Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

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wooded with cypress (Taxodium) and gum/tupelo (Nyssa). The ﬁnal freshwater type is the

riparian wetlands . These are usually forested wetlands along the sides of rivers and

streams. They are inundated on the average every one or two years in the United States,

when the rivers overﬂow their banks.

Wetlands are not just passive products of the hydrological conditions in which they

exist. In yet another example of the Gaia hypothesis at work, wetland plants exert control

over the hydrology. By accumulating peat and trapping sediment, they create land with

very little slope. The vegetation itself further obstructs the ﬂow of water. These factors

help dampen water-level variations. Trees reduce water loss by shading the water or

land surface and by increasing storage. Animals alter the landscape as well. Beavers, nota-

bly, create new wetlands by damming streams. Alligators in the Everglades excavate

‘‘gator holes’’ that provide refuge for all kinds of aquatic organisms during the dry season.

15.3.1 Hydric Soils

The U.S. Department of Agriculture has deﬁned the types of soil unique to wetl ands as

follows: ‘‘A hydric soil is a soil that is saturated, ﬂooded, or ponded long enough during

the growing season to develop anaerobic conditions that favor the growth and regeneration

of hydrophytic vegetation.’’ A hydric soil is considered a mineral soil if its organic matter

content is less than 20 to 35% dry weight (12 to 20% organic carbon content). The exact

percentage used depends on the frequency of saturation and the clay content. Otherwise,

the soil is considered an organic soil (see Table 15.15).

TABLE 15.14 Comparison of Wetland Types and Their Characteristics

Bog Peat-accumulating wetland; no signiﬁcant inputs or outputs; supports

acid-loving plants, especially sphagnum

Bottomland Lowlands along streams and rivers; periodically ﬂooded ﬂoodplains

Fen Peat-accumulating wetland with inputs from surrounding uplands

Peatland Any wetland with an accumulation of peat

Vernal pool Shallow, wet meadow, intermittently ﬂooded in winter and spring, but usually

dry for most of the summer and fall

North American usage

Marsh Wetland that is frequently or continuously inundated and is dominated by

emergent herbaceous vegetation

Muskeg Extensive bogs or peatlands, mainly in Canada and Alaska

Playa Marshlike ponds in the southwestern U.S.

Pothole Marshlike ponds in the northern prairie, many formed by glaciation

Slough Swamp or system of shallow lakes in north or midwestern U.S.; also a slowly

ﬂowing swamp or marsh in the southeastern U.S.

Swamp Wetland dominated by trees or shrubs

Wet meadow Grassland with water surface near but usually not above soil surface

Wet prairie Intermediate between a marsh and a wet meadow

European usage

Marsh Non-peat-accumulating wetland with mineral soil

Mire Any peat-accumulating wetland

Moor Same as peatland

Reedswamp Marsh dominated by Phragmites (common reed), especially in eastern Europe

Swamp Forested fen or reedgrass-dominated wetland

Source: Mitsch and Gosselink (1993).

538 ECOSYSTEMS AND APPLICATIONS

Organic matter gives soil lower density and higher cation-exchange capacity (CEC).

The higher CEC does not translate into high nutrient content , due to the typically

lower pH, which leaches mineral salts. Furthermore, organic complexation reduces the

bioavailability of the nutrients that are present.

Hydric organic soil may be classiﬁed as a muck , in which two-thirds of the organic

material is decomposed (or humiﬁed) and less than one-third of the plant ﬁbers are

identiﬁable as such. If less than one-third of the organic matter is humiﬁed and two-thirds

of the plant ﬁbers are still identiﬁable, the soil is considered a peat.

The molecular diffusivity of oxygen through water is about four orders of magnitude

less than in air. Thus, inundation usually leads to anaerobic soil conditions, except pos-

sibly in a thin surface layer. Oxygen is usually depleted within hours or days of inunda-

tion. Further degradation of organic material can continue, although with reduced

efﬁciency and velocity. Instead of oxygen, alternative electron acceptors are used, such

as nitrate, manganic, ferric, sulfate, or carbonate ions, in order of decreasing metabolic

efﬁciency.

