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

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In the fall turnover the phosphate is mixed back into surface layers, where it is available to

plants and algae. With the transport of oxygen back to the depths, the iron and sulfur are

oxidized again, and the cycle repeats. The process can be summarized by the following

reaction, in which oxic conditions push the reaction to the right and anoxic conditions

force it to the left.

4FeðIIÞS þ 4PO

3

þ 4H

þ 9O

, 4FeðIIIÞPO

þ 4SO

2

þ 2H

The phosphate concentration falls in the epilimnion thr oughout the summer stratiﬁca-

tion as phytoplankton take it up, then die and settle toward the bottom. Phytoplankton are

highly efﬁcient at taking up phosphate, even at very low environmental concentrations.

The Monod half-rate coefﬁcient for phosphate uptake is about 1 to 3 mg/L PO

-P.

Thus, the uptake mechanism is rarely saturated unless pollution increases the concentra-

tion greatly. Differences in ability to compete for phosphorus may contribute to the suc-

cession of algal species during the summer stratiﬁcation. Algae also exhibit luxury

consumption of phosphorus, in which they store as much as 20 times the amount needed

for cell division in cellular bodies cal led polyphosphate granules. Asterionella can store

100 times its immediate needs. By such hoarding they can continue their growth after the

water column has been depleted of phosphorus.

Phosphorus is recycled rapidly in the water column. Zooplankton eat the phytoplank-

ton, then excrete ph osphorus at the rate of 10% of their body stor e daily. Half of this is

phosphate and is readily reused. The other half is organic phosphate. Phytoplankton have

the unusual capability to excrete the extracellular enzyme alkaline phosphatase. This

acts in the water, outside the phytoplankton cell, to break the bond between the phosphate

and the rest of the organic molecule. This represents a risky investment for the phyto-

plankton, since other species may beneﬁt from the cost they bear for excreting the

enzymes. When phosphorus is plentiful, the enzyme production slows considerably.

Diatoms need large amounts of silica (SiO

) for their cell walls. Abou t half of the dry

weight of diatoms is silica. An absence of silica can result in replacement by cyanobacter.

Lake turnover Stratification

Fe(III)PO

Epilimnion

Hypolimnion

Fe(II)S

Oxic sediment Anoxic sediment

Figure 15.19 Phosphorus interaction with sulfur and iron in lakes. (Based on Horne and Goldman,

1994.)

528

ECOSYSTEMS AND APPLICATI ONS

Rivers typically have about 13 mg SiO

per liter, derived from weathering of feldspars.

Lakes range from 0.5 to 60 mg/L.

Iron, besides its role in the phosphorus cycle of lakes, is often a limiting nutrient. In the

eplimnion much of the iro n is chelated with organics. Some algae can only use inorganic

iron, others only the chelated form, and othe rs use both. Cyanobacter produce powerful

iron-chelating organics, which limit their competitors’ access to iron.

Eutrophication produces changes in the community species structure. As nutrient

levels increase, the dominant algal species shift from diatoms to green algae to cyanobac-

ter (Figure 15.20). In the more extreme forms of cultural eutrophication the phytoplankton

growth exceeds the ability of zooplankton to control them. More bacteria and detritus

feeders are found. Respiration by the plants at night or by bacteria degrading dead plants

can deplete oxygen, affecting higher organisms including ﬁsh. Species diversity and

richness may increase in moderate eutrophy, then decrease in the extreme form, although

abundance continues to increase. At such high abundance these blooms may produce

phytotoxins affecting higher forms of life, such as ﬁsh or cattle that drink the water.

