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

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15.2.2 Biota

Aquatic organisms are classiﬁed according to their habitats. The plankton are small free-

ﬂoating organisms. Planktonic plants, called phytoplankton, include cyanobacter, green

algae, and diatoms. Planktonic animals are zoopl ankton and include protozoans, small

crustaceans such as Daphnia species and larval forms of other animals. Organisms

such as duckweed that ﬂoat on the surface are called neuston. Active swimmers that

live in the pelagic zone, such as ﬁsh, are called nekton. The community of microscopic

bacteria, fungi, and small plants and animals that live attached to submerged surfaces such

as rocks are referred to as aufwuchs. A subset of these are the periphyton, which are

algae that grow attached to surfaces. (Aufwuchs and periphyton are what makes sub-

merged rocks so slippery to step on.) All organisms that live on the bottom or within

the sediments, whether in the littoral or profundal zones, are called benthic organisms,

or simply the benthos.

Lotic ecosystems have planktonic organisms only in the slowest of rivers. Periphyton

dominate over phytoplankton, and benthos over zooplankton.

Prokaryotes Three types of autotrophic bacteria are typically found in lakes: cyanobac-

ter, chlorobacter, and chemoautotrophs. Cyanobacter (blue-green bacteria) are found in

the epilimnion. Chlorobacteria peaks just below the thermocline, their numbers decreasing

with depth. If the hypolimnion becomes anoxic, sulfate-reducing bacteria will appear to

produce hydrogen sulﬁde. However, most of the bacteria are heterotrophs. Heterotrophic

concentrations peak in the thermocline and near the sediment. In highly polluted streams

the ﬁlamentous bacteria Sphaerotilus can form lush gray mats waving in the current .

One of the notable roles of cyanobacter, espec ially Aphanizomenon, is in the ﬁxation of

atmospheric nitrogen. This allows it to grow whe n nitrogen limitations apply to other

algae. Some cyanobacter, such as Oscillatoria or Microcystis, cannot ﬁx N

. The latter

depends on ammonia recycled by animal or bacterial excretion or by solubilization

from the sediments. In rivers, Nostoc or Rivularia may ﬁx considerable amounts of

nitrogen.

Gas vacuoles enable cyanobacter to ﬂoat to the surface during the calm night, where

they can be found forming a scum at dawn. As the sun rises higher, the cyanobacter can be

damaged by ultraviolet radiation, so they tend to sink by midmorning. Anabaena does this

by collapsing gas vesicles by osmotic pressure which has increased as a result of glucose

formed by photosynthesis. Colonies of Microcystis and Aphanizomenon, as well as ﬁla-

ments of Anabaena, also regulate their buoyancy by manufacturing glycogen, which is

about 1.5 times as dense as normal cell material. At night they use up the glycogen

and start to rise again. By moving up and down, the algae not only control their light

exposure but take advantage of nutrients farther below the surface.

Cyanobacter (Figure 15.5) make chemicals that inhibi t feeding by zooplankton. They

make compounds that have been known to poison cows and pigs that drank from

eutrophic sources of water. Cyanobacter and some actinomycetes also produce geosmin

and methyl isoborneol, the most common causes of taste and odor problems in drinkin g

water. These compounds are contained in the cells, but released when the cells break

down.

Green Plants Algae are plantlike protists (Figure 15.6). They are distinguished from

plants in that they do not have stems, roots, or leaves. They can be unicellular, colonial

508 ECOSYSTEMS AND APPLICATIONS

(forming usually small groups of cells), or ﬁlamentous. Some, such as the dinoﬂagellates,

are motile, enabling them to migrate in response to light. Periphyton are usually green

algae, cyanobacter, or diatoms. The long, bright green ﬁlaments attached to rocks in

streams are often cyanobacter. Other ﬁlamentous forms are green algae such as Uloth rix

and Cladophora.

Diatoms (Figure 15.7) have an advantage over other plants in that their cell wall, being

made of silica, requires about 12 times less energy to manufacture than cellulose. (Cya-

nobacter, which are prokaryotes, make cell walls of peptidoglycan, which requires more

energy than cellulose. The ability of cyanobacter to control their light input by controlling

buoyancy compensates them for this added cost.) However, diatoms are denser as a result,

so under low-turbulence conditions they may settle out of the photic zone and be replaced

by cyanobacter or dinoﬂagellates.

