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

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spruce, ﬁr, and pine. Dense shade inhibits other plants, but the trees support small seed-

eating animals. Larger animals are less common, as they require broadleafed plants for

food. A form of coniferous forest called the temperate rain forest exists in humid coastal

regions of North America from northern California to Alaska. The tree needles degrade

slowly, building into a thick detritus mat on the forest ﬂoor.

The temperate deciduous forest occupies milder climate zones with ample rainfall.

The canopy cover is less impenetrable, and smaller trees and shrubs (the understory) can

grow. A large number of subdivisions exist, based on the type of domin ant hardwoods

that are present. Nuts, berries, and broadleaf plants support a well-developed food

pyramid. The rate at which litter falls to the forest ﬂoor increases toward the equator,

up to 1100 g/m

per year.

Where rainfall is insufﬁcient to support the deciduous forest, a grassland will be found

instead. A variety of types occur, depending on rainfall, as this type of ecosystem grades

into desert. Moister regions support tall grasses and sod-forming grasses. More arid areas

are characterized by short bunchgrasses. Grasses produce a large amount of litter and

therefore organically rich soil. Most of the biomass is below ground. Large grazing

animals are typically present, such as bison, gazelles, and kangaroos.

Grassland ecosystems are often maintained by ﬁre, without which they would be suc-

ceeded by shrubland or forest. Forest can give way to grassland if ﬁre follows an extended

drought. Once the grasses are established in the resulting opening, ﬁres become more

frequent, due to the rapid production of leaf litter. This makes it difﬁcult for trees to

reestablish themselves.

Fire occurs naturally and can be a force in preserving an ecosystem. Even in forests,

periodic ﬁres conﬁned to the understory (surface ﬁres) eliminate deadwood, which other-

wise could fuel a much more devastating crown ﬁre. A crown ﬁre destroys the mature

trees as well as seeds and organic matter in the soil. A surface ﬁre can release nutrients

and stimulate germination of tree seeds. Pines tend to be more resistant to ﬁre than

hardwoods. Some mature coastal pine forests are called a ﬁre climax ecosystem.

Woodlands tend to give way to shrublands under maritime inﬂuence. The chaparral is

a shrubland that dominates in warm areas that have plenty of moisture in winter but are

very dry in summer.

The desert includes regions with less than about 25 cm (10 in.) of rain per year,

depending on the temperature. Primary productivity is proportional to rainfall, the limit-

ing factor for this ecosystem. The dominant plants are either rapi d-growing annuals, cacti,

or specially adapted desert shrubs. The creosote bush dominates the hot deserts of south-

eastern North America. This plant exhibits allelopathy, in which it inhibits nearby com-

petition chemically. In cooler northwestern deserts, sagebrush dominates. Desert animals

are specially adapted to conserve water. Lizards are suited to the desert because they

excrete dry uric acid instead of urea with water as mammals do. Nevertheless, some

small rodents survive by excreting a very concentrated urine and by nocturnal life-styles.

Irrigation of deserts can make them productive for agriculture. However, since the eva-

poration rate will be high, salts can slowly accumulate in the soil unless extra water is

provided to remove the sal ts by runoff and inﬁltration. (In the Central Valley of California,

selenium from irrigation and natural groundwater sources is accumulating in the soil. It is

thought that the productive life of this important agricultural region may be limited to a

matter of decades by this problem.)

Seasonal tropical forests occur in places such as south Asia, which has monsoon rains

and a pronounced dry season, during which the trees lose their leaves. Tropical rain

498 ECOSYSTEMS AND APPLICATIONS

forests occur where there is more than 200 cm of rain per year, distributed thr oughout the

year. They are typically found at low elevations near the equator. They tend to be very rich

in both plant and animal species. Some tracts of a few acres can have more species of trees

than are found in all of Europe. A 6-square mile area of Panama was found to have at least

20,000 species of insects, compared to several hundred in all of France. Ants, termites,

moths, and butterﬂies are important. A larger proportion of both plants and animals

live in the upper layers of the forest, compared to temperate forests. In one forest, 50%

of the mammals live in the trees. Plants that grow supported by other plants, with no roots

reaching the ground, are called epiphytes. Many animals have formed symbiotic relation-

ships with epiphytes. An important feature of tropical rain forests is the rapid cycling of

nutrients within the ecosystem. Litter is decomposed rapidly, and the nutrients soon reab-

sorbed by the plants. Therefore, most of the nutrients are stored in the biomass, and the

soil remains poor in nutrients. The rich growth of the rain forests made the early settlers

think the soil must be rich. Many attempts at agriculture failed for a lack of this und er-

standing.

