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Building a resilient eelgrass population in Great Marsh

Funding for this project was provided by MassBays to the University of New Hampshire (2013) and to the Town of Essex (2014)

Eelgrass (Zostera marina) is ecologically and economically valuable to coastal waters and is the focus of many resource management initiatives in Massachusetts. The Great Marsh Area of Critical Environmental Concern (ACEC), a state-designated area that includes the water bodies of Plum Island Sound and Essex Bay, once contained acres of lush, thriving eelgrass beds that were wiped out by the mid-1900s. With funding from MassBays grant program in 2012, the University of New Hampshire (UNH) developed a site suitability model to identify areas with good potential for the re-establishment and growth of eelgrass and began test-transplanting eelgrass at the most suitable sites in Plum Island Sound. 

Transplanting genetically different eelgrass donor populations in Plum Island Sound

With a 2013 MassBays grant, UNH conducted a test of the model in Plum Island Sound. Eelgrass shoots from multiple donor sources were transplanted at four of the most suitable sites in Plum Island Sound identified in 2012. Results were mixed. Shoots transplanted between June and August did not survive while those transplanted after September in the central and southern portions of Plum Island Sound are still thriving. The low success rate of the summer transplants was attributed to multiple stressors including warm waters, poor water clarity, and bioturbation from a hyper-abundant European green crab (Carcinus maenas) population. 

Based on the results, further investigation is needed before large-scale restoration is implemented. The report (2013) recommended additional test-transplanting in the southern part of Plum Island Sound where waters are well-flushed and where results from the fall transplants were more promising. The report also recommended studying the population structure of the European green crab to inform management of the species and advise future eelgrass restoration and management initiatives in this system. 

In 2014 the team coordinated with the Town of Essex and expanded test transplanting to Essex Bay (Conomo Point) where a new self-established bed, the first in the ACEC in over 75 years, had just been documented. Eelgrass grew successfully at multiple sites in Essex Bay but tailed in Plum Island Sound.

Data on the population structure of the European green crab indicated hyper-abundance throughout the Great Marsh system. Robust baseline information is needed to understand the dynamics of green crab and is a critical next step in bringing a thriving eelgrass population back to the waters of the Great Marsh ACEC. 

A more comprehensive description of the study and conclusions as well as detailed management recommendations are available in the reports below. 

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