Salt Marsh Ecosystem In Cape Cod Face Many Threats
Alan McStravick for redOrbit.com – Your Universe Online
One of the iconic ecological features of Cape Cod in Massachusetts is the dominant form of coastal wetland in the region called salt marshes. Most typically, a salt marsh can be found behind a barrier beach and within estuarine systems. They have, over the previous 3,000 years, come into existence as a direct result of sea-level rise. Vegetation in the area primarily consists of saltmeadow cordgrass and, in lower marshes, smooth cordgrass.
Animal diversity is fairly wide in this ecosystem, as well. Whether you take note of the northern harrier and the least tern, two rare and protected bird species, or you look at the abundance of crab and reptile life, the salt marshes are home to many species that rely on their continued health. Even humans rely on the marshes, as they act as a form of ecological sponge, filtering pollutants and excessive nutrients from water runoff in the region.
Unfortunately, Cape Cod and its marshes are in distress. And the blame for the distress, it appears, can be very evenly spread between humans and some of the animals that call the marshes home.
To be more specific, the marshes are in a state of recession thanks, it is believed, to a series of drainage ditches that were dug by the Works Progress Administration (WPA) during the economic depression of the 1930´s combined with an excessive amount of recreational fishing in the area. Tyler Coverdale and his colleagues at Brown University have published their findings online in this month´s ESA journal Frontiers in Ecology and the Environment.
The WPA came into the area with the best of intentions when they announced their plans to dig runoff ditches in the region. The purpose of doing so was to help control the growing population of mosquitoes in the area. The WPA drainage ditches, once hardly visible, have expanded to open channels — some nearly 30-40 meters wide — with muddy, exposed edges.
Paired with the now apparent WPA misstep is the overfishing by recreational fishermen. Whether on purpose or unwittingly, many of these sport fishers have removed too many of the predator fish from the ecosystem. By doing so, the fish further down the food chain are allowed to explode, in terms of population size. A ripple effect is felt throughout the ecosystem that has finally resulted in uncomfortable environmental changes for human residents.
One of the lower echelon food chain beneficiaries is the native purple marsh crab (Sesarma reticulatum). And it is S. reticulatum who is posing the most dire problem for the ecosystem now. This crab is known to burrow in the mud along the inner shorelines of the marshes. It is no mystery why they chose to inhabit this area, either. Their diet consists almost exclusively on low marsh cordgrass (Spartina alterniflora). S. alterniflora is a tall and fast-growing form of cordgrass that lines the edges of the marsh.
In addition to being breakfast, lunch and dinner for the purple marsh crab, S. alterniflora is invaluable for the purpose of protecting the ecosystem against the friction of tides and storms. As it disappears, due to the increased population of crab, the region has been watching the soft banks erode right out from under other plants that call these marshes home. The erosion of the soft banks has instigated an expedited retreat of the water lines that are moving further back into the marsh. The disappearance of the protective cordgrass, resultant from the unchecked population increase of the purple marsh crab, has taken a visible toll on the developed areas of the Cape.
The marsh ecosystem is comprised of both high and low marsh regions. The purple marsh crab relies on the low marsh due to its abundance of tidal creek edges. The crabs will not venture into the inner marsh region, despite the fact S. alterniflora has a cousin that resides there. Spartina patens, along with other high marsh plants, is the dominant flora in the inner marsh.
The most stunning aspect of the cordgrass die-off is its tight locality. The affected regions can be located within 3/4 of a mile of healthier, seemingly unaffected marshland. It was the pinpointing of this concentrated die-off that led the Brown University team to zero in on recreational fishing as a possible culprit for the drastic change to the ecosystem.
“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.” In addition to presenting an eyesore to both residents and tourists alike, the retreat and dissolution of this ecosystem is proving to be a substantial loss of a valuable ecological resource.
According to the study authors, marshes present an excellent model system for the observation of the intersection of human impacts that can trigger environmental degradation. They claim this is because these ecosystems have long been an area of exploit by humans over the centuries, if not thousands of years. In more modern study, they are exceptionally easy to monitor from both aerial and satellite images.
“Marshes are one of the most heavily utilized resources worldwide,” said Coverdale. “They are easily accessible, and provide shellfish, fuel, baitfish and opportunities for recreational anglers. A lot of those harvests are probably sustainable.” But he is interested in the tipping points at which use of the marsh becomes unsustainable. The revelation of the slumbering menace of the mosquito ditches raises the prospect of other submerged impacts that may surface under the influence of new, contemporary pressures.
One doesn´t even have to go back a full 100 years to see the stark changes that have occurred, ecologically, to Cape Cod. As use of the land saw a shift from agriculture to tourism, it was the local chamber of commerce that decided to fund an effort to draw off areas of standing water through drainage ditches in the hopes of limiting the mosquito population. Of course, looking back, the program was most probably very ineffective at controlling mosquito-borne disease. But the ditch program was responsible for putting many people back to work, all of whom employed that long fabled New England work ethic. This was evidenced by the nearly 1500 miles of ditches that were dug across the long and low-lying peninsula. To this day, the Cape Cod Mosquito Control Project continues and is funded and operated by the Barnstable County Department of Health and the Environment.
Initially, the ditch program really only registered a very minor impact to the marshes, especially when you compare it to other forms of development that were springing up around the same time. After World War II, Cape Cod was witness to its most rapid period of development, seeing a 3-fold increase in the region´s population between the years of 1940 and 1976. It was in the mid-seventies that an ecological and economic awareness was reached regarding the overall benefit of the marshland. As a result, strict limitations on further development were enacted.
The environmental impact of the ditches to the marsh seemed dwarfed in comparison to the development of roads, houses, restaurants, marinas and other hallmarks of a modern coastal community. However, the WPA remodeling project, which often would relocate native marsh species, in combination with the influx of a human population that enjoyed recreational fishing, the marsh ecosystem began to succumb to pressures, throwing the entire region into ecological imbalance.
In addition to a strong New England work ethic, the Cape Cod residents, according to Coverdale, also possess an exceptional conservation ethic. Current residents are able to remember how the Cape looked just last century, when their parents lived there. To say they are unhappy with the changes may be an understatement. Coverdale says, for this reason, he sees no conflict between ecologists and fishermen.
“People enjoy catching fish today, but they come back year after year. They want to see the fish there tomorrow,” Coverdale said. It is this shared desire of both residents and the Cape´s long-time visitors that will enable for the finding and implementation of a solution. Coverdale offers up the necessity for a catch and release program as the first step toward ensuring fishing the Cape becomes a sustainable activity. Catch and release would also be the most clearly evident way for the local Cape communities to retain their fishing heritage.