choptank River and in the Little Choptank River. These spots were noted to have moderately salty waters.

These losses are largely attributed to the continued decline in widgeon grass, which fluctuates from year-to-year as the species responds rapidly to impacts from extreme weather and changes in water quality.

“It is becoming imperative that we learn more about the life history and biology of this species, as our hope of continued underwater grass recovery in much of the Bay is becoming increasingly tied to the success or failure of this one species,” Patrick said in a released statement.

Like grasses on land, underwater grasses need sunlight to survive. When the waters of the Chesapeake Bay become clouded with algae blooms or suspended sediment, sunlight cannot reach the bottom habitat where these grasses grow. 

While healthy grass beds can trap and absorb some nutrient and sediment pollution— thus improving water clarity where they grow—too much pollution can cause grass beds to die off. 

High water temperatures, turbulence from strong storms and drought can also affect the growth and survival of underwater grass beds.

However, because bay grasses are sensitive to climate conditions and pollution but quick to respond to water quality improvements, their abundance is a good indicator of Bay health.

What Does the Underwater Grass Population Look Like in Our Area?

According to the news release, in the Virginia portion of the mid-to-lower Bay, underwater grass abundance did particularly well in the Potomac and Rappahannock rivers, as well as in the Mobjack Bay. 

The Potomac River increased from 6,631 acres in 2019 to 7,534 in 2020, while the Rappahannock, including the Corrotoman River just north of Urbanna, increased from 1,789 acres in 2019 to 2,204 acres in 2020. 

In the Mobjack Bay, underwater grass acreage increased from 5,027 acres in 2019 to 7,080 in 2020. 

What Can People Do to Help Keep the Grass from Declining More?

According to Director of Communications for the Chesapeake Bay Program, Rachel Felver, residents who live on a tidal tributary of the Bay, or the Chesapeake itself, should not rip out or mow grasses.

For boaters, make sure to slow down and trim your motor when you are navigating through a grass bed.

Experts are also seeking help in collecting data on underwater grasses. Volunteers can pitch in by becoming a Chesapeake Bay SAV Watcher. This community science-based program asks volunteers to document everything from the types of underwater grass species they see to the presence of shoreline erosion.

And there are ways to help on the Homefront as well. Homeowners can minimize pollution that may enter the Bay by using Bay-friendly pesticides and fertilizers, planting trees to reduce erosion and ensuring home’s downspouts drain onto grass or gravel to lessen stormwater runoff. 

For more ways to help the Bay, check out CBP’s How-To’s and Tips.

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