Hydrology and Physiography of Groundwater and Wetland Habitats

Hydrology and Physiography of Groundwater and Wetland Habitats

FIGURE 4.1 (Left) A stream leaving a limestone cave on the south island of New Zealand (photo by W. Dodds). (Right) A cypress wetland (photo courtesy ...

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FIGURE 4.1 (Left) A stream leaving a limestone cave on the south island of New Zealand (photo by W. Dodds). (Right) A cypress wetland (photo courtesy of L. Johnson).

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Hydrology and Physiography of Groundwater and Wetland Habitats Habitats and the Hydrologic Cycle Movement through Soil and Groundwater Wetlands Summary Questions for Thought

Identification of aquatic habitats is generally based on landscape geomorphology and hydrology. The hydrologic cycle describes the movement of water across the land and, in combination with other geological processes, determines many of the physical characteristics of the habitat. This chapter provides some detail on the hydrologic cycle and then discusses the physical geology of two aquatic habitats closely tied to terrestrial habitats: groundwaters and wetlands. Chapters 5 and 6 continue this theme with respect to streams, rivers, and lakes. The reader should be aware of the linkages between the aquatic habitats across the landscape, even though the different habitats are considered in separate chapters. Understanding physiography can provide a starting point for description of the abiotic factors that drive aquatic ecosystems.

HABITATS AND THE HYDR0LOGIC CYCLE The definition of aquatic habitats can be based on geology and the hydrologic cycle, or the way water moves through the environment. Temporal

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4. Hydrology and Physiography of Groundwater and Wetland Habitats

and spatial variations in movement and distribution of water are called hydrodynamics. In order to understand how water moves across the surface of the continents, the links between terrestrial and aquatic ecosystems, as well as the links among different aquatic ecosystems, must be examined. Chapter 1 included a brief description of a global water budget with respect to water availability to humans. This chapter provides a more detailed consideration of the hydrologic cycle and hydrodynamics. Aquatic habitats can be considered at a variety of spatial and temporal scales; the organism or process of interest dictates the scale chosen for study. For example, microbes can be influenced by proximity to a grain of sand, but ecosystem processes dominated by microbes can be altered by position in a watershed. Changes in small-scale microbial communities can occur over periods of hours, but changes in ecosystems can take decades to millions of years. An indication of the range of habitats and scales that can be used as a framework to link hydrodynamics to aquatic ecology is presented in Table 4.1. Basic physical properties of water movement were discussed previously; now I explain how water moves across the landscape. Weather patterns result in widely varied quantities of precipitation around the world (Fig. 4.2). Precipitation, in combination with factors that influence return of water to the atmosphere, dictates how much water enters aquatic systems (Fig. 4.3). Precipitation can either be intercepted by vegetation or fall directly on nonvegetated surfaces. Water can return to the atmosphere by direct evaporation or by transpiration through plants. Tran-

TABLE 4.1 Habitat Classification by Time and Distance Scalesa Habitat

Time range

Distance range

Examples

Microhabitat

1 second–1 year

1 m–1 mm

Fine particles of detritus, sand and clay particles, surfaces of biotic and abiotic solids in the environment

Macrohabitat

1 day–100 years

1 mm–1 km

Riffles and pools in streams, rivers, and underground rivers; logs; macrophyte beds; pebbles; boulders; and position on lakeshore

Local habitat

1 month–1,000 years

1 mm–100 km

Lake, regional aquifer, stream or riffle reach, shallow lake bottom

Watershed

1–106 years

1–10,000 km

Areas feeding small streams to the basins of large rivers, including lakes, aquifers, and streams within boundaries

Landscape

10–107 years

10 –10,000 km

A mosaic of local habitats or watersheds

Continent

10,000–109 years

10,000 km

A composite of large drainage basins and aquifers

a

Note that this classification is only one way to divide a natural continuum.

Habitats and The Hydrologic Cycle

FIGURE 4.2

Global precipitation patterns. Mean monthly precipitation (in millimeters per month) for 1998 (from the Global Precipitation Climatology Center).

spiration and evaporation together are called evapotranspiration. If water is not lost to evapotranspiration, it infiltrates (flows down into) the soil and can be stored as soil moisture (or as ice in polar or high-altitude areas). Water that is not stored percolates down through the soil layers into groundwater, flows across the surface, or flows through the shallow surface soils to streams. The amount of precipitation required to cause runoff is greater when the temperature is high because potential evapotranspiration is high.

FIGURE 4.3

The hydrologic cycle.

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4. Hydrology and Physiography of Groundwater and Wetland Habitats

Low temperatures and rates of evapotranspiration characterize polar regions. Thus, wetlands, lakes, and streams can form with moderate amounts of precipitation. Human activities can alter global patterns of precipitation and the hydrologic cycle in unpredictable ways. As the earth warms in response to increases of greenhouse gasses (primarily CO2), evaporation and precipitation will likely increase worldwide, but variability and distribution will also change. How such changes will influence local weather patterns is uncertain. Because freshwater habitats are influenced greatly by the balance between precipitation and evapotranspiration, predicting the impacts of the greenhouse effect and global change on specific habitats is difficult. The strongest effects will likely occur in areas that are currently arid or where precipitation is equal to or less than potential evapotranspiration (Schaake, 1990).

