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Mean temperature (°C)

FIGURE 5.17 Relationship between species richness (±2 standard errors) and mean temperature developed from approximately 300 sites in Michigan's Lower Peninsula. Arrows denote breakpoints between cold-, cool-, and warm-water streams. (Reproduced from Wehrly et al. 2003.)

frequently exceed their upper tolerance levels (25°C) during spawning migration. Salmon distribution and cool-water temperature patterns were strongly correlated in a warm stream but only weakly in a cold stream, further evidence that local-scale thermal patchiness provided needed habitat (Torgersen et al. 1999).

Finally, temperature controls the metabolism of all producers and ectothermic consumers in fluvial ecosystems, so it will strongly affect numerous ecosystem functions. Rates of photosynthesis and microbial activity are strongly temperature dependent (Sections 12.31), as are the metabolism and growth of macroinverte-brates and fish. Secondary production, which is the amount of new consumer biomass added per unit time, is estimated from the product of species-specific growth rates and standing crop biomass. As Figure 5.18 illustrates, daily growth rate of aquatic insects increases markedly with temperature, and so warmer ecosystems are more productive overall (Benke 1993).

Polypedilum spp. ^^^

Polypedilum spp. ^^^

FIGURE 5.18 Daily growth rates (mg mg1 day1) as a function of temperature for three aquatic insects found on snag habitat in the Ogeechee River, Georgia, and reared in stream-side artificial channels. Insects include the midge Polypedilum, the black fly Simulium, and the mayfly Baetis. (Reproduced from Benke 1993.)

Temperature (°C)

FIGURE 5.18 Daily growth rates (mg mg1 day1) as a function of temperature for three aquatic insects found on snag habitat in the Ogeechee River, Georgia, and reared in stream-side artificial channels. Insects include the midge Polypedilum, the black fly Simulium, and the mayfly Baetis. (Reproduced from Benke 1993.)

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