Trophic Dynamics in Aquatic Ecosystems

Abstract

The species populations within an ecosystem can be organized according to feeding levels (trophic levels). Photoautotrophs, which produce organic matter from inorganic materials using solar energy, are the first level (algae, aquatic vascular plants). The second trophic level consists of herbivores, which also may be called grazers (including many types of zooplankton and other invertebrates). Herbivores are eaten by primary carnivores (carnivorous zooplankton and many fish). In some cases, a secondary carnivore level may also be very important (large fish that eat primary carnivores). A diagram of the feeding connections of all organisms in an ecosystem or part of an ecosystem is called a food web. The food web includes a grazer food chain, which passes from photoautotrophs to top carnivores. The food web also contains a detrital food chain, which is based on fecal matter, dead organisms, dissolved organic matter, and particulate matter coming from the watershed. Often the trophic position of a species is intermediate between two trophic levels, because it feeds partially at one trophic level and partially at another trophic level. In this case, the species is assigned a fractional trophic level (e.g., 2.5). Species populations that have similar feeding and predation relationships sometimes are grouped together as trophic guilds for modeling purposes. The passage of energy or materials through a food web can be estimated from use of field data to calibrate a model showing the transfer of energy, carbon, or other substances through the food web. Efficiency of trophic transfer (proportion of energy or mass passed from one trophic level to the next) can be used in comparing food webs across ecosystems or within specific parts of an ecosystem. Trophic transfer efficiency may be related to physical conditions. In open water, for example, food chains tend to be longer because all plants are small and must be eaten by small herbivores, which support small carnivores that are eaten by large carnivores. Food-web analysis explains why some food webs are more efficient than others, and why some feeding levels suppress others. Thus, food-web analysis promotes an understanding of feeding relationships within ecosystems.