Human Importance and Impact

Coastal and estuarine ecosystems are some of the most heavily impacted systems on the Earth [17]. In the United States alone, nearly 40% of the population lives on the coast and this is expected to rise by nearly 10% each decade [18]. Humans have drained, diked, and diverted water away from coastal marshes both for agriculture and for development; it is estimated that 50% of the coastal marshes in the United States are gone [19] and the loss has accelerated recently [20]. In addition to direct human impact, marshes are highly susceptible to climate change via sea-level rise, terrestrial flooding, and extreme disasters in the form of hurricanes and tsunamis. Thus, coastal marshes occur at the nexus that includes important ecosystem services, chronic and extreme climate effects, and the sustainability of human health and welfare. In the following section, some of the important ecosystem services, some of the ongoing human threats, and some of the attempts at restoration of coastal marshes are described.

Ecosystem Services of Coastal Marshes • Human Impacts and Restoration

Salt and brackish marshes provide important and valuable ecosystem services to humans, because they are the result of coastal and terrestrial processes. Barbier et al. [1] reviewed these services in some detail and provided economic values, when available, from the literature. They identified seven main services, namely, raw materials and food, coastal protection, erosion control, water purification, maintenance of fisheries, carbon sequestration, and tourism and recreation. One of the most understudied and likely undervalued services provided by coastal marshes is protection from waves and storm surge caused by large storms and hurricanes. Marshes are able to reduce the velocity, height, and duration of incoming waves by increasing the drag of water across the vegetated surface [21]. The value of this service will surely increase given that more frequent and intense storms are projected to occur with climate change [22].

Salt marshes can also act as natural filters by purifying the water entering estuaries from watersheds [23]. This water often contains high nutrients and pollutants that can cause “dead zones” or extremely low oxygen conditions. When water passes over the vegetation in marshes, it traps sediment, stimulating plant growth and the uptake of nutrients. In some cases, coastal marshes have been used for wastewater treatment, saving significant amounts of money compared to conventional municipal treatment [24]. Finally, because coastal marshes are one of the most productive ecosystems on the Earth, they are also important for carbon sequestration [25], an ecosystem service that is likely to become more important as C02 continues to rise globally [26]. Because marshes have extremely low oxygen soils, dead plant matter does not easily decompose and can be stored in the form of peat for hundreds of years [25].

Human Impacts and Restoration

Despite the variety of ecosystem services provided by salt and brackish marshes, humans have had considerable negative impacts on these systems [19,27,28]. Loss of coastal marshes can be attributed to a number of factors that vary over local, regional, and global scales [28]. At local scales, marshes have been converted for agriculture and development purposes through draining, diking, or diverting water away from the marsh. Some major cities, including Boston, San Francisco, and London, are built on filled or drained wetlands. Another local-scale impact from humans is that of non-native invasions. For example, Spartina spp. have been introduced into estuaries worldwide, transforming mudflats into marshes [29,30]. This reduces habitat for seagrass, oysters, fish, and foraging birds. At regional scales, extraction of groundwater, oil, and gas has caused subsidence, leading to the submergence and erosion of hundreds of square kilometers of salt marsh habitat in the Chesapeake Bay, San Francisco Bay, and Gulf of Mexico. Finally, at global scales, sea-level rise caused by warming temperatures threatens coastal marshes if they are unable to maintain their elevation above sea level through sediment accretion and/ or if brackish marshes become more saline.

Generally, success at restoring coastal marshes once they are damaged is difficult and for the most part, has only been attempted at local scales [31,32]. Restoration of old marshes that were converted for agricultural use typically involves reestablishing the tidal inundation regimes that were diverted or blocked. For example, the removal of dikes allows saltwater to flow back into the fields; killing the vegetation; and reestablishing the connection with estuarine sediment, seeds, and animals. Slowly, coastal marsh vegetation can establish itself and start to build the marsh back to its former elevation. However, the restoration of marshes can be a very slow process, even with the intentional planting of vegetation, because the biological engineering of the habitat requires a certain density of plants that ameliorates the harsh physical conditions and promotes sediment accretion [32]. The establishment of marsh plants as ecosystem engineers in the restoration of marshes is particularly important to provide the foundation for the colonization of other species and thus the promotion of species diversity [33]. For this reason, if marshes are destroyed at very large spatial scales due to subsidence, erosion, or sea-level rise, their restoration will be virtually impossible without the reestablishment of the proper functional dynamics between vegetation and sediment [34].

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