Soil organic matter originates mostly from plant biomass. In most marshes, only a

small portion (10 to 40%) of the plant matter is consumed by herbivores, entering the

grazing food chain. The remainder fuels the detritus food chain. However, anaerobic

conditions in the soil greatly reduces the rate of decomposition, even to the point

where the rate of detritus mineralization is less than its production. Such wetlands are

called peat accumulating. Peat can accumulate to great depths as the marsh builds up

on previous layers of accumulation. Decomposition rates have been measured for

Typha in freshwater marshes. Most show a half-life for degradation ranging from 67 to

365 days, with a median of 198 days.

The shift of equilibrium toward more reduced conditions can be measured by a para-

meter called the redox potential. It is analogous to the pH scale for acid–base equilibrium

and is measured in a similar way using an electrode, usually connected to a standard pH

meter. Redox potential is often measured in millivolts. When oxygen is present, the redox

potential is around þ400 to þ700 mV. Under extreme reducing conditions it drops as low

TABLE 15.15 Properties of Mineral and Organic Soils in Wetlands

Property Hydric Mineral Soil Hydric Organic Soil

Organic matter, percent dry weight Less than 20 to 35% More than 20 to 35%

Organic carbon, percent dry weight Less than 12 to 20% More than 12 to 20%

pH Approximately neutral Acidic

Bulk density (g/cm

) 1.0–2.0 Typically 0.2 to 0.3, although

sphagnum moss peat can be

as low as 0.04

Porosity Low (45–55%) High (about 80%)

Hydraulic conductivity High Low to high

Water-holding capacity Low High

Nutrient availability High Low

Cation-exchange capacity Low, consisting of High, but consisting mostly of

major cations hydrogen ions

Example wetlands Riparian forest, some Peat-accumulating wetlands

marshes

Source: Based on Mitsch and Gosselink (1993).

WETLANDS

539

as 400 mV. Table 15.16 shows the redox potentials associated with redox transforma-

tions. For example, the table shows that iron will tend to be in the ferric form when the

redox potential is above about 120, and in the ferrous form below that value. The exact

redox potential for each transformation depends on the pH.

The sequence of redox reactions shows that the products of the most extreme reducing

conditions are the gases hydrog en sulﬁde and methane. The latter forms swamp gas

or marsh gas, which is formed only when the other electron acceptors are depleted.

Methane production rates were measured as high as 440 mg C/m

 dayas an annual

average in a tidal freshwater marsh in Lousisiana. A swamp in Michigan had an annual

production of 100 mg C/m

 day. Rice paddies in various locations were measured at 36

to 385 mg C/m

 day  Salt marshes produce less methane, possibly because the seawater

provides sulfate, which is a preferential electron acce ptor over carbon dioxide.

Hydric mineral soils that are ﬂooded all or most of the time form layers of bluish-gray

reduced iron called gleys. If the ﬂoodin g is intermittent, red or black spots called mottles

form in the gley. Mottles are made of oxidized iron or manganese. Gleys and mottles are

formed by microorganisms that require organic matter and anaerobic conditions.

Ferrous iron is very soluble and may diffuse to the roots of plants, where it is oxidized

by oxygen supplied by the plant. This results in a coating of ferric hydroxide on the roots,

which may limit nutrient uptake. If the dissolved ferrous iron seeps into an aerobic envir-

onment such as a pond, iron bacteria may oxidize it. It may then precipitate and form into con-

centrated deposits. This is the basis of the ‘‘bog-iron’’ ore deposits that are mined today.

15.3.2 Hydrophytic Vegetation

Living things can often be used as environmental indicators, due to their function as inte-

grators of environmental conditions. That is, they respond to conditions over a period

time rather than just reﬂecting current conditions. That is why wetland vegetation is an

important clue to identifying a wetland. Wetlands are not always wet, and wetland soil is

not always readily identiﬁable as a hydric soil.

