Chapter 1.2.1: Watershed Elements
Module 1: Basics of Watershed Management
1
Technical Annex
The Food Web and Decomposition
Food webs are a part of the natural system in watersheds. Within a food web (see figure
1), organisms interact and, in the process, may directly or indirectly affect other
organisms. The example pictured below is a simple, aquatic food web; when the whole of
the watershed’s terrestrial components are also considered, food webs can be very
complex with numerous interactions among land-based and water-based species [3].
Figure 1: A Simplified Food Web in a Watershed
Food webs also recognise the different roles species play by terming them producers
(organisms that generate food, primarily through photosynthesis), consumers (first-order
consumers are vegetarians, second-order consumers feed on first-order consumers, etc.),
and decomposers (which feed on dead tissue and return nutrients and energy to other
parts of the cycle), among other terms [3].
Decomposition involves the reduction of energy-rich organic matter (detritus), mostly by
micro organisms (fungi, bacteria, and protozoa) to CO2, H2O and inorganic nutrients.
Through this process they both release nutrients that become available for other
organisms and transform organic material into energy usable by other organisms [3].
The nitrogen cycle decomposition occurs in the waters prior to sedimentation. In the
headwater reaches of streams, external sources of carbon from upland forests are a
particularly important source of organic material for organisms and the decomposition of
microscopic particles occurs very rapidly. The bacteria and fungi modify the organic
material through decomposition and make it an important food source for invertebrate and
vertebrate detritivores, thereby reinserting these nutrients and materials into the
Chapter 1.2.1: Watershed Elements
2
watershed’s aquatic and terrestrial food webs. Decomposition involves the following
processes (see figure 2) [3]:
ˆ The
leaching
of
soluble
compounds from dead organic
matter
ˆ Fragmentation
ˆ Bacterial and fungal breakdown
ˆ Consumption of bacterial and
fungal organisms by animals
ˆ Excretion
inorganic
animals
of organic
compounds
and
by
ˆ Clustering of colloidal organic
matter into larger particles
The process of death and
consumption,
along
with
the
leaching of soluble nutrients from
the decomposing substrate, release minerals contained in the microbial and detrital
biomass. This process is known as mineralisation [3].
Figure 2: A Simplified Decomposition Process
Definitions of Terms used in the Figures:
Flocculation refers to a process by which fine particulates in water are caused to clump
together into floc. The floc may then float to the top or settle at the bottom of the water [4].
Organic matter (or organic material) is matter which has come from a recently living
organism; is capable of decay, or the product of decay; or is composed of organic
compounds [5].
Dissolved organic matter is dissolved (changed from solid to liquid) molecules derived
from degradation of dead organisms or excretion of molecules synthesised by organisms
[2].
Particulate matter (or fine particles), are tiny particles of solid or liquid suspended in a gas
[6].
Epilithic algae are algae that grow attached to rocks. They are important because they act
as a food source for several organisms higher on the food chain and they play a role in
dissolved atmospheric nitrogen fixation via symbiotic bacteria. As a result they serve as a
"base" for the food web of lakes and streams [1].
References and Sources for Further Reading
[1]
Brainmass.com, 2007: Biology.
http://www.brainmass.com/content/samples/biology.php
[2]
Fishbase.org, 2007: Glossary.
http://www.fishbase.org/Glossary/Glossary.cfm?TermEnglish=dissolved%20or
ganic%20matter
[3]
O Keefe, T.C., Elliott, S.R., Naina, R.J., Norton, D. 1997: Introduction to watershed
ecology. Watershed Academy Web. EPA.
http://www.epa.gov/watertrain/ecology/
[4]
Wikipedia, 2007: Definition Floccolation. http://en.wikipedia.org/wiki/Flocculation
Chapter 1.2.1: Watershed Elements
3
[5]
Wikipedia, 2007: Definition Organic Matter.
http://en.wikipedia.org/wiki/Organic_matter
[6]
Wikipedia, 2007: Definition Particulate Matter. http://en.wikipedia.org/wiki/Particulate