Cultural eutrophication can be controlled by eliminating nutrient loading, reducing

nutrients in the lake, or by addressing the symptoms, whether they be low dissolved

oxygen or plant accumulation. Nut rients can be eliminated by using advanced wastewater

treatment or by diverting the wastewater to another watershed. In the case of Lake Tahoe,

a highly oligotrophic lake on the border between California and Nevada, the wastewater is

both treated and pumped over the mountains surrounding the lake. This has eliminated

enough phosphorus so that the lake has changed from being nitrogen limited to being

phosphorus limited. However, signiﬁcant nitrogen loading still results from atmospheric

deposition of nitrogen oxides ﬁxed by combustion in automobile engines and wood-

burning stoves.

Dredging of lakes is used to remove phospho rus and nitrogen stored in sediments.

Phosphorus can be precipitated from the water column by adding alum. This also removes

suspended solids including bacteria, increasing the clarity of the water. Swimmers in

small recreational lakes can tell if this treatment has recently been applied, because the

alum ﬂocs are easily resuspended by waving the hand over the bottom.

Various

Cyanobacter

Chrysophyceae

Chlorophyceae(green algae)

Bacillariophyceae(diatoms)

Cryptophyceae

Dinoflagellates

ULTRAOLIGOTROPHIC (mm

/L biomass)

51110

RELATIVE ABUNDANCE (%)

Various

Cyanobacter

Chrysophyceae

Chlorophyceae (green algae)

Bacillariophyceae (diatoms)

Cryptophyceae

Dinoflagellates

HYPEREUTROPHIC

Figure 15.20 Changes in algal species composition with degrees of eutrophication. (From Connell

and Miller, 1984.)

FRESHWATER ECOSYSTEMS 529

Algae can be killed or inhibited by application of 500 mg Cu/L using copper sulfate.

Phytoplankton will be killed, although any taste and odor or toxic compounds may

remain. Cyanobacter are especially sensitive to copper. Fish and zoopl ankton are rela-

tively insensitive, but algae can develop resistance. In the case of macrophytes, physical

removal both eliminates the nuisance and takes nutrients out of the system.

The problem may be low dissolved oxygen in the hypolimnion. In this case, mechan-

ical mixing can be used to destratify the lake, mixin g the oxygen-rich epilimnion and the

deﬁcient hypolimnion. Sometimes it is desired not to destroy the stratiﬁcation: for exam-

ple, to support cold-water ﬁsh such as tro ut. In this case a variety of hypolimnetic aera-

tion techniques can be applied in which hypolimnion water usually is pumped to the

surface, aerated, then returned below the thermocline. Oxygen can also be dissolved

directly into the water of the hypolimnion.

15.2.7 Organic Pollution of Streams

The ﬁrst successes of modern environmental engineers and scientists were in protecting

human and environmental health from the effects of discharging human wastes (sewage)

to streams. The concerns wer e transmission of waterborne disease and gross degradation

of the receiving water body due to excessive organic loading. Sewage discharge affects

streams by adding toxins, pathogens, suspended solids, nutrients, and readily biodegrad-

able organic carbon. These produce a cascade of effects, mostly related to reduced oxygen

concentration in the water column. The oxygen depletion, in turn, is due to augmented

heterotrophic oxygen consumption in the water column and in the sedime nts and to

chemoautotrophic oxidation of ammonia. Recall that the nitrogenous oxygen demand

of typical domestic wastewaters can be very high, since nitriﬁcation of each milligram

of NH

-N to NO



-N consumes 4.57 mg of oxygen.

DO Sag Equation If the organic pollution comes from only a single point source, the

stream gradually recovers with distance downstream in a fairly predictable way. The mas-

ter variable in this situation is oxygen. The changes in oxygen can be described by a

model involving the following variables with their typical units (note that g/m

is equiva-

lent to mg/L):

C dissolved oxygen concentration (also called DO; g/m

saturated DO, or oxygen concentration the water would have if it were in

equilibrium with the atmosphere. Values decrease with temperature, sali-

nity, and altitude. Example values in pure water at sea level range from

14.6 g/m

at 0



C to 9.2 g/m

at 20



L biochemical oxygen demand (BOD; g/m

D DO deﬁcit, the amount the DO is below saturation; equal to C

 C.