MOVEMENT THROUGH SOIL AND GROUNDWATER Water can either flow across the surface of soil (sheet flow) or move down into porous soils (infiltration). Sheet flow often ends up directly in stream channels, whereas infiltration can percolate to groundwater. Several regions below the surface of the soil that receive infiltration have been described (Fig. 4.4). The dry or moist sediments below the surface soil layers form the unsaturated zone (also called the vadose zone). The depth of the unsaturated zone can vary from zero (where groundwater reaches surface water) to more than 100 m (in some deserts). The capillary fringe is the area where groundwater is drawn up into the pores or spaces in the sediment by capillary action. This zone is generally 1 m or less above the water table, which is defined as the top of the region where virtually all of the pore space is filled with groundwater. Below the water table is the groundwater habitat. A continuous groundwater system is called an aquifer; I use this definition in the book, but some authors use the term aquifer only for

FIGURE 4.4

Various subsurface habitats.

Movement Through Soil and Groundwater

groundwater reservoirs that are useful to humans. The dynamic zone of transition where both surface water and groundwater influences are found is referred to as the hyporheic zone. This zone forms a transitional habitat (ecotone) where there is a change between groundwater and surface water organisms. A hyporheic zone can be found between groundwater and wetlands, streams, or lakes (Gibert et al., 1994). Much of the water flow from land into the world’s oceans is from rivers. However, some areas, such as the southeastern United States, are characterized by large discharges of groundwater into marine waters (Moore, 1996). Likewise, the ecology of streams (Allan, 1995; Jones and Holmes, 1996; Brunke and Gonser, 1997), wetlands (Mitsch and Gosselink, 1993), and lakes (Hagerthey and Kerfoot, 1998) can be influenced by groundwater (Freckman et al., 1997). Thus, knowledge of groundwater flows and processes is integral to the study of aquatic systems. Soil texture and composition determine how rapidly water percolates into groundwater habitats. Impermeable layers, such as intact layers of shale or granite, do not allow water to flow deeper. In very fine clays or those with large amounts of organic material, the rate of percolation can be very low. In contrast, gravel and sand have relatively rapid water flow (Table 4.2). Infiltration capacity partially determines the proportion of water that flows off the surface and the quantity that enters groundwater or the aquifer. The rate of which water percolates into an aquifer is referred to as the rate of recharge. Infiltration rate can have important practical consequences. For example, groundwaters can be contaminated when sewage sludge is disposed of on cropland if infiltration rates are high enough that contaminants enter groundwater. Thus, infiltration rate is an important aspect of determining sewage application levels (Wilson et al., 1996). Once water enters groundwater, permeability determines the potential rate of flow (hydraulic conductivity) and is variable and dependent on geology. Water will flow slowly in fine sediments and more rapidly where large channels exist (e.g., in limestone aquifers with channels and unconsolidated sediments with large materials such as cobble). Hydraulic conductivity is partially dependent on the Reynolds number (see Chapter 2) because viscosity is high and flow is slow when Reynolds numbers are small (i.e., when sediment particles are small). Darcy’s law can express the rate at which water moves through aquifers. This law states that the flow rate in porous materials increases with increased pressure and decreases with longer flow paths. The law is used to

TABLE 4.2 Representative Particle Sizes and Hydraulic Conductivity of Various Aquifer Materialsa Material

Particle size (mm)

Hydraulic conductivity (m d 1)

Clay Silt Coarse sand Coarse gravel

0.004 0.004–0.062 0.5–1.0 16–32

0.0002 0.08 45 150

a

Data from Bowen (1986).

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4. Hydrology and Physiography of Groundwater and Wetland Habitats

METHOD 4.1. Sampling Subsurface Waters Sampling groundwater and water in the vadose zone is more technically demanding than sampling streams or lakes. To sample water from the vadose zone, lysimeters are used (Fig. 4.6). These samplers have a ceramic cup on the end that absorbs water from the surrounding soil when the lysimeter is placed under vacuum (Wilson, 1990). Wells are generally used to sample groundwaters, but these can cover only a small part of the habitat. Shallow, temporary wells may be installed by hand where the water table is close to the surface and there are unconsolidated sediments. Deeper sampling requires well drilling machinery. When wells are drilled, samples of the pore water can be collected and sediments can be removed from the drilling apparatus. A split-spoon sample is commonly used in such cases, in which the drill bit takes a core in its center as it cuts downward. The drill bit is removed and split, and the core can be analyzed. After a well is drilled, a casing is inserted through the length of the well with slots or screens placed in the region from which water is to be removed (Schalla and Walters, 1990). The outside of the well is then packed with sand fine enough to keep the sediments from the aquifer from entering and plugging the well when water is removed (Fig. 4.6). A problem with the fine packing materials used at the base of wells is that or-

