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

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Fish tend to feed selectively in the pelagic, littoral, or benthic zones. Food sources

include phytoplankton, zooplankton, detritus, or other ﬁsh. Cannibalism is not uncom-

mon, especially by adults on the young. Pelagic ﬁsh usually feed at the surface. Shad her-

ring, whiteﬁsh, and minnows feed almost entirely on zooplankton. Some of the large

predators wi ll also depend on zooplankton in their early life stages. Tilapia feeds on the

blue-green alga Microsystis aeruginosa. This makes it a candidate for producing meat on

manned space ﬂight, since the algae can also help close the nitrogen cycle on spacecraft.

Most ﬁsh live for many years, so the biomass is not subj ect to seasonal cycles. How-

ever, survival of the young produced in a single year, the cohort, depends on predation,

the availability of food for the young (usually plankton), and environmental factors. In the

presence of intense predation, juvenile ﬁsh may restrict their activity to macrophyte beds

for protection. Others may venture into deep water to feed on zooplankton in the early

hours of the day, retreating to the littoral zone when daylight makes them easy targets

for predatory ﬁsh. Others move from the depths to the surface and back in a similar way.

Floods and droughts may affect the availability of nesting sites. In tropical rivers, which

are much less affected by human activity than other rivers of the world, some ﬁsh depend on

the ﬂoodplains to a great degree. They often breed there and feed heavily during ﬂoods.

When there are a large number of species with similar food requirements, species often

evolve to avoid direct competition by specializing in a subset of the available food. This is

called resource partitioning. In the case of ﬁsh, for example, several species that eat zoo-

plankton may restrict their hunt to particular habitats. One species of ﬁsh may only eat

organisms from the benthic zone, others only from the pelagic surface, and others only

from the littoral zone.

Some ﬁsh are anadromous, meaning that they live most of their lives in salt water but

spawn in fresh water. The best known example is the salmon, which remember the

‘‘smell’’ of the stream that they hatched in after several years in the ocean. They even

remember the odor associated with each tributary they passed as a hatchling, and follow

them back in reverse order. The six species of Paciﬁc salmon die after spawning; Atlantic

salmon and steelhead trout return year after year. The movement of grown ﬁsh can also be

viewed as a transport of nutrients from the sea to the relatively nutrient-poor headwater

streams.

TABLE 15.9 Some Common Families of Fish

Family Examples

Lampreys Sea lamprey

Anguillids Freshwater eel

Herrings Alewife, herring

Salmonidae Salmon, trout, whiteﬁsh

Cyprinids Minnows, carp

Catostomids Suckers

Ictalurids Catﬁsh

Cyprinodontids Killiﬁsh

Gastersteids Sticklebacks

Percichtyidae Striped bass

Centrachids Sunﬁsh, largemouth black bass

Cichlids Tilapia

Gadidae Cod

Source: Horne and Goldman (1994).

518 ECOSYSTEMS AND APPLICATIONS

Other ﬁsh are catadromous—they live in fresh water but return to the sea to spawn.

The eels of the North Atlantic are a remarkable example of catadromous ﬁsh. For centu-

ries it was observed that the adults all left their streams in the fall for the ocean, and in the

spring a large upstream migration of matchstick-sized elvers appeared. Finally, in the

early part of the twentieth century it was realized that all the eel s from North America

and Europe swam to the Sargasso Sea near Bermuda. There they spawned and the eels

died. Over three years the larvae drift with the Gulf Stre am and metamorphose into elvers

and swim up the estuaries and streams of the North Atlantic. The males stay in the estu-

aries and the females continue upstream. They remain for 8 to 15 years before returni ng to

their place of hatching. The North American and European species are distinct from each

other, although they breed in overlapping areas of the Sargasso Sea and sort themselves

out for the reverse migration.

Many human activities have disrupted ﬁsh populations around the world. The ﬁrst

apparent effect is usually on populations used for food supply. Direct chemical alteration

by pollution can result in eutrophication or toxic exposures. Both can cause ﬁsh kills, the

former by deoxygenation, the latter by direct toxic effect. Fish can tolerate dissolved oxy-

gen levels down to about 4 to 5 mg/L. If not severe, eutrophication can increase ﬁsheries.

