Depression-era drainage ditches emerge as sleeping threat to Cape Cod salt marshes
Jan24

Depression-era drainage ditches emerge as sleeping threat to Cape Cod salt marshes

Contemporary recreational fishing combines with old WPA project to hasten marsh die-off By Liza Lester, ESA communications officer   CAPE Cod, Massachusetts has a problem. The iconic salt marshes of the famous summer retreat are melting away at the edges, dying back from the most popular recreational areas. The erosion is a consequence of an unexpected synergy between recreational over-fishing and Great Depression-era ditches constructed by Works Progress Administration (WPA) in an effort to control mosquitoes. The cascade of ecological cause and effect is described by Tyler Coverdale and colleagues at Brown University in a paper published online this month in ESA’s journal Frontiers in Ecology and the Environment. “People who live near the marshes complain about the die-off because it’s not nice to look at,” said Coverdale. “Without cordgrass protection you also get really significant erosion, retreating at sometimes over a meter a year.” The die-back is ugly, but it is also a substantial loss of a valuable ecological resource. When fishermen hook too many predatory fishes out of the marsh’s ecosystem, the fishes’ prey go on fruitfully multiplying, unchecked. The reverberations down the food chain can result in uncomfortable environmental changes for human residents. The problem for Cape Cod is the native purple marsh crab (Sesarma reticulatum), which burrows in the mud along the inner shorelines of the marshes, and dines almost exclusively on the tall and fast-growing low marsh cordgrass (Spartina alterniflora) that lines the marsh edges. The tall and sturdy cordgrass is an essential buffer against the friction of tides and storms. Without it, soft banks erode out from under the other plants and the water line retreats farther and farther back into the marsh. The unchecked multitudes of purple marsh crabs have taken a visible toll on the developed areas of the Cape. By 2008, 50 percent of the creek banks in the marsh had worn back. Old drainage ditches have expanded from nearly invisible threads to open channels – some nearly 30-40 meters wide – with muddy, exposed edges. The purple marsh crabs need tidal creek edge habitat to thrive, and do not venture into the inner heart of the marsh, where a shorter cordgrass species (the closely related, but squattier Spartina patens) and other high marsh plants dominate. The old WPA mosquito ditches also fulfill the crabs’ habitat requirements. Once benign, the ditches nucleated dramatic reconstruction of the landscape when fishing severely diminished the numbers of blue crab, striped bass, and smooth dogfish, causing a boom of purple marsh crabs. One of the remarkable features of the cordgrass die-off is its tight locality. Some areas of undeveloped marsh as close as a...

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