Underwater grasses in the Chesapeake Bay continue upward climb

An estimated 82,937 acres of underwater grasses were distributed throughout Chesapeake Bay and its tidal tributaries in 2023, marking a 7% increase from the previous year. Experts believe that it is likely that more underwater grasses, also known as submerged aquatic vegetation (SAV), grew in the Chesapeake in 2023 than the mapped acreage suggests. The SAV Restoration and Monitoring Program at the Virginia Institute of Marine Science (VIMS) directly mapped 79,234 acres of underwater grasses using aerial and satellite imagery, while an additional 3,703 acres were estimated to occur in areas along a portion of the Potomac River that was unmapped due to security restrictions.

This marks a 45% attainment of the ultimate goal of achieving and sustaining the 185,000 acres of underwater grasses needed for a restored Bay, and a 61% achievement of the target to restore 130,000 acres by 2025. These are positive gains, but they do not offset the decline of underwater grasses that occurred in 2019, making it unlikely that the Chesapeake Bay Watershed Agreement outcome will be met by 2025. In 2019, underwater grasses in the Chesapeake Bay experienced a rapid decline following two years of above average rainfall and heavy river flows.

The Chesapeake Bay is grouped into four different salinity zones to better reflect the various communities of grasses that are found in the Bay. The gains observed in 2023 can largely be attributed to increases in widgeon grasses and sago pondweed in the mesohaline (moderately salty waters) zone, which suffered heavy losses during the decline experienced in 2019, as well as increases in eelgrass in the polyhaline (very salty waters) zone.

"I'm glad to see the Bay's SAV continue to recover following losses in 2019, and despite some declines in the upper Bay, the Susquehanna Flats are holding strong, and the news is positive," said Brooke Landry, chief for the Living Resource Assessment Program at the Maryland Department of Natural Resources and chair of the Chesapeake Bay Program’s SAV Workgroup. “I am confident that we will continue to make progress on restoration, especially with species like Horned pondweed and Sago pondweed becoming so abundant in the mid-Bay. Their skyrocketing expansion in recent years, according to observations that my colleagues and I are making, along with data collected by Chesapeake Bay SAV Watchers Program volunteers, is exciting to see since both plants offer such beneficial ecosystem services.”

A large decrease in grasses was noted in the oligohaline (slightly salty) zone in 2023 with losses concentrated in the Gunpowder and Middle rivers, as well as in creeks and coves stemming from the adjacent central channel of the Bay. These losses may have been influenced by phytoplankton blooms and suspended sediment concentrated along the mainstem that were observed in the spring and summer of 2022.

• Tidal Fresh: Underwater grass beds increased 3% from 19,185 acres in 2022 to 19,804 acres in 2023, representing a 96% achievement of this zone’s 20,602-acre goal.
  
• Oligohaline (slightly salty): Underwater grass beds decreased 54% from 7,145 acres in 2022 to 3,429 acres in 2023, representing a 33% achievement of the zone’s 10,334-acre goal.
  
• Mesohaline (moderately salty): Underwater grass beds increased 21% from 30,932 in 2022 to 37,961 acres in 2023, representing a 32% achievement of the zone’s 120,306-acre goal.
  
  
• Polyhaline (very salty): Underwater grass beds increased 12% from 19,200 acres in 2022 to 21,743 acres in 2023, representing a 65% achievement of the region’s 33,647-acre goal.

In 2023, the Polyhaline (very salty) zone experienced the largest amount of underwater grasses since 1997. Experts believe this is attributable in part to the La Nina climate cycle, which has impacted the Chesapeake Bay region over the past few years, bringing with it below-average rainfall and cooler temperatures. Some species of underwater grasses—like eelgrass—cannot grow in waters that are too warm. Positive effects of water quality improvements are also a possible cause of the meadow expansions.

“It’s been exciting to observe some tremendous expansion in some areas of the Polyhaline with eelgrass growing at depths we haven’t seen in decades.” says Christopher J. Patrick, director of the SAV Monitoring and Restoration Program at VIMS. “We often call these plants the canary in the coal mine for the Chesapeake, as they tell us a lot about how the Bay is doing.”

Underwater grasses are greatly impacted by the weather. Excess precipitation–as well as the pollution that heavy flows bring into the Chesapeake–can cause grass beds to die off. However, healthy grass beds can help trap and absorb nutrient and sediment pollution, helping to improve water clarity.

Underwater grasses also provide critical habitat for the Bay’s critters, including blue crabs. The loss of underwater grass habitat means the loss of nursery grounds for blue crabs. In the 2024 Bay-wide Blue Crab Winter Dredge Survey, the population of juvenile blue crabs was 138 million, an increase of 22 million from the previous year.

Since underwater grasses are sensitive to pollution but quick to respond to improvements in water quality, their abundance is an excellent indicator of the health of the Chesapeake Bay. Research has shown that nutrient reductions made under the Chesapeake Bay Total Maximum Daily Load have played a critical role in the overall recovery of underwater grasses, as documented by the aerial survey beginning in 1984. However, continued and accelerated nutrient reductions will be necessary to facilitate the continued recovery of underwater grasses, particularly as impacts from land use changes and climate change continue to grow.

In 2023, local highlights of underwater grass abundance included:

• Susquehanna Flats: Despite losses throughout the upper Bay, underwater grasses in the Susquehanna Flats continue to be resilient and maintain their impressive expanse, now approaching 11,000 acres.
  
• Elk River: After exceeding its goal repeatedly since 2000, no underwater grasses at all were mapped in the Elk River in 2023.
  
• Western Shore Tributaries: Underwater grasses that grow in the freshwater portions of the mid- and southern-Bay tributaries along the Western Shore of the Chesapeake, as well as those freshwater grasses in the Upper Bay, experienced declines in 2023. Experts believe this decrease may have resulted from residual impacts due to the phytoplankton blooms and suspended sediments observed in the upper Bay in 2022 and increased salinity resulting from below average river flows.
  
• Mid-Chesapeake Bay: Underwater grasses in tributaries of the mid-Bay did exceptionally well in 2023, bolstered by the expansion of Sago pondweed, a robust plant that now dominates the Choptank and Severn rivers, as well as the eastern portion of the Bay. These observations were made through the Chesapeake Bay SAV Watchers Program and confirmed by Bay scientists and riverkeepers.
  
• Mobjack Bay: This region of Virginia’s portion of the Chesapeake hit a new record for SAV coverage with the most underwater grasses observed in 2023 since surveying began.
  
• Rappahannock River: Underwater grasses in the lower Rappahannock River increased 43% between 2022 and 2023 and achieved 90% of its restoration target of 1,700 acres.

To learn more about the progress the partnership is making toward achieving and sustaining underwater grasses in the Chesapeake Bay and its tidal tributaries, please visit ChesapeakeProgress.