Instream primary production and terrestrial production that enters streams as allochthonous material will either be utilized by stream hetero-trophs or exported from the system, usually downstream and possibly to the oceans. Organic matter can accumulate over relatively short periods, on the timescale of weeks to months, and storage on or within the streambed and on banks and floodplains can occur on the timescale of years to decades. Storage of organic matter depends on flow variation, as material tends to accumulate during low flows and be exported by high flows. Averaged over long periods, storage is thought to be negligible, at least for streams of low order. Thus, factors that influence the relative rates of conversion of organic C to CO2 versus transport largely determine what fraction of organic matter is mineralized within stream ecosystems, and this is expected to differ among organic matter compartments. A high rate of utilization relative to transport indicates that organic matter is contributing to stream metabolism and the stream ecosystem is efficient in its processing of organic C inputs. The opposite result indicates that most organic matter is exported downstream and the stream ecosystem is inefficient.

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