Hydrophytic vegetation are macrophytic plants that grow in areas where soil satura-

tion or inundation occurs with a frequency and duration sufﬁcient to exert a controlling

inﬂuence on the plant species present. They are distingui shed from upland plants by hav-

ing one or more adaptations to wetlands conditions. The adaptations have been classiﬁed

as morphologic, physiologic, or reproductive.

Morphologic adaptations are changes in structure (see Table 15.17). Most are

responses to anaerobic soil conditions. One of the most important of these is the formation

TABLE 15.16 Redox Couples and Corresponding Redox Potentials

Approximate Redox

Potential for the

Element Oxidized Form Reduced Form Transformation (mV)

Oxygen O

O 400 to 700

Nitrogen NO



(nitrate), NO



(nitrite) N

O, N

,NH

250

Manganese Mn

4þ

(manganic) Mn

2þ

(manganous) 225

Iron Fe

3þ

(ferric) Fe

2þ

(ferrous) 120

Sulfur SO

2

(sulfate) S

2

(sulﬁde) 75 to 150

Carbon CO

250 to 350

Source: Based on Mitsch and Gosselink (1993).

540 ECOSYSTEMS AND APPLICATIONS

TABLE 15.17 Morphologic Adaptations to Wetlands Conditions

Buttressed tree

trunks

Tree species (e.g., Taxodium distichum) may develop enlarged trunks in

response to frequent inundation. This adaptation is a strong indicator of

hydrophytic vegetation in nontropical forested areas.

Pneumatophores These modiﬁed roots may serve as respiratory organs in species subjected to

frequent inundation or soil saturation. Cypress knees are a classic example,

but other species (e.g., Nyssa aquatica, Rhizophora mangle) may also develop

pneumatophores.

Adventitious roots Sometimes referred to as ‘‘water roots,’’ adventitious roots occur on plant stems

in positions where roots normally are not found. Small ﬁbrous roots

protruding from the base of trees (e.g., Salix nigra) or roots on stems of

herbaceous plants and tree seedlings in positions immediately above the soil

surface (e.g., Ludwigia spp.) occur in response to inundation or soil

saturation. These usually develop during periods of sufﬁciently prolonged

soil saturation to destroy most of the root system. (Caution: Not all

adventitious roots develop as a result of inundation or soil saturation. For

example, aerial roots on woody vines are not normally produced as a response

to inundation or soil saturation.)

Shallow root

systems

When soils are inundated or saturated for long periods during the growing

season, anaerobic conditions develop in the zone of root growth. Most species

with deep root systems cannot survive in such conditions. Most species

capable of growth during periods when soils are oxygenated only near the

surface have shallow root systems. In forested wetlands, wind-thrown trees

are often indicative of shallow root systems.

Inﬂated leaves,

stems, or roots

Many hydrophytic species, particularly herbs (e.g., Limnobium spongia,

Ludwigia spp.) have or develop spongy (aerenchymous tissues in leaves,

stems, and/or roots that provide buoyancy or support and serve as a reservoir

or passageway for oxygen needed for metabolic processes.

Polymorphic

leaves

Some herbaceous species produce different types of leaves, depending on the

water level at the time of leaf formation: for example, Alisma spp. Produce

strap-shaped leaves when totally submerged, but produce broader, ﬂoating

leaves when plants are emergent. (Caution: Many upland species also produce

polymorphic leaves.)

Floating leaves Some species (e.g., Nymphaea spp.) produce leaves uniquely adapted for

ﬂoating on a water surface. These leaves have stomata primarily on the upper

surface and a thick waxy cuticle that restricts water penetration. The presence

of species with ﬂoating leav es is strongly indicativ e of hydrophytic vegetation.

Floating stems A number of species (e.g., Alternathera philoxeroides) produce matted stems

that have large internal airspaces when occurring in inundated areas. Such

species root in shallow water and grow across the water surface into deeper

areas. Species with ﬂoating stems often produce adventitious roots at leaf nodes.

Hypertrophied

lenticels

Some plant species (e.g., Gleditsia aquatica) produce enlarged lenticels on the

stem in response to prolonged inundation or soil saturation. These are thought

to increase oxygen uptake through the stem during such periods.