The major processes affecting oxygen levels in streams are:

U oxygen uptake rate (g/m

per day); a combination of heterotrophic

(organic carbon) and autotrophic (ammonia) biochemical oxidation.

B benthic oxygen demand (g/m

 day), due to heterotrophic and auto-

trophic oxidation in the sediment. Also called sediment oxygen demand

(SOD).

530

ECOSYSTEMS AND APPLICATI ONS

R reaeration rate (g/m

pre day); the rate at which oxygen enters the water

from the atmosphere by physicochemical mass transfer.

P speciﬁc photosynthetic oxygen production rate (g/m

day), by green

plants and algae.

At the point that a parcel of water mixes with the discharge, the mixture has a DO of C

and a BOD of L

. The DO at this point corresponds to an initial deﬁcit D

¼ C

 C

.As

the parcel continues downstream, its DO changes in response to a balance between the

sources of oxygen, R and P , and the depletion by U and B:

þ P  U 

ð15:2Þ

where h ¼ V=A is the average depth of the water column, or the volume of a section of

a stream, V, divided by its p lan area, A. Note from the units that reaerati on and benthic

consumption are normalized to the area, since they represent transport of oxygen across

the top and bottom surfaces of the water column, respectively.

At the same time that oxygen is disappearing according to equation (15.2), the BOD in

the water column is being depleted as oxygen is consumed by the uptake process:

¼U ð15:3Þ

Much can be learned from a simpl iﬁed model that neglects photosynthesis and benthic

oxygen demand. Recall that rivers tend to be allochthonous to begin with, and this will

be more so for polluted streams. Benthic demand may be quantitatively important, but it

behaves similarly to the water column BOD and does not greatly change the qualitative

behavior of the model. Thus, only U and R remain in the model. Oxyg en uptake can be

described as a ﬁrst-order decay:

U ¼ k

L ð15:4Þ

The parameter k

is the rate coefﬁcient for decay of BOD. This is sensitive to the tem-

perature and biodegradability of the BOD . The k

values must be determined experimen-

tally either in the laboratory or, preferably, in ﬁeld experiments. To measure k

in the ﬁeld,

BOD bottles containing samples of river water are submerged directly in the stream being

modeled for 2 to 6 hours. Initial and ﬁnal DO measurements yield a rate of oxygen con-

sumption, U. Dividing by the BOD yields a value for k

. Biodegradation rate coefﬁcients

have been measured for numerous industrial chemicals in laboratory experiments (see

Appendix B).

Usually, parallel experiments will be done with clear glass bottles and with black-

painted ones. The difference between the light and dark bottles gives an estimate of phy-

toplankton photosynthesis, and the dark bottle measures respiration. This is done either to

validate the assumption of neglecting photosynthesis or to quantify it for including in the model.

Combining equations (15.3) and (15.4) and solving the resulting differential equa-

tion produces the typical exponential or ﬁrst-order decay of BOD in the stream

(Figure 15.21a):

L ¼ L

ðk

tÞ

ð15:5Þ

FRESHWATER ECOSYSTEMS 531

Note that the distance downstream, x, in Figure 15.21 corresponds to the distance traveled

in time, t, at a stream velocity, v, by the transformation x ¼ vt.

Reaeration is also ﬁrst order, with respect to the DO deﬁcit. That is, the rate at which

oxygen enters the water is assumed to be proportional to how far the water is below

saturation:

¼ k

D ¼ k

ðC

 CÞð15:6Þ

The parameter k

is the coefﬁcient of reaeration. It is controlled by turbulent mixing in the

water. Correlations are available to estimate the value of k

from hydraulic parameters of

the stream such as its depth, velocity, and hydraulic slope.

