Decomposition in Aquatic Ecosystems


Adapted from The Boquet River Association


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:




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


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, 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.