15.1.1 Forest Nutrient Cycles

Besides participating in global biogeochemical cycles, ecosystems cycle nutrients

internally. When the ecosystem is harvested, such as in logging, the nutrients must be

replaced if the productivity of the ecosystem is to be sustained. For example, a forest

ecosystem receives nutrient input from the atmosphere and from weathering of the

bedrock. Atmospheric input includes nitrogen that is ﬁxed by nitriﬁcation as well as

nitrogen oxides from natural and human-made sources, and from dust and aerosols.

Forests export nutrients to the atmosphere and by streamﬂow. Stre ams leach mineral

nutrients and carry away biomass that has fallen in. Microbes in anaerobic environments

such as bog sediments eliminate nitrogen by denitriﬁcation and sulfur by reduction to

S. Trees also emit ammonia, H

S, and organic compounds such as terpenes. The

organic emission is famous for causing the haze in the Great Smokey Mountains of the

southeastern United States and for giving President Ronald Reagan a reason to discount

anthropogenic pollution, saying that ‘‘trees poll ute.’’ (Actually, it is true that plants may

emit more hydrocarbons to the environment than comes from human activities. Aspen and

oak leaves emit as much as 2% of the carbon produced by photosynthesis as isoprenes.

The isoprenes may play a role in protecting photosynthetic membranes. However, photo-

chemical smog found in urban air is caused not only by hydrocarbon emissions, but by

nitrogen oxides produced together with them.)

The ﬂuxes of nutrients within the forest are usually much greater than the external

ﬂows. These ﬂuxes amount to an internal recycling. Nutrients taken up by trees are

returned to the soil by leaf fall. Biodegradation by saprophytes releases the nutrients,

whereupon they are again absorbed by the roots. The nutrient cycles operate faster in

warmer climates, as biomas s in the litter decays more rapidly. Plants develop thick root

systems close to the surface to intercept nutrients before they leach away. As a result,

more than half of the organic carbon in temperate forests is in the soil and litter; whereas

in tropical rain forests the ﬁgure is 10 to 25%. Thus, tropical soils tend to be very poor in

nutrients. As a result, harvesting the trees to convert a tropical forest to farmland often

leaves the soil depleted of nutrients. Thus, ironically, these highly productive lands

become unproductive in their new use. Temperate forest lands, on the other hand,

make productive farmland.

TERRESTRIAL ECOSYSTEMS 499

When leaves and other detritus falls to the forest ﬂoor, nutrients are removed by leach-

ing of soluble components, consumption by soil invertebrates, and mineralization by bac-

teria and fungi. Rate of degradation depends largely on the lignin content of the litter. For

example, the highly digestible mulberry was found to lose 64% of its weight in a year on

the ground in eastern Tennessee. Oak degraded by 39%, beech only 21%. Pines and

conifers also degraded slowly. Lignins break down much more slowly than cellulo se or

hemicellulose, the other components of wood. Few bacteria can do the job. Most lignin

decomposition is done by the white rot fungi, Phanerochaete chrysosporium.

Nutrient relationships have been studied using large-scale long-term experiments. One

of the best known is the Hubbard Brook Experimental Forest. Stream and rainfall ﬂow and

composition were measured for a number of watersheds there (Table 15.2). A net loss of

calcium, magnesium, potassium, sodium, aluminum, sulfate, silica, and bicarbonate are

presumably balanced by inputs from weathering of bedrock. Amm onium, nitrate, and

chloride showed greater inputs than outputs. The nitrogen species may also be lost by

transformation.

About 60% of the rainfall that falls on undistur bed Hubbard Brook watersheds even-

tually leaves via the streams. The area is underlain by a tight bedrock formation, so losses

to groundwater are negligible. Thus, most of the loss is by evaporation and transpiration.

