WESTERLY — A consortium of scientists is studying methods for mapping eelgrass at Napatree Point. They hope to develop and refine techniques for documenting eelgrass beds and for distinguishing eelgrass from macroalgae, or seaweed, which can resemble eelgrass when viewed from a distance.
Eelgrass, a narrow-bladed sea grass, is a key component of healthy marine ecosystems. It provides habitat for invertebrates, young fish and shellfish, which in turn are food for many seabirds. Of the 15 eelgrass species, only one, Zostera marina, is found in Rhode Island, and the largest patch in the state, more than 80 acres, is growing in Little Narragansett Bay, between the Napatree Point Conservation Area and Sandy Point.
The Napatree eelgrass study represents some of the most notable research described in the Watch Hill Conservancy's annual "State of Napatree" report issued this month.
"It is not surprising why The Watch Hill Conservancy is interested in submerged aquatic vegetation in Little Narragansett Bay,” said Janice Sassi, manager of the Napatree conservation area. “Many of the iconic species on Napatree, for example osprey and terns, require healthy near-shore ecosystems. Eelgrass is one of our most important habitats off Napatree.”
Two participants in the study were University of Rhode Island professor Peter August, from the URI Department of Natural Resources, who serves as the conservation area’s science adviser, Michael Bradley, an eelgrass researcher, also of URI.
Bradley has undertaken three statewide eelgrass mapping initiatives in Rhode Island since 2006, collecting aerial photographs of beds from Napatree to Little Compton, and another bed in Long Island Sound.“Part of the goal is to to try and establish status and trends,” he said.
Developing consistent and effective assessment methods, August said, would result in a more accurate picture of the health of the eelgrass beds.
“The problem is, if there are differences, are the differences because the eelgrass is actually changing, or are the differences because different people mapped them?” he asked. “This is why we have Mike, because one guy is doing it in Rhode Island for all these years; we don’t have any personnel bias.”
For last summer’s Napatree eelgrass study, researchers chose a small area: a 10-acre patch growing in the shallow waters off the northwest corner of Sandy Point. In addition to URI and the Watch Hill Conservancy, collaborators included Caitlin Chaffee, coastal policy analyst at the Coastal Resources Management Council; Nicole Rohr, assistant director of the Coastal Institute; and Bryan Oakley, associate professor of environmental geoscience at Eastern Connecticut State University.
For one day last July, the team, which included Napatree naturalists and Grant Simmons of the Watch Hill Fire District, hit the water to document the eelgrass and anything else they came across.
“This one patch was easy to get to and clearly a mix of macroalgae and eelgrass,” August said.
The macroalgae growing with the eelgrass were identified by URI seaweed researcher Lindsay Green-Gavrielidis.
“We studied that bed to fine tune some of our techniques,” Bradley said. “We wanted to basically bombard this questionable area with all the mapping tools we had, so we took an underwater video camera linked to a GPS, we had a Quadcopter Phantom 4 drone. We flew at 400 feet above the bed and we took still pictures and then we also took continuous video over that patch. We also put people in the water with Gopros.”
The video GPS provided precise locations for the images.
“With that coordinate, we can put a dot on the map and we can know whether it’s pure macroalgae, or pure eelgrass,” August said.
The still photographs taken from the drone were a disappointment, but the video images and Gopro photos were much better, providing an accurate assessment of the eelgrass there.
“What we found at that bed versus what had been mapped in 2012, it confirmed my suspicion that this particular bed had been overestimated for a number of years,” Bradley said.
“That was a spinoff conclusion,” August added. “The goal of this was, what kinds of technologies can we use going forward when we do eelgrass mapping in the future to provide Mike and the people doing this the very best imagery and information to know whether it’s eelgrass or algae.”
Next summer, the study will move to the 80-acre site.
“We’re going to try and fine tune where the eelgrass is and where the eelgrass isn’t,” Bradley said. "It’s virtually impossible to see the bottom, so the only way to see what’s down there is to send someone down there. It’s a difficult area to navigate in a boat because it gets shallow so quickly.”
Sassi said she was grateful to have a team of scientists donating their expertise and time to the mapping project.
"One of the great assets I have in managing Napatree is the amazing group of project partners that we work with,” she said. “The eelgrass ground-truthing project is a terrific case in point. Experts from a number of agencies volunteered their time to determine the most accurate way to map eelgrass.”
More information on the eelgrass study and the State of Napatree report are available at: www.tinyurl.com/ntpca-son