in Aquatic Ecosystems
Adapted from The
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
from organism to organism. A food chain in a stream might look like
the food chain does not stop there. Most organisms are food for more
other type of organism within the same system. A food web
number of overlapping food chains and is usually a more accurate
feeding patterns in an ecosystem. Food webs illustrate the
of organisms within an ecosystem.
Decomposers are also a part of the
All living organisms take up and use nutrients. They can be thought of
living nutrient warehouses. Decomposers break down dead organic
release nutrients into the soil or water. These nutrients continue the
producers use them to grow. The major decomposers are bacteria and
Decomposers may even become food themselves when they are attached to a
of detritus that is eaten.
The following is from
Water on the Web
physical, chemical, and biological characteristics of lakes are
variable. Lakes vary physically in terms of light levels, temperature,
water currents. Lakes vary chemically in terms of nutrients, major
contaminants. Lakes vary biologically in terms of structure and
well as static versus dynamic variables, such as biomass, population
and growth rates. There is a great deal of spatial heterogeneity in all
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
similar to a forest ecosystem where, for example, a variety of physical
variables (light, temperature, moisture) vary from the soil up through
most fundamental set of properties of lakes relates to the interactions
light, temperature and wind mixing. The absorption and attenuation of
the water column are major factors controlling temperature and
Photosynthesis provides the food that supports much of the food web. It
provides much of the dissolved oxygen in the water. Solar radiation is
major source of heat to the water column and is a major factor
patterns in the lake basin and water movements.
organisms influence (and are influenced by) the chemistry of the
environment. For example, phytoplankton extract nutrients from the
water and zooplankton
feed on phytoplankton. Nutrients are redistributed from the upper water
lake bottom as the dead plankton gradually sink
lower depths and decompose. The redistribution is partially offset by
active vertical migration of the plankton.
break down organic matter. They are sinks for plant and animal wastes,
also recycle nutrients for photosynthesis. The amount of dead material
lake far exceeds the living material. Detritus is the organic fraction
dead material, and can be in the form of small fragments of plants and
or as dissolved organic material. In recent years, scientists have
that zooplankton grazing on detritus and its
bacterial community represent an additional important trophic pathway in lakes.
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.
decomposers can be found in all biological zones of a lake, although
the dominant forms in the lower hypolimnion
is an abundance of dead organic matter. Oxidation of organic matter by
decomposers (respiration) in the hypolimnion
responsible for the depletion of dissolved oxygen over the course of
summer, potentially leading to anoxic conditions (no dissolved oxygen).
is no source of oxygen in the hypolimnion
oxygen lost through decomposition. Stratification prevents atmospheric
from being mixed deeper than the thermocline,
is usually too dark for photosynthesis. Consequently, a large volume of
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
oxygen in the hypolimnion. The resulting
anoxia has a
profound effect on both the chemistry and the biology of the lake.