The short version
Massachusetts mapped its eelgrass from the air three times between 1994 and 2007. Charles Costello of the state Department of Environmental Protection and William Judson Kenworthy of NOAA analyzed the results. Eelgrass shrank in 30 of the 33 embayments mapped at the start and end of the period, and some lost it entirely. Waquoit Bay was one of them. The study showed that earlier reports of eelgrass loss in Waquoit Bay and Buzzards Bay were part of a statewide decline.
What the researchers did
Eelgrass grows in shallow, clear water and shows up as dark patches against sandy bottoms in aerial photos. MassDEP’s eelgrass mapping program photographed the coast during the summer growing season, at low tide in calm, clear conditions, and checked the maps in the field with underwater video.
- The first mapping, in 1994–1996, covered 46 of the state’s 89 coastal embayments plus seven open-water areas.
- Subsets were mapped again in 2000–2002 and 2006–2007, using aerial photos and later digital imagery.
- The authors calculated yearly rates of change for each embayment and region over each interval. Field checks showed the maps correctly identified eelgrass more than 85% of the time at first and 94% by the final mapping.
What they found
- A large resource. The first mapping found about 4,846 hectares of eelgrass in the 46 embayments and about 9,477 hectares in open waters such as Nantucket Sound and Cape Cod Bay.
- Widespread decline. Of 33 embayments mapped over the full period, 30 lost eelgrass. Together they lost 774 hectares, about 20.65% of their eelgrass. The median rate of decline was about 2.9% a year.
- Waquoit Bay lost it all. Mapped eelgrass in Waquoit Bay fell from 10.23 hectares to 5.95 hectares and then to zero by the final mapping. The neighboring Hamblin and Jehu Ponds and Great and Little Rivers fell from 29.91 hectares to 6.04, one of the fastest long-term loss rates in the state.
- The south coast was hit hardest. Many of the embayments that lost all their eelgrass were on Cape Cod and Buzzards Bay, where the authors link the losses to rapid growth and excess nutrients from the land.
- Recovery is possible. In Boston Harbor, after sewage treatment was upgraded and the outfall moved 15 km offshore, eelgrass grew at 29.2% a year in the second interval, reversing earlier losses.
What it means for Waquoit Bay today
This study documents, with state mapping, the final disappearance of eelgrass from Waquoit Bay between the early 2000s and 2006–2007. It also adds a worry for recovery. When whole embayments lose their eelgrass, there are fewer nearby meadows to supply seeds and new plants. The authors warn that recovery may be limited in semi-enclosed embayments that lack nearby sources of new plants.
The Boston Harbor result is the hopeful part: once nutrient inputs were cut, eelgrass came back quickly. The authors conclude that seagrasses can either thrive or expand their populations if water quality is managed properly
(Costello & Kenworthy, 2011). That is the logic behind the nitrogen limits for Waquoit Bay, which use eelgrass recovery as a key measure of success (see our explainer on the state’s threshold report).
Read the paper
Costello, C. T., & Kenworthy, W. J. (2011). Twelve-Year Mapping and Change Analysis of Eelgrass (Zostera marina) Areal Abundance in Massachusetts (USA) Identifies Statewide Declines. Estuaries and Coasts, 34(2), 232–242. https://doi.org/10.1007/s12237-010-9371-5
License: © Coastal and Estuarine Research Federation 2011, published by Springer. The article is not openly licensed, so we link to the publisher’s page rather than hosting a copy.
Drafted with AI assistance from the linked source.
