Definitions and Classification

Wetland Environments
James S. Aber

Table of Contents
Introduction Definitions
Classification Peatland
References

Introduction

What is a wetland, and what distinguishes wetland from other types of environments? One simple way to answer this question is to name different types of wetlands with words adopted from various languages--bog, fen, mangrove, marsh, moor, muskeg, pan, playa, sabkha, swamp, tundra, etc. Many of these names are united under the general term mire nowadays, but that still does not specify the basic characteristics of wetlands.

A definition for what is a wetland often depends who is asking the question and what development or study is proposed for a particular wetland site. The fact that wetlands may dry out from time to time complicates the attempt to define wetlands in a simple fashion. In fact, some wetlands may be dry more often than they are wet. With applications ranging from urban real estate to scientific nature preserve, a great many points of view may be expressed for proper definitions, classification, and management techniques for wetland environments.

Wetland definitions

Many definitions for wetlands have been proposed and utilized over the years--see FW definitions. Among the most widely accepted definitions is that of Cowardin et al. (1979), which was adopted by the U.S. Fish and Wildlife Service.

Land where an excess of water is the dominent factor determining the nature of soil development and the types of animals and plant communities living at the soil surface. It spans a continuum of environments where terrestrial and aquatic systems intergrade.

This definition comprises three aspects--water, soil, and organisms, which are accepted by wetland scientists as the basis for recognizing and describing wetland environments. The three basic components of wetlands are summarized below (Schot 1999, Charman 2002).

  1. Ground water -- Ground water (water table or zone of saturation) is at the surface or within the soil root zone during all or part of the growing season.

    Bird's eye view over Nigula Bog, southwestern Estonia. Notice the abundance of pools that indicate ground water is near the surface throughout the bog. A mineral island (left rear) rises above the bog and supports deciduous forest. Kite aerial photo date 9/01, © J.S. Aber.

  2. Hydric soils -- Soils are characterized by frequent, prolonged saturation and low oxygen content, which lead to anaerobic chemical environments where reduced iron is present.

    Bird's eye view over Cheyenne Bottoms, central Kansas. View of soil exposed in mudflat during late-winter, low-water conditions. A - moist soil of mudflat, B - salty crust on dry mudflat, C - cattail marsh. Cattle tracks crisscross the open mudflat. Kite aerial photo date 3/02, © J.S. Aber

  3. Specialized vegetation -- Plants adapted for growing in standing water or saturated soils, such as moss, sedges, reeds, cattail and horsetail, rice, mangroves, cypress, cranberries, etc.

    Spike rush growing in a small pothole surrounded by prairie grass and forbs. Flint Hills, east-central Kansas. Photo date 6/02, © J.S. Aber
    Horsetails (dark shoots) growing beside a spring-fed stream. Spring Creek, near Cuchara, Sangre de Cristo Mountains, southern Colorado. Photo date 8/02, © J.S. Aber

This triad is the modern approach for wetland definition under many circumstances that include greatly different environments. Notice that water quality is not specified--salinity varies from fresh, to brackish, to marine, to hypersaline. Acidity may span the entire range of naturally occuring pH values. Depth of standing water in pools and hollows is usually too deep to walk through but too shallow to swim in. However, tidal flats are flooded daily, and many wetlands experience deeper floods from time to time. Emergent vegetation ranges from heavily forested swamps to nearly bare playas and mudflats. The inclusion of rice, the world's most important crop, shows that wetlands may be cultural as well as natural features.

Wetlands are present in all climatic and topographic settings around the world. Wetlands are relatively common in tropical and temperate lowlands. Alpine wetlands are also fairly common, as mountains typically receive more precipitation than do adjacent lowlands. Even deserts have wetlands supported by drainage from adjacent mountains, ground water, or infrequent storm runoff. Much of the Arctic region turns to wetland during the brief period of summer melting. The rich diversity of wetland environments requires a flexible definition.