mathematically describe the flow of groundwater and infiltration through the vadose zone (Bowen, 1986). The amount of water that can be held in sediment is given by its porosity, or the volume fraction of pores and/or fractures. Higher flows are often found in higher porosity sediments because more porous materials tend to have more channels through which water can pass. For example, gravel and sand pack with large spaces between the particles for water to flow through. This packing results in large connected channels. Exceptions to this relationship exist; high-porosity sediments may have a low hydraulic conductivity when a large proportion of the pores are dead ends and not involved in flow. An example of this is carbohydrates excreted by microbes. These extracellular products have a high proportion of water and many microscopic pores but allow little if any flow through them because the pores are small and the Reynolds number precludes flow at such high viscosity. The microbial excretions can lower flow through sediments (Battin and Sengschmitt, 1999). Porosity may also not be related directly to flow rates because of uneven distribution of pore sizes and the tortuosity, or average length of the flow path between two points, which varies as a function of type of material (Sahimi, 1995). It can be difficult to determine velocity and direction of groundwater flow. If the elevation of groundwater at one site is lower than at the sec-

Movement Through Soil and Groundwater

ganisms larger than those able to pass though the packing material cannot be sampled. Thus, without specially designed wells (e.g., like those in Fig. 4.6 but without screens or fine packing materials), groundwater ecologists may miss significant components of the groundwater fauna. Material is packed in the hole outside the casing above the slotted portion to form a seal. Otherwise, water could move vertically into the aquifer from the surface or between aquifer layers and contaminate the sample from the desired depth. Bentonite is commonly used for this sealing because it is relatively chemically inert and swells when wetted. Once a well is installed, it must be developed. Developing entails removal of a large volume of water and sediments in the water to ensure that the well flows clearly and supplies water representative of the aquifer. The well must be sampled regularly, with several well volumes removed in each sampling, so water will not become stagnant. During sampling, several volumes of water in the casing must be removed before the actual sample is collected to ensure that the water sampled is from the aquifer outside the well. Various pumps and bailers are available for sampling groundwater. The type of analysis to be performed on the samples collected should be known before selecting the system. For trace metal analysis, pumps without metal parts that may contaminate samples are used. When organic materials are to be analyzed, pumps that do not use oil are essential because the oil may contaminate the water samples.

ond, and the two are hydraulically connected, then it is assumed that water is flowing from the higher to the lower site. The difference in elevation between the two sites is known as hydraulic head. Releasing a tracer at the upper site and monitoring its appearance at the lower site can indicate water velocity directly. Groundwater habitats can be divided into a variety of subhabitats (Fig. 4.4). For example, aquifers can flow through regions of continuous homogeneous substrata (even distribution of permeable substrata such as sand, clay, or gravel) with little obstruction. Other aquifers can occur where many alternate flow pathways exist because heterogeneous distribution of impermeable materials in the subsurface results in variable flow patterns and directions (e.g., aquifers with large rocks embedded in fine sediments or with patches of low-hydraulic-conductivity material interspersed among highhydraulic-conductivity materials). An aquifer between two impermeable layers is confined. Complicated groundwater flow patterns make determining the fate and source of waters difficult. Heterogeneous flow patterns are of concern when considering groundwater contamination problems because such patterns interfere with assessing the extent of the problem and attempts to clean up contaminants. Methods are available for sampling groundwater (Method 4.1), but the number of samples that can be collected is limited relative to those from surface water habitats.

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4. Hydrology and Physiography of Groundwater and Wetland Habitats

FIGURE 4.5 Water moves through groundwater and across the surface of the land in the hydrologic cycle (after Leopold and Davis 1996; drawn by Sarah Blair).

FIGURE 4.6

Some equipment used for sampling soil water and groundwater. (A) A slotted well casing packed with filtering materials allows sediment-free water to be sampled. (B) A vacuum sampler (lysimeter) relies on negative pressure to extract pore water from soil. The lysimeter is put under vacuum for sampling, and then it is put under pressure so the sample flows out of the sampling tube.

