Dataset: Eelgrass density and reproduction at Curlew Beach in Nahant, MA and Niles Beach in Gloucester, MA from samples collected by SCUBA in 2014

SCUBA was used to sample Zostera marina in late summer 2014 from two coastal eelgrass meadows in the Gulf of Maine, USA, separated by approximately 48 km: Curlew Beach in Nahant, MA (hereafter CB) and Niles Beach in Gloucester, MA (hereafter NI). Samples were collected from three depths at each site: the center of the meadow (mid), and approximately 5 m from the inshore and offshore edges (shallow and deep, respectively); depth of the shallow, mid and deep samples was approximately 1, 3 and 5 m MLLW, respectively. At each depth, 8-10 flowering shoots, separated by at least 2 meters, were collected. To compare shoot density and morphology between sites and among depths, all aboveground biomass was harvested from 0.0625-m² quadrats set adjacent to these focal flowering shoots, and back in the lab, the number of vegetative and flowering shoots were recorded. In both meadows, eelgrass cover was patchy along the deep transect, and quadrats were deliberately set to capture areas where eelgrass was present; thus, deep quadrats inflate any area-based estimate of total eelgrass density but describe the local density of interacting shoots. To estimate seed production, the total number of spathes per flowering shoot was recorded along with the number of seeds per spadix on the focal shoot and up to two additional flowering shoots from the associated quadrat (n = 10-24 flowering shoots per depth).