EFFECTS OF INVASIVE GREEN CRABS ON SALT MARSHES IN MAINE

The invasive European Green Crab (Carcinus maenus) exploded in numbers during the 2012-13 oceanographic warming in the Gulf of Maine, resulting in significant predation on juvenile clams and decimation of eelgrass beds on the Maine coast. In 2013 we noted severe dieback caused by widespread clipping of Spartina alterniflora low-marsh grass, denudation of the surface and increased surficial erosion. In addition, lateral burrowing by the crabs into peat scarps enhanced erosion by waves, tides, ice, and marsh-flap collapse. We established stake arrays and Surface Elevation Transects (SETs) in: 1) the Webhannet River Marsh, Wells; 2) Casco Bay; and 3) Damariscotta River Estuary. Overall 8 sites have stake arrays, and 4 coincident sites have SETs. These sites range 118 kilometers along the SW and WC Maine coast, have mean tidal ranges of 2.67 to 2.83 m, similar low wave activity, limited human impact, and significant differences in water temperatures and winter ice formation. The larger project involves direct study of the crabs with traps, and continuous measurements of T and S. Webhannet River marsh sites are on a broad backbarrier marsh >2 m thick, while the Casco Bay and Damariscotta River sites are fringing marshes <1.5 m thick. Stake arrays provide 5 replicate measurements per locality, and were established spring-summer of 2014, and measured 3-5 times before December. Measurements document inconclusive average retreats or advances, in the decimeter range, over this short time span. The stake arrays are intended to be longer-term monitoring systems. However, we documented peat flap and other block failure in photographs. SETs show a pattern of accumulation on the marshes 5 – 18 mm over the summer, and a partial deflation/decay in the first fall. We expect a long-term accumulation of ca. 2 mm/yr, equivalent to local relative sea-level rise. Green crab activity may change those background trends.