If a watershed is logged, streamﬂow increases drastically. Nitrate concentration in the

streams also increases greatly, since nitriﬁcation proceeds normally in the soil, but uptake

by trees is eliminated. As with anions in general, nitrate is weakly bound to soil and so is

rapidly leached out in runoff.

15.1.2 Soil Ecology

Engineers are trained to think of soil just as a structural material. In fact, one of the ﬁrst

acts of developing a property is removal of the topsoil, the part mos t associated with the

biosphere. The biomass in it makes it unstable and unpredictable from a structural point of

view. To the biologist the soil is a living material, intimately connected with living things,

and both created by and sustaining them.

TABLE 15.2 Annual Mineral Balances for Several Watersheds of the Hubbard Brook

Experimental Forest, 1963–1969

Input by Rain Output in Stream Stream Conc.

Mineral (kg/ha) (kg/ha) Net (kg/ha) (mg/L)

Calcium 2.6 11.8 9.2 1.58

Magnesium 0.7 2.9 2.2 0.39

Potassium 1.1 1.7 0.6 0.23

Sodium 1.5 6.9 5.4 0.92

Aluminum Very small 1.8 1.8 0.24

Ammonium 2.7 0.4 þ2.3 0.05

Nitrate 16.3 8.7 þ7.6 1.14

Sulfate 38.3 48.6 10.3 6.40

Dissolved silica Very small 35.1 35.1 4.61

Bicarbonate Very small 14.6 14.6 1.90

Chloride 5.2 4.9 þ0.3 0.64

Source: Krebs (1994).

500 ECOSYSTEMS AND APPLICATIONS

Soil is the part of terrestrial ecosystems with the slowest turnover time (Table 15.3).

This makes it the part that regenerates slowest if disturbed. A forest ﬁre can kill all above-

ground life in an area, but if the soil is not deeply burned, the forest can soon grow back.

But if erosion carries away the topsoil with its organic matter and nutrients, the area may

remain desolate for a long time. Thus, one of the most important goals for conservation-

ists should be preservation of an ecosystem’s soil.

The mineral portion of soil is produced by weathering of bedrock, either locally or at a

distant location followed by transport by wind or water. Weathering is caused by physical,

chemical, and biological factors. Physical weathering is caused mainly by the expansion

of water when it freezes in cracks. Chemical weathering is caused by water dissolving

the more or less soluble components, and by acids from atmospheric carbon dioxide

(which forms carbonic acid in water) or from acid rain. Biological weathering contri-

butes to chemical weathering by adding more CO

plus organic acids to the water.

Roots can help break up rocks by penetrating and forcing open cracks. Lichens produce

acids that dissolve rocks.

A well-developed soil is divided into three main layers, or horizons (Figure 15.2). The

A horizon, often called the topsoil, is the uppermost portion of the soil. The A horizon is

rich in organic matter from decayed plants and animals. It is the site of most of the bio-

logical activity in the soil. Below this is the B horizon, which is lower in organic content

and contains soil part icles that are less weathered than the A horizon. The B horizon is a

zone in which minerals, clay particles, and to a lesser extent organic matter are deposited

after leaching from the A horizon. Next comes the C horizon, which is made up of par-

tially broken-down and weathered rocks from which the soil of the upper layers is formed.

Below this is the bedrock. Some classiﬁcations include the layer of decaying plant and

animal matter above the A horizon, called the O horizon, which has little mineral matter.

The portion of the soil that is subject to the inﬂuence of plan t roots is called the rhizo-

sphere or root zone.

When rocks weather, less soluble components such as silicates remain as solid parti-

cles. Particles larger than 20 mm are classiﬁed as sand; between 2 and 20 mm as silt, and

particles smaller than 2 mm as clay. Soil with a preponderance of larger particles drain

well, holding less moisture and nutrients. Clay particles tend to retain both. Mineral sur-

faces tend to have a net negative charge that enables them to adsorb cationic nutrient ions

such as Ca

2þ

,Mg

2þ

,Na

,NH

, and H

. Anionic nutrients such as NO



,SO

2



, and HCO



are not strongly adsorbed and leach more easily from soil. PO

3

is an

exception since it forms very low solubility precipitates and adsorbs to mineral surfaces

containing iron, aluminum, or calcium.