Exotic species disrupt the food chain. They may be introduced deliberately or inadver-

tently. The p eacock bass, Cichla ocellaris, was introduced to a lake in Panama as a

sport ﬁsh. Subsequently, 11 other species suffered serious declines, and gulls and herons

lost their main food sources. The loss of several larvae feeders has resulted in an increase

in mosquitoes in the area.

Inadvertent introductions may occur as a result of constructed waterways such

as canals. The alewife and the sea lamprey were introduced to the Great Lakes in

this way after having been limited by Niagara Falls. The sea lamprey, Petromyzon

marinus, is a parasitic eel that attaches itself by its mouth to the side of a ﬁsh and

feeds on the ﬁsh’s body ﬂuids. The alewife, Alosa pseudo harengus, competes with

lake herring for zooplankton. The lampreys killed off the larger ﬁsh of the popular

salmonid species, forcing ﬁshermen to take smaller sizes. This gradually elim inated the

breeding stock, until the ﬁshery suddenly collapsed. Overﬁshing also caused depletion of

some ﬁsheries, such as sturgeon, which were not related to the introduction of exotic

species.

On the other hand, construction of dams has created barriers to ﬁsh that need to

migrate, such as the salmon. Dams may also ﬂood breeding grounds. Flood control elim-

inates those species that depend on ﬂooding to breed. Development of the river shore

eliminates overhanging trees which shade and cool, as well as provide falling insects

that some ﬁsh depend on, and leaf fall which supplies the detritivores.

Fisheries management is a set of strategies to ame liorate some of these problems.

Fish ladders are artiﬁcial cascades that can be constructed alongside dams to provide

routes for migrating ﬁsh. Reservoir releases are controlled with downstream ﬁsheries in

mind, controlling ﬂow and temperature. Gravel beds are constructed in streams to furnish

ﬁsh nesting sites. Competition by overabundant centrarchids in reservoirs can be con-

trolled by lowering the reservoir level at the appropriate time to strand nesting areas.

Log jams and fallen trees (snags) serve as sources of invertebrates for migrating ﬁsh,

as well as resting places in migration. They used to be cleared routinely, but now that

their role is understood, they are left undisturbed by ﬁsheries managers. Finally, nutrients

are sometimes introduced deliberately to increase primary productivity, ultimately

increasing the productivity at higher levels of the food chain as well.

FRESHWATER ECOSYSTEMS 519

Another important tool of ﬁsheries management is regulation of ﬁshing. A rule of

thumb for harvesting ﬁsh is that the maximum sustainable yield is obta ined if about

one-third of the population is taken in each reproductive period. This has been found

in experimental systems to result in an equilibrium biomass that is slightly less than

50% of the unexploited ﬁshery. The optimum was found to be independent of the carrying

capacity. However, the situation is sometimes complicated by interactions with other spe-

cies. Harvesting a single species can tip the balance toward a competitor. For example,

heavy ﬁshing of the Paciﬁc sardine (Sardinops caerulea) resulted in an increase in the

less valuable anchovy (Engraulis mordox).

Natural disturbances to ecosystems tend to show their strongest effects at the lowest

trophic levels. The disturbance tends to dampen as it goes up the food chain, since

each level often has alternative food supplies. On the other hand, removal of top carni-

vores tends to have impacts throughout the food chain, even disturbing phytoplankton

populations. Fishing usually concentrates on carnivorous species and thus is an example

of the latter type of disturbance.

15.2.3 Succession in Lakes

The epilimnion has plenty of oxygen since it is in contact with the atmosphere. It is also

fairly productive since it has more sunlight available. However, biomass production gra-

dually reduces nutrient availability during the growing season. Sedimentation of dead

organisms transports the nutrients to the hypolimnion or to the sediments. Degradation

reduces oxygen and releases nutrients in the hypolimnion. However, twice a year in the

temperate zone, in spring and fall, these differences disappear as temperature changes

destroy the stratiﬁcation. The resulting mixing is called turnover (not to be confused

with the idea of turnover time in nutrient cycling). The turnover returns nutrients to the

surface and oxygen to the depths.

