Differential responses to UVR by bacterioplankton and phytoplankton from the surface and the base of the mixed layer

SUMMARY 1. We tested the influence of ultraviolet radiation (UVR) and shallow stratification on phytoplankton and bacterioplankton from the surface and the base of the mixed layer in two boreal lakes in north-western Ontario, Canada.
2. We measured phytoplankton biomass and production, bacterioplankton production and plankton respiration after transplantation under three solar radiation treatments: ambient radiation (Photosynthetically active radiation (PAR) + ultraviolet-A (UVA) + ultraviolet-B (UVB)), minus UVB (PAR + UVA) and PAR only. We repeated this experiment on three occasions in each lake during the summer.
3. Solar stress (measured as reduced growth and photoinhibition) was generally only found in the 'base phytoplankton' (i.e. originating from the base of the mixed layer). No inhibition of photosynthesis by UVB exposure was found in near-surface phytoplankton. On the other hand, production of near-surface bacterioplankton was reduced following a 4-h UVR exposure but had increased after a 48-h exposure to both UVA and UVB compared with the PAR only treatment.
4. Negative effects of UVR on phytoplankton and bacterioplankton were not ubiquitous. We emphasise the importance of conducting experiments repeatedly, particularly those which test the effects of UVR on different community assemblages from different lakes.     

Grazing rates of Calanus finmarchicus on Thalassiosira weissflogii cultured under different levels of ultraviolet radiation

UVB alters photosynthetic rate, fatty acid profiles and morphological characteristics of phytoplankton. Copepods, important grazers of primary production, select algal cells based upon their size, morphological traits, nutritional status, and motility. We investigated the grazing rates of the copepod Calanus finmarchicus on the diatom Thalassiosira weissflogii cultured under 3 levels of ultraviolet radiation (UVR): photosynthetically active radiation (PAR) only (4 kJ-m(-2)/day), and PAR supplemented with UVR radiation at two intensities (24 kJ-m(-2)/day and 48 kJ-m(-2)/day). There was no significant difference in grazing rates between the PAR only treatment and the lower UVR treatment. However, grazing rates were significantly (～66%) higher for copepods feeding on cells treated with the higher level of UVR. These results suggest that a short-term increase in UVR exposure results in a significant increase in the grazing rate of copepods and, thereby, potentially alters the flow rate of organic matter through this component of the ecosystem.     

Vertical mixing within the epilimnion modulates UVR-induced photoinhibition in tropical freshwater phytoplankton from southern China

1. The importance of vertical mixing in modulating the impact of UVR on phytoplankton photosynthesis was assessed in a tropical, shallow lake in southern China from late winter to mid‐spring of 2005. 2. Daily cycles of fluorescence measurements (i.e. photosynthetic quantum yield, Y) were performed on both ‘static’ and in situ samples. Static samples were of surface water incubated at the surface of the lake under three radiation treatments – PAB (PAR + UVR, 280–700 nm), PA (PAR + UV‐A, 320–700 nm) and P (PAR, 400–700 nm). In situ samples were collected every hour at three different depths – 0, 0.5 and 1 m. 3. The general daily pattern was of a significant decrease in Y from early morning towards noon, with partial recovery in the afternoon. Samples incubated under static conditions always had lower Y than those under in situ conditions at the same time of the day. 4. Under stratified conditions, no overall impact of UVR impact could be detected in situ when compared with the static samples. Further rapid vertical mixing not only counteracted the impact of UVR but also stimulated photosynthetic efficiency. 5. Based on these measurements of fluorescence, the mixing speed of cells moving within the epilimnion was estimated to range between 0.53 and 6.5 cm min⁻¹. 6. These data show that mixing is very important in modulating the photosynthetic response of phytoplankton exposed to natural radiation and, hence, strongly conditions the overall impact of UVR on aquatic ecosystems.     

Effects of solar ultraviolet radiation on the periphyton community in lotic systems: comparison of attached algae and bacteria during their development

The effects of solar ultraviolet radiation (UVR) on the development of a periphyton community were studied in an outdoor artificial stream apparatus. Algal biomass, species composition, and bacterial cell density were measured under full sunlight and non-UVR (photosynthetically active radiation [PAR]-only) conditions. Attachment of algae was detected on days 6–9. Although the chlorophyll-a concentration under non-UVR conditions was 2–4 times that under full sunlight (PAR + UVR) throughout the experiment, neither net algal growth rate nor species composition differed significantly between the two light conditions. The relative carotenoid pigment contents of attached algae in the PAR + UVR condition were 1.1–1.3 times those in the non-UVR condition. Rates of increase of bacterial cell densities under the PAR + UVR condition were depressed by solar UVR for the first few days, although there were no apparent differences in the rates of increase between the light conditions later in the experiment. The small effect of UVR on the development of this periphyton community may be attributable to low UV flux at this study site and to the experimental conditions under which the algae were kept: a high physiological state with high nutrient conditions. Attached bacteria and algae that colonize substrata first are likely to be sensitive to solar UVR, and the negative effects of UVR are mitigated by the development of a periphyton community.     

