The objectives of this section are to:
Wetlands are so variable that their appearance and boundaries fluctuate over time (Figure 1 - Example of a Wetland. Photo by R.P. Brooks). These dynamic changes are what makes wetlands difficult to recognize. Yet, these same characteristics are what make wetlands so productive and diverse. Wetlands are unique. They are one of the few habitats that are protected and regulated by state and federal agencies. The following definition of wetlands is used for regulatory and permitting purposes:
"Those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal conditions do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs and similar areas."
This definition states the three important features of wetlands. Look at the three underlined words - these define a wetland.
Water: Hydrology is the driving force in a wetland. The water in a wetland may either be contributed from above or below ground. The hydrologic evidence will necessarily be present at some time during the year, but it may not be obvious. The presence of water at or near the surface, or other indicators, such as flood debris and water stained leaves, clearly indicates that wetlands are present.
Note: Students will observe water levels during every visit.
Soils: Wet or hydric soils develop recognizable characteristics after being saturated for several weeks. A mixture of brown and gray mottles near the surface usually means the water table fluctuates during the year and may support wetland vegetation. Dark gray soils are usually very wet or hydric. Lists of hydric soils have also been developed by county offices of the National Resources Conservation Service.
Vegetation: Wetland plants, called hydrophytes, are the most obvious indicator that one is standing in a wetland area. Numerous field manuals are available to help identify wetland plants. The U.S. Fish and Wildlife Service developed a list of plants that occur in wetlands for various regions of the country. In this list, wetland plants are assigned to several categories depending on how often they are found in wet areas. For example, some species are called obligates because they are almost always found in standing water or saturated soils. Other species might be classified as facultative because they can adapt to either wet or dry conditions. (See the National Wetlands Inventory description of plant indicator status in Appendix 1).Wetland Functions and Values
What do wetlands do? A function is a process. It is something the wetland does. Some functions can be measured or quantified, others are more difficult to measure.
Wetlands are an essential component of the environment. They are purifiers. They perform functions such as improving water quality by transforming chemicals or trapping sediment or preventing erosion. Wetlands perform a useful, free service in cleaning our waters, but these biological systems can be degraded and destroyed to the point where they are no longer able to perform these functions.
Wetlands have social and economic values associated with them. These values are something the wetland provides to humans. Some functions can also be thought of as values, such as flood control. When wetlands are destroyed, flooding is more prevalent and the costs of these floods can reach up to billions of dollars.
Shoreline Stabilization and Erosion Control
Wetlands protect shorelines from the erosive action of waves. The dense root masses of emergent and woody hydrophytes stabilize river banks and shorelines. On banks and beaches where native vegetation has been removed or damaged by livestock grazing, the soil is quickly washed away (Figure 2 - Soil erosion caused by lifestock. Photo by R.P. Brooks). Eroded sediments carried downstream can cloud the water and smother the larvae of trout, bass, frogs, or reduce the penetration of light needed for aquatic plants.Flood Control
Wetlands are important for flood control, acting as buffers during spring thaws and heavy rains. Wetlands adjacent to rivers and streams absorb the excess waters of floods and delay its release. By slowly releasing water, wetland basins and their associated vegetation can reduce the incidence of flooding downstream.
Sediment Trapping and Nutrient Removal
Wetlands trap pollutants and sediment that might otherwise contaminate streams and reservoirs. They can prevent toxins that have been dumped on the ground from seeping into groundwater. Plants are one of the important components of this function. Plants slow down the flow of water allowing silt to settle. Plants are also able to absorb nutrients and toxins. These nutrients are broken down and cycled through the food web as animals consume the plants. Plants use photosynthesis to provide food for themselves. The process supplies oxygen to the ecosystem, and the plants become food for other life forms. This is called "primary productivity".
Wildlife and Fisheries
Wetlands are important as breeding and spawning grounds for waterfowl, fish and amphibians (Figure 3 - Heron. Photo by R.P. Brooks). They provide protective cover and food for migrating birds, muskrat, moose and other wildlife species. Many species of juvenile finfish and shellfish are raised in wetlands before entering lakes and oceans. Aquatic invertebrates, such as snails, clams, crayfish and aquatic insects, are abundant in wetlands. These invertebrates are essential links in the food web that support waterfowl, furbearer and fish populations.
Thus, recreational hunters, trappers, and anglers rely on wetlands, both directly and indirectly, to provide adequate numbers of harvestable species.
