Wetland Ecology
Baldomero Prado
Often conjuring images of dank, smelly, mosquito-infested wastelands, upon closer look, wetlands are actually biologically diverse and productive ecosystems. Home to a variety of plant life, including floating pond lilies, cattails, cypress, tamarack, and blue spruce, wetlands support diverse communities of invertebrates, which in turn support a wide variety of birds and other vertebrates. Primary consumers from crustaceans, mollusks, and aquatic insect larvae to muskrats, geese, and deer rely on the abundance of algae, plants, and detritus for food. Wetlands also support a variety of carnivores, including dragonflies, otters, alligators, and osprey. Thus, wetlands of the world maintain biologically diverse communities of ecological and economic value.
Origin
Definition
Citation US Fish and Wildlife Service (USFWS)
...lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water. Wetlands must have one or more of the following three attributes: 1. at least periodically, the land supports predominately hydrophytes; 2. the substrate is predominately undrained hydric soil; and 3. the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season of each year. Cowardin et al. 1979 Ramsar Convention on Wetlands
Areas of marsh, fen, peatland, or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish, or salt including areas of marine water, the depth of which at low tide does not exceed 6 meters. Finlayson & Moser 1991 National Research Council (NRC) The minimum essential characteristics of a wetland are recurrent, sustained inundation or saturation at or near the surface and the presence of physical, chemical, and biological features reflective of recurrent, sustained inundation or saturation. Common diagnostic features of wetlands are hydric soils and hydrophytic vegetation. NRC 1995
Table 1: Excerpts from three wetland definitions distinguishing wetland habitats from other ecosystems
What may seem like a relatively straightforward task, developing a precise definition for wetlands presented some difficulty and resulted in many different definitions (Table 1). Part of the difficulty arises from the diversity of wetland types that exist around the world, from salt or brackish water coastal marshes and mangroves to inland freshwater swamps, peatlands, riparian wetlands, and marshes. Furthermore, as transitional areas, wetlands can possess characteristics of both terrestrial and aquatic ecosystems while also possessing characteristics unique unto themselves. Despite the diversity of wetland types, all wetlands share some common features. To be considered a wetland, an area must have:
Many ecologically and economically important species call wetlands home for at least part of their lives. For instance, commercially important fishes and shellfish, including shrimp, blue crab, oysters, salmon, trout, and seatrout rely on, or are associated with, wetlands. Wetlands are also critical habitat for migratory birds and waterfowl, including ducks, egrets, and geese. In fact, more than one-third of the species listed as threatened or endangered in the United States live solely in wetlands and nearly half use wetlands at some point in their lives (USEPA 1995). As such, many wetlands are often recognized as important conservation or restoration targets.
While covering only 6% of the Earth's surface, wetlands provide a disproportionately high number of ecosystem services, in addition to maintaining biodiversity. For instance, wetlands also mitigate floods, protect coastal areas from storms, improve water quality, recharge groundwater aquifers, serve as sinks, sources, or transformers of materials, and produce food and goods for human use. When evaluating the economic value of these various functions, Costanza et al. (1997) concluded that the economic value provided by wetland ecosystems exceeded that provided by lakes, streams, forests, and grasslands and was second only to that provided by coastal estuaries.
Increasing recognition of the value and importance of wetland ecosystems over the last century led to the creation of laws, regulations, and plans to restore and protect wetlands around the world. In the US, wetlands protection largely falls under the Clean Water Act of 1972, which requires permits for dredging and filling activities in most US wetlands and monitors water quality standards. Initiatives such as the "no-net-loss policy," which was recommended by the National Wetlands Policy Forum in 1988, aim to limit further wetland loss in the US, requiring wetland creation, restoration, or mitigation to offset wetland losses due to human activity. With mitigation, wetlands are created, restored, or enhanced to offset or replace wetland loss due to development. The Ramsar Convention, an international treaty aimed at conserving wetlands, requires member countries to develop national wetland policies, to establish wetland reserves, and to designate one or more wetlands as an area of international importance. All these efforts are designed to protect or conserve wetlands and the ecosystem services they provide.