Movement Through Soil and Groundwater

Where the groundwater impinges on the surface, a stream, lake, or wetland forms (Fig. 4.5). The hyporheic zone represents the interface through which materials are exchanged between surface and groundwater. This zone may include interstitial water of sediments below lakes and wetlands, gravel bars in rivers, sand below streams, and many other benthic habitats in aquatic systems. As with any habitat classification, the distinction between the hyporheic zone and groundwater is unclear because the zone is transitional and varies over space and time and it depends on whether material transport or habitats of organisms are considered (Gibert et al., 1994). For example, hyporheic zones that are formed by river action can be quite complex because of erosion and deposition that naturally occur in the stream channel (Creuzé des Châtélliers et al., 1994). The importance of hyporheic zones has become apparent to aquatic ecologists (Danielopol et al., 1994; Gounot, 1994; Allan, 1995; Stanley and Jones, 2000), and methods have been developed to quantify their connections with surface water (Harvey and Wagner, 2000). Groundwater is located worldwide, but the depth of the aquifer below the surface and the amount in the aquifer can vary across the landscape. In porous substrata, wells can yield a large amount of water. In some areas, such as those underlain by solid rock, groundwater yields can be very low. Examination of the distribution of large aquifers demonstrates heterogeneity in the types of aquifers found in the United States

FIGURE 4.7

Principal large aquifers used by humans of the conterminous United States. 1, Semiconsolidated sand aquifers; 2, unconsolidated gravel and sand aquifers; 3, basaltic and volcanic rock aquifers; 4, sandstone and carbonate rock aquifers; 5, sandstone aquifers; and 6, carbonate rock aquifers. Sand and gravel aquifers north of the dark line are mainly glacial deposits. Localized aquifers may occur in areas that are not shaded (image courtesy of the U.S. Geological Survey).

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4. Hydrology and Physiography of Groundwater and Wetland Habitats

(Fig. 4.7). Some areas have extensive continuous aquifers (e.g., the lower Mississippi valley and the High Plains) and others have more sparse, localized aquifers (the Rocky Mountain region). Groundwater in many of these Sidebar 4.1. aquifers is being depleted at rates faster than Mining the Ogallala Aquifer the rate of recharge. Perhaps the most famous example of this is the Ogallala or The High Plains or Ogallala Aquifer stretches High Plains Aquifer (Sidebar 4.1). Groundfrom Nebraska to the southern tip of Texas (Fig. water depletion is commonly associated 4.8). The aquifer underlies 450,000 km2 and has with irrigated land worldwide. an estimated thickness of up to 300 m and an One of the major types of groundwater estimated water volume of 4000 km3. The aquifer habitat is found in limestone regions with supplies 30% of all irrigation water in the United rough land surface called karst topographies States (Kromm and White, 1992a). Mean (White et al., 1995). Understanding specifics recharge rate is 1.5 cm per year, and withdrawal of karst aquifer hydrology is important in rates average about 10 times this rate. Precipiassessing the impacts of humans on groundtation to land above the aquifer is less than that waters (Maire and Pomel, 1994). Large required to support the crops that are irrigated channels can form in these habitats because from the aquifer (i.e., potential evapotranspirathe water can dissolve the limestone. If the tion exceeds precipitation). Annual withdrawals water subsides, caves are left (Figs. 4.1 and exceed the total annual discharge of the Col4.5). Pools and streams in limestone caves orado River (Kromm and White, 1992b). Some provide one groundwater habitat in which regions of the aquifer are very thick and can the geological formation allows humans to support withdrawals for decades. In many redirectly interact and sample the subsurface gions the water table has dropped far enough habitat. Hydrology of karst aquifers is very that it is not economically feasible to use the complex, in part because it is difficult to groundwater for irrigation (Kromm and White, predict the pathways of limestone dissolu1992b). Water is being withdrawn at greater tion (Mangin, 1994). than sustainable rates, so the withdrawals can be referred to as "mining" the aquifer. In addition to loss of economic uses, there are ecological impacts as the water table is drawn deeper under ground. Depletion of the groundwater has caused decreased water supply and stream and river flow has disappeared in many regions (Kromm and White, 1992b). For example, the Arkansas River loses water to the aquifer because agricultural activity has lowered the water table, and now it only flows during floods (Fig. 4.9). The loss of flow has negative impacts on migrating waterfowl that use the river and decreases the ability of the river to dilute and remove pollutants. Conserving the remaining water makes good economic and ecological sense. It remains to be seen if more efficient irrigation technology and dryland farming will allow the same level of economic productivity as was made possible in the region during the past few decades by exploiting the High Plains Aquifer for irrigation water.

WETLANDS Wetlands are crucial habitats for many types of plants and animals (e.g., migratory waterfowl) and provide many ecosystem services, including flood control and the improvement of water quality. Wetlands are used to treat wastewater in many places. In addition, wetlands are globally important as natural sources of methane to the atmosphere (see Chapter 12), and this trace gas plays an important role in the regulation of climate (Schlesinger, 1997). Wetland sediments are valuable because they preserve a long-term record of environmental conditions, and sediments in peat bogs are mined for use in gardens (Fig. 4.10). The study of wetlands is relatively new compared to that of lakes because such study falls between the traditional disciplines of limnology and terrestrial ecology.

Wetlands

The extent of the High Plains Aquifer. Zones of depletion 20 m depth are shown in dark (data from the U.S. Geological Survey).