Figure 15.5 Cyanobacter: ðaÞ Aphanizomenon; ðbÞ Anabaena; ðcÞ Oscillatoria; ðdÞ Polycystis;

ðeÞ Nostoc; ðf Þ Spirulina. (From Standard Methods, 12th ed. # 1965 American Public Health

Association.)

Figure 15.6 Green algae: ðaÞ Pediastrum; ðbÞ Selenastrum; ðcÞ Coelastrum; ðdÞ Scenedesmus;

ðeÞ Spirogyra; ðf Þ Microspora; ðgÞ Ulothrix. (From Standard Methods, 10th ed. # 1955 American

Public Health Association.)

FRESHWATER ECOSYSTEMS 509

Dinoﬂagellates (Figure 15.8) can swim several meters per hour. They use their motility

to swim upward in the morning to collect more light for photosynthesis, and then down-

ward in the afternoon to avoid predation. In a condition called the red tide, dinoﬂagellates

sometimes accumulate to nuisance levels in lakes, estuaries, and oceans, coloring the sur-

face of the water blood-red. They can be toxic to ﬁsh and invertebrates and irritating to the

skin of humans exposed by swimming. Examples of red tide genera include Noctiluca and

Gymnodinium in marine systems and Peridinium and Ceratium in lakes.

In the open sea, the condition has been associated with salinity fronts where estuarine

water converges with ocean currents. This leads to stratiﬁcation between the estuarine

water on top and the more saline water below. These waters may be comple mentary in

the sense that each contains nutrients that may be limit ing in the other. Algae grow pre-

ferentially at the boundary, where there will be some mixing. Dinoﬂagellat es have the

advantage of being able to move between the two layers, taking advantage of both.

Large plants are called macrophytes. Most are from the plant kingdom, although some

are large algae (Table 15.4 ). Roots may be used for obtaining nutrition from the sediment

or may be primarily for attachment. Nutrients can be obtained directly from the water.

Besides their obvious role as primary producers, macrophytes are important as shelter

and substrate for other organisms. Periphyt on, which in this case are considered epiphytic,

grow on the stems and leaves of the macrophytes. Protozoans roam the epiphytic cultures,

and snails graze among them. Amphibians and small ﬁsh ﬁnd shelter from predatory ﬁsh

and birds. The macrophytes contribute greatly to the detritus food chain as dead plants

settle to the bottom. Freshwater angiosperms will grow only at shallow depths. Deeper

plants consist of ferns, mosses, and large algae. Emergent plants are those that grow

Figure 15.7 Diatoms: ðaÞ Surirella; ðbÞ Asterionella; ðc; dÞ Gomphonema; ðeÞ Tabellaria; ðf ; gÞ

Navicula; ðhÞ Eunotia; ðiÞ Cymbella ; ðjÞ Fragilaria; ðkÞ Gyrosigma. (From Standard Methods, 10th

ed. # 1955 American Public Health Association.)

510

ECOSYSTEMS AND APPLICATI ONS

out of the water. Some plants have only reproductive parts that are emergent. Many plants

that are primarily emergent occur in marshe s and are discussed in Section 15.3.

Zooplankton and Invertebrates Aquatic ecosystems can have all the trophic levels at

microscopic size, as we are accustomed to seeing on a large scale in terrestrial ecosys-

tems. The zooplankton include protozoans, small crustaceans, rotifers, and other small

invertebrates (Figures 15.9, 15.10, and 15.11). These may have roles of herbivore and pri-

mary and secondary carnivore. The nymph or larvae of many insects are aquatic and have

similar roles.

Protozoans are widely dispersed, and tend to be most abundant in waters with large

amounts of organic matter (Table 15.5). They feed on detritus and other single-celled

organisms, whether bacteria, algae, or other protozoans. Some are obligate parasites,

with hosts ranging from algae to ﬁsh. The presence of parasites can change the species

composition of an aquatic ecosystem, such as by bringing an algal bloom to an end.

