Coastal squeeze of salt marshes refers to any situation in which the coastal margin, which is the area buffering land and sea is “squeezed” between the fixed boundary of the land, and the rising sea level. Numerous studies regarding rising sea level exist, though there is only a select few that directly address the problem of coastal squeeze on tidal marshes. Past 200 years has seen an accelerated and unprecedented loss of natural wetlands due to direct and indirect human activity. A reported 50% or salt marshes are lost or degraded worldwide (MEA 2005, UNEP 2006). Despite ongoing restoration efforts around the world, the overall demands for more housing and associated development activities will lead to more loss of active wetlands. An increasing population, coupled with agricultural development and advances in technology have an ever greater impact on wetlands.
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Humans have been utilizing wetlands and their resources since the onset of organized civilization. Documented use of salt marshes for ecosystem services date back to the Neolithic in some areas (Knotterus 2005), and the origin of civilization is said to be the Mesopotamian tidal marshes, or the Fertile Crescent. (Sanlaville 2002) Nevertheless, wetlands have been undergoing mass degradation worldwide, with losses in the United States beginning with the arrival of early settlers in the late 16th century. Studies show that the United States has lost 54% of its original 87 million hectares of wetland, and that number is continually dwindling. (Tiner 1984) This loss of wetlands can only be expected to get worse with rising sea level which will drown and squeeze out many coastal marshes. Current predictions expect sea level to rise by 40cm by 2080, producing massive impacts on coastal wetland.
Human Use
The enclosure of coastal land, namely salt marshes, has been a way of artificially creating productive land for many centuries. Beginning with the colonization of the United States by Dutch and English settlers in the late 17th century, salt marshes were used by humans for a variety of uses, their primary use being grazing of livestock, and harvesting of salt hay to be used as livestock feed and fodder. Salt marshes during this period of time were often artificially diked, filled, planted and tilled to create an alterable and ideal landscape for agricultural use. This widespread drainage was most prevalent in the southern colonies of the United States. As the industrial revolution began in the 19th century, agriculture began to move across the US via Westward Expansion, and an increasing immigrant population and the need for urban expansion yielded a new population which had little connection to the land. In this time period, salt marshes were increasingly converted to “usable” space, such as housing and industrial factories. (Bromberg- Gedan & Sillman 2009) The public perception of salt marshes shifted from that of a fertile agricultural land, to a “menace to health and life”, a “trackless wasteland” that must be converted to a usable landscape. In this time period, a large portion of the United States’ salt marshes were converted into other types of environments, often urbanized.
It was not until the late 1960’s that the value of salt marshes was recognized again by the United States’ public and governmental population, and these concerns have deepend over time as repeated environmental and economic disasters validate the predictions of the 60’s and 70’s. Scientific perspectives towards wetland science are shifting to contain: a wider recognition of the consequences of wetland degradation, opportunities for wetlands to deliver improvements through integrated development, a focus through the conservation movement, and more recognition of ecosystem services within policy frameworks. (Maltby, 2009) The concept of “wise use” of wetlands, enacted through the Ramsar Convention of 1970, was a major leap forward in the preservation of coastal wetlands, with its regulations and goals still being used in coastal policy today. (Maltby 2009)
Though human society has made leaps and bounds in the preservation of salt marshes, the current rate of loss is estimated at 1-2% per year worldwide. (Butler, 2010) The diked coastal floodplain of the US is about 50,000 km in size, much of which would have been coastal wetlands, and while restoration efforts are in place, it is not enough to counteract the loss worldwide. Models suggest that future coastal wetland loss through sea level rise will reach 5-20% of current wetlands by 2080, while urban development will continue to pressure wetlands. The global biodiversity outlook suggests that this coastal squeeze may cause coastal wetland systems to be reduced to narrow fringes by 2100, or lost entirely. (Figure 1)
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Figure 1: Anticipated future changes to salt marshes as sea level rises. (Titus 1991)
The Importance of salt marshes
“To stand at the edge of the sea, to sense the ebb and flow of the tides, to feel the breath of a mist moving over a great salt marsh, to watch the flight of shore birds that have swept up and down the surf lines of the continents for untold thousands of years, to see the running of the old eels and the young shad to sea, is to have the knowledge of things that are nearly eternal as any earthly life can be.”- Rachel Carson, Under the Sea Wind, 1941
National academy of Sciences defines wetlands as: “ecosystems that depend on constant or recurrent, shallow inundation or saturation at or near the surface of the substrate. The minimum essential characteristics of a wetland are recurrent, sustained inundation or saturated at or near the surface and the presence of physical, chemical, and biological features reflective of recurrent, sustained inundation or saturation. Common diagnostic features of wetlands are hydric soils, and hydrophytic vegetation. These features will not be present where specific physio-chemical, biotic, or anthropogenic factors have removed them or prevented their development.” (Natural Resource Council, 1995) Technically, wetlands can occur in any area in which precipitation is larger than losses from evaporation and drainage, but are dependent on how humans choose to use them. Since the colonization of the United States, wetlands have been steadily decreasing.