TABLE 15.3 Average Turnover Time (years) for Organic Matter (OM) and Nutrients

in Forest Soil

Region OM N K Ca Mg P

Boreal coniferous 353 230.0 94.0 149.0 455.0 324.0

Boreal deciduous 26 27.1 10.0 13.8 14.2 15.2

Temperate coniferous 17 17.9 2.2 5.9 12.9 15.3

Temperate deciduous 4 5.5 1.3 3.0 3.4 5.8

Mediterranean 3 3.6 0.2 3.8 2.2 0.9

Source: Krebs (1994); original source Cole and Rapp, 1981, in Dynamic Properties of Forest Ecosystems,D.E.

Reichle (Ed). Cambridge University Press, Cambridge.

TERRESTRIAL ECOSYSTEMS

501

The capacity of soil to adsorb cations is called its cation-exchange capacity (CEC).

The CEC describes the soil’s capacity to serve as a reservoir for nutrients for plants.

Because clay particles have a large surface area per unit mass, they have a relatively

high CEC. For example, the CEC of kaolinite clay ranges from 3 to 15 milliequivalents

(mEq) per 100 g. Montmorillonite clay, which has about an order-of-magnitude smaller

particle size than kaolinite, has a CEC of 80 to 100 mEq per 100 g. The CEC of a whole

soil will be strongly affected by its clay content as well as by organic matter (see below).

The relative strength of attraction of cations to clay particles is

> Ca

2þ

> Mg

2þ

> K

> Na

Thus, acidic conditions displace the other ions from the soil, allowing them to be leached.

A low pH can also cause charge reversal on clay particles, enabling adsorption of anionic

nutrients. In a process called podsolization, acidity breaks down the clay particles in the

A horizon, precipitating their soluble salts in the B horizon. This reduces the CEC of the

soil and therefore its capacity to hold nutrients. Podsolization commonly occurs in con-

iferous forests in cold regions, where the heavy fall of needles produce organic acids as

they decay. This type of soil is called spodisol.

In low-lying tropical regions such as the Amazon basin, warm, moist conditions result

in faster weathering than erosion. As a result, soil accumulates to great depths. Since the

Figure 15.2 Soil horizons.

502

ECOSYSTEMS AND APPLICATI ONS

soil at the surface is so far from the parent mineral, the clay is leached out, leaving iron

and aluminum oxides. These minerals do not retain nutrients as well as clay and can con-

solidate into a hard, concretelike material. This is called laterite soil,orlatisol.

Soils with about 20% clay and 40% each of sand and silt are called loam. Increasing

the fraction of one or the other component produces soils classiﬁed as ‘‘sandy loam,’’

‘‘clay loam,’’ ‘‘silty clay loam,’’ and so on. Soil with a clay content above 40 to 60%

has a low permeability to water, which makes it a poor substrate for plants. A very

sandy soil does not retain water well and has too low a CEC to store signiﬁcant quantities

of nutrients.

Thirty to ﬁfty percent of soil volume is occupied by pore space. This fraction is cal led

the porosity. If soil that is initially saturated with water is allowed to drain freely by grav-

ity, a point is reached when the weight of the water is balanced by capillary forces holding

the water in the pores. This point is called the ﬁeld capacity. The moisture content can

drop further by the action of air drying or by absorption by plant roots. The water content

can be described in terms of the soil water potential or suction pressure, which

expresses the pressure needed to extract the moisture from the soil. Free pure water

has a soil water potential of zero.

The maximum pull most plants can exert on soil water is 1.5 MPa (abou t 15 atm).