Multitrunks or

stooling

Some woody hydrophytes characteristically produce several trunks of different

ages or produce new stems arising from the base of a senescing individual

(e.g., Forestiera acuminata, Nyssa ogechee) in response to inundation.

Oxygen pathway

to roots

Some species (e.g., Spartina alterniﬂora) have a specialized cellular arrange-

ment that facilitates diffusion of gaseous oxygen from leaves and stems to the

root system.

Source: U.S. Army Corps of Engineers (1987).

WETLANDS

541

of aerenchyma, which are airspaces within stem, leaf, and root tissues that provide a dif-

fusional route of oxygen transfer from the surface to the roots. They are formed by cell

separation during maturation or by breakdown of existing cells. The porosity of wetlands

plants can be up to 60% vs. 2 to 7% in normal plants. A plant will increase its porosity in

response to ﬂooding.

Some trees produce pneumatophores,orair roots, which protrude above the ground

some distance from the trunk of the tree and are thought to promote gas exchange with the

atmosphere. An example are the ‘‘cypress knees’’ found in southern U.S. swamps.

Adventitious roots are those that come from unusual locations, such as leaf nodes or

in a circle around the base of the tree. The red mangrove (Rhisophora spp.) form arched

adventitious prop roots. Adventitious roots and pneumatophores may have small pores

called lenticels that provide a pathway for oxygen to reach the roots.

Some of the oxygen that plants transport to their roots affects the soil surrounding

them. In a process called rhizosphere oxygenation, plants create a microaerobic envir-

onment around their roots in an otherwise anaerobic soil. Spartina alt erniﬂora forms

brown deposits around its roots because of precipitation of iron and manganese when

they are oxidized. In the vicinity of mangrove prop roots or pneumatophores, the redox

potential was higher and the sulﬁde concentration was three to ﬁve times lower than in

mud deposits at a greater distance. Thus, the role of oxygen transport to the roots seems

not only to provide metabolic oxygen but also to protect the roots from toxic minerals.

Examples of plants with morphological adaptations are given in Table 15.18.

Physiological adaptations to anaerobic soil conditions are not readily identiﬁable in

the ﬁeld. However, they further illustrate the differences between wetlands and other

plants (Table 15.19) . Reproductive adaptations increase the chance of a plant becoming

established in a wetlands area (see Table 15.20).

Salt marsh plants also need adaptations to survive in salt water. Exposure to salt water

can be harmful both because of toxicity from salts such as sodium and because osmotic

forces can cause lethal water loss. One protective mechanism is for their cell membranes

to form a selectively permeable barrier, similar to an ultraﬁlter. The sap of some plants can

have salt contents on the order of 3% of seawater salinity. However, no cell membr ane is

perfectly selective. Thus, the cells also selectively excrete harmful salts, especially

sodium. Many salt marsh grasses, such as Spartina, act ually form salt crystals on their

leaf surfaces from these excretions. The cell prevents water loss by maintaining enough

internal solutes in the form of potassium and organics. Although it has not been documen-

ted, it is thought that tidal freshwater marshes are even more productive than tidal salt

marshes, because they receive the same nutrient and energy subsidies but do not have

the salt stresses to deal with.

Salt marsh plants are more likel y to use the C

photosynthesis pathway than upland

plants. Recall that C

plants use CO

more efﬁciently than the more common C

plants.

This allows them to keep their stomates closed more, with the result that they lose less

water by evapotranspiration. The C

adaption helps plants survive arid conditions. The

water potential of saline water can be as low as the soil in dry climates; despite the

large amount of water present, it can be just as unavailable as in a desert. The C

mechan-

ism uses malate to store CO

. Thus, malate is involved in both respiration and photosynth-

esis in wetland plants, and its use is another indicator of hydrophytic vegetation.

Examples of C

wetlands plants are Spartina alterniﬂora, S. townsendii, S. foliosa,

Cyperus rotundus, Echinochloa crusgalli, Panicum dichotomiﬂoru, P. virgatum, Paspalum

distichum, Phragmites communis, and Sporobolus cryptandrus.

