/ The effects of docks on eelgrass beds were measured using plant population characteristics (shoot density, canopy height, and growth rates), light levels, and an assessment of eelgrass bed quality in Massachusetts estuaries. Eelgrass populations were impacted under and directly adjacent to docks, as shown by depressed shoot density and canopy structure. Severe impacts can cause fragmentation of eelgrass beds, thus contributing to large-scale declines in estuaries such as Waquoit Bay. Impacts were fewer under docks supported by piers than under floating docks, and taller piers resulted in fewer impacts. Based on this investigation, we conclude that docks should be greater than 3 m in height above the bottom in areas with tidal ranges less than 1 m to allow enough light to sustain eelgrass beds under the docks. In addition to dock height, orientation and width were also found to be important factors affecting eelgrass. Narrow docks with a north-south orientation can best ensure the long-term survival of eelgrass under and near the dock. KEY WORDS: Docks; Shade; Eelgrass beds; Zostera marina; Seagrass