If the soil water potential falls below that level, it is considered to be below the perma-

nent wilting point for that soil, and plants in that soil will be irreversibly wilted. A typical

loam containing a fairly even distribution of sand, silt, and clay has a saturation of about

45 g of water per 100 g of dry soil. This corresponds to a porosity of 25%. Its ﬁeld capa-

city is about 32 g per 100 g dry weight (d.w.; about 16% by volume), and the wilting point

is about 7 g per 100 g d.w. (about 3% by volume). Thus, the amount of usable water in soil

at the ﬁeld capacity is about 25 g per 100 g d.w. (9% by volume). Soils with higher clay

content have more of their water bound at the 1.5 MPa water potential, and therefore

have a higher wilting point.

The bedrock is the ultimate source of most of the minerals for the ecosystem. Carbon

and nitrogen, of course, come mostly from the atmosphere, and mineral input from the

atmosphere, from dust, can also be signiﬁcant. The bedrock provides the macronutrients

phosphorus, potassium, calcium, magnesium, and sulfur, p lus the micronutrients boron,

copper, chlorine, iron, manganese, molybdenum, and zinc. The importance of bedrock

is dramatized by the example of serpentine soils, which overlie serpentine rock, a mag-

nesium iron silicate. These soils are deﬁcient in calcium, nitrogen, phosphorus, and

molybdenum, and high in magnesium, nickel, and chromium. Serpentine soils have

poor plant coverage. What plants exist are characterized by specially adapted vegetation

that tolerates these conditions. These plants cannot compete in normal soils, and normal

plants cannot survive serpentine conditions.

Living things affect soil properties in other ways besides causing biological weather-

ing. Living things in the soil mix and burrow, changing its structure, and living things

contribute organic matter. Soil organic matter consists of plants and animals and the pro-

ducts of their degradation. Most of it is from plant material such as leaf litter in various

states of decay. The conversion of dead biomass to soil organic matter is called humiﬁca-

tion. Biological materials that are degraded to the point of being unrecognizable are

called humic substances (or humus). They represent about 5 to 10% of the dry weight

of topsoil.

Humic substances are divided into three groups, based on whether they are soluble

in acids, bases, or neither. The groups are called fulvic acid, humic acid, and humins.

TERRESTRIAL ECOSYSTEMS 503

Fulvic acid is the fraction that is soluble in strong acid and base solutions; humic acid is

soluble in acids but insoluble in bases; humins are insoluble in both. None of these have a

deﬁnite chemical structure. They are dominated by randomly polymerized phenolic rings

with a variety of side groups, many of them acidic, and by numerous cross-links. Molar

masses range from about 700 to 400,000, with fulvic acids tending to be the smallest

molecules and humins the largest. Their structure makes them resistant to further bio-

degradation. The presence of large numbers of acidic groups, such as hydroxyl and

carboxylate, makes humic substances a substantial component of the CEC of soil.

Humiﬁcation occurs in several stages. First, organic polymers such as cellulose, hemi-

cellulose, and lignin are broken down to monomers such as phenols, quinones, amino

acids, and sugars. In the second stage of biodegradation, the monomers polymerize due

to spontaneous reactions and due to enzyme catalysis, producing the humic substances.

Following humiﬁcation, the process of mineralization slowly converts humic substances

to inorganic materials. In natural ecosystems the formation of humic substances tends to

be in equilibrium with their mineralization.

Humiﬁcation is caused mainly by saprotrophic bacteria and fungi. Sugars, fats, and

proteins are decomposed fairly readily. Cellulose breaks down more slowly, and lignin

even more slowly. Lignin may be the source of the phenolic rings that form the major

building block of humic substances. As was mentioned previously, the fungus P. chrysos-

porium plays a major role in lignin degradation. The saprotrophic microbes are also

important for the organic compounds they release to the soil environment. These include

inhibitory compounds such as antibiotics, and stimulating compounds such as vitamins

and essential amino acids.

In some ecosystems, organic matter accumulates to high levels. Grasslands produce

large amounts of biomass. Humiﬁcation proceeds rapidly, but mineralization is slow.

Forests, on the other hand, recycle nutrients more efﬁciently. Thus, grasslands typically

have ﬁve to 10 times as much soil organic matter as forests. This makes them particularly

suitable for conversion to agriculture. Oxygen limitations in wetlands may slow both

humiﬁcation and mineralization. Biomass can accumulate to great depths in wetlands,

forming peat. Coal is formed when peat is buried under heat and pressure over geologic

time.