Wetlands provide critical habitat for many threatened and endangered species. In Pennsylvania, the American bittern, king rail, Blanding's turtle, and eastern tiger salamander are but a few of the species of special concern that are dependent on wetlands.
Wetland plant communities provide the shelter and food upon which wetland fauna depend (Figure 4 - Wetland Plant. Photo by R.P. Brooks). Wetlands also provide homes for many species of rare plants including the spreading globe flower, whorled pogonia, carnivorous sundews and pitcher plants. In some instances, plant products are removed from wetlands. These include renewable supplies of blueberries and cranberries. Wetlands are also used as sources of timber and peat. The commercial production of berries, timber, or peat will invariably alter the hydrology of the system.
Recreation and Education
Wetlands provide a host of recreational and educational activities. They are living classrooms ready to teach lessons in plant and animal diversity. They are places of scientific research and artistic inspiration. Birders, photographers, musicians, students, hunters and anglers are all people that have benefited from the natural beauty of wetlands.
Types of Wetlands
(adapted from Wetlands and Wildlife, Brooks et al. 1993)
The term wetlands is actually an all-encompassing name for a very diverse and variable group of habitats (Figure 5 - Emergent wetland. Photo by R.P. Brooks). This variety is due to regional and local differences in climate, soils, topography, hydrology and water chemistry. These factors control to a large extent the amount and kind of vegetation in the area. Can you name a type of wetland?
U. S. Fish and Wildlife Classification
In the 1970's the U.S. Fish and Wildlife Service (USFWS) designed a classification scheme to assist in a new effort called the National Wetlands Inventory. This new product, commonly called Cowardin classification (Cowardin et al. 1979), has become the most widely recognized and commonly used detailed classification for wetland ecosystems in the United States (Figure 6). The five major wetland systems recognized are: marine, estuarine, lacustrine, riverine and palustrine.
The marine system includes open ocean and coastlines, and is generally limited to beaches, rocky shores, and deep, saltwater habitats. Estuarine wetlands are coastal brackish areas such as tidal salt marshes, intertidal mudflats and coastal rivers. Lacustrine wetlands include deep freshwater areas such as lakes and reservoirs and the fringing wetlands. The Riverine system includes freshwater rivers, stream channels, and immediately adjacent wetlands. The Palustrine (meaning "marshy") system contains the majority of vegetated freshwater wetlands including marshes, swamps, bogs and wet meadows.
Emergent, Shrub, and Forested Wetlands
Wetlands are further characterized by either the amount of water in the area (nonvegetated wetlands) or the type of dominant plants in the area (vegetated wetlands). Types of nonvegetated wetland include the open water portions of lakes, ponds, rivers and streams. Vegetated wetlands can be broken into three main types: emergent wetlands, shrub wetlands and forested wetlands (Figure 6).
Emergent wetlands, commonly called marshes and meadows, are dominated by herbaceous (nonwoody) plants such as grasses, sedges, and forbs (broad-leaved plants) that "emerge" from the water. Shrub wetlands, commonly called shrub swamps or thickets, are dominated by low, woody plants such as willow, alder, buttonbush and meadowsweet. Forested wetlands, known as wooded swamps or bottomland forests, are dominated by large trees over 6 meters (20 feet) tall. These trees include species such as red and silver maple, willow, pin oak, black ash, slippery elm, eastern hemlock, spruce and tamarack.
Many inland wetlands are a mosaic of several wetland types. Indeed, it is rather common to find nonvegetated and vegetated wetlands adjacent to each other. Emergent wetland communities are often present as fringing wetlands along lakes and other water bodies, or may include clumps of woody plants.
Bogs are a unique mosaic of wetland types found in the northeastern and northwestern corners of Pennsylvania. Most of these originated during the last ice age when small, round "kettle ponds" were formed. Large blocks of ice were left behind as the glaciers retreated northward. Bogs do not have flowing water, but may have open water depending on how far succession has progressed. Plants associated with bogs include sphagnum moss, cranberry, blueberry, pitcher plant, leather-leaf, sheep laurel, black spruce and tamarack (Figure 7 - Example of a Bog. Photo by R.P. Brooks)
Hydrogeomorphic (HGM) classification is a promising new classification method developed by Dr. Mark Brinson from East Carolina University in Greenville, NC. This system classifies wetlands based on three characteristics: geomorphic setting (location of the wetland within the surrounding landscape), water source and transport (precipitation, surface and near-surface water flow and groundwater discharge), and hydrodynamics (direction and strength of water flow within the wetland). These characters are responsible for maintaining many of the functions performed by wetland ecosystems (Brinson 1993). This systems focuses on the abiotic factors that influence such functions as chemistry of water, habitat maintenance, and water storage and transport (Brinson 1993). The Cowardin classification system relies primarily on the structure and composition of the vegetative cover. The HGM approach does not mean to disregard the importance organisms play in the ecosystem, but rather intends to develop a better understanding of the relationship between organisms and the environment (Brinson 1993). Wetland functions are closely tied to HGM class.
