INVASIVE GREEN CRAB IMPACTS ON SALT MARSHES IN MAINE - SUDDEN INCREASE IN EROSION POTENTIAL

The Maine coast is experiencing an explosive population growth of the invasive European Green Crab (Carcinus maenus), similar to an outbreak in the mid 1950’s. Green crabs are implicated in the widespread destruction of juvenile clams, eelgrass beds, and possible impacts on mussels and lobsters, with critical consequences for seafood industries as well as ecosystem health. In 2013 we noticed severe dieback of low salt marshes in the Damariscotta River Estuary. In addition, we observed green crab molts in much greater density than we have observed over decades at this marsh. Neckles (2013) is studying eelgrass dieback in southern Maine also attributed to green crabs. Salt-marsh dieback in Cape Cod (e.g., Johnson, 2010; Hight, 2013) is thought to be caused by a different crab, the purple marsh crab (Sesarma reticulatum), which is not (yet) found in the northern Gulf of Maine. We hypothesize that green crabs could be causing the widespread clipping of Spartina alterniflora low-marsh grass, resulting in denudation of the surface and increased surficial erosion. Subsequent discussions with researchers and stakeholders, including at a Maine Sea Grant sponsored summit in December, proved widespread effects are already underway, including a massive population of green crabs burrowing like termites into marshes in the Brunswick region, causing lateral and internal erosion. The cause of the population explosion is unknown: hypotheses include warmer winters and/or a population shift caused by more recent invasions by northern European populations that are more aggressive and cool temperature adapted. Beyond the biological and ecological importance of these environments, salt marshes are critical in the bluff erosion/stability cycle in Maine (Kelley and Hay, 1986; Kelley et al., 1989; Kelley and Dickson, 2000). Rapid dieback and consequent marsh erosion could lead to greater extents of bare bluff exposed to wave and ice erosion, resulting in accelerated land loss.