Experiments have shown that invertebrates play a major role in humiﬁcation. Important

groups include protozoans, nematodes, ostracods, snails and slugs, and earthworms. They

act not primarily by digesting the biomass, but more by breaking up the biomass into

smaller pieces, increasing its surface area, and by grazing the bacteria and fungi, which

increases their activity. This is similar to the role of invertebrates found in biological

wastewater treatment processes.

Humic substances are a major source of CEC in soil and increase its water-holding

capacity as well as its nutrient capacity. Humic substances form chemical complexes,

called chelation, with metal ions. This increases the bioavailability of the metals. Metals

may be less toxic when chelated. For example, copper toxicity to phytoplankton depends

on the free copper ion concentration. In marine environments the same copper concentra-

tion will be less toxic close to shore, where humic substances are more available, then in

the open ocean. Soil organic matter also improves the soil structure, especially its

friability. Friability is the ability of a soil to crumble. This property makes it easier for

roots to penetrate. The increased CEC and friability are the primary reasons that we add

composted manure to gardens and agricultural ﬁelds, not because of the nutrients they

carry.

504 ECOSYSTEMS AND APPLICATIONS

Agriculture removes organics by harvesting. This removes nutrients and reduces the

generation of humic substances. Tilling increases erosion, removing existing topsoil. Add-

ing fertilizers compensates for nutrient loss, but not for soil structure. A new development

is no-till farming, in which weeds are controlled by the use of chemical herbicides

instead of plowing. Crop residues and winter cover are left on the ﬁeld as a mulch,

which controls soil temperature and evaporation of moisture. The residue s add to the

production of humus and reduce erosion.

Agriculture, deforestation, and overgrazing all expose bare soil and increase the rate of

erosion. Some estimate that as much as 1% of Earth’s topsoil is lost each year. By one

estimate, 25% of all farmland in the United States is losing topsoil at a rate exceeding a

‘‘tolerable’’ 1 in. every 34 years.

Humic and fulvic acids lower the pH of the soil. Another organic acid produced by

degradation of biomass is oxalic acid, (COOH)

. Oxalic acid can form complexes with

otherwise insoluble iron and aluminum. The complex then leaches to lower soil horizons,

where the metals are precipitated upon oxidation of the oxalic acid to CO

. Oxalic acid

also catalyzes the chemical weathering of rocks. Oxidation of sulﬁde minerals such as

iron pyrite produces sulfuric acid. Soil pH was once thought to be a major factor control-

ling plant distribution. However, now it is known that although some plants have narrow

pH tolerances, many are not strict. Even in some of those with strict limitations, it may be

due to the effect of pH on nutrient availability rather than the effect of pH on the plant

directly.

Animals are an important com ponent of the soil community (Figure 15.3). Nematodes,

annelids, and insects form the base of the detritus food web, along with fungi and bacteria.

They aid in humiﬁcation by digesting and reducing the size of biomass particles. Nema-

todes, or roundworms, are microscopic worms that are very abundant in soil. Up to 3 bil-

lion can be found 1 m

of soil. Ants and earthworms are important for their role in

aeration of the soil, which they do by their burrowing activity. Mammals such as moles

and shrews also form large burrows.

Earthworms are particularly important to the soil. Besides their function of aerating the

soil by burrowing, they process the soil through their gut. Finally, they mix or till the soil

by ingesting soil at depth, then discharging it at the surface when they come out at night.

The discharged material can often be seen as twisted strands of soil, called castings, on

the surface. Darwin spent a signiﬁcant amount of time studying the role of earthworms in

the years after he published The Origin of Species. He found that rocks in a ﬁeld gradually

became buried as soil was moved from underneath to atop.

Soil typically has 10

to 10

bacteria per gram. Most are associated with the decaying

biomass in the O and A horizons. In the A horizon, more are associated with roots of

plants than in free soil. Bacteria and fungi that perform the initial degradation of biomass

include Pseudomonas, Bacill us, Penicillium, Aspergillus, and Mucor. About 10 to 33%

of the bacteria are Actinomycetes, especially the genera Streptomyces and Nocardia.

