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Wetlands

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What are the trends in the extent and condition of wetlands and their effects on human health and the environment?

The United States has many types of wetlands, which include marshes, swamps, bogs, and similar marine, estuarine, or freshwater areas that are periodically saturated or covered by water. Wetlands are an integral part of the landscape because they provide habitat for a diverse array of plants and animals, act as buffers to flooding and erosion, and serve as key links in the global water and biogeochemical cycles.

In terms of extent, wetlands currently cover 5.5 percent of the surface area of the contiguous 48 states, with freshwater wetlands accounting for nearly 95 percent of the current wetland acreage and marine and estuarine wetlands accounting for the remaining 5 percent.13 Condition is somewhat harder to measure, as it reflects a combination of physical, chemical, and biological attributes. To be in healthy condition, however, a wetland should generally demonstrate good water quality and support native plant and animal communities, without the presence of invasive non-indigenous species. A healthy wetland should not show signs of stress related to substantial degradation or cumulative effects of smaller degradations, and should be free of modifications that restrict water flow into, through, or out of the wetland, or that alter patterns of seasonality.

Wetlands can be classified by many different attributes. First, they can be divided by degree of salinity—freshwater, marine, or estuarine. Wetlands also may be classified based on dominant vegetation type. For example, swamps are dominated by trees and shrubs, while marshes are characterized by non-woody, emergent (vertically oriented) plants like grasses and sedges. Other characteristics used to classify wetlands include soil type, water source, and the length of time a given wetland is saturated.

The structure and function of any given wetland will be governed by a combination of interrelated factors, including topography, underlying geology (e.g., mineral composition), the abundance and movement of water (hydrology), and weather and climate. These factors ultimately determine which plant and animal species will thrive in a given wetland.

All wetlands share a few basic physical, chemical, and biological attributes. By definition, all wetlands are saturated or covered by water at least periodically, and wetland vegetation is adapted to these conditions. Thus, wetlands are like sponges, with a natural ability to store water. Wetlands also tend to have highly developed root systems that anchor trees and other vegetation in place. This web of roots not only holds the soil in place, but also filters pollutants out of the water as it flows through.

Because of their physical, chemical, and biological properties, wetlands serve many important environmental functions. They play an important role in improving natural water quality by filtering pollutants. This function is particularly important to human health because it may affect the condition of waters used as a source of drinking water—a topic described in greater detail in Section 3.6. Wetlands also act as a buffer to protect the shoreline from erosion and storm damage. Because of their sponge-like capacity to absorb water, wetlands slow the water’s momentum and erosive potential and reduce flood heights. During dry periods, the “sponge” releases water, which is critical in maintaining the base flow of many surface water systems.

Wetlands are also among the most biologically productive natural ecosystems in the world. Microbial activity in wetlands enriches the water and soil with nutrients. As the interface between terrestrial and aquatic ecological systems, wetlands provide food and habitat for many plant and animal species, including rare and endangered species. Because of these functions, wetlands support a number of human activities, including commercial fishing, shellfishing, and other industries, as well as recreation, education, and aesthetic enjoyment.

In addition, wetlands play a role in global biogeochemical cycles, particularly those driven in part by the microbial processes that occur in wetlands (e.g., the mineralization of sulfur and nitrogen from decaying plants and the methylation of mercury). Plant growth in wetlands provides a “sink” for many chemicals including atmospheric carbon. If a wetland is disturbed or degraded, these cycles can be altered and some of the chemicals may be released.

The extent of wetlands can be affected by a variety of natural stressors, such as erosion, land subsidence, changes in precipitation patterns (e.g., droughts), sea level change, hurricanes, and other types of storms. However, the vast majority of wetland losses and gains over the last few centuries have occurred as a result of human activity.14 For years, people have drained or filled wetlands for agriculture or urban and suburban development, causing habitat loss or fragmentation as well as a decline in many of the other important functions outlined above, such as improving water quality. Conversely, other human activities may increase the extent of wetlands—for example, creating shallow ponds or re-establishing formerly drained or modified wetlands on farmlands.

Wetland extent may influence condition, as wetland loss may result in added stress to remaining wetlands. For example, if fewer wetlands are available to filter pollutants from surface waters, those pollutants could become more concentrated in remaining downgradient wetlands. Wetland loss and fragmentation also lead to decreases in habitat, landscape diversity, and the connectivity among aquatic resources (i.e., fragmented wetlands essentially become isolated wildlife refuges). Thus, stressors that affect extent may ultimately affect condition as well.

Wetland condition also reflects the influence of stressors that affect topography, hydrology, climate, water condition, and biodiversity. For example, human modifications such as pipes and channels can alter the topography, elevation, or hydrology of wetlands, while withdrawal of ground water or upstream surface waters can directly reduce inflow. Natural forces and human activities (e.g., hurricanes, sea level change, and certain agricultural and forestry practices) can also affect wetlands through increased erosion or sedimentation. Pollutants in ground water and fresh surface waters that flow into wetlands may be toxic to plants and animals, and may also accumulate in wetland sediments. In addition, invasive species can alter the composition of wetland communities. Some of the most well-known invasives in the U.S. are wetland species, including plants such as phragmites and purple loosestrife and animals such as the nutria (a South American rodent introduced to the Chesapeake and Gulf states).

Another key stressor to wetlands is conversion from one wetland type to another. Although conversion can occur naturally through plant succession (such as marshes turning into forested wetlands over time), human activities can cause more drastic changes, such as clearing trees from a forested wetland, excavating a marsh to create an open water pond, or introducing certain invasive species (e.g., the nutria, which converts tidal marsh to open water by removing vegetation). Even if wetland extent is not altered, conversion from one type to another has a major ecological impact by altering habitat types and community structure.

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