Threats to Eelgrass
Despite the obvious value of eelgrass ecosystems, eelgrass beds are threatened by a number of anthropogenic perturbations. Declines in seagrass populations have been linked to physical disturbances (i.e., dredging, construction, shellfishing, mooring chains, propeller damage), turbidity (i.e. topsoil runoff, activities that resuspend sediments), pollution, competition with marine invasive species, and eutrophication as a result of nutrient loading. During aerial surveys with LightHawk, we have documented the destruction of eelgrass habitat in Cape Cod Bay from several of these disturbances.
Mooring Chain Scars
The circular bare patches in the aerial image taken of Provincetown Harbor occur when the heavy chain attached to boat moorings drags around the mooring, denuding the area of eelgrass. The second photograph shows an underwater view of a mooring scar with a mooring chain lying in a bare, sandy area surrounded by eelgrass.
Marine Invasive Species and Nuisance Algae
Several species of algae compete with eelgrass for habitat, light, and nutrients. Tunicates and other encrusting marine invertebrates can also destroy eelgrass habitat. They are often agressive, fast-growing species, and once they become established, will often dominate the area, overgrowing and shading out eelgrass. Some of the invasive species we have documented in Cape Cod Bay include Codium fragile, Botryllus schlosseri, Botrilloides violaceus, Styella clava, and Didemnum sp.
Some methods of commercial shellfishing, such as hydrauic clamming, can destroy eelgrass habitat. Although it is illegal to employ this method of shellfishing in areas colonized by eelgrass, these photographs show evidence that these laws aren’t always obeyed.