Azotobacter and some of the anaerobic Clostridium speci es can ﬁx atmospheric nitrogen

in free soil.

Chemolithotrophs include the nitriﬁers Nitrospira, Nitrosomonas, Nitrobacter, and the

Thiobacillus, the last of which oxidizes inorganic sulfur and ferrous iron. In agriculture,

nitriﬁcation can increase leaching of nitrogen. Addition of nitriﬁcation inhibitors

increases crop yield 10 to 15% and reduces groundwater pollution by nitrate.

Most kinds of fungi can be found in soil, usually in the top 10 cm, where they may

constitute a major fraction of the soil biomass. Protozoans also occupy the upper layers;

TERRESTRIAL ECOSYSTEMS 505

populations are typically from 10

to 10

per gram of soil. They function as predators on

bacteria and algae. Algae exist mostly on the surface, but they also have been found to

exist heterotrophically at depths up to 1 m. Mycorrhyzal fungi are an important group, as

has been discussed (see Section 10.7.4).

15.2 FRESHWATER ECOSYSTEMS

The three basic types of freshwater ecosystems are:

Lentic Slow-moving water, including pools, ponds, and lakes

Lotic Rapidly-moving water, such as rivers and streams

Wetlands Areas the soil is saturated or inundated at least part of the time

Wetlands are considered separately in Section 15.3, where we also consider salt

marshes. The science of inland waters is called limnology. The specialized science of

Figure 15.3 Soil community. (From Raven et al., 1992.)

506

ECOSYSTEMS AND APPLICATI ONS

rivers and streams is called potomology (which has the same root as hippopotomus, which

in turn means ‘‘river horse.’’) Limnology and potomology include the physics and chem-

istry of the respective systems. Here, of course, we emphasize the biology.

15.2.1 Aquatic Environments

The lacustrine (lake) environment can be divided into categories by total depth and by the

depth within the water column, by light availability, and by habitats. The shallow envir-

onment along the shore where rooted aquatic plants (including completely submerged

plants) can grow is called the littoral zone. Deeper water is called the limnetic or pelagic

zone. The upper layer reached by sunlight and therefore dominated by phytoplankton is

called the photic zone. Below the photic zone, where light is attenuated to less than 1%, is

the profundal zone. This is dominated by heterotrophic organisms subsisting on food that

falls from above.

Deep waters may also be physically divided by thermal stratiﬁcation in the winter and

summer (Figure 15.4). Because water has a maximum density at 4



C, colder water in the

winter and warmer water in the summer form a distinct layer that ﬂoats. The upper layer is

called the epilimnion, the lower layer is the hypolimnion. Each of these layers is fairly

uniform and, especially in the epilimnion, well mixed. These layers are separated by

an area of rapid temperature and density change called the thermocline. The density

gradient is very stable and prevents mixing between the two layers. The thermocline

protects the profundal zone from the impacts of pollution. The littoral zone, on the other

hand, is more susceptible because of its proximity to sources of pollution. The stratiﬁca-

tion shown in Figure 15.4 is typical of sufﬁciently deep lakes in summer in the temperate

zone. Stratiﬁcation varies signiﬁcantly with season and local climate.

The basic characteristic of riverine (river or stream) environments is the velocity of

ﬂow. Rifﬂes are relatively fast, shallow, and turbulent, whereas pools are laminar in

ﬂow, approaching lakes in their charact eristics. Rivers generally lack the stratiﬁcation

found in lakes. Flow greatly affects dispersal of organisms and import and export of nutri-

ents. Systems (whether rivers or lakes) that depend on their own productivity are called

autochthonous. Those that depend on imported energy or nutrients are allochthonous.

Lakes tend to be autochthonous. Rivers are more likely to be allochthonous, obtaining

their organic matter from terrestrial sources such as leaf litter and other detritus in runoff.

Compensation

depth

Thermocline

Profundal zone Photic zone

Epilimnion

Hypolimnion

Pelagic zoneLittoral zone

Figure 15.4 Lake zones classiﬁed by depth, light, and stratiﬁcation. (Based on Horne and

Goldman, 1994.)

FRESHWATER ECOSYSTEMS 507