FIGURE 4.8

FIGURE 4.9 Discharge of the Arkansas River as it flows through a region of the High Plains Aquifer that has been utilized heavily for center-pivot irrigation since the 1960s. Note that the logarithmic scale of discharge is modified so that it reads 0.1 m3 s1 when the river is dry. The river downstream of the aquifer has flowed only during periods of flooding since the early 1970s; it was almost never dry prior to that (data from the U.S. Geological Survey).

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4. Hydrology and Physiography of Groundwater and Wetland Habitats

FIGURE 4.10

Peat moss removed from a wetland (photo courtesy of Loretta Johnson).

One of the problems with studying and managing wetlands is defining them. Although distinct delineation is difficult between a wetland and a very shallow pond, or a slow shallow side channel of a stream, the problem of finding a definition for wetland lies more in deciding what is a wetland versus what is terrestrial habitat. These definitions generally depend on the plants that are present (often water-loving plants, called hydrophytes) and soils with characteristics, related to constant inundation (hydric soils), particularly anaerobic conditions.. What is legally considered a wetland has particular importance with respect to the requirements for wetland preservation. Policymakers are realizing the central importance of wetlands as wildlife habitat and key features of ecosystem function. Pressure from environmentalists has been for more inclusive definitions of wetlands. Agriculturists, developers, and others want more freedom to develop and drain both seasonally wet regions and permanent wetlands. Consequently, numerous definitions of wetlands have been developed by scientists, policymakers, and others (Sidebar 4.2). There is no single, indisputable, ecologically sound definition for wetlands because wetland types are very diverse (Sharitz and Batzer, 1999). In many areas of the world, wetlands have been drained, filled in, or considered useless land. In the United States, 70% of the riparian (near rivers) wetlands were lost between 1940 and 1980, and more than half of the prairie potholes (shallow glacial depressions in the northern high plains that form vital habitat for waterfowl) as well as the Florida Ever-

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Wetlands

TABLE 4.3 Conversion and Losses of Various Wetland Types Including Agricultural and Urban Uses from the Mid-1970s to the Mid-1980s in the United Statesa

Wetland type

Amount in mid-1970s

Amount in mid-1980s

Total loss Urban (gain) Agricultural land use

Deep water Other

Conversion to other wetland types

Swamps Marshes Shrubs Ponds Total

223000 98000 63000 22000 406000

209000 99000 62000 25000 396000

14000  1000 1000  3000 11000

200  0  0  0 200

5200 3000 1700  300 200

4000 1500 1000  900 5600

240 150  0  0 390

4360 350 1700 1800 4600

a

Deep water represents conversion to lakes or reservoirs and conversions are placed in the “other” category if they are not to agriculture, urban, or deep water. Values are given in thousands of square kilometers. Positive values indicate a net gain (data from Dahl et al., 1991).

glades have been lost since pre-European times (Mitsch and Gosselink, 1993). This degree of loss is typical for all types of wetlands in the United States. Twenty-two states have lost more than half of their wetlands in the past 200 years (Fig. 4.11). The amount of loss and interconversion among wetland types has been great (2.5% lost over a period of 10 years) and driven primarily by agriculture. The creation of small ponds from wetlands is an important aspect of human activities (Table 4.3). Peatlands are under pressure throughout the world as a source of peat moss for gardening. In Southeast Asia, existing wetlands have been modified or new

56 85 85

52 91

60

50

50

87 67

90

87

56

81 59

72 59 50

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FIGURE 4.11

States that have lost 50% or more of their wetlands since 1780, labeled with percentage lost (data from Dahl et al., 1991).

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TABLE 4.4 Major World Wetland Typesa Type

Description

Tidal salt marsh

A halophytic grassland or Mid- to high latitude, on dwarf brushland on intertidal shores riverine sediments worldwide influenced by tides or other water fluctuations Wetland close enough to Mid- to high latitude, in coast to experience tidal regions with a broad influence but above the coastal plain reach of oceanic saltwater

Tidal freshwater marsh

Distribution

Mangrove

Tropical and subtropical, coastal, forested wetland

25° north to 25° south worldwide

Freshwater marsh

A diverse group of inland wetlands dominated by grasses, sedges, and other emergent hydrophytes; includes important types, such as prairie potholes, playas, and the Everglades

Worldwide

Northern wetland

Bogs and peatlands characterized by low pH and peat accumulation

Cold temperate climates of high humidity, generally in Northern Hemisphere

Deepwater swamp Fresh water most or all of the season, forested

Southeast United States

Riparian wetland

Worldwide

a

Wetland adjacent to rivers

After Mitsch and Gosselink (1993).

