Decomposition
in Aquatic Ecosystems
Adapted from The
http://www.boquetriver.org/adoptaqecosys.html
Because
of
their ability to manufacture food green plants are the base of all food
chains. A food chain describes the sequence of energy as it moves
along
from organism to organism. A food chain in a stream might look like
this:
algae>mayfly>trout>osprey
Of
course
the food chain does not stop there. Most organisms are food for more
than one
other type of organism within the same system. A food web
describes a
number of overlapping food chains and is usually a more accurate
description of
feeding patterns in an ecosystem. Food webs illustrate the
interconnectedness
of organisms within an ecosystem.
Decomposers are also a part of the
food web.
All living organisms take up and use nutrients. They can be thought of
as
living nutrient warehouses. Decomposers break down dead organic
materials and
release nutrients into the soil or water. These nutrients continue the
cycle as
producers use them to grow. The major decomposers are bacteria and
fungi.
Decomposers may even become food themselves when they are attached to a
piece
of detritus that is eaten.
The following is from
Water on the Web
http://waterontheweb.org/under/lakeecology/03_lakevariability.html
The
physical, chemical, and biological characteristics of lakes are
extremely
variable. Lakes vary physically in terms of light levels, temperature,
and
water currents. Lakes vary chemically in terms of nutrients, major
ions, and
contaminants. Lakes vary biologically in terms of structure and
function as
well as static versus dynamic variables, such as biomass, population
numbers,
and growth rates. There is a great deal of spatial heterogeneity in all
these
variables, as well as temporal variability on the scales of minutes,
hours, diel (day/night), seasons, decades,
and geological time.
Though lakes vary in many dimensions they are actually highly
structured,
similar to a forest ecosystem where, for example, a variety of physical
variables (light, temperature, moisture) vary from the soil up through
the
canopy.
Perhaps
the
most fundamental set of properties of lakes relates to the interactions
of
light, temperature and wind mixing. The absorption and attenuation of
light by
the water column are major factors controlling temperature and
potential photosynthesis.
Photosynthesis provides the food that supports much of the food web. It
also
provides much of the dissolved oxygen in the water. Solar radiation is
the
major source of heat to the water column and is a major factor
determining wind
patterns in the lake basin and water movements.
Aquatic
organisms influence (and are influenced by) the chemistry of the
surrounding
environment. For example, phytoplankton extract nutrients from the
water and zooplankton
feed on phytoplankton. Nutrients are redistributed from the upper water
to the
lake bottom as the dead plankton gradually sink
to
lower depths and decompose. The redistribution is partially offset by
the
active vertical migration of the plankton.
Decomposers
break down organic matter. They are sinks for plant and animal wastes,
but they
also recycle nutrients for photosynthesis. The amount of dead material
in a
lake far exceeds the living material. Detritus is the organic fraction
of the
dead material, and can be in the form of small fragments of plants and
animals
or as dissolved organic material. In recent years, scientists have
recognized
that zooplankton grazing on detritus and its
associated
bacterial community represent an additional important trophic pathway in lakes.
Decomposers
Decomposers, which include bacteria, fungi, and other microorganisms,
are the
other major group in the food web. They feed on the remains of all
aquatic
organisms and in so doing break down or decay organic matter, returning
it to
an inorganic state. Some of the decayed material is subsequently
recycled as nutrients,
such as phosphorus (in the form of phosphate, PO4-3)
and
nitrogen (in the form of ammonium, NH4+) which are readily
available
for new plant growth. Carbon is released largely as carbon dioxide that
acts to
lower the pH of bottom waters. In anoxic zones some carbon can be
released as
methane gas (CH4). Methane gas causes the bubbles you may
have
observed in lake ice.
The
decomposers can be found in all biological zones of a lake, although
they are
the dominant forms in the lower hypolimnion
where there
is an abundance of dead organic matter. Oxidation of organic matter by
the
decomposers (respiration) in the hypolimnion
is
responsible for the depletion of dissolved oxygen over the course of
the
summer, potentially leading to anoxic conditions (no dissolved oxygen).
There
is no source of oxygen in the hypolimnion
to replace
oxygen lost through decomposition. Stratification prevents atmospheric
oxygen
from being mixed deeper than the thermocline,
and it
is usually too dark for photosynthesis. Consequently, a large volume of
organic
matter from a variety of sources (e.g., wastewater, sinking algae,
dying macrophytes, and organic sediment
washed in from the
watershed) leads to faster oxygen depletion and often complete removal
of
oxygen in the hypolimnion. The resulting
anoxia has a
profound effect on both the chemistry and the biology of the lake.