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Where Have All the Salt Marshes Gone?By Meredith Haas We don’t know as much as we thought about salt marshes, and we could be killing one of the most productive ecosystems on Earth without even realizing it, according to Mark Bertness, Brown University biology professor. “You hear all about global warming, but salt marsh degradation is what will kill the polar bears,” Bertness said in his presentation on salt marsh organization and dynamics at this year’s National Estuarine Research Reserve meeting in Newport. “Without strong conservation and management, native New England marshes, plus their societal services, could be lost during our lifetime.” It is estimated that salt marshes have a food production that is 20 times higher than the open ocean, according to the Georgia Nature Conservancy. Salt marshes serve as the base of the marine food chain and as nurseries for wildlife critical to fishery stocks. They are also important transition zones between land and water, protecting coastlines from erosion and filtering harmful pollutants from run-off. Destroying these ecosystems reduces food availability in all systems throughout the world and increases pollutant concentrations, which can even be carried to the Arctic by globe-trotting ocean currents. The biggest threat to New England salt marshes, according to Bertness, is shoreline development created by agriculture, road and housing developments, or anything that removes the natural woodland buffers between marshes and uplands. More than 20,000 acres of coastal habitat disappear each year due to shoreline development, according to a report by the Pew Commission in 2002. “New England salt marshes are in more trouble than we thought,” Bertness said. “What you do in your backyard and locally matters.” Culverts and drainage pipes from roads or rail beds disrupt natural flooding cycles, which salt marsh plants and animals rely on, changing the natural landscape into one in which native species cannot survive. The largest impact from shoreline development, however, is the removal of woody vegetation, Bertness said. These areas of woody vegetation, also referred to as buffer zones, prevent erosion and aid marsh systems in filtering harmful terrestrial run-off from roads, fertilized lawns, or agricultural areas. Removing these buffer zones increases the amount of freshwater and pollution from run-off that enter salt marsh systems. The amount of oil equivalent to the Exxon Valdez spill enters coastal waters every eight months due to excess runoff, according the Pew Commission. In response to such physical changes, native vegetation and its natural distribution are disrupted. This may have severe effects on food web relations by altering consumer and vegetation (primary production) dynamics in salt marsh communities, which Bertness said may force ecologists to reevaluate how they look at community ecology. “Much of what we learned as undergrads may be wrong,” he said. According to Bertness, scientists have historically thought that salt marsh systems are structured by physical conditions, or bottom-up factors such as salinity and nutrient levels. Human disturbances through overharvesting and eutrophication, however, are changing marsh systems into systems controlled by top-down factors such as consumer control by insects, crabs, geese, and other species that feed on marsh vegetation. “The old paradigm was that consumers played a small role in the regulation of primary production,” Bertness said. “Now we are seeing that top-down pressures from consumer control are having a greater effect on marsh organization.” Salt marshes are constructed with a vertical zonation, or spatial segregation of vegetation based on plant competition and physical conditions. The distribution of plants in lower marsh zones is influenced more by the physical stresses of frequent exposure to tidal fluctuations, whereas plants in the higher marsh zones are influenced more by competition for nutrients. Eutrophication, however, has relieved the below-ground competition for nutrients, evolving the system to above-ground competition for light. The invasive common reed, Phragmites australis, now dominates New England marshes because it is better adapted to low salinity and high nutrient conditions. Shoreline development accelerates the invasion of Phragmites, which in turn drives most of the native plant community to local extinction. For the past two decades, Bertness has been researching the dynamics and organization of New England salt marsh plant communities in order to implement better management and conservation. His current research, which is funded by Rhode Island Sea Grant, examines 30 different sites throughout the Bay. Experimental fertilized plots were compared to unfertilized control plots to determine if increased nutrients triggered consumer control in marsh systems. Bertness and his colleagues found that Phragmites flourished more readily in fertilized plots than in plots without fertilization. Increases in plant biomass resulting from fertilization increased the rate of herbivory. Rhode Island salt marshes, in particular, are under increasing consumer pressure from insects, such as grasshoppers, leafhoppers, and beetles, as well as from marsh crabs. These animals reproduce and consume in response to increases in marsh vegetation. Marsh vegetation, however, cannot be replenished as quickly as it is consumed, resulting in patches of marsh left bare like mudflats. Bertness is also currently investigating the effects of a relatively unknown nocturnal marsh crab, Sesarma reticulatum, found in southern New England marsh systems. “They’re not even in the common field guide,” he said. This species, according to Bertness, may be largely responsible for leaving barren mudflats in Cape Cod’s salt marsh communities, which was one of the first areas in New England to report marsh dieback in 2002, according to the U.S. National Park Service. “It’s a potential disaster in the beginning stages at our doorstep,” Bertness said. In other research along the East Coast and in northern regions around the Hudson Bay, as well as in Chile and Argentina, Bertness has found that these marsh systems have also experienced increases in herbivory due to nutrient increases and higher consumer densities, resulting in various degrees of marsh dieback. Nearly 50 percent of Rhode Island salt marshes have been lost to human development since the colonial period and, of those remaining, over 90 percent have been heavily affected by human activity, according to Bertness. He feels that a better understanding of the organization and dynamics of natural communities will lead to better conservation of salt marshes. “The damage is already done, but we can manage it properly so we stop the process of marsh degradation,” Bertness said. “It may take decades or centuries for salt marshes to fully recover into their original state.” —Meredith Haas is a 2007 URI Marine Biology and Journalism graduate and served as a Rhode Island Sea Grant Communications Intern. |
Rhode
Island Sea Grant |
Coastal Institute |