Stream metabolism

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Stream metabolism, also known as aquatic ecosystem metabolism in lakes, can be expressed as net ecosystem productivity (NEP), the difference between gross primary productivity (GPP) and ecosystem respiration (ER). Analogous to metabolism within an individual organism, stream metabolism represents how energy is created (primary production) and used (respiration) within an aquatic ecosystem. In net heterotrophic ecosystems, GPP:ER is <1 (ecosystem using more energy than it is creating); in net autotrophic ecosystems it is >1 (ecosystem creating more energy than it is using) (Odum 1956). A net heterotrophic ecosystem often means that allochthonous (coming from outside the ecosystem) inputs of organic matter, such as leaves or debris are needed to fuel ecosystem respiration rates, because respiration is greater than production within the ecosystem. However, it is important to note that autochthonous (coming from within the ecosystem) pathways may also remain important in heterotrophic ecosystems. A net autotrophic ecosystem, conversely, has available primary productivity (from algae, the main primary producer in aquatic ecosystems) to fuel upper trophic levels such as insects or fishes (but again, allochthonous pathways may still be important to these systems).

Stream metabolism can be influenced by a variety of factors, including physical characteristics of the stream (slope, width, depth, and speed/volume of flow), biotic characteristics of the stream (abundance and diversity of organisms ranging from bacteria to fish), light and nutrient availability to fuel primary production, water chemistry and temperature, and natural or human-caused disturbance, such as dams, removal of riparian vegetation, nutrient pollution, wildfire or flooding.

Measuring stream metabolic state is important to understand how disturbance may change the available primary productivity, and whether and how that increase or decrease in NEP influences foodweb dynamics, allochthonous/autochthonous pathways, and trophic interactions. Metabolism (encompassing both ER and GPP) must be measured rather than primary productivity alone, because simply measuring primary productivity does not indicate excess production available for higher trophic levels. One commonly used method for determining metabolic state in an aquatic system is daily changes in oxygen concentration, from which GPP, ER, and net daily metabolism can be estimated.

Disturbances can affect trophic relationships in a variety of ways, such as simplifying foodwebs, causing trophic cascades, and shifting carbon sources and major pathways of energy flow (Power et al. 1985, Power et al. 2008). Part of understanding how disturbance will impact trophic dynamics lies in understanding disturbance impacts to stream metabolism (Holtgrieve et al. 2010). For example, in Alaska streams, disturbance of the benthos by spawning salmon caused distinct changes in stream metabolism; autotrophic streams became net heterotrophic during the spawning run, then reverted to autotrophy after the spawning season (Holtgrieve and Schindler 2011). There is evidence that this seasonal disturbance impacts trophic dynamics of benthic invertebrates and in turn their vertebrate predators (Holtgrieve and Schindler 2011, Moore and Schindler 2008). Wildfire disturbance may have similar metabolic and trophic impacts in streams.

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