Geomorphology

Vegetation

Ecosystem importance

Form where sediment input exceeds land subsidence in regions with gentle slopes

Salt-tolerant grasses and rushes/periphyton

Highly productive, serves as nursery area for many commercially important fish and shellfish

Area with adequate rain or river flow, with a flat gradient near coastline

High plant diversity, including algae, macrophytes, and grasses

Highly productive, many bird species; often close to urban communities and susceptible to human impact Forms in areas protected Halophytic trees, shrubs, and Exports organic matter to from wave action, other plants; generally coastal food chains, including bays, estuaries, sparse understory physically stabilizes leeward sides of islands coastlines; may serve as a and peninsulas nutrient sink Widely varied Reeds such as Typha and Wildlife habitat, can serve as Phragmites; other grasses nutrient sink such as Panicum and Cladium, sedges (e.g., Cyperus and Carex); broad-leaved monocots (Sagittaria spp.); and floating aquatic plants Forms in moist areas where Acidophilic vegetation, Low-productivity system lakes become filled in or particularly mosses, but where bay vegetation also sedges, grasses, and spreads and blankets; often reeds a terrestrial ecosystem Varied Bald cypress (tupelo or pond Can be low nutrient or high cypress), black gum nutrient; can serve as a nutrient sink In floodplains of rivers in High diversity of terrestrial Can provide key wildlife regions with high water plants habitat and productivity, table particularly in more arid regions; can act as an essential nutrient filter

TABLE 4.5 Examples of Expected Ecosystem Functions of Wetlands Based on Hydrodynamic Characterizationa

Primary water source

Climate

Geomorpological aspects

Important quantitative attributes

Functions that can relate to ecological properties

Significance of function or maintenance of characteristic

Precipitation

Humid

Poor drainage

Precipitation exceeds evapotranspiration during most of year so soils waterlogged

Soil constantly waterlogged, leading to peat formation and sediment anoxia

Low plant productivity related to anoxic sediment keeping plants from soil sources of nutrients; plants rely on nutrients in precipitation only

Surface flow from flooding river

Mesic–humid Floods occur at least annually

Frequency and height of floods and position of wetland an index of connectivity to river

Allows continued high production and high habitat heterogeneity

Groundwater influx

Mesic

Aquifer permanence; yield of springs and seeps dominates hydrologic throughput

Overbank flow creates influx of nutrients and moves sediments (changes physical structure) Groundwater supplies nutrients and flushes habitat; habitat often very stable

a

Based on Brinson et al. (1994).

Groundwater springs and seeps often at bottom of slopes or stream margins; some sediments must be permeable to allow influx

High plant production; stable plant community

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4. Hydrology and Physiography of Groundwater and Wetland Habitats

wetlands have been created to allow for rice culture (Grist, 1986), and these rice paddies have fed billions of people over the centuries. The decline in wetlands is global; for example, a large percentage of wetlands have been lost in United States (54%), Cameroon (80%), New Zealand (90%), Italy (94%), Australia (95%), Thailand (96%), and Vietnam (99%). Wetlands are distributed worldwide (Fig. 4.12), with large areal coverage in northern Europe, northern North America, and South America. The processes that form these wetlands vary (Table 4.4). A classification system for wetlands has been proposed to allow assessment of wetland functions (Brinson et al., 1994). The wetlands can be classified by geomorphology, hydrology, climate, nutrient input, and vegetation. (Table 4.5). Four broad geomorphic classifications that can be used are riverine, depressional, Sidebar 4.2. coastal, and peatland. Depressional formaDefinitions of Wetlands tion processes will be described more fully in Chapter 6, and formation of riverine wetSeveral definitions of wetlands have been lands is discussed in Chapter 5. chronicled by Mitsch and Gosselink (1993) and Hydrologic regimes of wetlands can be by the Committee on Characterization of Wethighly variable or fairly constant. Hydrolands (1995). The definition used often depends logic regime forms probably the most imon the requirements of the user. portant abiotic template that influences wetUnited States Fish and Wildlife Service: Wetland ecology (Wissinger, 1999). Important lands are lands transitional between terrestrial characteristics include permanence, preand aquatic systems where the water table is dictability, and seasonality. For example, usually at or near the surface or the land is permanence controls the ability of large covered by shallow water. Wetlands must have aquatic predators to inhabit a wetland. The one or more of the following three attributes: presence or absence of these predators then (i) at least periodically, the land supports pristructures the invertebrate and vertebrate marily hydrophytes; (ii) the substrate is precommunity. dominantly undrained hydric soil; and (iii) the Wetlands can receive any of three substrate is nonsoil and saturated with water sources of water: precipitation, surface waor covered by shallow water at some time. ter, or groundwater. Hydrodynamic characCanadian National Wetlands Working teristics include fluctuations in water level Group: Wetland is defined as land having the and direction of water flow. When rivers water table at, near, or above the land surface flow through wetlands, water moves unidior which is saturated for a long enough period rectionally. Tidal wetlands have bidirecto promote wetland or aquatic processes as tional flow. Hydrologic regimes of riparian indicated by hydric soils, hydrophytic vegetawetlands are characterized by sporadic tion, and various kinds of biological activity flooding with unidirectional flow, followed which are adapted to the wet environment. by extended periods of stagnation. ConserSection 404 of the 1977 United States Clean vation of wetlands clearly requires underWater Act: The term "wetlands" means those standing of hydrology. For example, in riverareas that are inundated or saturated by surine wetlands hydrodynamic characteristics face or groundwater at a frequency and durarelated to links to the river channels and getion sufficient to support, and that under normal omorphology are important components of circumstances do support, a prevalence of vegconservation (Bornette et al., 1998a; Galat etation typically adapted for life in saturated et al., 1998). soil conditions. Wetlands generally include Some wetlands have high hydrologic swamps, marshes, bogs, and similar areas. throughput (minerotrophic) and others are mainly fed by precipitation and have

