Mollusk community recovery in a restoring estuary on lower Cape Cod, Massachusetts, U.S.A.

Thelen, Brett Amy
Rachel Thiet, PhD
Department of Environmental Studies
Historic human-imposed tidal flow restrictions at many Cape Cod estuaries have resulted in a dramatic alteration of estuarine community structure and function. Esat Harbor, a 291-ha coastal lagoon and salt marsh in Truno, MA, was artificially isolated from Cape Cod Bay in 1868. After this diking, salinity in the East harbor system decreased to near freshwater levels, the waters became hypereutrophic with large blooms of nitrogen-fixing cyanobacteria, and estuarine fish and invertebrate populations declined precipitously. Cape Cod National Seashore and cooperating state and local agencies restored partial tidal flow to East Harbor in 2002; since then, East Harbor has experienced substantial increases in salinity, and native flora and fauna have begun to return to the system. The objective of this study was to obtain baseline information on marine mollusk populations recolonizing East Harbor. Using a combination of benthic cores and direct searching, we surveyed 50 plots throughout the estuary in July and August 2005. Sampling was spatially stratified within three areas that varied markedly in salinity and distance to Cape Cod Bay: Moon Pond, the central lagoon, and the northwest cove. We detected 16 mollusk species in East Harbor as a whole; the four most abundant species were Mya arenaria (softshell clam), Littorina spp. (periwinkle), Mytilus edulis (blue mussel), and Mercenaria mercenaria (northern quahog). We found significant differences in species richness and density of these species among the three regions; diversity and density were both highest in Moon Pond, which has a direct connection with water sources of seawater and marine biota, and lowest in the northwest cove, which receives high freshwater discharge. These findings demonstrate the effectiveness of tidal restoration efforts at Cape Cod National Seashore and carry implications for the potential expansion of commercial and recreational shellfishing on lower Cape Cod, which encompasses approximately one-third of all historically tide-restricted marshes in Massachusetts.