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Light and nutrients play pivotal roles in determining the growth of autotrophs, yet the potential for synergistic interactions between the two resources in algal communities is poorly understood, especially in stream ecosystems. In this study, light and phosphorus were manipulated in large experimental streams to examine resource colimitation and synergy in stream periphyton. Whole-stream metabolism was simultaneously limited by light and phosphorus. Increasing the supply of either light or phosphorus resulted in significant increases in primary production and the transformation of the streams from heterotrophy to autotrophy. Resource-driven changes in periphyton community structure occurred in concert with changes in production. Algal assemblages in highly shaded streams were composed primarily of small diatoms such as Achnanthidium minutissima, whereas larger diatoms such as Melosira varians predominated at higher irradiances. Phosphorus enrichment had relatively little effect on assemblage structure, but it did substantially diminish the abundance of Meridion circulare, a diatom whose mucilaginous colonies were conspicuously abundant in phosphorus-poor, high-light streams. Bacterial biomass declined relative to algal biomass with increases in primary productivity, regardless of whether the increases were caused by light or phosphorus. Synergistic effects on primary production appeared to occur because the availability of one resource facilitated the utilization of the other. Light increased the abundance of large diatoms, which are known to convert high concentrations of nutrients into primary production more effectively than smaller taxa. Phosphorus enrichment led to the replacement of Meridion circulare by non-mucilaginous taxa in phosphorus-enriched streams, and we hypothesize that this change enabled more efficient use of light in photosynthesis. Higher ratios of chlorophyll a : biomass in phosphorus-enriched streams may have also led to more efficient photon capture and higher photosynthetic rates. Synthesis. Our results underscore the potential for resource colimitation, even in habitats where a single resource is as strongly limiting as is light in shaded streams. The capacity of autotrophic communities to respond to more than one limiting resource suggests that prevailing single-resource models of ecosystem productivity are overly simplistic. [ABSTRACT FROM AUTHOR]

1. Streambed light regimes change dramatically when riparian trees gain leaves in spring and lose them in autumn. This study examined the effect of these changes on periphyton photosynthetic characteristics, primary production, and light utilisation efficiency in two eastern Tennessee streams. 2. Photosynthesis–irradiance responses were measured at intervals covering leaf emergence and abscission in spring and autumn. Photosynthetic efficiency (αchl ) increased with declining streambed irradiances during spring leaf emergence, but returned to pre-emergence values after autumn leaf fall. The onset of photosaturation (I k ) displayed the opposite pattern, decreasing during leaf emergence and increasing after leaf fall. Both αchl and I k were closely associated (P < 0.01) with daily integrated streambed irradiance, as were periphyton carotenoids. Internal shading by photoprotective carotenoids is hypothesised to account for lower αchl when streambed irradiances are high. 3. An in situ shading experiment confirmed that the temporal changes observed in periphyton photosynthetic characteristics and carotenoids were primarily the result of changing light levels and not other environmental factors (e.g. nutrients, temperature). 4. Daily chlorophyll-specific primary production (PP chl ) was calculated with P –I models and recorded streambed irradiances. In both streams, PP chl was the highest in early spring when trees were leafless, and then declined markedly as leaves emerged, reaching a minimum in summer. PP chl increased after leaf abscission, but was still lower than it was in early spring, when the sun was higher and daylength was longer. A hyperbolic tangent equation fit to PP chl and daily integrated irradiance (r 2 =0. 85) suggested that primary production was light saturated at 4–8 mol m–2... [ABSTRACT FROM AUTHOR]

Summary 1 This laboratory study examined the effect of a gradient of UV-B radiation (280–320 nm) on photosynthesis and food quality of periphyton, the trophic base of many freshwater benthic communities. Four irradiances of UV-B (0, 0.6, 1.2, and 2.3 W m-2) were delivered by UV-B lamps (313 nm peak irradiance) over a 13-day period in the first experiment and over a 4-h period in the second experiment. These irradiances were roughly equivalent to 0, 1, 2, and 4 times the ambient biologically effective (DNA) midsummer, midday UV-B irradiance in Tennessee. 2 Rates of photosynthesis and photosynthetic pigments were significantly reduced by irradiances greater than ambient during the 13-day experiment, suggesting that food supply rates to grazers would be depressed by increases in current UV-B levels. Effects on community structure were minor, but mean diatom cell size decreased at higher UV-B irradiances. 3 Irradiated periphyton was fed in surplus to juvenile snails (Physella gyrina) in the first experiment as a bioassay for food quality. Snail growth was the same on all four diets, suggesting that UV-B did not affect food quality. Nitrogen and phosphorus content of the periphyton were not affected by UV-B, either. 4 Photosynthesis by low-biomass periphyton in the second experiment was significantly depressed by irradiances above ambient after only 4 h. Photosynthesis by the high biomass periphyton was not significantly affected by UV-B, suggesting that self-shading reduced UV-B effects. [ABSTRACT FROM AUTHOR]

Two sweetpotato [Ipomoea batatas (L.) Lam] genotypes (TU-82-155 and NCC-58) were grown hydroponically and subjected to a temporary loss of lighting in the form of 14 days of prolonged darkness compared with a lighted control under standard daily light periods to determine the impact on growth responses and storage root yield. Vine cuttings of both genotypes were grown in rectangular channels. At 65 days after planting, lights were turned off in the treatment chambers and replaced by a single incandescent lamp, providing between 7 and 10 µmol-m-2·s-1 photosynthetic photon flux (PPF) for 18 hours, and the temperature lowered from 28/22 °C light/dark, to a constant 20 °C. Plants remained under these conditions for 14 days after which the original light level was restored. Growth chamber conditions predark included, a PPF mean provided by 400-W metal halide lamps, of 600 ± 25 pimol·m-2-s-1, an 18-hour light/6-hour dark cycle and a relative humidity of 70% ± 5%. The nutrient solution used was a modified half-Hoagland with pH and electrical conductivity (EC) maintained between 5.5-6.0 and 1000-1200 µS·cm-1, respectively, and was adjusted weekly. Storage root number and fresh weight were similar regardless of treatments. Plants exposed to prolonged darkness produced 10.5% and 25% lower fibrous root fresh and dry mass, respectively, but similar foliage yield and harvest index (HI). 'NCC-58' produced an average of 31% greater storage root yield than that of 'TU-82-155' but the number of storage roots as well as % dry matter (%DM) were similar. 'NCC-58' also produced 31% greater fibrous root dry weight, whereas 'TU-82-155' produced a 44% greater HI. The significant interaction between prolonged darkness and cultivars for %DM of the storage roots showed that DM for 'TU-82-155' was 18.4% under prolonged darkness and 17.9% in the light. That for 'NCC-58' was 16.4% under prolonged darkness compared with 19.4% (14.8% greater) for plants that were not subjected to prolonged darkness. The evidence that there were no adverse impacts on storage root yield following the exposure to prolonged darkness suggests that the detrimental effects were below the detectable limits for these cultivars in response to the short perturbation in the available light and that sweetpotatoes would be hardy under short-term failure situations. [ABSTRACT FROM AUTHOR]