The movement, distribution, and quality of water is the primary factor influencing wetland structure and function. To be classified as a wetland, the presence of water must contribute to the formation of hydric soils, which are formed under flooded or saturated conditions persisting long enough for the development of anaerobic conditions during the growing season (NRCS 1998). Water conditions in wetlands can vary tremendously with respect to the timing and duration of surface water inundation as well as seasonal patterns of inundation.
In coastal wetlands, tidal influence drives the movement and distribution of water and can range from permanent flooding in subtidal wetlands to less frequent flooding in others, with changes in water level occurring daily or semi-daily. Inland wetlands, which lack daily tidal influences, can also be permanently flooded on one extreme or intermittently flooded on the other extreme, with fluctuations over time often occurring seasonally. It is the balance of water inflows and outflows, or the water budget (Figure 1), as well as the geomorphology and soils that determine the timing, duration, and patterns of flooding in a wetland.
For most wetlands, the sources of inflows (e.g., precipitation, surface flow, groundwater flow, tides) and outflows (e.g., evapotranspiration, surface flow, groundwater flow, tides) change over time. As such, hydrology is rarely stable but fluctuates over time resulting in pulsing hydroperiods. Hydrologic pulses can alter productivity along a flooding gradient by altering the extent of flood subsidies and stresses in a wetland (Figure 2). When subsidies are high but stress is relatively low, pulses can promote productivity by introducing water, sediments, and nutrients while also removing waste materials and toxins.
Flooding can affect the physiochemistry of wetlands in various ways. Water can introduce or remove sediment, salt, nutrients or other materials from wetlands, thereby influencing its soil and water chemistry. Hydrology also influences the structure and function of wetland ecosystems through its influence on species richness, productivity, rates of organic matter accumulation, and nutrient cycling. Hydrology may restrict species richness in areas subject to long-term flooding while enhancing it in areas with variable or pulsing hydroperiods. Similarly, productivity is typically lower in permanently flooded, stagnant wetlands, or in drained wetlands than in slow-flowing or seasonally flooded wetlands (Conner & Day 1982). The anaerobic conditions created under these inundated or flooded conditions often limit decomposition rates, thereby promoting organic matter accumulation in soils, and can alter reduction-oxidation reactions controlling nutrient transformations in wetland soils.
Figure 2: Subsidy-stress model illustrating the relationship between ecosystem productivity and wetland hydrology along a flooding gradientProductivity is low when flood pulses are minimal and water is stagnant, as well as when pulses are frequent and intense. When flood pulses are intermediate in frequency and intensity, productivity is maximized. 2011 Nature Education Reproduced from Odum et al. 1995. All rights reserved.
The inundation or saturation of wetland soils by water leads to the formation of anaerobic conditions as oxygen is depleted faster than it can be replaced by diffusion. The rate of oxygen loss in flooded soils can vary depending on other soil conditions, such as temperature and rates of microbial respiration. In most wetlands, small, oxidized layers of soils may persist on the surface or around the roots of vascular plants, but generally, anaerobic, or reduced, conditions prevail.
The prevalence of anaerobic conditions in wetlands has a tremendous impact on their biogeochemistry, with important implications for carbon, nitrogen, phosphorus, iron, manganese, and sulfur transformations. Wetlands can function as sources, sinks, or transformers of these materials, depending on inflows, outflows, and internal cycling rates. One of the most important biogeochemical cycles in wetlands is the nitrogen cycle, and while the potential transformations are not unique to wetlands, the dominance of anaerobic transformations does set wetlands apart from other ecosystems. One such anaerobic transformation is denitrification, in which nitrate is lost to the atmosphere via conversion to nitrogen gas or nitrous oxide by bacteria (Mitsch & Gosselink 2007). In many wetlands, nutrient availability is dramatically altered by agriculture or other practices that increase nutrient loading, contributing to changes in ecosystem structure and function. Through processes like denitrification and plant uptake, wetlands can help remove some of this excess nitrogen introduced to wetland and aquatic ecosystems.
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