Combining (15.4) and (15.6) with (15.2) and (15.4) yields a differential equation

which can be solved for initial conditions L

and D

, resulting in the Streeter–Phelps

Tubifex

Chironomids

Ascellus

Algae

NO3–

PO4–3

NH4+

BOD

Spheaerotilus

Bacteria

Protozoa

Figure 15.21 Changes downstream from a point of organic loading such as a sewage discharge.

(Based on Connell and Miller, 1984.)

532

ECOSYSTEMS AND APPLICATI ONS

or DO sag equation (D ¼ C

 C is the DO deﬁcit):

D ¼

 k

ðe

k

 e

k

ÞþD

k

ð15:7Þ

Figure 15.21a shows the shape of this curve. Qualitatively, the DO initially decreases

rapidly when U > R= h . However, U is decreasing as L decreases (less food available

for respiration), and R increases as C decreases farther below saturation. Ultimately, if

the initial pollution is not too great, these two rates will approach each other, and the

curve bottoms out. The DO deﬁcit at this point is called the critical deﬁcit, D

, and occurs

at a distance downstream called the critical time of travel, t

 k

1 

ðk

 k



ð15:8Þ

The critical deﬁcit can then be found by substituting the result of equation (15.8) into

(15.7).

As the BOD continues to decrease farther downstream, the stream starts to recover. If

the critical DO concentration, C

¼ C

 D

is low enough, it can form a barrier to the

movement of ﬁsh or other oxygen-sensitive organisms. The critical concentration is often

used for regulatory purposes to indicate the seriousness of the pollution. In the regulatory

process of waste load allocation, models of the river are manipulated to determine how

much pollution must be reduced at the source to meet some minimum DO standard at the

critical point.

Other Chemical Changes and Effects Other changes occur as one travels downstream

of the discharge. Biological effects are described below. Chemical change s include

increases in nutrient concentrations. As shown in Figure 15.21b, ammonia increases

due to mineralization of organic nitrogen and raw ammonia in the wastewater. The ammo-

nia is gradually depleted as nitriﬁcation converts it ultimately into nitrate. Th e nitrate may

be removed by phytoplankton uptake, and then either stays in the water column as organic

nitrogen or is settled to the benthic zone. If the bent hic zone is anoxic, nitrate may be

removed from the water column to use to oxidize buried organics by denitriﬁcation.

Sedimentation of suspended solids near a discharge can smothe r benthic organisms.

Sedimentation of suspended solids from the wastewater, plus biomass produced by growth

on dissolved and colloidal material, can biodegrade in the sediment. The oxygen con-

sumed in this way contributes to the benthic oxygen demand. Besides removing oxygen

from the overlying water column, benthic oxygen demand reduces oxygen availability to

the benthos. Benthic invertebrates are particularly sensitive.

Oxygen enters the sediment in a mass transport process similar to oxygen transport

from the atmosphere to the water in reaeration. In this case the rate of transport is con-

trolled by diffusion through the pores of the sediment and by the concentration in the

water. If the oxygen demand is high enough or the rate of transport sufﬁciently low,

the oxygen will only penetrate to a certain depth before it is consumed completely.

Thus, there will be an oxic layer covering an anoxic laye r, where denitriﬁcation replaces

oxygen respiration. The depth of transition between the oxic and anoxic layers is

controlled by a balance between the oxygen concentration in the water and the benthic

oxygen demand.

FRESHWATER ECOSYSTEMS 533

At greater depths in the sediment, nitrate may be depleted and anaerobic conditions

would prevail. Under these conditions respiration uses sulfate and carbonate as electron

acceptors, producing H

S and methane. The H

S can diffuse back up through the sedi-

ment and enter the water column, where it can exert a toxic effect. The LC

of H

(the concentration that kills half the organisms in a laboratory test) to several freshwater

invertebrates has been measured to range from 11 to 1070 mg /L. Some zoobenthos can

live in anoxic sediments because they circulate water containing oxygen mechanically

from the surface of the sediment.