Wetland occur over a wide range of environments, from the arctic to the tropics, from coastal areas to secluded intercontinental areas. The total wetland area on earth has been estimated to be approximately 6% of its total land surface at a minimum, as many countries do not have comprehensive inventories of identified wetlands. (Mitsch and Gosselink, 2000) In North America, specifically the USA and Canada, there is an estimated 14.2 million hectares of wetlands (Scott and Jones 1995). The wetlands of the United States span the entire east coast, and are also incredibly extensive along the Gulf of Mexico, but less common on the steeper, rockier Pacific coast. This paper will focus on the salt marshes of the Eastern United States, namely New England. Complex interactions take place within these ecosystems, in which the biotic and abiotic world are fully linked. The interactions that take place within these environments provide the basis for the delivery of goods and services from these ecosystems. The provision of these services, however, is reliant on the maintenance and protection of these ecosystems. Benefits from ecological processes that occur in wetlands are not always obvious, and for this reason, they tend to be ignored by humans when decisions are made to alter wetlands. As stated in The Wetlands Handbook, “Wetland functions are the result of ecological processes that are necessary for the self-maintenance of the ecosystems, and occur without human intervention.” (Maltby 2009)
Wetlands protect and maintain water quality by providing a filter for sediments and excess nutrients, essentially purifying water in connected water resources, such as oceans, lakes, and rivers, which are used by humans for recreational activities, and drinking water. Nutrients, toxins, and sediments enter the wetland environment via runoff, which in urban areas can contain very high levels of toxic materials which could contaminate the water supply, if not for filtration via marshes and wetlands. Scientists have estimated that wetlands may remove between 70% and 90% of the world’s entering nitrogen (Reilly 1991, Gilliam 1994), in addition to the removal of pathogens, toxic metals such as lead and copper, surface water pollutants, and other nutrients such as phosphorus. Salt marshes alone sequester more carbon in their soils than any other temperate biome partially due to the unique microbes that live in these environments, sequestering roughly 771 Billion tons, the same amount that is currently in our atmosphere. In addition to this, they contribute 1% or more to the annual global loss of fixed nitrogen via microbially mediated denitrification. (Schuster & Watson 2007)
The hydrology of a particular wetland environment controls every factor of the ecosystem, including nutrient cycling, biogeochemical processes, species biodiversity, and filtration. (Maltby 2009) Coastal Wetlands are not only an interface between land and sea, but also an interface between groundwater, surface water, and atmospheric moisture. Wetlands process key ecosystem elements such as nitrogen, carbon, and phosphorus, and thus are the basis of ecosystem functioning and balance. It is this balance that maintains the supply of wetland products and service that are valuable to humans and other species alike. However, like many environments, this balance is fragile, and the removal or addition of one key element could alter the way the entire ecosystem functions.
Salt marshes provide many valuable ecosystem services which must be preserved. (Table 1) Salt marshes act as natural filters that purify water entering the estuary (Mitsch and Gosselink 2008). As water passes through marshes, it slows due to friction of grasses. Suspended sediments are then deposited on the marsh surface, facilitating nutrient uptake, and filtering the water. This filtration is very valuable to human drinking water, as displayed In Louisiana, where treatment of wastewater attained capitalized cost savings of $785 to $15,000/acre compared to municipal treatment. (Breaux 1995) Marshes are an important storm buffer, and provide many resources such as fish, sand, gravel, hay, and shellfish to humans.
Table 1 Values of ecosystem services of tidal marshes
Ecosystem service Examples of human benefits
Average value
(Adj. 2007 $a ha−1 year−1)
Disturbance regulation Storm protection and shoreline protection $2824
Waste treatment Nutrient removal and transformation $9565
Habitat/refugia Fish and shrimp nurseries $280
Food production Fishing, hunting, gathering, aquaculture $421
aw materials Fur trapping $136
Recreation Hunting, fishing, birdwatching $1171
TOTAL $14,397
Table 1: Estimated monetary value of ecosystem services marshes provide on an annual basis (Gedan & Bromwell, 2009)
Dollar values were adjusted for inflation from original data, presented in 1994 dollars (Costanza et al. 1997). The adjustment was done with the U.S. Department of Labor Inflation Calculator, which uses the Consumer Price Index to correct values through time. Please see Costanza et al. (1997) for valuation methods and note that this valuation method is not universally accepted by economists, see Bockstael et al. (2000)
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Coastal Squeeze in Marshes
Coastal squeeze, as defined by the Environment Agency of England is “The reduction of intertidal (mean low water spring tide to mean high water spring tide) habitat as a consequence of sea level rise and the action of flood defenses. If sea levels rise without flood defenses in place, the inter-tidal area is able to gradually move inland over time and there is no net loss of habitat. With defenses or other constraints present, the movement inland of the high water line is impeded, but the low waterline moves shoreward, which leads to a loss of the inter-tidal delta.” (Figure 2) (Black and Veatch, 2006) It occurs when landward conversion is not able to take place. Landward conversion takes place when the lower limits of salt marsh habitats are eroded, and the eroded sediments are re-deposited further landwards. This process is often referred to as habitat ‘rollover’.