Spring turnover brings nutrients and light together with a warming environment. This

initiates successional changes in populations. A sequence of species goes through boom-

and-bust cycles. Each ﬂourishes temporarily as it enjoys a competitive advantage over the

others. Eventually, it depletes some necessary reso urce or comes under attack by zoo-

plankton predators or fungal parasites , and the population crashes. This leaves the ﬁeld

for another opportunist, which can best compete in the resulting environment.

The largest total algal populations occur shortly after turnover. Large, sudden increases

in phytoplankton populations are called algal blooms. During an algal bloom, cell counts

on the order of millions per liter can color the lake green. The diatom Asterionella is often

the ﬁrst algae to bloom as longer days in spring increase the sunlight available. It is faster

growing than its competitors Fragilaria, Tabellaria, and the cyanobacter, and it performs

luxury uptake of phosphorus, up to 100 times its immediate requirement. However, it is

susceptible to fungal attacks, which can give Fragillaria and Tabellaria a temporary

advantage. If the ﬁlamentous cyanobacter Oscillatoria gains a foothold, it can shade

Asterionella, outcompeting it for sunlight. Ultimately, silica limits Asterionella blooms

(Figure 15.14). Unless some other nutrient becomes limiting, Asterionella stops growing

when the silica concentration falls below 0.5 mg/L.

Because they can ﬂoat on the surface, cyanobacter may gain an advantage when tur-

bidity limits light to the green algae. A cyanobacter bloom may continue into the summer

as nitrate and ammonia are depleted, since they can ﬁx atmospheric nitrogen. Eventually,

nutrients become depleted even for the cyanobacter. Iron is often one of the limiting

520 ECOSYSTEMS AND APPLICATIONS

Log of number of phytoplankton cells

Total cells

Various

diatom

species

Diatoms,

dinoflagellates

and

cyanobacter

Small

flagellates and

cyanobacter

0 50 100 150 200 250 300 350

ear

Zooplankton grazing rate

Crustaceans,

rotifers,

protozoans,

etc.

Spring

turnover

Fall

turnover

Thermal stratification

Temperature

Nutrients

Figure 15.14 Typical algal succession. (Based on Horne and Goldman, 1994.)

FRESHWATER ECOSYSTEMS 521

nutrients. It is needed for the nitrogenase enzyme for nitriﬁcation. Anabaena and Micro-

cystis often form late summer–autumn blooms as nutrients are recycled from sediments or

the hypolimnion.

When the fall turnover comes, large numbers of the diatom Melosira can be resus-

pended from the sediments, becoming the dominant alga over the winter period. During

the summer they settle back to the sediment before nutrient limitation begins to affect them. This,

plus their tolerance of anoxic conditions, enables them to survive in the sediment.

Phytoplankton growth responds most strongly to light availability. But zooplankton

feed at a rate strongly related to the temperature. Thus, they often peak later than the phy-

toplankton from the spring bloom, bringing it to an end. The elimination of phytoplankton

brings about the clearwater phase, during which the lake is most transparent. This initial

peak in zooplankton is usually dominated by cladocerans such as Daphnia or Bosmina.

Other animals present in this peak include predatory rotifers, carnivorous and herbivorous

copepods, and protozoans. Herbivorous rotifers follow the clearwater phase as they can

eat larger and harder-to-digest algae, as well as toxic or repulsive cyanobacter rejected by

the cladocerans.

15.2.4 Microbial Loop

Many of the aquatic organisms are omnivorous, complicating application of the concept

of trophic levels. Figure 15.15 shows a food web for a small lake with organisms or

groups arranged by trophic level. This lake is managed for ﬁsh production and thus

receives an artiﬁcial energy input. Notice the large loss to the system from the emergence

of chironomid insects.

As has been mentioned, most of the nutrients in the photic zone of lacustrine and mar-

ine ecosystems tend to be tied up in living things. This would place a severe limitation on

further growth except for the existence of an efﬁcient detrital recycling process. The

Water Air or

Land

Phytoplankton

or C

Adult insects

emerge into air

Chironomid larvae

(partially benthic)

730,000

7400

900

1000

Trophic Level

Sunfish

Lepomis

Chaoborus larvae

(predatory insect)

Adult insects

emerge into air

Zooplankton

Humans

Bass

Terrestrial Insects

(grasshoppers, worms)

200

Figure 15.15 Energy food web for a managed lake. The values represent kilocalories per square

meter per year. (From Horne and Goldman, 1994.)

