01 READING Streams Energy Resources
Transcript of 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,