542 ECOSYSTEMS AND APPLICATIONS

15.3.3 Wetlands Animals

Animals exhibit specialized adaptations for wetlands, as well. Many benthic invertebrates,

such as some nematodes, crabs, and clams, have either higher concentrations of oxygen-

transporting pigments, or pigments with higher oxygen afﬁnity than usual. Some organ-

isms are effective at conserving oxygen. Fiddler crabs (Uca spp.) can maintain their

resting respiration rates at oxygen levels down to 5 to 15% of saturation. At lower oxygen

TABLE 15.18 Partial List of Species with Known Morphological Adaptations for Wetlands

Conditions

Species Common Name Adaptation

Acer negundo Box elder Adventitious roots

Acer rubrum Red maple Hypertrophied lenticels

Acer saccharinum Silver maple Hypertrophied lenticels; adventitious roots

(juvenile plants)

Alisma spp. Water plantain Polymorphic leaves

Alternanthera philoxeroides Alligatorweed Adventitious roots; inﬂated, ﬂoating stems

Avicennia nitida Black mangrove Pneumatophores; hypertrophied lenticels

Brasenia schreberi Watershield Inﬂated, ﬂoating leaves

Cladium mariscoides Twig rush Inﬂated stems

Cyperus spp. (most species) Flat sedge Inﬂated stems and leaves

Eleocharis spp. (most species) Spikerush Inﬂated stems and leaves

Forestiera acuminata Swamp privet Multitrunk, stooling

Fraxinus pennsylvanica Green ash Buttressed trunks; adventitious roots

Gleditsia aquatica Water locust Hypertrophied lenticels

Juncus spp. Rush Inﬂated stems and leaves

Limnobium spongia Frogbit Inﬂated, ﬂoating leaves

Ludwigia spp. Waterprimrose Adventitious roots; inﬂated ﬂoating stems

Menyanthes trifoliata Buckbean Inﬂated stems (rhizome)

Myrica gale Sweetgale Hypertrophied lenticels

Nelumbo spp. Lotus Floating leaves

Nuphar spp. Cowlily Floating leaves

Nymphaea spp. Waterlily Floating leaves

Nyssa aquatica Water tupelo Buttressed trunks; pneumatophores;

adventitious roots

Nyssa ogechee Ogechee tupelo Buttressed trunks; multitrunk; stooling

Nyssa sylvatica var . biﬂora Swamp blackgum Buttressed trunks

Platanus occidentalis Sycamore Adventitious roots

Populus deltoides Cottonwood Adventitious roots

Quercus laurifolia Laurel oak Shallow root system

Quercus palustris Pin oak Adventitious roots

Rhizophora mangle Red mangrove Pneumatophores

Sagittaria spp. Arrowhead Polymorphic leaves

Salix spp. Willow Hypertrophied lenticels; adventitious roots;

oxygen pathway to roots

Scirpus spp. Bulrush Inﬂated stems and leaves

Spartina alterniﬂora Smooth cordgrass Oxygen pathway to roots

Taxodium distichum Bald cypress Buttressed trunks; pneumatophores

Source: U.S. Army Corps of Engineers (1987).

WETLANDS

543

levels (down to 2% of saturation) they can further reduce their respiration to survive.

Clams can respire anaerobically for a time with their shells closed.

The simpler animal s are osmoconformers; they adjust their cell osmolarity in response

to the external salinity. More complex animals are osmoregulators, which maintai n their

internal environment in the face of external changes. The difference depends on the ability

of the organism to excrete salt. In crabs it also depends on the permeability of the shell to

salt. Crabs such as Cancer are always submerged; they have a more permeable shell and

are an osmoconformer. Crabs that spend part of their time out of the water, such as Uca

spp., are excellent osmoregulators. Their shel ls are relatively impermeable to salt. Other

species fall in between.

The annelids are divided into two groups. The polychaetes are osmoconformers. Eggs

and sperm are released into the water, where fertilization occurs. Oligochaetes and

TABLE 15.19 Physiological Adaptations to Wetlands Conditions

Accumulation of malate Nonwetland species concentrate ethanol, a toxic by-product of

anaerobic respiration, when growing in anaerobic soil conditions.

