page_title: River ecosystem
section_title: Energy inputs
hierarchical_section_title: River ecosystem / Trophic relationships / Energy inputs
caption_attribution_description: English: Canadian Waterweed, Elodea canadensis (with enlarged cross section of a single whorl). In this instance, the leaves are relatively long and narrow (four times longer than wide). There are also specimens with stockier leaves. Deutsch: Sprossteil sowie vergrößerter Querschnitt eines einzelnen Blattquirles der Kanadischen Wasserpest (Elodea canadensis). Die Blätter sind bei diesem Exemplar vergleichsweise lang und schmal (viermal so lang wie breit); es gibt auch wesentlich gedrungenere Exemplare mit kürzeren Blättern.
context_page_description: River ecosystems are flowing waters that drain the landscape, and include the biotic interactions amongst plants, animals and micro-organisms, as well as abiotic physical and chemical interactions of its many parts. River ecosystems are part of larger watershed networks or catchments, where smaller headwater streams drain into mid-size streams, which progressively drain into larger river networks.
  River ecosystems are prime examples of lotic ecosystems. Lotic refers to flowing water, from the Latin lotus, meaning washed. Lotic waters range from springs only a few centimeters wide to major rivers kilometers in width. Much of this article applies to lotic ecosystems in general, including related lotic systems such as streams and springs. Lotic ecosystems can be contrasted with lentic ecosystems, which involve relatively still terrestrial waters such as lakes, ponds, and wetlands. Together, these two ecosystems form the more general study area of freshwater or aquatic ecology.
  The following unifying characteristics make the ecology of running waters unique among aquatic habitats.
  Flow is unidirectional.
  There is a state of continuous physical change.
context_section_description: Energy sources can be autochthonous or allochthonous.
  Autochthonous (from the Latin "auto" = "self) energy sources are those derived from within the lotic system.  During photosynthesis, for example, primary producers form organic carbon compounds out of carbon dioxide and inorganic matter.  The energy they produce is important for the community because it may be transferred to higher trophic levels via consumption. Additionally, high rates of primary production can introduce dissolved organic matter (DOM) to the waters. Another form of autochthonous energy comes from the decomposition of dead organisms and feces that originate within the lotic system.  In this case, bacteria decompose the detritus or coarse particulate organic material (CPOM; >1 mm pieces) into fine particulate organic  matter (FPOM; <1 mm pieces) and then further into inorganic compounds that are required for photosynthesis. This process is discussed in more detail below.
  Allochthonous energy sources are those derived from outside the lotic system, that is, from the terrestrial environment.  Leaves, twigs, fruits, etc. are typical forms of terrestrial CPOM that have entered the water by direct litter fall or lateral leaf blow.  In addition, terrestrial animal-derived materials, such as feces or carcasses that have been added to the system are examples of allochthonous CPOM.   The CPOM undergoes a specific process of degradation. Allan gives the example of a leaf fallen into a stream. First, the soluble chemicals are dissolved and leached from the leaf upon its saturation with water. This adds to the DOM load in the system. Next microbes such as bacteria and fungi colonize the leaf, softening it as the mycelium of the fungus grows into it.  The composition of the microbial community is influenced by the species of tree from which the leaves are shed (Rubbo and Kiesecker 2004).  This combination of bacteria, fungi, and leaf are a food source for shredding invertebrates, which leave only FPOM after consumption.  These fine particles may be colonized by microbes again or serve as a food source for animals that consume FPOM.  Organic matter can also enter the lotic system already in the FPOM stage by wind, surface runoff, bank erosion, or groundwater.  Similarly, DOM can be introduced through canopy drip from rain or from surface flows.
caption: Pondweed is an autochthonous energy source
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