Ultimately, the organic matter becomes oxidized and no longer affects the river. How-

ever, many of the nutrients remain. In fact, degradation of organic-rich sediments may

release previously stored nutrients. Thus, the high organic-loading condition of the stream

can give way to a eutrophic situation, with photosynthesis exceeding the baseli ne level

upstream of the discharge.

The Streeter–Phelps equation is a steady-state solution. If it is necessary to take photo-

synthesis into account, P should be modeled with a diurnal function. This would result in

DO concentrations that oscillate, reaching a minimum in the morning. It is even possible

to achieve supersaturation ðC > C

Þ during sunny days if the amount of pollution is not

great. The addition of organic pollution increases the ratio of heterotrophic to photosyn-

thetic activity simply by providing food for the heterotrophs. However, photosynthesis

may also be reduced directly by the effect of increased turbidity in the water, which

can limit light availability.

Individual and Ecological Effects Fish and larger invertebrates breathe through gills

and have a circulatory system to bring oxygen-enriched blood to the tissues. When oxy-

gen becomes depleted, these organisms can increase the pumping of water over the gills in

order to extract more. Organisms such as ﬁsh that can readily move in resp onse to oxygen

have been found to avoid areas where concentrations have dropped below 3 to 5 mg/L.

Small invertebrates such as zooplankton obtain their oxygen by diffusion through the

skin, without specialized organs; some do not even have circulatory systems. Some organ-

isms are adapted to low-oxygen conditions. Annelid worms have special high-capacity

hemoglobin. Chironomid larvae have a high concentration of hemoglobin, giving them

a red color. Table 15.12 shows some of the oxygen limitations observed experimentally

with aquatic animals.

Systems with large organic carbon input are called saprobic, as opposed to eutrophic,

which refers primarily to excess nitrogen and phosphorus. In eutrophic waters primary

production exceeds respiration. In saprobic waters the reverse is true. As organic pollution

TABLE 15.12 Dissolved Oxygen Limits for Several Organisms

Organism Temperature (



C) Type of Test DO (mg/L)

Brown trout (Salmo trutta) 6.5–24 Limiting concentration 1.28–2.9

Coho salmon (Oncorhynchus kisutch) 16–24 Limiting concentration 1.3–2.0

Rainbow trout (Salmo gairdnerii) 11.1–20 Limiting concentration 1.05–3.7

Worm (Nereis grubei) 21.7–26.3 28-day LC

2.95

Worm (Capitella capitata) 21.7–26.3 28-day LC

1.50

Amphipod (Hyalella azteca) — Threshold 0.7

Amphipod (Gammarus fasciatus) — Threshold 4.3

Source: Connell and Miller (1984).

534 ECOSYSTEMS AND APPLICATIONS

levels increase, population shifts occur. Clean waters are dominated by ﬁsh and green

algae. The Shannon–Weaver diversity index for benthic invertebrates in clean water is

greater than 3. At moderate saprobic levels, the diversity index drops to between 1 and

3 and the dominant population shifts to oligochaetes such as tubifex. Seriously polluted

waters have these as well as increasing numbers of ciliates and ﬂagellate protozoans, and

a diversity index less than 1.

A succession occurs with time in saprobic lakes that is similar to what occurs down-

stream of a wastewater disc harge. A pure lake begins in a xenosaprobic state, correspond-

ing to oligotrophic. If a large pollution discharge occurs, it becomes polysaprobic. As the

organic matter is degraded and the lake recovers, it passe s through intermediate stages

termed mesosaprobic and oligosaprobic. At this point the organic matter has been elimi-

nated, but nutrients remain. Primary productivity may exceed respiration, and the lake

passes into a eutrophic phase. The same terms are applied to rivers at various stages of

recovery from pollution.