Under such conditions, many hydrophytic species produce high

concentrations of the nontoxic metabolite malate instead, and

unchanged concentration of ethanol, thereby avoiding accumulation

of toxic materials (e.g., Glyceria maxima, Nyssa sylvatica var.

Biﬂora).

Increased levels of

nitrate reductase

Nitrate reductase is an enzyme involved in conversion of nitrate

nitrogen to nitrite nitrogen, an intermediate step in ammonium

production. Nitrate ions can accept electrons as a replacement for

gaseous oxygen in some species, thereby allowing continued

functioning of metabolic processes under low soil oxygen

conditions. Species that produce high levels of nitrate reductase

include Larix larcina.

Slight increases in

metabolic rates

Anaerobic soil conditions effect short-term increases in metabolic

rates in most species. However, the rate of metabolism often

increases only slightly in wetland species. Examples species

include Larix laricina and Sencio vulgaris.

Rhizosphere oxidation Some hydrophytic species (e.g., Nyssa aquatica , Myrica gale) are

capable of transferring gaseous oxygen from the root system into

soil pores immediately surrounding the roots. This prevents root

deterioration and maintains the rates of water and nutrient

absorption under anaerobic soil conditions.

Ability for root growth

in low oxygen tensions

Some species (e.g., Typha angustifolia, Juncus effusus) have the

ability to maintain root growth under soil oxygen concentrations as

low as 0.5%. Although prolonged (>1 year) exposure to soil oxygen

concentrations lower than 0.5% generally results in the death of

most individuals, this adaptation enables some species to survive

extended periods of anaerobic soil conditions.

Absence of alcohol

dehydrogenase (ADH)

activity

ADH is an enzyme associated with increased ethanol production.

When the enzyme is not functioning, ethanol production does not

increase signiﬁcantly. Some hydrophytic species (e.g., Potentilla

anserina, Polygonum amphibium) show only slight increases in

ADH activity under anaerobic soil conditions.

Source: U.S. Army Corps of Engineers (1987).

544 ECOSYSTEMS AND APPLICATIONS

leeches, on the other hand, are osmoregulators. Fert ilization is internal, and the resulting

zygote is then enclosed in a cocoon.

15.3.4 Hydrology and Wetlands Ecology

The ﬂow of water through a wetland provides many beneﬁts. Foremost, it is often a source

of nutrients from other ecosystems. It ﬂushes toxins away. It facilitates dispersal of organ-

isms. If the ﬂow is tidal, it acts as a selection agent, inhibiting organisms that cannot tol-

erate the variation in water levels. In these functions, ﬂow does work for the ecosystem

and constitutes an energy subsidy to it. Wetlands with signiﬁcant ﬂow are termed open.

Ecosystems without signiﬁcant ﬂow are called closed.

Flow can also have a negative effect as a source of stress and causing an export of

biomass in the form of plant litter that is washed away by tides or ﬂoods. However, the

evidence suggests that watersheds that include wetlands tend to export more biomass

while retaining more nutrients. Salt marshes tend to export biomass in the form of

plant litter at an order-of-magnitude higher rate than freshwater marshes, about 100 to

200 g/m

 yr. This reduces the net primary productivity and provides an allochthanous

input to downstream ecosystems.

Wetlands cycle nutrients similarly to terrestrial ecosystems. Two major differences are

that the sediments tend to be anaerobic, and nutrients can be lost by export or burial in

peat d eposits. Ana erobic sediments cause a loss of nitrogen by denitriﬁcation, but phos-

phorus is made more soluble, and thus more available. However, if the wetland is open, it

may not be as dependent on nutrie nt recycling as terrestrial ecosystems.

