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Light limitation in a stream ecosystem: responses by primary producers and consumers

Heavy shade presents serious challenges for primary producers and food- limited 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: (1) in situ manipulation of light and snail density to test the effects of light limitation and grazing; (2) construction of photosynthesis-irradiance (P-I) curves to test for shade ad- aptation 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 WOC 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/) and low (50 individuals/) 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 com- petition 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-I curve: snail growth increased almost linearly with increased light and then leveled off at a photon flux density of -'~7 mol.m-2.d-l. If this curve primarily reflects rates of food supply, then periphyton production and grazer growth in WOC and similar streams is light-limited at a photon flux density <7 mol.m-2-d-'. 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.