Wetlands

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low hydrologic throughput (ombrotrophic). Compared with lakes and streams, water loss by plant transpiration is usually more important to the hydrology of wetlands, which tend to be shallow and densely vegetated. Tidal action can influence wetland hydrology where wetlands are contiguous with the ocean, as in coastal estuaries and mangrove swamps. Climate interacts with hydrology to constrain hydrodynamics and the plants found in wetlands. Given the hydrogeomorphic properties, wetlands can then be classified by function (Table 4.5). Those wetlands that are fed by constant flows of groundwater may be influenced little by seasonal factors that control surface water flows. At the other extreme, seasonal wetlands, such as playas or riparian wetlands, can fill during wet seasons and remain dry throughout the rest of the year. 1985 United States Food Security Act: The Wetlands can be further categorized by term "wetland," except when such term is part nutrient input (eutrophic or oligotrophic), of the term "converted wetland," means land salinity, pH, and other water chemistry. Bethat (i) has a predominance of hydric soils; (ii) cause of the variety of hydrogeomorpholgy, is inundated or saturated by surface or climate, and other factors, vegetation can groundwater at a frequency and duration sufrange from dense forest to tundra or from ficient to support a prevalence of hydrophytic macrophytes to trees (Table 4.4). A variety vegetation typically adapted for life in satuof different subhabitats also occur within rated soil conditions; and (iii) under normal cireach wetland, depending on the degree and cumstances does support the prevalence of duration of inundation and the water depth such vegetation. 1995 Committee on Wetlands Characteriza(Fig. 4.13). tion, U.S. National Research Council: A wetWetlands may be strongly influenced by land is an ecosystem that depends on constant global change because the water level in many or recurrent, shallow inundation or saturation wetlands is highly sensitive to changes in rates at or near the surface of the substrate. The of precipitation and evapotranspiration. Such minimum essential characteristics of a wetchanges may directly affect input and output land are recurrent, sustained inundation or satof water, or they may indirectly affect weturation at or near the surface and the presland water levels by altering the height of the ence of physical, chemical, and biological groundwater table. A warmer climate will features reflective of recurrent, sustained inproduce higher evapotranspiration rates, undation or saturation. Common diagnostic which can lower water levels even if precipifeatures of wetlands are hydric soils and hytation rates remain constant. Given the comdrophytic vegetation. These features will be plex and variable hydraulic characteristics present except where specific physicochemiacross the different types of wetlands, the cal, biotic, or anthropogenic factors have remagnitude and direction of the changes in moved them or prevented their development. these habitats are not easy to predict. Some other words historically used to deIn addition to evapotranspiration, biotic lineate wetlands or specific types of wetlands: factors may also influence wetland hydrolThe term wetland has only been in regular use ogy. Human activities have strong influences by scientists since the mid-1900s. Terms used on wetland hydrology. A large-scale examprior to this, or to indicate specific types of ple of this is the effort to manage the Everwetlands, include bog, bottomland, fen, marsh, glades in Florida (Sidebar 4.3). Other factors mire, moor, muskeg, peatland, playa, pothole, that may be important include beavers, allireedswamp, slough, swamp, vernal pool, wet gators, and other large freshwater vertemeadow, and wet prairie. brates. For example, beavers have altered the geomorphology of entire valleys (Naiman

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4. Hydrology and Physiography of Groundwater and Wetland Habitats

FIGURE 4.12

Distribution of wetlands across the world. Lighter regions have a higher density of wetlands (Cogley, 1994).

FIGURE 4.13

Classification of some subhabitats in two wetland types; associated with still open water (A) and associated with a slow-moving stream (B) (from Cowardin et al., 1979).

Summary

et al., 1994). Alligators in the Everglades construct holes that keep the wetland from becoming completely dry during times of low precipitation and serve as refugia for fishes, snails, and turtles.