Besides tolerance of low oxygen levels, the animals that proliferate in saprobic

conditions include detrital feeders that feed as ﬁlterers or collectors. Examples are certain

caddis ﬂies and mayﬂies, chrionomids, and clams. Grazers also are successful and include

certain other caddis ﬂies and mayﬂies, and snails. Predators such as leeches and midges

respond to the increasing saprobic community. Table 15.13 shows some selected organ-

isms that are found in various saprobic conditions. Although the term mesosaprobic is

TABLE 15.13 Tolerance of Organisms to Organic Pollution

Taxonomic Oligosaprobic Mesosaprobic Polysaprobic

Group (Unpolluted) (Moderately Polluted) (Highly Polluted)

Cyanobacter Oscillatoria Chamasiphon

Diatoms Nitzschia palea Cocconeis placentula

Gomphonema parulum

Green algae Chlamydomonas Chlamydomonas Chlamydomonas

Ulothrix Ulothrix

Scenedesmus Chaetophora

Turbellaria Polycelis nigra Polycelis cornuta

Annelids Tubifex tubifex Lubricus, Stylaria Chaetogaster

Hirudinea Erpobdella octoculata

(leeches) Helobdella stagnalis

Glossiphonia spp.

Crustaceans Daphnia Daphnia

Cyclops Cyclops

Asellus aquaticus

(water louse)

Sialis lutaria (alder ﬂies)

Insects Eristalis tenax Chironomous plumosus Stoneﬂies and mayﬂies

(rat-tailed maggot) (all species)

Chironomous plumosus Caddis ﬂies Chironomous spp.

Hydropsyche spp.

Anabolia, Molanna spp.

Fish Eel, stickleback, carp, Trout, pike, perch

goldﬁsh

Source: Connell and Miller, 1984.

FRESHWATER ECOSYSTEMS

535

associated with ‘‘moderately polluted,’’ this condition can occur naturally if there are

large allochthonous inputs.

15.3 WETLANDS

Wetlands have experienced a remarkable turnaround in the public mind. Not long ago

they were derided as swamps and mires, sources of disease, impediments to travel, and

a waste of real estate. If they could not be drained for agriculture or development, the only

practical use for them was as a location for garbage dumps. If at all possible, they were

eliminated by draining or ﬁlling. Between the late eighteenth century and the mid-1980s,

it is estimated that over 50% of the wetlands in the lower 48 states of the United States

have been lost.

Today they are regarded as natural treasures. They are protected by strict laws, and

their importance is touted in newspaper articles and elementary school curricula. What

has changed is public recognition of the ecological value of wetlands. Salt marshes are

among the most productive ecosystems in the world (Table 15.1). They are the breeding

ground for numerous animals that range far beyond the marsh, including shorebirds and

pelagic ocean ﬁsh. Mig ratory birds use them as stopping-off points, where they feed and

restore their energy reserves before continuing their journey.

Although wetlands occupy only about 2% of Earth’s land area, they store 10 to 14% of

its organic carbon in their soil. When wetlands are dewatered, such as by draining or by a

decrease in precipitation, the organic matter is exposed to increased amounts of oxygen.

As a result, the organic matter becomes oxidized to CO

. This is one of the ‘‘positive feed-

back mechanisms’’ that may aggravate the effect of global warming.

Wetlands mitigate storm ﬂows and ﬂooding and serve as sourc es for groundwater

recharge. Surface water passing through a marsh can be reduced in nutrient level and

other contaminants. They have traditionally been, and continue to be, used as a source

of natural resources, including lumber, moss, shellﬁsh, waterfowl, and pelts. Finally, as

can be attested to by the birders and hunters who venture into them, they have aesthetic

value. It is fortunate that nature made its most productive ecosystem so inaccessible to humans,

so that wetlands hav e survived e ven in the midst of the world’s most densely populated cities.

Some consider wetlands to be merely an ecotone, an intermediate zone between terres-

trial and deepwater ecosystems. They can also be viewed as a stage in the successional

change from open water to terrestrial ecosystem (recall the discussi on of eutrophication in

Section 15.2.6). However, wetlands have a number of unique characteristics. Wetlands are

deﬁned by their water, soil, and plants. Speciﬁcally:

 Water is present at a depth either above the surface (but not so deep as to preclude

the presence of rooted plants that emerge from the water) or below the soil surface

but within the root zone.

