Reference: Gehman ALM, McLenaghan NA, Byers JE, Alexander CR, Pennings SC & Alber M (2018) Effects of small-scale armouring and residential development on the salt marsh-upland ecotone. Estuaries and Coasts. 41: S54-S67. (doi: 10.1007/s12237-017-0300-8)
Build It & They Will Come? Not Exactly
Residing along the coastline has tremendous value for many people. Coastal communities liken themselves to the foods, recreation and the views living by the ocean can provide. In fact, humans have been living near the ocean for over a millennium. However, the boundary separating where ocean ends and land begins has always been dynamic. Beaches and coastal marshes have migrated seaward in eras of lower sea level as well as migrated landward in times of rising seas such as the present day.
In the past, it was assumed that shoreline migration could be managed by constructing barriers such as concrete walls. In theory, this plan would enhance the safety and protection of coastal homes, while still allowing a seaside ecosystem to persist. 14% of the shoreline in the continental US is armored. The armoring of shorelines in the US is expected to increase as both sea level and coastal community densities increase in the future.
There is more and more evidence supporting that armoring isn’t an effective strategy to mitigate erosion. Such structural boundaries may reduce the risk of on-site flooding, but waves are then bounced back into the ocean and can disrupt land further down the shoreline. Boundaries may also limit the movement of organisms across natural shorelines, reduce overall biodiversity, and reduce the resiliency of the ecosystems persisting.
Looking out into the Sea
Gehman and colleagues surveyed 20 locations along the Georgia coastline. Each location was selected based on its proximity to land with an armored coastline, an unarmored coastline, or a forested coastline. This allowed the researchers to investigate how human residency may impact the quality of coastal habitats on a spectrum of development. A house that is situated along the coast may have a 1-3 meter wall separating the upland from the marsh (armored), whereas other residential homes did not install a barrier and a more natural progression of coastline to upland development is observed (unarmored). Lastly, a forested site is composed of a salt marsh and a forest ecosystem upland. Forested sites were used to identify any patterns or progressions of coastal habitat associated with human development.
As coastline armors appear to be important when viewed through the lens of the inhabitants, it’s important to study the effects of human development (i.e. housing and physical barriers) on the local habitat. Within each site, along the transition from salt marsh to upland, the researchers studied the local flora, fauna, and physical conditions to examine how small-scale barriers may impact the landscape.
In coastal ecosystems, elevation is an important physical parameter that may influence the surrounding environmental conditions. For example, low elevations are typically flooded more often, resulting in higher levels of salinity in the water and soils. Much of the biota along the coastline is stratified based on its ability to tolerate saline conditions. The researchers determined that elevations of armored sites were significantly lower than unarmored or forested sites. Looking at historical data, the researchers had no reason to believe that these differences in elevation were in existence before the armored-barriers were installed decades ago.
Differences in biological diversity didn’t necessarily persist between site types. In fact, within each site you were bound to observe several grasses (Spartina sp. & Juncus sp.) in addition to snails, Littoraria sp., and several species of crabs. However, the densities of the biota somewhat differed among the sites. For example, unarmored sites had larger areas without vegetation, whereas greater amounts of vegetation may be witnessed in armored and forested sites. Sites with lower elevation tended to be dominated by Spartina more so than at sites with higher elevations. The squareback marsh crab, Armases sp., is a species that mainly occupies the coastal-upland boundary. The density of these crabs was the greatest at the forested sites, followed by the unarmored sites; the fewest were found in the armored sites.
Conclusions and Implications
The authors suggest that the observed low elevations of the armored sites are the result of a blockade of upstream sediment. The lack of upstream sediment being transported to the marsh may limit the total amount of sediment reaching the coastline, preventing the marshes from accumulating soil and remaining low in elevation This has been documented before and is known as a “coastal squeeze”.
Sea level continues to rise, and the southeastern US is expected to experience large development in the near future. The defending of coastal infrastructure is at a crossroads. As shorelines become increasingly dense with people, personal (and policy) decisions will be made whether to move away or stick it out on the coast. One option when staying put is to build physical barriers between residences themselves and the coastline. Research by Gehman and colleagues suggests that these barriers can create subtle changes in both the biotic and abiotic makeup of the coastal-upland boundary.
Currently, the southeastern US has lower population densities along the coast, but these numbers are expected to grow, and only a limited portion of the coastline is armored. The authors suggest this region of the US is a critical area where proactive policy and management strategies could be enacted to prevent deleterious effects observed with further armoring.