http://www.eloisegroup.org/themes/climatechange/images/coastal_squeeze.jpg
Figure 2: Illustration of the process of coastal squeeze due to seawalls. (Pontee 2011)
Causes of Coastal Squeeze
There are many different driving factors of coastal squeeze, including sea level rise, waves, storm activity, sediment supply, and sediment mobility. Coastal erosion is the main factor in coastal squeeze, as it would not be able to occur if it were not for sea level rise and the resulting need for a migration of the salt marsh landscape. Natural causes of coastal squeeze are loss of the total wetland area by coastal erosion and inundation, change in forest or beach structure via natural disasters or erosion, migration rather than overall loss, and the accretion of new beach or land. There are also a number of anthropogenic causes of erosion, which tend to be more localized than natural causes. Oftentimes, the erosion is caused by sand and gravel extraction from beaches, the construction of piers or breakwaters, which interrupt sediment transport, and the construction of floodwalls and ditches, which prevent coastal retreat. (Pontee 2011) . Examples of anthropogenic coastal ‘defenses’ include seawalls, which are large concrete structures, bulkheads, which are retaining walls made of wood (not only do they block landward migration, but also often release toxins into the water), and revetment, which is a sloping structure of rocks which decreases the shallow water refuge of an intertidal zone. (Butler 2007) While the width of coastal environments varies natural on an annual, or even a month to month basis, the result of anthropogenic coastal squeeze are typically long term, if restoration actions are not taken.
Relation to Climate Change
Climate change can affect salt marshes in a number of different ways, namely through sea level rise, particularly when sea walls prevent marsh vegetation from moving upward and inland. With predicted rates of sea level rise, coastal accretion may very well fail to keep pace with this accelerated rise if a critical threshold is crossed, and marsh vegetation is drowned. (Kirwan and Guntenspergen 2009) However, sea level rise does not always lead to the loss of marsh areas, because some marshes experience a process called vertical accretion, in which sediments accumulate vertically, helping the marsh to maintain their elevation with the respect to sea-level where the supply of sediment is sufficient. This is only possible, however, in areas in which the sediment is available. In areas where the sediment supply is more limited, marshes are more susceptible to coastal squeeze, which may lead to their eventual drowning. Vertical Accretion is not always good, however, because after a certain height, marshes will no longer be regularly inundated by the tide, and this accretion will form a natural barrier. (Temmerman et al 2004) Rising levels of Co2 may affect salt marsh plants and limit their response to coastal squeeze, and temperature change could potentially alter the geographical distribution of salt marshes in temperate and arctic latitudes. (Chapman 1977) However, temperature may change too quickly for many marshes to migrate and adjust. Recent evidence suggests that hurricane intensity and frequency is also anticipated to increase with climate change, causing higher rates of erosion in tidal marshes, increasing the rate of coastal squeeze. (IPCC 2007)
Case Studies
Abbots hall: Abbots hall Farm is located within the Blackwater Estuary of Essex, England. It consists of farmland, dry grassland, salt tolerant grassland, and existing marsh areas. The main factor in the increasing problem of coastal squeeze in this area was the 3.8km of sea wall along the north bank of the Salcott Channel, a main marsh creek. The saltmarsh was on the seaward side of this wall, thus subjecting it to coastal squeeze by limiting it migration landwards. However, the Coastal Realignment project breached the wall in many areas, so as to provide area for new saltmarsh to form inland. While wall still remains at the ends of the farm as property markers, the resulting intermittent, unrestricted areas of marsh now have the potential to remain healthy and viable in the future. While this is an improvement, the Essex coastline still contains over 400 miles of sea wall, built to enclose saltings to improve grazing. Though these are not expected to be taken down in the near future, the loss of wetlands may prompt a dire need to do just so. (Figure 3)
Figure 3: Rising seas causing a narrowing shoreline on the Blackwater Estuary, Essex (Doody, 2004)
The Gulf coast Region of the United States, which includes vast marshes such as the Florida Everglades, is experiencing some of the highest wetland loss rates in the United States, largely because of human interference. The Everglades region of Florida contains a wide array of wetland environments, including sawgrass prairies, salt marshes, tree islands, and mangrove forests. The Southern Florida Project for Flood Control and Other Purposes of 1948 created many canals, floodgates, and levees to reduce flood risks to agriculture, transportation, and urban development. However, they interfered with natural hydrological processes in the area, and in return, actually reduced their natural capacity to mitigate flooding. (Robert Twilley 2007)
Future Expectations and Conclusions
Global climate change is expected to affect can and atmospheric circulation, sea level rise, the intensity of hurricanes, the magnitude of precipitation, and sea surface and air temperatures. (IPCC 2007) Under normal conditions, salt marshes adjust to these conditions, but under the increased pressures of population rise and urbanization, combined with the ever quickening rate of climate change, salt marshes may not be able to adapt fast enough to the changes occurring around them. The future hydrology of salt marshes will all depend on these factors. In the case of a widespread depletion of salt marshes, we can anticipate the effects to worsen globally.
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