U.S. postage stamp sheet depicting the Pacific Coast rain forest.
Forested wetland showing typical plants, animals, and water.
U.S. postage stamp sheet depicting the Arctic tundra environment.
Lichens and low-growing plants beside a shallow stream channel.

Wetland classification

Given the diversity of wetland environments, many classification schemes have been proposed and utilized over the years. Cowardin et al. (1979) suggested a hiearchy that consists of wetland systems, subsystems, and classes--see
FW classification. At the highest level are five wetland systems (Schot 1999).

  1. Marine -- Open ocean, continental shelf, including beaches, rocky shores, lagoons, and shallow coral reefs. Normal marine salinity to hypersaline water chemistry; minimal influence from rivers or estuaries. Where wave energy is low, mangroves, mudflats or sabkhas may be present.

    High-energy, rocky, marine shoreline at Pacific Grove, California. Photo date 11/02, © J.S. Aber.

  2. Estuarine -- Deepwater tidal habitats with a range of fresh-brackish-marine water chemistry and daily tidal cycles. Salt and brackish marshes, intertidal mudflats, mangrove swamps, bays, sounds, and coastal rivers. Drowned coasts, where supply of river sediment is insufficient to infill estuary basin.

    Red mangroves on the lee side of Key Largo, Florida. These mangroves grow in seawater in Flordia Bay on the protected northwestern side of the Florida Keys. The stilt roots are exposed here at low tide. Photo date 12/75, © J.S. Aber.

  3. Riverine -- Freshwater, perennial streams comprised of the deepwater habitat contained within a channel. This restrictive system excludes floodplains adjacent to the channel as well as habitats with more than 0.5‰ salinity.

    Spring-fed stream at the head of Trinchero Creek, Sangre de Cristo Mountains, southern Colorado. Mounds of brook cress in full flower line the banks of the spring. Photo date 7/03, © J.S. Aber.

  4. Lacustrine -- This system includes inland water bodies that are situated in topographic depressions, lack emergent trees and shrubs, have less than 30% vegetation cover, and occupy at least 20 acres (8 ha). Includes lakes, larger ponds, sloughs, lochs, bayous, etc.

    Reed beds grow in fresh, shallow water on the margin of Endla Lake, east-central Estonia. Bare birch trees occupy mineral soil in the background of this autumn view. Photo date 9/00, © J.S. Aber.

  5. Palustrine -- All non-tidal wetlands that are substantially covered with emergent vegetation--trees, shrubs, moss, etc. Most bogs, swamps, floodplains and marshes fall in this system, which also includes small bodies of open water (< 20 acres), as well as playas, mudflats and salt pans that may be devoid of vegetation much of the time. Water chemistry is normally fresh but may range to brackish and saline in semiarid and arid climates.

    Cattails and alkali bulrush on the margin of a marsh at Stillwater National Wildlife Refuge, Carson Sink, Nevada. Photo date 7/03, © J.S. Aber.

Each of these systems may interface with each other or with dryland and deep ocean habitats, so that an environmental continuum exists across the Earth's surface. Flows of materials and energy occur within wetlands and between wetlands and other habitats; a holistic approach is necessary to investigate, understand, develop, and manage wetlands successfully.

The primary emphasis of this course deals the palustrine system of wetland environments. This system includes several classes that are recognized on the basis of substrate conditions or dominant vegetation cover (Schot 1999). Palustrine wetland classes include rock bottom, unconsolidated bottom, aquatic bed, unconsolidated shore, moss/lichen wetland, emergent wetland, scrub/shrub wetland, and forested wetland. Common wetland terms and types and described in the following table.