 The soil is anaerobic within at least part of the root zone during a signiﬁcant part of

the growing season.

 The ecosystem is capable of supporting plants that are specially adapted for wet

conditions and has no plants that are ﬂood intolerant.

A more precise deﬁnition is necessary for legal purposes. This is discussed in Section

15.3.7. There are numerous terms for speciﬁc types of wetlands in both common use

536 ECOSYSTEMS AND APPLICATIONS

and scientiﬁc use. Sometimes, the terms marsh or swamp are used interchangeably with

wetland. However, these terms both have more speciﬁc meanings. Some examples are:

Bog: a peat-accumulating wetland with little or no inﬂow or outﬂow. Most of its

nutrients come from precipitation. Its waters tend to be acidic, and as a result it

supports mosses primarily, such as sphagnum moss.

Fen: a peat-accumulating wetland that receives some ﬂow and nutrients from

surrounding uplands. It supports some vegetation similar to marshes.

Marsh: a wetland that is freq uently or continuously inundated and has emergent plants

that are specially adapted to saturated soil conditions. In Europe this term excludes

wetlands that are peat accumulating.

Moor: in European use, any wetland that accumulates partially decayed organic

matter.

Muskeg: large expanses of bogs, such as in northern Canada and Alaska.

Riparian wetlands (or ﬂoodplains): occasion ally ﬂooded lands along rivers that are

dry during parts of the growing season.

Swamp: in the United States, a wetland dominated by shrubs or trees. In Europe, a

forested fen or wetland dominated by reedgrass (Phragmites, or common reed).

Vernal pool: an intermittently inundated wet meadow that is usually dry during the

summer and fall.

Table 15.14 shows how several of these wetland types ﬁt along the continuums for

plant type, water ﬂow, soil organic matter content, pH, and nutrient condition. A rheo-

trophic wetland is one that receives signiﬁcant inﬂow vs. an ombrotrophic wetland

that receives little ﬂow. All of the characteristics in Table 15.14 relate to input of nutrients

and whether the wetland accumulates peat. The oligotrophic wetlands become acidic

because products of degradation are not washed away. Combined with anaerobic condi-

tions in saturated soils, this limits biodegradation. Instead, the partially degraded biomass

accumulates, forming peat.

Wetlands can also be classiﬁed as to whether they are coastal or inland, tidal or non-

tidal, freshwater or saline, intermittent or permanent. Tidal salt marshes are found all

along the U.S. Atlantic and Gulf coasts. They are dominated by the grass Spartina and

the rush Junca. Moving inland the salt marsh gives way to tidal freshwater marshes.

Tidal freshwater marshes are dominated by both grasses and broadleafed aquatic plants.

In tropical and subtropical regions, such as southern Florida and parts of Louisiana and

Texas, the tidal freshwater marsh, in turn, gives way to the mangrove swamp. The man-

grove swamp is dominated by the salt-tolerant mangrove tree, such as the red mangrove

Rhizophora or black mangrove Avicennia. All of these coastal marshes require protection

from ocean storm action and are frequently found behind barrier islands.

There are four major types of inland wetlands in North America. Freshwater marshs

occur throughout the continent and consist of shallow water, thin peat deposits, and

diverse emergent plants, including cattails, arrowheads, pickerel-weed, reeds, and grasses

and sedges. They include the prairie potholes of the Dakotas, the Great Lake marshes, and

the Everglades of Florida. The northe rn peatlands include bogs and fens from the north-

ern midwestern and northeastern United States and central and eastern Canada. Many are

ﬁlled-in lake basins, in ﬁnal stages of natural eutrophication (Section 15.2.6). The south-

ern deepwater swamps have standing water through most of the growing season and are

WETLANDS 537