By transplanting eelgrass, scientists aim to restore balance to Great Bay

A scientist holds a clump of eelgrass
Transplanted eelgrass plants are more likely to take than new seeds. (Amanda Gokee | New Hampshire Bulletin)

This story was updated June 8, 2021 to state that current wastewater treatment efforts reduce nitrogen to less than 5 milligrams per liter.

Great Bay is a complex, convoluted system. That’s the first thing that Melissa Paly will tell you about it. She’s been a waterkeeper there for five years, working with the Conservation Law Foundation to advocate for the health of the ecosystem.

Seven tidal rivers feed into the estuary that connects to the sea. People often think of rivers as flowing downstream; not so in the estuary, where water moves in all directions, following the push and pull of the tide that comes in and then recedes twice a day.

“Being in a complicated system complicates the pollution problem,” Paly said. And that problem has been plaguing these waters for decades. With 18 local wastewater treatment plants connected to the watershed, levels of nitrogen in the water soared. Wastewater isn’t the only pollutant – mills and roads are built near the water, and industrial buildings dot the shoreline.

Eelgrass, which is an indicator of the overall health of the estuary, started dying off in large swatches.

“Having lost so much of the eelgrass,” Paly said, “we’ve lost the balance.”

Only half as much eelgrass is growing in Great Bay compared to 20 years ago. And the plant, zostera marina, is a cornerstone species for the estuary – a habitat for young fish, a powerful carbon sink, and a water filter that can use nitrogen, a pollutant, to fuel its growth, removing it from the water. But now, as water quality has been improving, scientists and advocates say the time is right for eelgrass to make a comeback.

Two scientists in wetsuits stand at knee depth in an estuary while transplanting eelgrass
Alyssa Novak and Trevor Mattera of the Piscataqua Region Estuaries Partnership prepare to transplant eelgrass. (Amanda Gokee | New Hampshire Bulletin)

A pilot program run by Dave Burdick, an estuarine marine scientist out of the University of New Hampshire, and Alyssa Novak, a coastal ecologist working out of Boston University, is transplanting 2,500 eelgrass plants into five sites in the Great Bay estuary. They’re experimenting to see which planting methods work best and to learn more about the factors that influence whether the eelgrass grows well or dies off.  

“We want them to be growing, expanding, putting out more shoots,” Burdick said. “Some become reproductive. We hope to see little patches next year.”

They will know within a few weeks whether the transplants have taken to their new sites. On Tuesday, they planted at the mouth of Sagamore Creek. The water is shallow – just about 4 feet, although it fluctuates with the tides. The planting takes place at low tide to ensure that the eelgrass won’t be exposed when the water is at its lowest.

Novack has created a site suitability model with new data. The model looks at water quality parameters and generates locations where the seagrass may thrive. But, Novack said, “warmer water is changing things.”

Burdick said that as the water warms just 10 degrees Celsius, the anabolic requirements of eelgrass double.

“We’ve been learning a lot,” Burdick said. 

If the pilot program is a success and the eelgrass takes off, the scientists are considering scaling up in the project in the future – applying for bigger grants to fund the transplanting of more plants. The transplanting started last week, but the project has been years in the making, Paly said.

Eelgrass stores four times as much carbon as a forest on land, which has led to enthusiasm about the plant’s ability to help combat climate change. The plant also photosynthesizes under water and creates oxygen. When it’s healthy, it grows in lush meadows – and that’s what the researchers are hoping to see return to Great Bay.

Some of the transplanted material comes from an area that’s called the turning basin – where tugboats are used to turn around the large ships that bring materials to the industrial buildings along the shores of the estuary. With the size of ships getting bigger over time, there are now plans for the U.S. Army Corps of Engineers to dredge the area – the same area where a bed of eelgrass happens to grow. The dredging will destroy about an acre of grass, but some will get transplanted in other parts of the bay.

Half of the plant material used in the pilot program comes from this site. The other half comes from a site farther up one of the tidal rivers. This will allow the scientists to see if one kind does better than the other.

Paly, as a waterkeeper, said that for years conservation efforts in the estuary have focused on oysters – which she called filtration machines. Oysters, which filter between 30 and 40 gallons of water a day, have also faced a drastic reduction as water conditions worsened in the bay: about 90 percent of the population was lost, dropping from an estimated 25 million to just 1 to 2 million. The Nature Conservancy has been active in oyster restoration for the last decade, with mixed results.

But no one was paying much attention to eelgrass, except one man: Dr. Fred Short. Paly called him the grandfather of eelgrass science. The UNH ecology professor has been studying eelgrass since the 1970s. The long-term data sets he collected have helped scientists to determine overall trends in eelgrass population.

Without those data sets, how would you know if things are getting better or worse, Paly said.

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Now, there are scientists all over the world monitoring seagrass in a monitoring program called SeagrassNet that includes information from about 120 different sites. Paly said many of the problems are similar, even in different parts of the world. For example, eelgrass needs a lot of light to grow. When eelgrass populations are healthy, a strong mat of roots holds sediment in place, but once their critical mass starts to decline, the sediment can more easily get stirred up, making the water murky and dark.

Scientists are able to do the experiments now in part because improvements to the local wastewater treatment plant are decreasing the amount of nitrogen that gets into the water.  

The wastewater treatment plant for the city of Portsmouth is right on the water, and for years wastewater that was getting treated at minimal levels was deposited straight into the estuary. Recently, the town approved a $90 million upgrade to the facility. Before the upgrade, 30 milligrams of nitrogen per liter ended up in the waterways after it received primary treatment. Now, the wastewater goes through an advanced secondary treatment, decreasing that number to less than 5 milligrams per liter.

Seagrass can deal with nitrogen in small quantities and actually use it to grow. But if levels of nitrogen get too high, it can spur algae growth. When algae dies, it depletes the oxygen in the water, making it hard for other species to survive.

And, Paly said, it’s not just about one plant. It’s about the health of an ecosystem.

“This is what makes the region a place people want to live,” she said.

Great Bay estuary, looking out toward Sagamore Creek
Great Bay estuary, looking out toward the planting site at the mouth of Sagamore Creek. (Amanda Gokee | New Hampshire Bulletin)