Costello and Kenworthy. 2011. Twelve-year mapping and change analysis of eelgrass areal abundance in Massachusetts identifies statewide declines. Est and Cst. 34, 2: 232-242.
Aerial mapping 1994-1996 (t1) in 46 of 89 embayments, and portions of 7 open-water near-shore areas.
open water = 9477 ha
embayments = 4846 ha
Aerial mapping 2000-2002 (t2) in 29 of the original 46 embayments; and again in 2006-2007 (t3) in the same 29 plus four more of the original 46 (total of 33).
Change analysis by embayment between t1 and t2, t2 and t3, and t1 and t3.
The paper has detail regarding change assessments for specific embayments, which is particularly valuable. Approximately 20% of the eelgrass in Massachusetts has been lost over the study period; five embayments lost eelgrass altogether (Waquoit Bay, Buttermilk Bay, Great/Perch Ponds, Eel Pond, and Three Bays). The median rate of decline across four regions ranged from -2.21% per yr to -3.51% per yr and the largest increase was seen in Boston Harbor between t2 and t3 (29.22% per yr). This coincides with restoration activities in Boston Harbor, though the paper doesn't describe the influence of those activities on the results. The overall conclusion is fairly dire: at this rate we'll have no eelgrass in 50 years.