01 READING Streams Energy Resources

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C H A P TE R 3

Energy

Resources

Int rod uct ion

In st re am ( Autoc h thonous) Energy S ource s

Te rrestrial (A llochthonous) En er gy S our ces

D is so lv ed O rga nic Matte r

A Prel ude to Food W ebs

Recom mended Rea di ng

I N T R O U T I O N

Chapters 1 and 2 set the stage for our understanding of the biological corn-

munities found in r ivers and streams. The various physical and chemical

factors in a river act as limiting or controlling factors to biological activity

beca use they influence which species can survive, and thr ive, in a particular

location. The amount and diversity of energy supplies are another important

factor that determines which and how many organisms will be found in a

particular stream. In this chapter, and elsewhere in this book, we will explore

the various sources of energy for aquatic food webs-how it is produced, and

the different ways it becomes available to consumer organisms.

An important first distinction is that between

autot roph s,

which

produce their own energy from inorganic matter, and

heterotroph s,

which

derive their energy from autotrophs. With a very few exceptons, such as

certain deep-sea bacteria that use a process of

chemos ynth es is

to derive

energy from hydrogen sulphide, autotrophs are plants. Organic carbon

3. En ergy Resour ces

33

compounds are formed out of carbon dioxide and other inorganic matter,

capturing the energy of sunlight via the process of

photosy nthes is.

We call

this

primar y

prod uction beca use it creates new organic matter from inor-

ganic precursors. Al organisms unable to synthesize energy from inorganic

matter, obtain energy by consuming the organic matter formed by primary

producers. These are heterotrophs or consurners, and this includes

animals, bacteria, fungi, and protozoans. When consumer organisms grow

and reproduce, adding biomass to their populations, we cal this seconda ry

pr od uction .

Virtual y al life on earth derives its energy from the sun, via

primary production. Autotrophs and heterotrophs use that energy to do

metabolic work, and in 'the process convert the energy contained in

organic carbon compounds back into inorganic matter.

In riverine food webs, al energy originates from primary production,

but not necessarily from aquatic plants. In many instances, organic matter

from terrestrial primary production enters the stream, is utilized first by

mcrobes, and then, as a microbe-rich amalgamation, is consumed by

other heterotrophs. The flow of energy through the food webs of stream

communities is complex. In many types of rivers and streams, aquatic

plants are important energy sources; in others, terrestrial plant production

is very important and can be the dominant energy supply to running

waters. The important categories of aquatic plants are the algae, the mosses,

and the true vascular plants usually referred to as

mac roph ytes

beca use of

their large size. Important t errestrial sources of energy include leaves,

fruits, or other terrestrial plant materials which fall into or are blown into

the stream. This nonliving organic matter is referred to as

detritu s,

or

pa r-

ticul ate organ ic matter

(POM). This same material also decomposes on the

forest floor, enters the soil water, and eventual y enters the stream as dis-

solved organic matter (DOM). Because aquatic plants also break down into

POM and release DOM, it can be quite difficult to ascertain where this

matter originated. We would like to know, however, whether the energy

base of the riverine food web carne primarily from aquatic primary pro-

duction within the stream itself, or whether it was heavily subsidized by

energy inputs from outside. Stream ecologists refer to energy produced

within the stream channel as

autochthonous

production. Organic matter

produced outside the stream which falls, blows or leaches into the stream

channel is known as

alloc ht honous

production.

To better understand the complexities of aquatic food webs, we also

need to define how energy moves from one food web level to another, or

in other words how the organisms feed. Principal feeding or

troph ic

roles

are

herb ivo ry

(plant feeders),

carnivo ry

(organisms which feed on other

anmals), and

detritivo ry

(detritus feeders).

Omnivory

refers to animals

feeding on more than one leve. A close study of Fig. 3.1 will help readers

understand these terms and their relationships to each other.

