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Edwards and Meyer (1987)

a For seasonal trends in P/R see Figure 12.13 b Seasonal data show P/R < 0.5 throughout year instream primary production in this data set is notable and perhaps surprising. In accord with expectations of the greater importance of instream primary production as stream size increases, the P/R was positively correlated to stream order (Figure 12.14), and the correspondence was especially clear for river systems in Quebec and Oregon, which had multiple sites spanning a range of stream sizes. Note also that desert streams had high GPP and few litter inputs, reflecting their unshaded condition in unproductive landscapes; in contrast, blackwa-ter streams of the southeastern United States had lower P/R than expected from the overall data set and these are heavily shaded systems that receive high litterfall and floodplain inputs.

In addition to stream size and the terrestrial setting, ecosystem metabolism is likely to vary with hydrologic disturbance and stream channel retentiveness because these influence organic matter storage and thus benthic respiration. Frequent hydrologic disturbance may affect stream metabolism by scouring periphyton and biofilms from stone surfaces, and in more extreme cases through bed transport and up-ending of stones. Measurement of ecosystem metabolism in a sixth-order, gravel-bed Swiss River for 447 days using single-station oxygen curves showed strong effects due to bed-moving spates (Uehlinger and Naegeli 1998). Immediately after spates, primary production and ecosystem respiration both declined, and because production was most strongly affected, P/R ratios declined as well. Primary production recovered more rapidly in summer than in winter, whereas recovery of respiration showed less seasonal dependency. Respiration may be less affected by spates than is primary production because heterotrophic processing of organic matter within the streambed is likely to be less affected than autotrophic activity on the bed surface. Thus, depth of scouring, amount of organic matter storage within the streambed, and magnitude of the disturbance will determine the

• Deciduous forest o Deciduous forest, blackwater streams

■ Boreal forest

* Aridlands r Montane coniferous forest

□ Tundra a Tropical forest

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