产毒与不产毒铜绿微囊藻对模拟酸雨及紫外辐射的生理响应

为了比较研究酸雨与紫外辐射对淡水水体常见藻华蓝藻的生理学影响,选取铜绿微囊藻(Microcystis aeruginosa)产毒(FACHB-905)与不产毒(FACHB-469)株系作为实验材料,通过人工模拟酸雨,研究了不同p H处理后2藻株的光合生理变化以及对紫外辐射的敏感性的异同。实验设置3个p H梯度,p H7.10为对照组(正常培养基培养的藻体),两模拟酸雨处理组(p H5.65和p H4.50);两种辐射处理,可见光处理(PAR)以及全波长辐射处理(PAB)。研究结果表明,905藻株细胞粒径在各p H处理下都要显著高于469藻株,模拟酸雨处理显著降低了两藻株细胞的平均粒径及体积,但叶绿素含量显著提高;酸雨处理同时也引起细胞死亡率的增加,表现为藻体有效光化学效率显著降低,生长速率显著受到抑制,低p H下呈负增长,且这种抑制程度在469下更为显著。高的可见光以及紫外辐射处理,使两株系有效光化学效率随p H的降低而呈降低趋势,其中469藻株降低至更低的水平,且高光辐射以及紫外诱导的抑制率要显著高于905藻体,这可能与469藻株较低的光保护色素有关(较低的类胡萝卜素以及紫外吸收物质)。在未来全球变化背景下,不同种类的浮游植物对环境变化的响应及适应能力不同,可改变水体的群落结构和种群丰度,铜绿微囊藻905较469较强的耐受酸雨以及紫外辐射的能力,可能会使该株系在竞争力上占据优势。     

UV RADIATION,PHOSPHORUS, AND THEIR COMBINED EFFECTS ON THE TAXONOMIC COMPOSITION OF PHYTOPLANKTON IN A BOREAL LAKE1

We examined how UV radiation and phosphorus (P) affect the taxonomic composition, abundance, and biomass of phytoplankton in an oligotrophic boreal lake. We exposed phytoplankton to three different solar radiation regimes (PAR + UV‐A radiation [UVAR]+ UV‐B radiation [UVBR], PAR + UVAR, and PAR only) and to five levels of P. The biomass of small chrysophytes was reduced by 350% after exposure to PAR + UVAR + UVBR compared with PAR only. No other taxa were found to be negatively affected by exposure to UVBR. Several taxa (e.g. Chry‐ sochromulina laurentiana Kling) were sensitive to UVAR, whereas others (e.g. Tabellaria flocculosa (Roth) Kutzing) were not affected by UV radiation exposure. Principal components analysis ordination separated phytoplankton that were negatively affected by UV radiation and/or positively affected by P treatments (e.g. small chrysophytes, Cryptomonas rostratiformis, T. flocculosa) from those that generally were unaffected by either treatment (e.g. desmids, some Cyanobacteria). Richness, Shannon‐Weaver diversity, and evenness were significantly higher in phytoplankton communities shielded from UVAR and UVBR. The relationship between diversity and richness was positive in all phytoplankton samples except in those exposed to UVBR. Thus, UVBR‐exposed phytoplankton communities were dominated by a few species even though the number of taxa remained relatively unchanged. Consequently, alterations in the UV environments of lakes resulting from climate warming (e.g. drought) and land‐use change (e.g. increased P export) will likely promote shifts in the community composition of lake phytoplankton.     