The terms autotrophic and heterotrophic are also used by stream ecol-

ogists to describe the energy base of a s tream as a whole. Ecologists use the

production of oxygen by photosynthesis within a stream reach as an indi-

cator of the amount of organic matter produced and the loss of oxygen by

respiration as an indicator of the consumption of primary production by

anrnals and microbes. Thus, if a given stream reach produces more oxygen


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PART1. The Ecology 01 Rivers and Streams

Secondary consumers

. . . .

herbivory

. coagulation

, and

...~ precipitation

. . . . .

detritivory

Dissolved

organic

matter

~

exudates

Autochthonous

primary pr oduction

Allochthonous

primary production

Photosynthesis

F I G U R E

3.1 The sources o f energy in streams (autochthonous and allochthonous primary

production; DOM), energy pathways (dashed arrow

=

autotrophic; solid arrow

=

het-

erotrophic; dash-dot arrow = chemotrophic), and feeding processes (herbvory,

carnivory, detritivory, omnivory) within stream food webs. Coagulation and p recipitation

are the processes by which DOM is converted to POM, and decay is the mechanism by

which aquatic plant material is converted to POM after death.

than it consumes by respiration, it is autotrophic-it produces more energy

than it uses. Conversely, a heterotrophic section of river consumes more

oxygen than is produced within it and must depend on an allochthonous

source of energy to provide adequate energy for the stream community.

Obviously, food webs in strearns, or the pathways that energy follows as it

is produced, utilized, and degraded to basic elements, can be simple or

quite cornplex, with the latter being the more common.

IN S TR E A M A U TO C H TH O NO U S E N ER G Y S O UR C ES

Now let's look in more detail at each of these energy sources. Plants living

in rivers include the microscopic algae (although large colonies can be seen

with the naked eye) and cyanobacteria (photosynthetic bacteria formerly

known as blue-green algae; see Chapter 12) and larger plants or macro-

phytes. The latter includes the mosses and liverworts, and the true vascu-

lar plants, or angiosperms. The algae and cyanobacteria are the most

3. Energy Resources

important direct source of energy to heterotrophs, as most macrophytes

are unpalatable, although they do contribute to instream production of

DOM and POMo

In streams and rivers with sufficient nutrients (see Chapter 2), a suitable

stable substrate, and adequate sunlight, algae will proliferate. They occur as

single cells, colonies, or as long filaments. Three groups are particularly

important and are described in more detail in Chapter 12. Green algae are

common, generally palatable, and often conspicuous as long, green strands

usually attached to solid objects, such as rocks, and trailing in the water

current (Fig. 3.2). These are commonly, but mistakenly, called

moss

by

many people. The filaments are usually bright green and can grow to several

meters in length under favorable conditions. They usually take on a brown-

ish color as the filaments mature, either from senescence, or from being

covered with unicel lular algae called diatoms. Diatoms are truly the grasses

of the water, numerous in both numbers and variety and the most impor-

tant autochthonous energy source to s tream food webs. They are unicellu-

lar, bu t may occur singly or in fi laments or groups (Fig. 3.3). When you pick

up a stone or other object from the streambed, it usually has a brownish,

slippery coating on the sides exposed to sunl ight. This coa ting i s composed

of billions of diatom cells and, incidentally, is what you usually slip on

when you lose your foot ing whi le wading. Cyanobacteri a can be unicellu-

lar or colonial, often secrete a mucilaginous coat, and have the ability to

fix

atmospheric nitrogen into other forms of nitrogen that can be utilized

by other autotrophs. They probably are a less importan t energy source to

stream food webs than are green algae and diatoms.

