Sources of organic carbon (C) and thus energy to lotic ecosystems fall into two broad categories: autochthonous inputs by aquatic primary producers and allochthonous inputs of dead organic matter from terrestrial ecosystems. Heterotrophs - microorganisms, meiofauna, and macrofauna -decompose and consume these supplies of organic C, ultimately remineralizing some fraction of the total as carbon dioxide (CO2), but also exporting substantial quantities to downstream ecosystems (Figure 12.1). Indeed, utilization and export are the two principal fates of organic C in stream ecosystems; although some storage occurs on the timescale of months to years, storage is thought to be negligible on longer timescales.

This whole-ecosystem view brings into focus a series of topics that comprise the study of stream metabolism. Allochthonous inputs of coarse, fine, and dissolved organic matter (DOM) are important in many stream settings, especially in small streams with a forested riparian, where algal primary production tends to be light-limited. Thus, it is suggested that allochthonous inputs will dominate in forested headwaters, a view with much supporting evidence. Autochthonous production by algae and other primary producers is expected to be of greater importance at roughly fourth-through sixth-order streams, and then less so in larger, deeper, and more turbid lowland rivers (Figure 1.7). This shift between autochthony and allochthony is also a shift between reliance on internal versus external energy sources, and it is expected to vary along the river continuum and with landscape setting (Vannote et al. 1980, Minshall et al. 1983). A stream reach with a low level of primary production (P) and a high level of respiration (R) clearly is dependent upon external energy inputs, either from the adjacent terrestrial ecosystem or from upstream sources. Thus, the P/R ratio, or P minus R (referred to as net ecosystem production, NEP), serves as an index of the relative importance of internal versus external organic C inputs, and may be used as a measure of any metabolic transitions that occur along a river's length. Stream ecosystems where NEP is positive are likely to export organic matter to downstream locations. When NEP is negative the stream ecosystem is dependent upon external energy sources, usually allochthonous detritus from the land.

Lotic ecosystems are open, which means that they import organic matter from upstream and export it downstream, and lateral exchanges can be substantial wherever the river is connected to a floodplain. Thus, transport is an important process in stream energy flux, and this may be especially true of particulate organic matter

FIGURE 12.1 Simplified model of principal carbon fluxes in a stream ecosystem. Heavier lines indicate dominant pathways of transport or metabolism of organic matter in a woodland stream. Note that storage is omitted. (Modified from Wetzel 2001.)

(POM), where hydrologic variation and the presence of retention devices such as wood and boulders together determine whether POM resides in the stream system long enough for decomposition to occur. Stream ecosystem efficiency quantifies the extent to which all energy inputs are converted to CO2 versus exported downstream. By integrating information concerning energy inputs, the processes of decay and consumption, and the transport of organic C into and out of stream reaches, one can investigate the overall efficiency of stream ecosystems, and whether efficiency varies as a function of types of inputs or landscape and longitudinal position. At least for streams dominated by allochthonous inputs of terrestrial POM, current evidence suggests ecosystem efficiency is low, especially in headwater reaches.

Quantification of inputs (primary production, POM and DOM), rates of utilization and transport, and standing stocks (typically coarse and fine benthic OM) are the main components in the analysis of C flux. These topics were discussed in detail in Chapters 6 and 7 and are revisited here, with the primary emphasis on estimates of amounts and rates. To address the relative importance of autochthony versus allochthony, utilization versus transport, and stream ecosystem efficiency, investigators have used P/R and NEP, attempted to calculate all inputs and outputs of organic matter using a budgeting approach, and developed measures of C retention and processing expressed as distance or time in transport as an indication of ecosystem efficiency. Although some quantities are difficult to measure and the open nature of lotic ecosystems creates special challenges, recent findings from the study of stream ecosystem metabolism provide clear evidence that stream size and characteristics of the surrounding catchment are important determinants of organic C inputs and utilization.

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