In some cases, primary productivity has been positively correlated to ﬂow through a

wetlands or to tidal range; in other case s, to phosphorus input. On the other hand, it

has been found that freshwater marshes along Lake Erie exhibit an increase in producti-

vity when they are isolated from the lake surface waters by dikes. It is probably best to

think of wetland productivity as resulting from a combination of effects of nutrient input,

biomass export and burial, degradation, and energy subsidies, and that the effect of ﬂow is

generally positive, although it might be an indirect effect.

TABLE 15.20 Reproductive Adaptations to Wetland Conditions

Prolonged seed viability Some plant species produce seeds that may remain viable for 20 years

or more. Exposure of these seeds to atmospheric oxygen usually

triggers germination. Thus, species (e.g., Taxodium distichum) that

grow in very wet areas may produce seeds that germinate only

during infrequent periods when the soil is dewatered. (Note: Many

upland species also have prolonged seed viability, but the trigger

mechanism for germination is not exposure to atmospheric oxygen.)

Seed germination

under low oxygen

concentrations

Seeds of some hydrophytic species germinate when submerged. This

enable germination during periods of early-spring inundation,

which may provide resulting seedlings a competitive advantage

over species whose seeds germinate only when exposed to

atmospheric oxygen.

Flood-tolerant seedlings Seedlings of some hydrophytic species (e.g., Fraxinus pennsylvanica)

can survive moderate periods of total or partial inundation.

Source: U.S. Army Corps of Engineers (1987).

WETLANDS

545

Hydrology has a signiﬁcant effect on species distribution. Since relatively few plant

species are hydrophytic, the greater the duration or frequency of inundation, the lower

the plant species richness and diversity. In fact, it is common to see a single species dom-

inating a marsh, such as Typha (catt ail) or Phragmites in freshwater marshes or Spartina

in salt marshes. However, less intense ﬂooding can create a higher species diversity than

purely terrestrial ecosystems, since both hydrophytic and upland species may coexist in

this situation.

15.3.5 Wetlands Nutrient Relationships

In closed wetlands, nutrient input and biomass output are reduced. Instead of exporting

biomass, the wetland would tend to accumulate it in the form of peat. It is not true that all

wetlands are highly productive. Peat-accumulating wetlands tend to be oligotrophic, with

a consequently reduced gross primary productivity.

A major question about wetlands is their role in nutrient balances. A wetland is called a

source if exports of an element exceed inputs. In particular, many wetlands are sources of

organic carbon. A wetland is a sink if exports are less than inputs. Wetlands are consid-

ered transformers if they change the form of the element, such as by converting soluble

phosphorus into particulate, or ammonia into nitrate. Few generalizations have been

established to predict the nutrient balance of a wetland. Denitriﬁcation contributes to

the role of wetlands as a nitrogen sink, and assimilation makes it either a sink or a trans-

former for phosphorus. However, many studies have also found wetlands to function as

sources. Several multiyear studies found that wetlands used for wastewater treatment

would act as sources in some years and sinks in others.

Because wetlands have aerobic and anaerobic environments in close proximity, they

may be the major location for denitriﬁcation in the biosphere. The aerobic environment

provides nitrate. This can leach into the anaerobic zone to be converted to N

. Since such

a large proportion of nitrogen ﬁxation on Earth is anthropogenic, wetlands may play an

important role in maintaining the balance.

As discussed elsewhere, wetlands also have an important role in global carbon cycling.

Peat-accumulating wetlands are major sinks for carbon, providing negative feedback for

global warming. On the other hand, they are a source of atmospheric methane, which is

another greenhouse gas.

15.3.6 Major Wetland Types

Here we highlight several of the most extensive wetlands types, of the many that exist.

Our interest is in those most likel y to be affected by development activities.

Tidal Salt Marshes Salt marshes tend to form in protected areas such as in the shelter of

spits, bars, or islands, in bays, and in estuaries. Marshes may grow with sedimentation

supplied by rivers or tidal action and inﬂuenced by sediment-trapping effects of the

biota. In some places, sea-level rise results in an increase in the extent of salt marshes.

For example, sea level along the Atlantic coast of North America has been increasing

1 to 3 mm per year for about 4000 years. Marshes kept pace by accumulating peat, in

fact increasing their extent by growing both landward and seaward.