That slippery film on the surface of rocks and other hard substrates is

referred to as a biofitm, and while algae are an important component, we

F I G U R E

3.2 Filamentous green algae,SanJuan River,New Mexico. (Photo by C. E.Cushing.)


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P AR T 1. The Ecology

01

Rive rs and Streams

D ~ ~ ',

Me d O~

Cocconeis

x500 Achnanthes x500

~

Nitzschia x700

~~ E iJM

-; .: ~ ~

Melosira x500

Gomphonema

x150

c: :

= = = = = = = = = = = : : : : : = : = : = = = ~ : : : : : : ~

,~

Synedra x250

Navicula x500

F I G U R E

3.3 Common diatoms found in streams. (From Hynes, 1970, with permission.)

now know these biofilms are very complex microenvironments, Some

algae, often those in the lower layers, die and their cell contents leak into

the surrounding matrix, from which compounds diffuse away very slowly.

This provides an environment in which heterotrophic bacteria can obtain

energy from the breakdown of dead cells. In addition, actively photosyn-

thesizing cells produce exudates of organic compounds that also nourish

bacterial growth. The bacteria, in turn, convert these organic compounds

back into inorganic compounds, which are needed by algae for continued

photosynthesis. As Fig. 3.4 illustrates, both autotrophs and heterotrophs

benefit from their close association within the biofilm. Any external source

of DOM, perhaps from groundwater upwelling from the substrate, is

potentially available to the heterotrophic bacteria, and so the biofilm can

be seen to be an incredibly important region of energy production.

Autotrophy often dominates within biofilms, but they will form under

very low light or even in the dark. With the addition of microconsumers,

including protozoans, nematodes, and tiny crustaceans, the biofilm

becomes an entire ecosystem within itself.

Stream ecologists in North America traditionally have used the term

periph yton

peri

=

around, phyton

=

plants) to describe this complex

system, emphasizing the role of plants. In Europe, the German word

Aufwuchs refers to the same entity. It is a vital community in the ecology of

rivers and streams and an important food source for stream invertebrates.

In larger rivers, backwaters, and embayments, algae may be found sus-

pended in the water column. These suspended algae are termed

ph yto-

pLankton ( phyto

= plant, plankton = free floating), a term usually

associated with the algae found in lakes and oceans. There do not appear

to be any phytoplankton that are unque to rive rs, and they are sparse or

absent from fast-flowing streams, where the only likely source is cells dis-

lodged from the stream bottom. Substantial amounts of suspended algae

3. En ergy Resources

37

~

I

DDM-COM-POM

J j j I I j j

\: ~ \: \: \ (Algae)

- .. o .:- .. .:- ....t .:..o .:- ....... :-.

* , . . : - . .

,. J . : - J . . .

l... :-.

'.'@

lr~ 9:-~',li.~.'.', ..., .':~', ....-'... t .. . ': :- . ' .....O:-~ .... ~.. ... 0-.

. ~ 5 .

, ~ ~

- ; o- i .~ i -r :. . . . . . , , .

l lo < : > ' . .t.... o ~i . . lo lo .-.: o .@lo-t>:J.t ~ ~.

~~ : . ~ ; i ~ . b : . : :

J ~ : . : . : . : :

~ .: < : > : :

. , '@ :

:: .0 . :: ~ ~ .: :: .: .: : : ~ ,~ ~ . : : ~ : . , ; . _ ~

. : - : a : . : ; ; ._:

: :~.:.: .-....-; . .. .: .. ...: ... :.: o

.,Ci);:.:

Polysaccharide , . :.:

; ~ : , ; ~ : : :~~ ~ . :. : ~ ; : \ ~ ( : : :1 : : - : . ~ : ( ; ~ ~ . :\ - ;} { ;

~ : : - ) ( : : \ : \ ; i ; :~:~~tf::.,::~}

a

q.. ~ ...... ~

*

' . ,. , l

< > . ~ ~ . , .. . ~ . _ . . ~ . . . ~

,.... ')'.'.- -@ .

J ' -. ,~

.'';:'. ,.- ',' a } ~ _

J . - .

. * . - . . )._

b .. . , :. :, ~ . ?: ..:... : .. -. . . E:t . . o .. .. . .: . . . . . . . .. .. '. o . : .

