Seagrass beds are critical wetlands components of shallow coastal ecosystems throughout the state. Seagrass beds provide food and cover for a great variety of commercially and recreationally important fauna and their prey. The leaf canopy of the seagrass bed calms the water, filters suspended matter and together with extensive roots and rhizomes, stabilizes sediment.

Eelgrass, Zostera marina, is the most common seagrass present on the Massachusetts coastline. Because eelgrass has been recognized as a sensitive indicator of nitrogen loadings and because the distribution and abundance of eelgrass beds can be easily documented with aerial photographs, it is an ideal habitat to track to monitor overall ecosystem health. Many physical and water quality impacts from human activity may affect eelgrass abundance and distribution, and eelgrass is often considered a sentinel species for evaluating ecosystem health. The two principal human disturbances affecting eelgrass growth is declining water quality and physical disturbance.

The MassDEP Eelgrass layer, produced from data collected in 2001, is the second statewide mapping of the eelgrass resources along the coast. The data were compiled from similar methodologies as the earlier 1995 dataset. A similar third iteration of this statewide mapping is planned for the 2006-07 seasons. The purpose of the second statewide mapping was to determine areas where the Zostera marina resource was decreasing in area and cover. The methods used for the 2001 were similar to those used in the earlier 1995. One exception to this specification was the change in the delineatation of apparent areas of no Zostera within polygons of the resource (i.e. holes). This change in mapping convention was derived from a better understanding of the scientific literature of the dynamics of Zostera beds which describes subtle "infra" movement within beds in response to day-to-day wave action, currents and sand migration, and storm events.