Heavy shade presents serious challenges for primary producers and foodlimited herbivores in forest streams. In this study, we examined the response of periphyton and grazing snails (Elimia clavaeformis) to summer shade in White Oak Creek (WOC), a second-order stream in a Tennessee deciduous forest. Three experiments were performed: (I) in situ manipulation of light and snail density to test the effects of light limitation and grazing; (2) construction of photosynthesis-irradiance (P-/) curves to test for shade adaptation by periphyton; and (3) measurements of snail growth vs. irradiance, to quantify the indirect relationship between grazers and an abiotic constraint on photosynthesis. In the first experiment, light and snail densities were manipulated in a 2 X 2 factorial design: two light treatments were created by removing streamside vegetation from four sites in woe and by pairing each of these sites with an adjacent, shaded site; two snail density treatments at each site were created by adding snails at normal (970 individuals/m 2 ) and low (50 individuals/m 2 ) density to the two sides of Plexiglas channels. Snails at normal densities cropped periphyton biomass to low levels regardless of light regime, but periphyton productivity was higher at the open sites where snails grew faster and accumulated more lipid. Snail growth and lipid accumulation were strongly affected by intraspecific competition in both light regimes. In the second experiment, photosynthesis-irradiance curves for periphyton from shaded and open sites illustrated considerable shade adaptation: shaded periphyton was 2 times more efficient at low irradiance than was periphyton from open sites. Despite the greater efficiency of shaded periphyton at low irradiance, integrated primary production estimated with photosynthetic models was 4 times greater in the open because shade adaptation provided only partial compensation for the very low irradiances in the shade. In the third experiment, in situ snail growth again increased with decreasing shade. The growth vs. irradiance response resembled a P-1 curve: snail growth increased almost linearly with increased light and then leveled off at a photon flux density of =7 mol·m-2 ·d-1 • If this curve primarily reflects rates of food supply, then periphyton production and grazer growth in woe and similar streams is light-limited at a photon flux density <7 mol·m-2 ·d-1 • Bottom-up effects of light limitation were propagated very strongly in WOC, where the invertebrate fauna is dominated by a grazer that appears to escape top-down control.

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Relationships between toxicant exposure, physiological effects, and population-level responses were investigated in redbreast sunfish (Lepomis auritus) from a stream receiving chronic inputs of mixed contaminants. Elevated levels of detoxification enzymes, which provided evidence of direct toxicant exposure, were associated with low lipid levels, histopathological damage, and reduced growth for fish at the upper three sites in the contaminated stream. Decreased fecundity, exhibited by fish at the upper site, might have been due to the reduced capacity of the liver to manufacture yolk proteins. Reduction in lipid pools due to metabolic drains might have decreased the amount of physiological useful energy needed for growth resulting in smaller age-specific sizes of fish at the upper three sites. This approach for investigating relationships between contaminant exposure, physiological effects, and population-level responses such as growth and size distributions could serve as a model for designing biomonitoring studies and for stimulating further research to improve our ability to evaluate the ecological significance of chronic contaminant stressors on aquatic ecosystems.

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