Photosynthetically active and UV radiation act in an antagonistic way in regulating buoyancy of Arthrospira (Spirulina) platensis (cyanobacterium)

Buoyancy provided by gas vesicles has been suggested to play an important role in regulating vertical distribution and nutrient acquisition in cyanobacteria. However, little is known about how changes in UV radiation (UVR, 280–400 nm) would affect the buoyancy. We have shown here that the floatation activity of the economically important cyanobacterium Arthrospira platensis (D-0083) decreased with increased photosynthetic rates associated with increased photosynthetically active radiation (PAR), but it decreased less in the presence of UVR, which resulted in inhibitory effects. When the cells were grown under isoenergetic levels of solar PAR or UVR alone, they migrated downward under PAR but maintained buoyant under UVR. The buoyancy regulation of A. platensis depended on the exposed levels of PAR as well as UVR, which affected photosynthesis and growth in an antagonistic way. The buoyancy of A. platensis in water columns is likely to be dependant on diurnal photosynthetic performance regulated by solar radiation, and can hardly be considered as an active strategy to gain more energy during sunrise/sunset or to escape from harmful irradiation during the noon period.     

Erratum to: How do UV radiation, temperature, and zooplankton influence the dynamics of alpine phytoplankton communities?

Plankton in mountain lakes are confronted with generally higher levels of incident ultraviolet radiation (UVR), lower temperatures, and shorter growing seasons than their lower elevation counterparts. The direct inhibitory effects of high UVR and low temperatures on montane phytoplankton are widely recognized. Yet little is known about the indirect effects of these two abiotic factors on phytoplankton, and more specifically whether they alter zooplankton grazing rates which may in turn influence phytoplankton. Here, we report the results of field microcosm experiments that examine the impact of temperature and UVR on phytoplankton growth rates and zooplankton grazing rates (by adult female calanoid copepods). We also examine consequent changes in the absolute and relative abundance of the four dominant phytoplankton species present in the source lake (Asterionella formosa, Dinobryon sp., Discostella stelligera, and Fragilaria crotonensis). All four species exhibited higher growth rates at higher temperatures and three of the four species (all except Dinobryon) exhibited lower growth rates in the presence of UVR versus when shielded from UVR. The in situ grazing rates of zooplankton had significant effects on all species except Asterionella. Lower temperatures significantly reduced grazing rates on Fragilaria and Discostella, but not Dinobryon. While UVR had no effect on zooplankton grazing on any of the four species, there was a significant interaction effect of temperature and UVR on zooplankton grazing on Dinobryon. Discostella and Dinobryon increased in abundance relative to the other species in the presence of UVR. Colder temperatures, the presence of zooplankton, and UVR all had consistently negative effects on rates of increase in overall phytoplankton biomass. These results demonstrate the importance of indirect as well as direct effects of climate forcing by UVR and temperature on phytoplankton community composition in mountain lakes, and suggest that warmer climates and higher UVR levels may favor certain species over others.     

Exposure of eggs to solar UV‐B leads to reduced hatching rates in two sparid fishes,red sea bream Pagrus major and black sea bream Acanthopagrus schlegeli

Hatching success was examined under exposure to solar ultraviolet radiation (UVR) using filters to give three different light conditions [C1: UV‐B, UV‐A and photosynthetically active radiation (PAR), C2: UV‐A and PAR, C3: PAR] in red Pagrus major and black Acanthopagrus schlegeli sea bream. Hatching rate of both species was reduced by an exposure over a 2 day period to UVR and was not significantly different between two species under the three light conditions.     

Natural UVR Does Not Affect Decomposition by Aquatic Hyphomycetes

We performed field and laboratory experiments to evaluate the effect of solar radiation (UVR and PAR) on leaf litter decomposition, fungal biomass and sporulation rates, in the Andean Patagonia, where high UVR levels are common. Leaves of Alnus glutinosa exposed to three treatments, normal radiation (PAR + UVR), protected from UVR and protected from total radiation (SHADE) by plastic films lost 31–37% of their mass. Leaves of Nothofagus pumilio lost 61–64% of their mass under the same conditions. For both leaf species, differences in mass losses among treatments were not statistically significant. Sporulation rates were significantly lower in the SHADE treatment. Fungal biomass accounted for 6.2 to 7.1% of leaf mass, without significant differences among treatments. In the laboratory, leaf discs of A. glutinosa colonized by single species of aquatic hyphomycetes (Articulospora tetracladia, Flagellospora curta or Lunulospora curvula) and exposed to or protected from UVR did not differ in mass loss and sporulation rates. Pure cultures of two fungal species grew at the same rates when exposed to light (PAR and PAR + UVR) or to the SHADE. In summary, we found no evidences that current high levels of UV radiation affect litter decomposition mediated by aquatic hyphomycetes. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

IMPACTS OF SOLAR UV RADIATION ON THE PHOTOSYNTHESIS,GROWTH, AND UV‐ABSORBING COMPOUNDS IN GRACILARIA LEMANEIFORMIS (RHODOPHYTA) GROWN AT DIFFERENT NITRATE CONCENTRATIONS1