Rotifers are among the more interesting creatures to observe under a microscope as

they move and feed. Most are sessile (nonmotile); many are motile but attach themselves

Figure 15.8 The dinoﬂagellates Peridinium and Ceratium.

FRESHWATER ECOSYSTEMS 511

periodically to substrates b y several ‘‘toes.’’ A few have a ring of cilia that gives the

impression of circular motion (accounting for the name) as it sweeps in suspe nded matter

for food. Some are parasites or predators.

Arthropods include two important aquatic groups: the crustaceans and the insects. Both

have exoskeletons com posed of chitin. Most of the larger zooplankton are crustaceans

TABLE 15.4 Common Aquatic Macrophytes

Habitat Taxonomic Group Examples

Free-ﬂoating

Subtropical and tropical lakes, slow streams Monocot Pistia (water lettuce)

Monocot Eichhornia (water hyacinth)

Fern Salvinia (water fern)

Temperate ponds Fern Azolla (water fern)

Monocot Lemna (duckweed)

Rooted

Temperate marshes Monocot Phragmites (giant reed)

Dicot Rorippa (watercress)

Tropical marshes Monocot Scirpus (bulrush)

Monocot Papyrus (reed)

Dicot Victoria (water lily)

Ponds and slow streams Dicot Nuphar (water lily)

Moncot Potamogeton (pond weed)

Dicot Ceratophyllum (coontail)

Estuaries Dicot Myriophyllum (milfoil)

Monocot Zostera (eel grass)

Monocot Ruppia (widgeon grass)

Deep in oligotrophic lakes Lycosida Isoetes (quillwort)

Large algae Chara (stonewort)

Large algae Nitella (stonewort)

Moss Fontinalis (willow moss)

Source: Horne and Goldman (1994).

Figure 15.9 Protozoans. Amoebae: ðaÞ Amoeba; ciliates: ðbÞ Paramecium, ðcÞ Frontonia; stalked

ciliates: ðdÞ Carchesium, ðeÞ Epistylis, ðf Þ Vorticella; ﬂagellates: ðgÞ Peranema, ð hÞ Chilomonas,

ðiÞ Astasia. (From Standard Methods, 10th ed. # 1955 American Public Health Association.)

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ECOSYSTEMS AND APPLICATI ONS

(Table 15.6). Cladocerans, such as Daphnia, and copepods, such as Cyclops, combine

with protozoans to form the majority of aquatic zooplankton.

The presence of some of the insect larvae or nymphs are indicators of a healthy aquatic

environment (Table 15.7 and Figure 15.12). Examples include nymphs of stoneﬂies, may-

ﬂies, and caddisﬂy larvae. Recall that a nymph is an immature stage of that minority of

insects that go through gradual metamorphosis. It is usually similar in form to the adult,

except possibly lacking the adult’s wings, coloration, and sexual maturity. The immature

form of insects that go through complete metamorphosis, called a larva, may be very

different in form (often, a worm or caterpillar) and habitat from the adult. The caddisﬂy

larvae may be housed in a case made of bits of plant matter, as shown in Figure 15.12.

The insects occupy trophic levels between the primary producers and the ﬁsh. Hatching

of adult insects may occur en mass and results in a feeding frenzy among the ﬁsh.

Benthos The benthos contains o rganisms on and beneath the surface of the sediment.

Their food web is based on detritivores: bacteria, fungi, protozoans, and invertebrate ani-

mals. The source of the energy is the primary productivity of the water column, for lakes,

or the watershed, for rivers. Transport of pollutants through the pores of the sediments

cannot be predicted using the diffusion methods from groundwater pollution hydrology.

This is because burrowing organisms actively mix and transport chemicals in sediment

(and the sediment itself), move water through the sediment, and provide channels for

increased circulation and diffusion.

Two distinct benthic environments are the littoral and the profundal, according to

whether they are above or below the thermocline, respectively. The littoral zone tends

to have higher species diversity, although the profundal zone may have greater total num-

bers of organisms. The littoral benthos is subject to signiﬁcant daily and seasonal changes.

The profundal benthos may experience seasonal changes in oxygen in eutrophic lakes.