522

ECOSYSTEMS AND APPLICATI ONS

process known as the microbial loop involves the rapid uti lization by bacteria and pro-

tozoans of organic matter liberated as waste products by all of the other aquatic organisms

(Figure 15.16). This loop cycles nutrients largely independently of the phytoplankton,

enabling growth to continue in the summer when the algal blooms have disappeared.

The waste products are organics excreted by zooplankton and ﬁsh.

Another important source of soluble organic matter for the microbial loop are the

extracellular products of photosynthesis (ECPP). These are organics such as glyc olic

acid that are produced by photosynthesis and leaked continuously by phytoplankton.

The leakage has been measured to average from 7 to 41% of the carbon ﬁxation rate.

Nutrients are also recycled by bacterial and fungal degradation of dead organisms.

These mechanisms are very important in the low-productivity areas of oligotrophic

lakes and in the open ocean.

15.2.5 River Productivity

Riverine food webs are often dominated by allochthonous inputs from terrestrial sources.

Coarse particulate organic matter (CPOM) from fallen leaves, dry grasses, and so on,

are low in nitrogen. They require nutrients from the stream for their initial degradation.

Otherwise, they are not readily available to the food chain. Nutrient availability, and

therefore productivity, tends to be higher in hard-water areas (with high calcium and

magnesium) than in soft-water regions.

Degradation of the CPOM is accomplished by a coating of fungi and bacteria that

adheres to the surface. They break down the cellulose and lignin which cannot be digested

by animal s. Lignin is degraded mostly by fungi, including species adapted to function in

the winter cold, to take advantage of the annual leaf fall. Invertebrates ingest the CPOM,

but digest only the attached fungi and bacteria. The CPOM is excreted, and the coating

regrows. In repeated cycles of degradation, ingestion, and excretion, the CPOM is

Phytoplankton

Cladocerans

Rotifers

Copepods

Fish

Dissolved Organic Carbon

508

Ciliates

168

From loop

Heterotrophic

Microbial Loop

Conventional

Autotrophic/Heterotrophic

Food Chain

Flagellates

Bacteria

Figure 15.16 Carbon cycle in a lake during the summer. Values indicate grams carbon per square

meter per day, ignoring respiration and cannibalism. (Based on Horne and Goldman, 1994.)

FRESHWATER ECOSYSTEMS 523

gradually broken down into ﬁne particulate organic matter (FPOM) and then to dis-

solved organic matter (DOM) until it has entered the food chain completely.

The invertebrates that feed on the allochthonous input fall into four groups. Grazers

scrape bacteria, fungi, rotifers, and periphyton from surfaces. Examples of grazers are

snails and caddis ﬂies. Shredders cut and ingest pieces of leaves and stems which

form the CPOM, reducing them in size. They include the stoneﬂies, amphipods, and cray-

ﬁsh. Colle ctors and ﬁlterers take FPOM from the water. Clams pump water through ﬁl-

tration organs, expending considerable energy. Midge larvae use fanlike collecting

structures to ensnare particles. Caddisﬂy larvae spin long nets to capture FPOM from

the current. Most benthic invertebrates simply use bristles on their appendages or mouth-

parts. Worms feed on FPOM that has settled to the bottom in more quiescent areas.

Predators range from stoneﬂy larvae and dragonﬂy nymphs to ﬁsh and birds of prey.

Figure 15.17 shows a schematic food web for this ecosystem. This ﬁgure shows three

kinds of inputs. Direct light energy fuels the autochthonous primary productivity.

Dissolved organics may originate in decay products that run off from the land and provide

an allochthonous input for microorganisms. Large organic matter feeds another allochtho-

nous input to the cycle, involving the shredders that break the CPOM down to FPOM,

ultimately providing food for the scrapers and the ﬁlter feeders. Finally, of course, each

group of invertebrates is subject to predation by carnivores.

During the day, most invertebrates hide under or between rocks to avoid predation.

