Energy Considerations: The Flow Of Life Through
The River
The Leaf
A leaf suspended in the surface film is a thing
of beauty, poised to fulfill an essential role in the life of
the river. When it was still attached to its branch, it supplied
energy to the tree throughout spring and summer. In the fall,
leaves falling straight to the ground will rot, releasing various
elements and organic molecules to the recycling process, benefiting
next year’s tree growth. By falling into the river, especially
those of the freestone type, the leaf enters into another ecosystem,
and now supplies a portion of the energy needed by countless macro
invertebrates. Without the input of leaves in the fall, stream
insects could not complete their life cycles, and without these
essential life forms, there of course would be far fewer trout.
Leaves from all varieties of hardwood trees play a similar role,
but each species rots at a unique rate. The entire ensemble of
forest leaves acts like a king-sized time-release capsule of energy.
Shredders begin
the process of energy flow by reducing them into tiny bits, due
to their own unique feeding mechanisms. It may take a total of
over 200 days for the river’s assemblages of macro invertebrates
to completely consume all the leaves that fell into the river
the year before.
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Rotting Leaves |
The process of converting leaves to usable energy
starts the moment they enter the water and become trapped under
rocks, or collect in massive underwater leaf piles in quiet back
eddies. It is at this point that leaves begin to give up their
soluble nutrients through leaching.
Soluble components such as sugars, plant pigments and other organic
compounds provide an initial burst of food for in-stream microbes
(i.e., bacteria and fungi), allowing them to multiply and colonize
the surface of the leaf. Hyphomycetes
(water molds) are early settlers on the leaf surface, followed
by a succession of distinct but related fungal
and bacterial
communities, which attach to the leaf’s surface and digest
a portion of it. After several days to weeks, the leaf becomes
palatable to macro invertebrate communities that are as fussy
about what they eat as we are about enhancing our hamburgers.
Plain ones won’t do; we demand the special sauce, pickles,
cheese, and the sesame seed bun, too.
Once the microbes have “conditioned”
the leaf, shredder insects (second trophic
level) begin their work in earnest of reducing the particle
size of rotting leaves, now referred to as detritus. Fortunately,
shredders are sloppy eaters, and a significant amount of processed
material flows downstream. Filter
feeding species , including most net-spinning caddis larvae
and many burrowing mayfly species, now feast on this mana from
up-stream. Each fecal pellet produced by shredders is composed
of about 50% undigested, organically rich detritus,
contributing another source of energy to filter feeding communities
downstream from their feeding stations. The third trophic level
of the aquatic food
web is composed of some species of insects, most notably stonefly
nymphs, which feed on both shredders and filterers. Finally, the
fish (fourth trophic level) feed on all macro invertebrates. Scrapper
insects eat mostly algal growth on the undersurface of in-stream
rocks. They, too, are fodder for the fourth trophic level feeders.
What happens when this exquisitely balanced
food web is disturbed; in particular when trees are removed from
the stream bank? Taking leaves away from the river is the same
as siphoning fuel from a car. No one expects to drive as far after
that. So what happens to the river’s bio-productivity when
its energy source is depleted by over-foresting ? The answer is
simple; the river gets less “mileage” out of its “gallon
of gas”.
Remediation
can be as simple as planting trees where they once stood tall
along the banks. Granted, this is a long-term solution. It is
one that needs to begin with the concept that maintaining the
intactness of a river’s ecosystem is everyone’s responsibility.
If adopted as an environmental ethic for our generation, we would
have the rare opportunity to leave something truly extraordinary
behind for the next one to appreciate and care for. So, plant
a tree and save a trout!