The littoral benthos consist of relatively more herbivores, grazers, and ﬁlterers than the

profundal. Macrophytes can greatly increas e the available habitat. Wave action and the

proximity of the surface usually ensure an ample oxygen supply.

The profundal benthos consist mainly of worms and mollusks in four groups: oligo-

chaete worms (only in cold waters), amphipods (crustaceans, including mys id shrimp),

insect larvae, and clams (Figure 15.13). Most of these are detritus feeders, although

there may be some predators, such as the midges. Most of the food input to the profundal

benthos comes from the spring algae bloom, especially diatoms, which sink readily . The profun-

dal benthos can store large amounts of lipids to get them through the rest of the year.

Figure 15.10 Planktonic crustaceans: ðaÞ the cladoceran Daphnia; ðbÞ the copepod Cyclops;

ðcÞ the ostracod Cipridopsis. (From Standard Methods, 10th ed. # 1955 American Public Health

Association.)

FRESHWATER ECOSYSTEMS 513

Figure 15.11 Other invertebrates: (1) hydrachnid, or water mite; (2) water spider; (3) gasterotrich,

Chaetonotus; (4) coelenterate, Hydra; (5) tardigrade, Macrobiotus; (6) bryozoan, Plumatella; (7),

bristle worm, Nais; (8) sewage worm, Tubifex; (9) leech, Clepsine; (10) ﬂatworm, Planaria; (11)

colonial rotifer, Conochilus; (12) nematode worm; (13) freshwater sponge; (14) gommules and

spicules from the sponge. (From Standard Methods, 10th ed. # 1955 American Public Health

Association.)

514

ECOSYSTEMS AND APPLICATI ONS

The differences in adaptation between littoral and benthic organisms can be repre-

sented by a comparison of the littoral amphipod Gammarus pulex and the benthic chiro-

nomid midge Chironomus anthracinus. Midges are larvae of predatory gnats. They feed

on rotifers and burrow into the sediments at night. When the water is saturated, both

species use oxygen at the rate of about 250 mg/g. But for Gammarus the rate decreases

proportionally as oxygen saturati on in its environment decreases. At 30% of saturation its

respiration rate drops from to about 100 mgO

per gram of organism. The mi dge, on the

other hand, maintains a fairly constant respiration rate down to 30% saturation; 100 mg/g

TABLE 15.5 Some Important Aquatic Protozoans

Phyla Habitat Examples

Flagellates (some are algae) Lakes; oceans; some are Synura (Chrysomonad)*

parasites Ceratium (Dinoﬂagellate)*

Euglena*

Oikomonas

Ciliates Ponds, streams, detritus, Paramecium

some parasitic Vorticella

Amoeboid Most waters Difﬂugia

Globigerina (Foraminifera)

Actinosphaerium (Radiolaria)

Vampyrella (algal parasite)

Sporozoans (all parasitic) Aquatic organisms Henneguya (on ﬁsh)

Plasmodium (human malaria)

Source: Horne and Goldman (1994).

TABLE 15.6 Common Aquatic Crustaceans

Taxonomic Most Common

Group Habitat

Feeding Method Examples

Cladocera FW, P Predator Leptodora

(water ﬂeas) FW, P Filter Daphnia

FW, P Filter Bosmina

FW, P, B Filter Sida

FW, P, B Filter Chydorus

FW, S, P Predator Polyphemus

Copepoda FS, S, B Filter and parasitic Nitocra

FS, S, B, P Predator on animals Cyclops, Megacyclops

Predator on algae Thermocyclops, Eucyclops

FW, S, P Filter Limnocalanus, Boeckella, Diaptomus

Mysidacea FW, P Predatory and Misis

(opossum shrimp) detritovore

Amphipoda S, FW, B Predatory Gammarus

Astacus

Decapoda S, FW, B Predatory Pacifastacus

(freshwater crayﬁsh) Cancer

FW, freshwater; S, saline; B, benthic or attached; P, planktonic.

Source: Horne and Goldman (1994).

FRESHWATER ECOSYSTEMS

515

is not reached until about 5% of saturation. This is attributed to the presence of hemoglo-

bin in the blood of the midge, which improves the efﬁciency of oxygen transfer.