Solar ultraviolet radiation (UVR, 280–400 nm) is known to affect macroalgal physiology negatively, while nutrient availability may affect UV‐absorbing compounds (UVACs) and sensitivity to UVR. However, little is known about the interactive effects of UVR and nitrate availability on macroalgal growth and photosynthesis. We investigated the growth and photosynthesis of the red alga Gracilaria lemaneiformis (Bory) Grev. at different levels of nitrate (natural or enriched nitrate levels of 41 or 300 and 600 μM) under different solar radiation treatments with or without UVR. Nitrate‐enrichment enhanced the growth, resulted in higher concentrations of UVACs, and led to negligible photoinhibition of photosynthesis even at noon in the presence of UVR. Net photosynthesis during the noon period was severely inhibited by both ultraviolet‐A radiation (UVA) and ultraviolet‐B radiation (UVB) in the thalli grown in seawater without enriched nitrate. The absorptivity of UVACs changed in response to changes in the PAR dose when the thalli were shifted back and forth from solar radiation to indoor low light, and exposure to UVR significantly induced the synthesis of UVACs. The thalli exposed to PAR alone exhibited higher growth rates than those that received PAR + UVA or PAR + UVA + UVB at the ambient or enriched nitrate concentrations. UVR inhibited growth approximately five times as much as it inhibited photosynthesis within a range of 60–120 μg UVACs · g^?1 (fwt) when the thalli were grown under nitrate‐enriched conditions. Such differential inhibition implies that other metabolic processes are more sensitive to solar UVR than photosynthesis.     

Effect of solar ultraviolet radiation on growth in the marine macroalga Dictyota dichotoma (Phaeophyceae) at Helgoland and its ecological consequences

At Helgoland, in the North Sea, growth of the high sublittoral brown macroalga Dictyota dichotoma (Hudson) Lamoroux was examined in October (the time of tetraspore release) in an outdoor tank by exposing 2-day-old germlings to four solar radiation treatments achieved with different filter materials or an additional artificial light source: photosynthetically active radiation (PAR; 395–700 nm), PAR plus ultraviolet (UV)-A (320–700 nm), full solar spectrum, or solar spectrum plus artificial UV radiation (UVR). Based on length measurements over a period of 3 weeks, the growth rate in germlings strongly decreased in conditions with UVR compared to PAR: by 14% under PAR+UV-A, by 31% under the full solar spectrum and by 65% with additional UVR. Although growth rates of germlings under UVR were reduced mainly in the first week, the plants did not regain the size of the untreated plants even after 9 weeks. Regardless of the exposure, no defects in morphology or anatomy including the exposed apical meristem were detected, except for a reduction in cell division rates perhaps due to additional cost for photoprotective or repair mechanisms. Depending on the actual position of D. dichotoma plants in the natural habitat, individuals in high positions receive substantial amounts of the more harmful UV-B while those lower down might only receive UV-A during part of the day, thus the effect of UV-B on the growth of D. dichotoma will depend on its position in the field. The effects of tidal variation of the light climate and the implications of our results for the zonation of D. dichotoma are discussed. Received in revised form: 6 July 2000 Electronic Publication     

阳光紫外辐射对室内水培发状念珠藻生理特性的影响

发状念珠藻（Nostoc flagelliforme Bornet & Flahault）是一种重要的陆生经济蓝藻,室内培育出的原植体如何适应阳光辐射的问题尚需探讨。为此,作者将室内水培发菜置于阳光下培养,测定了其生长、有效光化学效率（F/Fm΄）和色素的变化。结果表明,较高的可见光（PAR,395－700 nm）和紫外辐射（UVR,280－395 nm）均导致水培发菜的F/Fm΄下降。第1天中午,PAR和UVR分别使F/Fm΄下降了54％和13％;傍晚,F/Fm΄有部分恢复。UVR对发菜适应阳光2d后的生长无负面作用。发菜在适应全阳光辐射期间,紫外吸收物质（Scytonemin和Mycosporine-like amino acids）含量不断增加,9d后,分别增加了124倍和9倍。这些紫外吸收物质的增加对发菜细胞降低光抑制,适应阳光辐射,起到了重要作用。本研究的结果可为水培发菜室外培养方法的建立提供一定的理论依据。     

Nutrient and Grazer-Mediated Effects on Picoplankton and Size Structure in Phytoplankton Communities

This study examines the factors which contribute to the abundance of algal picoplankton in lakes. A three-year field study of a meso-eutrophic lake was compared with observations from oligotrophic and highly eutrophic lakes in the region. Trophic state alone (oligotrophic vs. eutrophic) was not a good predictor of the importance of picoplankton; smaller cells were relatively abundant when phosphorus was limiting other phytoplankters, but also when nitrogen was in surplus. Subsequent field experiments found that picoplankton growth was stimulated by N, but not by P additions. This relationship was strongly affected by light and grazer levels. Grazers apparently mediate the effects of nutrient deficiency, and favor the growth of larger algal size classes, especially nanoplankton. The flux of P within experimental enclosures was controlled by picoplankton abundance under low nutrient conditions, but was a function of total phytoplankton biomass under P surplus.     

