Rescuing Eelgrass Beds: Researchers Plan Restoration In Pleasant Bay

by Alan Pollock
Eelgrass provides key habitat for juvenile fish and shellfish like scallops. COURTESY BOB ORTH Eelgrass provides key habitat for juvenile fish and shellfish like scallops. COURTESY BOB ORTH

PLEASANT BAY – If you can remember large, lush beds of eelgrass rippling just under the surface of waterways like Pleasant Bay, you’ve probably been around awhile. Since 1951, more than half of the eelgrass in Massachusetts waters has vanished — taking with it important fish and shellfish habitat, a buffer against erosion, and a vitally important mechanism for capturing carbon.

Researchers working with the Cape Cod National Seashore are studying how to rescue and restore these underwater meadows, even against the threat of rising sea surface temperatures.

The National Seashore recently announced that it received $112,000 in Inflation Reduction Act funds to support the effort. The money is part of climate restoration and resilience grants issued to seven national parks from Maine to North Carolina and will be used locally to study eelgrass beds in Provincetown, Wellfleet and in Pleasant Bay. The strategy is to identify healthy, climate change-resistant eelgrass beds that could eventually become donor sources, providing seeds or plants to be transplanted in areas that need them.

But it’s not enough to simply add new plants to eelgrass beds that are in decline, says Alyssa Novak, a coastal ecologist with Boston University who has been leading the work in Pleasant Bay. Novak has helped develop computer models that determine which eelgrass beds are most likely to survive given the rapid rise in sea surface temperature that’s already underway; in a worst-case scenario, the water could warm by nearly 2 degrees centigrade by 2050.

“Eelgrass meadows play a very significant role in our coastal environments,” Novak told last year’s Science in the Seashore Symposium. Their roots and rhizomes trap sand to reduce erosion and cycle nutrients to improve water clarity. They store carbon below ground twice as efficiently as terrestrial forests, she added. Eelgrass is also a health indicator for water quality. “It responds very rapidly to changes in environmental conditions,” Novak said.

Nitrogen from residential septic systems in the Pleasant Bay watershed has helped fuel a 55 percent loss of eelgrass beds in the system since 1951, amounting to about 9,000 acres. The nutrients spur the growth of phytoplankton, which shades sunlight from reaching the eelgrass on the bottom. While eelgrass restoration projects happened as early as the 1930s and ‘40s, they’ve had limited success.

“You can’t go into a system and expect a restoration to be successful if you still have stressors in the system or you don’t understand how you can manage them,” Novak said. Replanting eelgrass costs around $400,000 per acre, “and success rates are very variable,” she said.

For that reason, restoration areas need to be chosen very strategically. The computer models Novak used factored in the amount of sunlight reaching the bottom, depth, sediment, currents, nutrient levels, temperature and other factors for locations around Pleasant Bay, yielding a map of areas where replanted eelgrass is most likely to survive given worst-case temperature increases.

“We had about 400 acres that we could work with under worst-case scenarios,” she said. From that area, locations were omitted if they had conflicting uses like mooring fields, aquaculture installations, docks and piers. In the end, about 330 acres of the bay remained available. “This is actually a lot of area that we can work with in terms of restoration,” she said. The models further broke down the 330 acres into regions representing high, medium and low-priority areas.

The models also helped researchers identify areas that are immediately available for restoration efforts under current conditions, and that represents another 570 acres, she said.

When it comes to preserving eelgrass beds, “everybody can play a role,” Novak said. Homeowners should reduce their use of landscape fertilizer and make certain their residential septic systems are pumped yearly and functioning well. Boaters can comply with rules preventing the discharge of waste, obey no wake zones and avoid dropping anchor where eelgrass is visible. “It takes a long time for the bed to recover from it,” she said.