Other invertebrates include various worms and mollusks (Table 15.8). Included are

nematodes, oligochaetes, leeches, bivalves (clams), and snails. Most are detritivores.

Leeches are predators that attach to ﬁsh. Some nematodes are internal ﬁsh parasites.

Bivalves are becoming uncommon in American rivers. Snails feed on macrophytes and,

along with insect larvae, can control their abundance. Tubifex can become so abundant in

highly polluted streams that they form red mats on the bottom.

The zebra mussel, Dreissena polymorpha , is an exotic species imported to North

America from Europe. It probably hitched a ride in the ballast water of a ship that released

it near Detro it, Michigan, in 1985 or 1986. Since then it has been spreading rapidly, car-

ried by currents and shipping. Zebra mussels cause great ecological and economic harm

TABLE 15.7 Characteristics of Some Common Insect Larvae

Common Name and Order Habitat Food Habits Examples

Nymphs

Stoneﬂies (Plecoptera) Rapids Mainly Nemoura

carnivorous

Mayﬂies (Ephemeroptera) All waters Mainly Baetis, Ephemerella

herbivorous

Damselﬂies (Odonata) Slow and stagnant Carnivorous

Dragonﬂies (Odonata) Slow and stagnant Carnivorous Libellula

Water bugs (Hemiptera) All waters Carnivorous Notonecta (backswimmer)

Gerris (water strider)

Larvae

Caddisﬂy (Trichoptera) All waters Carnivorous

Water beetles (Coleoptera) Slow or stagnant Carnivorous Dytiscus (diving beetle)

Order Diptera (and family)

Mosquitoes Pools at surface Herbivorous Culex

Blackﬂies Rocks in rapids Herbivorous Simulium

True midges (chironomids) All waters Herbivorous Chironomus

Horseﬂies Beds in pools Carnivorous

Source: Needham and Needham (1962); Horne and Goldman (1994).

Figure 15.12 Insect larvae and nymphs: ðaÞ Ephemeroptera (mayﬂy); ðbÞ Diptera, Chironomus; ðcÞ

Trichoptera (caddisﬂy); ðd; eÞ Trichoptera case. (From Standard Methods,10thed.# 1955 American

Public Health Assoc., Inc.)

516

ECOSYSTEMS AND APPLICATI ONS

by occupying growing surfaces formerly used by other organisms and by growing on

intake structures of power plants and drinking water plants, obstructing their ﬂow.

Fish and Fisheries Fish occupy and often dominate all the trophic levels from detriti-

vore to herbivore to top carnivore (Table 15.9). They serve as prey to other ﬁsh and to

aquatic birds or other animals. Since streams receive most of their nutrient input from

terrestrial sources, harvesting of ﬁsh by b irds, bears, or humans represents a return of

some of those nutrients. Fish are the most motile of aquatic species; thus, they can live and

breed in different places, and move in response to changes in food supply or other conditions.

Freshwater ﬁsh are hypertonic: Their bodily ﬂuids are more saline than the water they

swim in. Thus, they tend to absorb water by osmosis and lose salts by passive diffusion.

To counter this, they discharge large volumes of dilute urine, and their cells absorb salt by

active transport. They do not drink.

Figure 15.13 Zoobenthos of the profundal zone. (From Horne and Goldman, 1994. # McGraw-Hill,

Inc. Used with permission.)

TABLE 15.8 Common Invertebrates Other Than Insects

Taxonomic Group Feeding Method Examples

Turbellaria (ﬂatworms) Carnivores Dugesia

Nematodes (roundworms) Carnivores, herbivores, parasites Dolichodorus

Annelida

Oligochaeta (segmented worms) Sediment grazers Tubifex (red worms)

Hirudinea (leeches) Carnivores, detritivores Haemopis (horse leech)

Mollusca

Gastropoda (snails) Grazer Limnaea (pond snail)

Planorbis (ramshorn snail)

Palacypoda (bivalves) Filterer Anodonta (swan mussel)

Crustacea

Malacostraca Detritivores Gammarus

(crayﬁsh, amphiphods) Astacus

Source: Horne and Goldman (1994).

FRESHWATER ECOSYSTEMS

517