Substratum

~ Fungalhypha

:~. Matrix

o~o Bacteria

*

* Enzymes

F I G U R E 3.4 The biofilm found as a surface slirne on s tones and other submerged objects

in streams. A polysaccharide matrix produced by the microbial community binds together

bacteria, algae, and fung i, and is inhabited by protozoans and micrometazoans, which

consume this material. Within the matrix, extracellular release and cell death result in

enzymes and other molecular products that are retained due to reduced diffusion rates.

(From Allan, 1995, with permission.)

@@ @ Cyanobacteria

may be found immediately downstream from reservoirs or lakes; however,

they will not proliferate under these conditions and are usually lost to the

stream within fairly short distances. They have been found to provide a

rich source of energy to filter-feeding organisms (see Chapter 4), resulting

in large concentrations of these animals for short distances below lakes or

reservoirs.

Whenever current washes algal populations downriver more rapidly

than they can reproduce, growth of phytoplankton populations is pre-

cluded. Hence slow currents and long river sections favor the greatest

abundance of phytoplankton. Embayments and floodplain lakes, which

can serve as reservoirs for phytoplankton, also can be important. Light and

nutrients also can be limiting to river phytoplankton. In tu rbid, well-

mixed rive rs, adequate Iight for photosynthesis might penetra te much less

than l m, yet f the river is lO-m deep, turbulence will ensure that the algal

cell spends much of its time at light levels too low for photosynthesis. In

general, the more lakelike the river, the more phytoplankton will develop,

whch is why impounded rivers often have the grea test algal blooms,

Sometimes reaching nuisance levels.


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PA RT 1 . Th e E c ol og y 01 R i ve rs a nd S tr ea m s

F I G U R E

3.5 Moss in an Oregon stream. (Photo by

J.

M. Lyford.)

Macrophytes, primarily true mosses and angiosperms, comprise

ano~her autochthonous energy source. Mosses (Fig. 3.5) are attached plants

havmg leaves, usually found in the colder, well-shaded areas of streams-

essentialIy the headwaters-although they have the adaptive characteris-

tics to grow in a variety of stream environments. These characteristics

inc~ude the ability to grow in low light and low temperature conditons, a

rapid rate o.f nutrier:t uptake, and a high resist ance to being dis lodged by

spates-agam, conditions found in headwater reaches of streams.

Common genera are

Fontina/is

and

Fisidens.

They grow attached to the

rocks on the streambed (see Fig. 13.1) and also forrn thick coatings on

rocks and logs along the stream banks. For some unknown reason, mosses

are perhaps the least studied of the majar constituents of stream ecosys-

tems-at least from an ecological contexto Mosses photosynthesize and

prod.uce organic matter and, in fact, may have higher rates of primary pro-

ducton that algae. However, because of their l imited distribution within a

given stream or river, they are of minor importance as an overall energy

source throughout the entire continuum of the river.

As a stream increases in size and the current, at least in places, tends

to decrease, silt will settle out and provide a suitable substrate for the

rooting and growth of large water plants. Here you will find common

genera of flowering plants, or angiosperms (Fig. 3.6), including

Potamogeton, E/odea, Ranunculus, Nuphar,

and others; in smaller springs and

brooks watercress, Nasturtium, is common. Plants may be sparsely distrib-

ute? or can form dense mats which clog watercourses, and they play

vanous roles in the ecology of streams: an energy source (both before and

after death), substrate for attachment of other organisms, cover from

3. E ne rg y Resources

9

F I G U R E 3.6 Macrophytes growing in slow-flowing reach of the Madison Rver, Wyoming.

(Photo by C. E. Cushing.)

predators, etc. Their role as an energy source in streams is probably more

important after they die and decay, when their remains break down and

become

fine particu/ate organic matter

(FPOM,

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01 READING Streams Energy Resources