SUMMARY 1. Freshwater habitats vary in scale from individual sediment particles to continental watersheds. The appropriate scale of investigation depends on the question being asked. 2. Water falls unevenly across the earth and evaporates or is transpired (evapotranspiration) at different rates depending on a variety of factors, including global weather circulation patterns, geography, and landscape level influences. The water that is not lost to evapotranspiration either flows across the surface of the land to streams and rivers or infiltrates the soil to groundwater. 3. The characteristics of the medium between soil and groundwater alter the rate at which water flows into the aquifer. Generally, water flows more slowly through fine-grained sediments. Once water enters an aquifer, the rate at which it moves through is also dependent on slope and the materials that make up the aquifer. Water flows very slowly between the pores of fine-textured sediments such as silts and clays or those with large amounts of organic materials and relatively rapidly in coarse gravel or limestone with large channels and pores. 4. The hydrodynamics of groundwater dictate its use as a water resource, the ecology of a unique biota, and interactions with other aquatic habitats. Efforts to clean up groundwater pollution also depend on knowledge of groundwater and soil characteristics, particularly flow characteristics. 5. Wetlands are distributed worldwide, provide important habitat for wildlife, provide vital ecosystem services (such as flood control and water purification), and are an important source of methane to the atmosphere. The types of wetlands that have been described are extremely variable and generally defined by the length of time they contain water, their vegetation, and the degree of marine influence. The geology of wetlands varies in different parts of the world, and there is no dominant process that leads to wetland formation worldwide. 6. Rice paddies constitute a type of wetland that feeds a large portion of the world’s human population. 7. The hydrology of many wetlands has undergone major changes due to human activity. Many wetlands have been drained and lost, and others are compromised severely. Thus, wetlands are among the most endangered habitats in the United States and throughout the world. 8. The Everglades in Florida provide a good example of the impact people can have on wetlands by altering hydrodynamics. Biota such as beavers and alligators can also alter wetland hydrology.

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4. Hydrology and Physiography of Groundwater and Wetland Habitats

QUESTIONS FOR THOUGHT Sidebar 4.3. Managing Hydrology of the Florida Everglades The Everglades and adjacent Big Cypress Swamp are parts of a large wetland area that covered more than 10,000 km2 of southern Florida prior to massive human modification during the past century (Gleason and Stone, 1994). This area is characterized by slowly flowing freshwater from the Kissimmee–Okeechobee–Everglades watershed. Movement of clean water through the wetlands is an ecosystem characteristic that is required to support the native flora and fauna. Over the years, canals and dikes were used to drain large areas in the watershed for agriculture, development, and supplying water to the Miami metropolitan area. For example, in one decade in the late 1800s, millionaire Hamilton Disston drained 20,235 ha for agriculture. By 1917, four large canals (380 km total length) had been enlarged by the U.S. Army Corps of Engineers. Because of these flood control and drainage practices, agricultural production and population increased dramatically in the region (Light and Dineen, 1994). In 1947, Everglades National Park opened, making official a desire to conserve at least part of the wetland. Today, the South Florida Water Management District in large part controls the drainage systems erected during the past 100 years. The system is extremely complex and includes more than 700 km of canals, nine large pump stations, 18 gated culverts, and 16 spillways (Light and Dineen, 1994). These control structures must be managed to ensure delivery of freshwater for agriculture and Miami drinking water, to control flooding, and to provide enough clean water to maintain the ecological systems of the Everglades National Park. Vari-

1. Why would understanding the hydrodynamics of groundwater be important when an oil spill occurs on land (e.g., leakage of a gasoline or oil storage tank)? 2. Why might understanding the application of Darcy’s law to sediments under a wetland be important when calculating a water budget for a wetland? 3. How could global warming alter

Questions for Thought

precipitation patterns throughout the world and the recharge of groundwater aquifers? 4. Do you know of any local wetlands that are endangered or have been drained in your lifetime? 5. How can temporary wetlands in arid habitats be extremely important to wildlife? 6. Why do extensive wetlands exist in the high Arctic, even though annual precipitation is similar to that in many temperate or tropical deserts?

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ations in climate that need to be considered in this management include dry and wet seasons and extreme weather such as hurricanes (Duever et al., 1994). To further complicate matters, agricultural runoff has had detrimental effects on the native sawgrass, the input of nutrient-enriched water to sensitive areas must be managed (see Chapter 17), and animal communities respond variably to different management approaches (Rader, 1999). Plans and actions to mitigate problems associated with altered hydrology and pollution in the Kissimmee–Okeechobee–Everglades are varied. The largest restoration project (up to the 1990s in North America) involves reversing the effects of channelization in the Kissimmee River. Biological effects of this restoration are discussed in Chapter 20. In other parts of the watershed, land is being purchased and water running off of agricultural areas is being treated to assist with nutrient removal. Nutrient pollution problems and solutions are described in Chapter 17. The tremendous economic stakes (billions of dollars) conflict with preservation of what is left of the natural environment. Given the large tax base of the region, this has led to a situation in which numerous hydrologists, modelers, and aquatic ecologists, among others, are paid by local governments to make decisions that minimize the human impact on the Everglades while attempting to maximize the human benefits. The problems are complex, but if they are not solved in the near future, the Everglades may be lost forever (Harwell, 1998). Although much important scientific information has been generated, still more is needed to provide the basis for rational management decisions about this important system.