Selected terms and types of wetlands. Based primarily on Mitsch and Gosselink (1993).
Bog Peat accumulation usually dominated by moss. Receives only direct precipitation; characterized by acid water, low alkalinity, and low nutrients.
Fen Peat accumulation; may be dominated by sedge, reed, shrub or forest. Receives some surface runoff and/or ground water, which has neutral pH and moderate to high nutrients.
Mire Used mainly in Europe to include any peat-forming wetland (bog or fen).
Marsh Permanently or periodically inundated site characterized by nutrient-rich water. In Europe, must have a mineral substrate and lack peat accumulation.
Playa Shallow, ephemeral ponds or lagoons that experience significant seasonal changes in semi-arid to arid climates. Often have high salinity or may be completely dry.
Slough Widely used term for wetland environment in a channel or series of shallow lakes. Water is stagnant or may flow slowly on a seasonal basis. Synonym--bayou.
Swamp Characterized by forest, shrub, or reed cover (fen). Particularly a forested wetland in North America. Depends on nutrient-rich ground water derived from mineral soils.
Wet meadow Open prairie, grassland or savannah with waterlogged soils but without standing water for most of the year.
Open water Deeper, normally perennial pools within wetlands and shallow portions of lakes and rivers. Typically home to submerged macrophytes.

Peatland classification

The classification scheme outlined above deals mainly with the surficial, two-dimensional aspects of wetlands. As most wetlands exist in topographic depressions, they tend to accumulate sediment through time. Furthermore, wetland soils are typically anaerobic, which often leads to the preservation of organic remains, namely peat. The term peatland denotes those wetlands in which substantial peat accumulation--at least one foot (30 cm)--has taken place (Charman 2002). The peatland substrate is in reality an organic structure built by biological activity. This gives peatland depth--a third dimension that contains a record of past environmental conditions.

Peat is intrinsic to many wetlands around the world. Peat is partly decomposed plant remains that consist of more than 65% organic matter (dry weight). Moss, grass, herbs, shrubs and trees may contribute to the buildup of organic remains, including stems, leaves, flowers, seeds, nuts, cones, roots, bark and wood. Through time, the accumulation of peat creates the substrate, influences ground-water conditions, and modifies surface morphology of the wetland. Several factors are considered important for classification of peatland types (Charman 2002).

From these diverse criteria, the water supply and nutrient status are the most fundamental elements for classifying peatlands. A gradient exists from nutrient-rich, alkaline water (fens) to nutrient-poor, acidic water (bogs), and so peatlands are generally divided in two broad classes (Charman 2002).

  1. Ombrotrophic -- Raised or blanket bogs that receive all water and nutrients from direct precipitation. Neither ground water nor runoff from surrounding land reaches the surface of the bog. Rain and snow provide the water source, and nutrients are derived from whatever blows in--dust, leaves, bird droppings and feathers, spider webs, animal fur, etc. Water chemistry tends to be acidic, and nutrients for plant growth are in short supply. Few plants can survive such extreme conditions, namely Sphagnum (peat moss) and pine.

    Vertical airphoto over Teosaare Bog, a small raised ombrotrophic bog in east-central Estonia. Reddish-brown vegetation is moss (Sphagnum sp.) in hollows and around pools. Dwarf pines grow on hummocks. White survey marker at bottom center of view is one meter square. Kite aerial photo date 9/01, © J.S. Aber.
    Closeup view of blanket bog at Rj˙pnafell, southern Iceland. Moss forms a spongy cushion over volcanic rocks. Swiss army knife for scale. Photo date 8/94, © J.S. Aber.

  2. Minerotrophic -- Fens located in depressions that receive surface runoff and/or ground-water recharge from surrounding mineral-soil sources. Nutrients are more abundant and water is more alkaline--conditions that are suitable for a wide range of plants and which give rise to greater floralistic diversity compared to bogs. The terms oligotrophic and eutropic refer to more nutrient-poor and more alkaline, calcium-rich fens respectively.

    Spring-fed fen, Sangre de Cristo Mountains, near Cuchara, southern Colorado. Oligotrophic conditions prevail, as ground water is derived from silica-rich sandstone bedrock. Note the diversity of vegetation. Photo date 8/02, © J.S. Aber.


References

Return to wetlands syllabus.

© Notice: Wetland Environments is presented for the use and benefit of students enrolled at Emporia State University. Any other use of text, imagery or curriculum materials is prohibited without permission of the instructor, J.S. Aber (2012).