However, at night many of these will join the current to migrate downstream. Together

with FPOM they form drift, which is an important food source for larger collectors

and ﬁsh. After a lifetime of drifting downstream, many insect larvae may emerge into

the air, then ﬂy upstream to lay their eggs, repeating the cycle.

According to the river continuum concept, the balance between allochthanous and

autochthanous input changes gradually from small headwaters to large downriver condi-

Large allochthonous input

(leaves, seeds, etc.)

Dissolved organic

allochthonous input

Light energy and

nutrient input

Micro- and macrophyte

autochthonous input

Bacteria and fungi

Predators

Bacteria and fungi

Shredders

(crayfish, stoneflies)

Filter feeders

(midge larvae, clams)

Scrapers and Grazers

(snails, caddisflies)

Coarse Particulate

Organic Matter

(CPOM)

Fine Particulate

Organic Matter

(FPOM)

Figure 15.17 Allochthonous food web with functional groups. (Modiﬁed from Horne and

Goldman, 1994.)

524

ECOSYSTEMS AND APPLICATI ONS

tions. As streams merge and move farther from their sources, FPOM increases relative to

CPOM and the shredders give way to collectors and ﬁlterers. Farther downstream the river

may acquire an autochthanous character based on attached algae and macrophytes, with

grazers and collectors dominating. Finally, the river may becom e more like a lacustrine

environment, dominated by collectors living off phytoplankton. The ratio of production to

respiration exceeds 1, whereas respiration dominates at other parts of the river. Diversity

is also greatest in this section. Figure 15.18 illustrates these changes with increase in river

024681012

Stream/River Order

FPOM/CPOM

25%

50%

75%

100%

024681012

Stream/River Order

Percent of total invertebrate population

Shredders

Grazers

Predators

Collectors

Leaves, twigs

Fragments

Refractory particles

& dissolved organics

1.5m 10m 60m (River width) 700m

Figure 15.18 River continuum concept. (Based on Horne and Goldman, 1994.)

FRESHWATER ECOSYSTEMS 525

order, which can be roughly described as the number of branches in the river counting

from the headw aters and incrementing the count whenever equal-order tributaries merge.

In large higher-order rivers, and especially in warmer climates, the low productivity in

the main channel may be compensated for by a large input from the periodically inun-

dated ﬂood plain along the banks. Flooding results in a large input of nutrients and detri-

tus. Many ﬁsh spawn in the ﬂoodplains, and rivers with large ﬂoodplains tend to have

more diverse ﬁsh populations.

15.2.6 Nutrients and Eutrophication in Lakes

Aquatic systems are also classiﬁed according to their status with respect to nutrients or

productivity (Table 15.10). Lakes that are relatively nutrient limited and low in produc-

tivity are called oligotrophic (‘‘poorly fed’’). Lakes with higher nutrient levels and higher

productivity are called eutrophic (‘‘well fed’’). An intermediate designation is called

mesotrophic.

Basically, an oligotrophic lake is one that would be perceived as ‘‘clean’’ vs. the rela-

tively murky eutrophic lake. However, eutrophic does not necessarily mean polluted.

Lakes proceed through a natural succession, starting as oligotrophic. Gradually, they

accumulate sediments and nutrients. The shoreline grows more and more macrophytes

as the lake becomes shallower. Eventually, they become marshes or wetlands, until

they are ﬁnally transformed into terrestrial ecosystems.

Although it occurs naturally, addi tion of anthropogenic nutrients hastens this process.

People contribute nutrients in the form of fertilizer runoff from lawns and agricultural

lands, human waste from either treatment plant discharge or indirectly from septic

tanks, and animal waste from pasture or feedlot runoff. In these cases the process is

termed cultural eutrophication . Oligotrophic lakes are preferred for numerous human