Wetlands classified into different HGM classes perform different functions and have different structures; wetlands in the same HGM class should have similar functions. Each wetland is different, but if they are grouped based on functions they perform we can focus on the processes fundamental to the sustained existence of these ecosystems. Developing a method to classify wetlands based on what they do will also lend a greater understanding to the relationship between ecosystem structure and function. Using both the Cowardin and HGM classification systems is very useful in distinguishing among wetland types.
The diagram (Figure 8) shows the subclasses, different types of wetlands, in Pennsylvania. This classification for Pennsylvania wetlands was developed using the HGM approach and a set of 63 reference wetlands by the CWC. The purpose of this classification system is to categorize actual wetland sites based on the idealized characteristics and functions of model wetlands, which are based in part on data collected from reference wetlands (Brooks et al. 1996). The HGM classification can be used to describe a subclass such as headwater floodplain (HF) and then the Cowardin system can further describe the vegetation type of that area. For example, a riverine headwater-floodplain (HF) could be dominated by shrubs (SS for scrub-shrub) or trees (FO for forest) or both (SS/FO).
Riparian depressions are often located at the juncture of hillside slopes and the valley floor, along rivers. The water in these depressions often flows into streams, but the source is more often groundwater than water flooding over the banks of the river or stream. Headwater floodplains are found along small tributaries in the watershed. These wetlands receive most of their water from overland flow during spring runoff and rainfall, and occasionally flooding. Mainstem floodplain wetlands are found along larger streams and rivers, and most of their water comes from overbank flow during floods. Slope systems are found along elevational gradients, some that are very gentle. The water for these systems can either come from a groundwater or surface water source. Figures 9 (Schematic of HGM Subclasses of Wetlands in Pennsylvania) and 10 (Vegetation Profiles of Wetland HGM Subclasses) provide a schematic of the HGM subclasses of wetlands in Pennsylvania.
Wetlands and Watersheds
A watershed is all the land area that contributes runoff to a particular body of water. It is a catch basin that guides all the precipitation and runoff into a specific river system. A watershed is the geographic locus of a water-driven dynamic that affects all living and nonliving things within its boundaries (Aquatic Project WILD 1992). Do you know what watershed you live in?
A watershed is a landscape unit formed around a network of streams (Figure 11 - Example of a Watershed). All the water in a watershed flows out at a single point and into another watershed. Watersheds are also called river basins, catchments or subwatersheds. Watersheds form a natural unit upon which to focus efforts aimed at understanding wetlands protection and restoration. Watersheds are constrained within identifiable boundaries and the primary defining factor, water, is measurable. Along the stream network, headwater and riparian wetlands form important buffers between landscape activities (such as development, agriculture and mining) and in-stream effects (such as acid mine drainage, erosion and runoff).
Determining the boundary of a watershed can be a challenging task and is normally done through the use of topographic maps and field surveys. The boundary of a watershed usually consists of topographic features, such as ridges, which direct the water flow in a certain direction. No surface water will flow across a watershed boundary and all flow within the boundary will drain to the watershed outlet (flow outside the boundary will be associated with a different watershed) (Ward 1995). Remember, water flows downhill!
Everything exists within a watershed, including wetlands. Since wetlands largely influence water quality, the quality of water within a watershed can be drastically affected by the number of wetlands existing within that watershed. To guard against water quality degradation, wetland protection efforts should take into account the larger watershed-scale. This is sometimes referred to as a landscape perspective and is simply a means of looking beyond the boundaries of the wetland of interest.
Perhaps the single most important thing to remember about watersheds is that they are hierarchical systems, connected to other watersheds as they are traced downstream. What affects a watershed in one place eventually affects other sites downstream. Damage often accumulates as water proceeds downstream. Most scientists feel it is far more economical to prevent contaminants from entering water systems than to clean up pollution after it takes place (Aquatic Project WILD 1992).
The internet is a tremendous resource for exploring topics on wetlands and watersheds. Appendix 2 provides a short list of web site addresses to help you get started on exploring different topics.