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The light:nutrient hypothesis posits that herbivore growth is increasingly constrained by low food quality as the ratio of light to nutrients increases in aquatic ecosystems. We tested predictions of this hypothesis by examining the effects of large seasonal cycles in light and nutrients on the mineral content of periphyton and the growth rate of a dominant herbivore (the snail Elimia clavaeformis) in two oligotrophic streams. Streambed irradiances in White Oak Creek and Walker Branch (eastern Tennessee, USA) varied dramatically on a seasonal basis due to leaf phenology in the surrounding deciduous forests and seasonal changes in sun angle. Concentrations of dissolved nutrients varied inversely with light, causing light : nitrate and light : phosphate to range almost 100-fold over the course of any individual year. Periphyton nitrogen and phosphorus concentrations were much lower than the concentrations of these elements in snails, and they bottomed out in early spring when streambed irradiances were highest. Snail growth, however, peaked in early spring when light:nutrient ratios were highest and periphyton nutrient concentrations were lowest. Growth was linearly related to primary production (accounting for up to 85% of growth variance in individual years), which in turn was driven by seasonal variation in light. Conceptual models of herbivore growth indicate that growth should initially increase as increasing light levels stimulate primary production, but then level off, and then decrease as the negative effects of decreasing algal nutrient content override the positive effects of increased food production. Our results showed no evidence of an inflection point where increasing ratios of light to nutrients negatively affected growth. Snail growth in these intensively grazed streams is probably unaffected by periphyton nutrient content because exploitative competition for food reduces growth rates to levels where the demand for nitrogen and phosphorus is small enough to be satisfied by even low levels of these nutrients in periphyton. Competition for limited food resources in habitats where herbivore densities are uncontrolled by predation or other mortality factors should strongly influence the potential for herbivores to be limited by mineral deficits in their food. Key words: algae; competition; Elimia clavaeformis; herbivore; light : nutrient hypothesis; nitrogen; phosphorus; snails; streams.

Simultaneous gradients of phosphorus and light were applied in experimental streams to develop quantitative relationships between these two important abiotic variables and the growth and composition of benthic microalgae. Algal biovolume and whole-stream metabolism responded hyperbolically to phosphorus enrichment, increasing approximately two-fold over the 5-300 [micro]g [L.sup.-1] range of experimental phosphorus concentrations. The saturation threshold for phosphorus effects occurred at 25 [micro]g [L.sup.-1] of soluble reactive phosphorus (SRP). Light effects were much stronger than those of phosphorus, resulting in a nearly ten-fold increase in algal biovolume over the 10-400 [micro]mol photons [m.sup.-2] [s.sup.-1] range of experimental irradiances. Biovolume accrual was light-saturated at 100 [micro]mol photons [m.sup.-2] [s.sup.-1] (5 mol photons [m.sup.-2] [d.sup.-1]). Light effects were diminished by low phosphorus concentrations, and phosphorus effects were diminished by low irradiances, but evidence of simultaneous limitation by both phosphorus and light at subsaturating irradiances was weak. Contrary to the light : nutrient hypothesis, algal phosphorus content was not significantly affected by light, even in the lowest SRP treatments. However, algal nitrogen content increased substantially at lower irradiances, and it was very highly correlated with algal chlorophyll a content. Phosphorus enrichment in streams is likely to have its largest effect at concentrations

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 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 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/[m.sup.2]) and low (50 individuals/[m.sup.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-I curve: snail growth increased almost linearly with increased light and then leveled off at a photon flux density of ≅7 mol[center dot][m.sup.-2][center dot][d.sup.-1]. 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 Key words: algae; competition; Elimia; grazing; herbivory; lipids; periphyton; photoinhibition; photosynthesis-irradiance; shade; snails; woodland stream., INTRODUCTION Abiotic constraints on primary production are strong influences in natural ecosystems. In terrestrial habitats, light, temperature, and precipitation determine the structure and production of plant communities (Ricklefs 1979). In [...]

Streams in deciduous forests undergo marked transitions from light-replete to light-limited ecosystems every spring when leaves emerge on streamside trees. During the course of leaf emergence and enlargement, shade from leaves on streamside trees can reduce photosynthetically active radiation (PAR) falling on the streambed from [is greater than] 1000 to [is less than] 30 [micro]mol [multiplied by] [m.sup.-2] [multiplied by] [s.sup.-1]. In this study, we examined the effects of leaf emergence at multiple levels in two headwater streams in eastern Tennessee. Primary production estimated from both photosynthesis-irradiance measurements of periphyton in the laboratory and whole-steam diurnal oxygen measurements decreased dramatically over the course of canopy closure. Monthly carbon fixation estimates for periphyton in White Oak Creek declined from 354 [micro]g C/[cm.sup.2] in April to 66 [micro] g C/[cm.sup.2] in June, while carbon fixation in Walker Branch declined from 495 to 168 [micro]g C/[cm.sup.2]. Periphyton photosynthesis became increasingly efficient at low irradiances ([Alpha] increased more than threefold) as ambient streambed irradiances declined, but this increase in efficiency only partially compensated for the photon scarcity caused by riparian shade. Ecological photosynthetic efficiency (percentage of incident PAR energy fixed by photosynthesis) estimated from static models, whole-stream measurements, and ambient PAR was a negative exponential function of incident PAR, increasing from [is less than] 0.3% to 2% during canopy closure. This increase was attributable to (1) inefficient use of the relatively high irradiances before leaf emergence, and (2) greater photoefficiency (increased [Alpha]) low irradiances after leaf emergence. Nutrient concentrations (dissolved nitrate and phosphate) in both streams increased coincident with leaf emergence, implying a cascade of shade effects through primary producers to abiotic components of the ecosystem. Shade effects also propagated to higher trophic levels: growth rates of grazing snails (Elimia clavaeformis) in both streams decreased substantially from April to June, consistent with modeled decreases in the productivity of their food resource (periphyton). Snail growth rates were almost zero in White Oak Creek and were negative in Walker Branch during summer when streambed PAR was lowest. The multilevel effects of leaf emergence reported in this study accentuate the importance of light variation in aquatic ecosystems and illustrate the close coupling between streams and their surrounding terrestrial ecosystems. Key words: carotenoids; chlorophyll a; ecological efficiency; Elimia clavaeformis; grazing; nutrients; periphyton; photoacclimation; photosynthesis-irradiance; primary production; snails; streams.

William Donahue and John Clare question the methodology of Walter Hill and Amy McNamara's 1997 study on the effects of ultraviolet (UV) radiation on freshwater benthic communities. The plexiglas shields that were used did not compromise the results of the experiment. They charged that the study only included biota adapted to high levels of UV, but the sites chosen offered a variety of UV environments.