Salt marshes are primarily nitrogen limited. The main available form is ammonium.

However, nonnutrient stresses may also be signiﬁcant. These can include salt stress and

546 ECOSYSTEMS AND APPLICATIONS

lack of drainage. If evaporation exceeds rainfall, portions of the marsh away from the

water may accumul ate salt. Even salt-tolerant plants must expend energy to obtain

water against the osmotic gradient.

Cordgrass (S. alterniﬂora) tends to dominat e in the lower marsh, near the water. S.

alterniﬂora has stiff leaves and occurs in two forms. The tall form is 1 to 3 m tall and

dominates in the extreme lower portions of the marsh. The short form is 17 to 80 cm

tall and is found in higher salinity conditions, farther from the water. Closer to land, S.

alterniﬂora gives way to the salt-tolerant species S. patens and Distichlis spicata (spike

grass). At the high-tide level will be found stands of Juncus gerardi or J. Roemerianus.

Finally, at the part of the marsh reached only by the highest spring tides, if rainfall

exceeds evaporation, salt-intolerant species such as Panic um virgatum and Phragmites

australis are found.

Salt marsh fauna may be grouped into one of three habitats. The aerial habitat con-

sists of the parts of the plants that are almost always above the water. It is similar to a

terrestrial habitat and is dominated by grazing insects and spiders that live by chewing

leaves or sucking the sap. Many birds feed off these insects as well as crabs, worms,

and ﬁsh. Wading bir ds such as egrets and herons, and waterfowl such as the black

duck, are residents of the salt marsh. The benthic habitat supports the detrital food

chain and includes fungi, bacteria, and invertebrates.

The aquatic habitat includes some of the benthic organisms, but by convention may

be deﬁned to refer mostly to aquatic vertebrates, especially ﬁsh. Most salt marsh ﬁsh live

at least part of their life cycles outside the marsh. Many ﬁsh and shellﬁsh spawn upstream

or offshore. The juveniles then ﬁnd food and shelter in the salt marsh, eventually continu-

ing their journey to their adult range. Over 90% of the commercially valuable ﬁsh and

shellﬁsh harvested off the Gulf and southeastern Atlantic coast of the United States

spend part of their life-cycles in the salt marsh. This fact is one of the major utilitarian

motives behind wetlands protection.

Freshwater Marshes Freshwater marshes have a wide variety of types, so it is more dif-

ﬁcult to make generalizations that apply to all. They range from tidal freshwater marshes

to inland freshwater marshes. The latter include prairie potholes of the northern prairie

of North America, the Everglades of Florida, the playas in arid areas of Texas and

New Mexico, the Great Lakes marshes, and riverine marshes distributed in most regions.

Hydrology is still a major controlling factor. The periodicity of inland marshes tends to be

seasonal, in contrast to the tidal cycling of salt marshes.

The vegetation tends to grow in zones from upland to deep water (Figure 15.22) . The

edge that is periodically ﬂooded is dominated by grasses. Where the water level is at or

just above the soil, one can ﬁnd sedges (Carex spp.), rushes (Juncus spp.), bulrush (Juncus

spp.), and the broadleafed monocots, the arrowheads (Sagittaria spp.). In deeper water,

cattails are common. The broadleafed cattail Typha latifolia is less ﬂoodresistant than

the narrow-leafed cattail Typha angustifolia. The latter will grow in depths up to 1 m.

Hardstem bulrushes (Scirpus acutus) occupies the deepest zone of emergent plants. At

greater depth the ﬂoating plants are found. These include water lilies (Nymphaea tuberosa

or T. Odorata) and water lotus (Nelumbo lutea). Also found at depths that preclude emer-

gent vegetation are the submersed hydrophytes, including coontail (Ceratophyllum

demersum), water mi llfoil (Myriophyllum spp.), pondweed (Potamogeton spp.) and

waterweed (Elodea canadensis). Interestingly, many temperate aquatic ecosystems

develop ﬂoating marshes, buoyed by trapped nitrogen and methane and plant material.

WETLANDS 547