Light Limitation in a Stream Ecosystem: Responses by Primary Producers and Consumers

Abstract

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.

Linking resource availability and habitat structure to stream organisms: an experimental and observational assessment

“…Rock Creek is narrower than the main stem (;4 m vs. 22 m wide), resulting in riparian vegetation shading more of the stream surface from incoming solar radiation: canopy closure was about 54% in the main stem and 86% in Rock Creek (P. M. Kiffney, unpublished data). This difference in canopy cover is important because the trophic productivity of many temperate forest streams is primarily limited by light, which is largely a function of riparian shading (Murphy et al 1986, Hill et al 1995, Kiffney et al 2003. Our field data support this link between light and trophic production.…”

Using benthic diatom assemblages to assess human impacts on streams across a rural to urban gradient

“…Triplicate sediment samples were collected from areas of low flow by inverted petri dish along the riverbed to collect the surface material. To minimize the effects of light availability on benthic diatom (Hill et al 1995), the selected sites had a canopy cover of 0-1.0 % and are saturated by light.…”

Spatial and seasonal variations in benthic algal assemblages in streams in monsoonal Hong Kong

“…Hill, 1996;Roberts et al, 2004;Fuller et al, 2008), but the consequences of enhanced light availability can be modified by grazing pressure, especially where grazers are numerous enough to consume any increases in productivity (e.g. Power, 1984;Rosemond, 1994;Hill et al, 1995;Hillebrand, 2005). Populations of algivorous Pseudogastromyzon myersi and Liniparhomaloptera disparis in shaded TPK were Fig.…”

“…Apart from abiotic influences, grazing by herbivores applies a pronounced top-down control on the benthic algae in streams under low-to moderateintensity disturbance (Feminella & Hawkins, 1995;Hill et al, 1995;Hillebrand, 2005). Algal biomass is reduced or limited by the presence of herbivores but other effects are possible also; for example, grazing fishes can also cause a shift from dominance by diatoms to upright filamentous cyanobacteria (Power et al, 1988;Gelwick et al, 1997;Abe et al, 2006).…”