TABLE 15.10 Comparison of Oligotrophic and Eutrophic Lakes

Criteria Oligotrophic Eutrophic

Phosphorus conc. (winter) P < 10 mg/L P > 20 mg/L

Organic matter Low High

Primary productivity GPP < 150 g C/m

yr GPP > 250 g C/m

yr

Chlorophyll a < 3 mg/L Chlorophyll a > 6 mg/L

Depth <15–25 m <10–15 m

Clarity Clear, blue, Secchi depth > 5 m Dark, turbid, Secchi depth < 3m

Hypolimnion dissolved >50% saturation <10% saturation (about 1 mg/L)

oxygen in summer

Bottom fauna Diversiﬁed with deepwater ﬁsh Low-DO-tolerant organisms;

no deepwater ﬁsh; carp

Bottom sediment Low organic matter content Organic rich muck, high in N

Characteristic alga or genera Green algae Cyanobacter (Anabaena,

Desmids (Staurastrum) Aphanizomenon, Microsystis)

Diatoms (Tabellaria, Cyclotella) Diatoms (Molosira,

Chrysophyceae (Dinobryon) Stephanodiscus, Asterionella)

Examples Lake Superior Western Lake Erie

Lake Geneva (Switzerland) Lake Lugano (Switzerland)

Lake Baikal (Russia)

Source: Berner and Berner (1987); Conell and Miller (1984).

526 ECOSYSTEMS AND APPLICATIONS

purposes, whether as a source of drinking water, food, or recreation. Among the problems

caused by cultural eutrophication: Large plant biomass can cause oxygen depletion at

night or upon death of the plants. High concentrations of algae cause taste and odor

problems in water supplies. Some algal blooms can be toxic to animals that drink the

water. Toxicity or deoxygenation can cause ﬁsh kills. Macrophytes can physically

choke the lake, interf ering with uses such as boating or swimming. Table 15.11 shows

several examples of lakes in different categories of nutrient loading.

Primary produc tivity is limited by nutrient availability, especially phosphorus.

Nitrogen is less often limiting because cyanobacter (blue-green bacteria) ﬁx N

. Never-

theless, dosages of nitrogen and phosphorus that individually stimulate a doubling in phy-

toplankton growth produce a ﬁvefold increase when added simultaneously. Other potential

limits to plant growth include silicon, CO

, iron, and molybde num.

Phosphorus concentration in a lake can be computed from material balance considera-

tions. Loss of phosphorus from the water column by sedimentation is usually accounted

for by a reaction term. The peak summer algae concentration, measured in terms of the

average chlorophyll a concentration, shows a strong log-log correlation with the phos-

phorus concentration in the water during the spring turnover. A typical such relation is

¼ 0:37 P

0:91

ð15:1Þ

where the phosphorus concentration ðP

Þ and the chlorophyll a concentration ðC

Þ are

both expressed in mg/L (see Example 14.2).

One source of phosphorus in the lake water column that is difﬁcult to quantify origi-

nates from the sediment. Phosphorus that has been deposited in the past may be liberated

again, producing what is known as internal loading. This phosphorus results from an

interaction with sulfur and iron in the hypolimnion and the sediments of lakes (see

Figure 15.19). When spring turnover brings oxygen to the depths, ferrous iron is oxidized

to ferric, which precipitates with phosphorus and settles to the sediment. During summer

stratiﬁcation, oxygen is limiting and anoxic conditions may form in the sediment. The ferric

iron and sulfate are reduced to ferrous and sulﬁde ions, respectively, liberating the phos-

phate. The sulﬁde precipitates the iron and phosphate is released to the hypolimnio n.

TABLE 15.11 Trophic Status of Several Lakes

Avg HRT Secchi Chloro-

Lake Status

Depth (m) (yr) NO

2

-N NH

-N P SiO

(m) phyll a

Superior O 149 184 220 <10 0.5 2000 11.3 0.9

Huron O 59 21 180 0.5 800 5–9 1.8

Michigan O–M 85 104 130 5 700 4.3 1.3

Erie E 1.4 3 23 1 31 2–4.4 5.4

Ontario M 86 8 40 100

Tahoe O 313 700 4 <2 2

Windermere 24 0.75 300–400 10 5

(UK)

George (Uganda) 2.4 0.34 nd <10 2

Baikal (Siberia) 730 3000

O, oligotrophic; M, mesotrophic; E, eutrophic; concentrations are summer measurements in mg/L; nd,

nondetectable.

Source: Horne and Goldman (1994).

FRESHWATER ECOSYSTEMS

527