Combined effects of light and nitrate supplies on the growth,photosynthesis and ultraviolet‐absorbing compounds in marine macroalga Gracilaria lemaneiformis (Rhodophyta), with special reference to the effects of solar ultraviolet radiation

It is well known that light and nutrients are essential to plants; however, there are few investigations in which these have been studied in combination on macroalgae, especially when solar ultraviolet radiation (UVR) is concerned. We cultured the red alga Gracilaria lemaneiformis (Bory) at different nitrate concentrations and light levels with or without UVR for 24 days. The results showed that nitrate supply markedly enhanced the growth and photosynthesis, increased the absorptivity of UV‐absorbing compounds (UVACs), and decreased photoinhibition in the presence of UVR. The thalli that received photosynthetically active radiation (PAR) treatment exhibited higher growth rates than those that received PAR + UVR at ambient or enhanced nitrate concentrations. However, under PAR + UVR treatment, the absorptivity of UVACs was higher than that of PAR and fluctuated with light levels. UVR was found to reduce the maximal net photosynthetic rate, apparent photosynthetic efficiency and light‐saturating irradiance while increasing the dark respiration rate, and inducing higher inhibition of growth and photosynthesis under high light versus under low light. Ultraviolet B significantly induced the synthesis of UVACs but led to higher inhibition on growth and photosynthesis than ultraviolet A.     

Effects of seston on ultraviolet attenuation in Lake Biwa

We examined the attenuation of underwater ultraviolet (UV) radiation and photosynthetically available radiation (PAR) in Lake Biwa, Japan, at offshore and inshore sites and under contrasting stratification and mixing regimes. There were large spatial differences in the water column transparency to both wavebands, despite little change in concentrations of dissolved organic carbon (DOC). The 1% of surface irradiance depth varied from 0.3 to 2.7 m at 305 nm, from 0.8 to 6.3 m at 380 nm, and from 2.3 to 12.8 m for PAR. Both PAR and UV transparency declined abruptly in the South Basin of the lake when a typhoon caused the resuspension of sediments. The water column ratio of UV to PAR increased by 30% at all stations over the course of a 3-week sampling period associated with the general increase in phytoplankton concentrations. At several sites, the diffuse attenuation coefficient for UV radiation deviated substantially from that predicted from UV-DOC models. A significantly positive linear relationship was found between UV attenuation (K _d determined with a profiling UV radiometer) and the beam attenuation coefficient at 660 nm as measured by transmissometer. These results indicate that scattering and absorption by particulate matter can reduce UV transparency to below that inferred from DOC concentrations, and that current UV-exposure models should be modified to incorporate this effect. Received: March 21, 2001 / Accepted: August 17, 2001     

The Impact of UV Radiation on Paramecium Populations from Alpine Lakes

Paramecium populations from a clear and a glacier‐fed turbid alpine lake were exposed to solar simulated ultraviolet (UVR) and photosynthetically active radiation (PAR) at 8 and 15 °C. The ciliates were tested for DNA damage (comet assay), behavioral changes, and mortality after UVR + PAR exposure. High DNA damage levels (~58% tail DNA) and abnormal swimming behavior were observed, although no significant changes in cell numbers were found irrespective of the lake origin (clear, turbid), and temperatures. We conclude that environmental stressors such as UVR and their effects may influence the adaptation of ciliates living in alpine lakes.     

Antarctic krill (Euphausia superba) acquire a UV-absorbing mycosporine-like amino acid from dietary algae

We hypothesised that Antarctic krill acquire UV-absorbing mycosporine-like amino acids (MAAs) from dietary algae, which produce MAAs in response to ultraviolet (UV) irradiation. To test this hypothesis, we grew cultures of Phaeocystis antarctica that had been grown under either photosynthetically active radiation (PAR, 400-750 nm) plus UV irradiation (UVR, 280-400 nm), or else PAR-only. Algae grown under PAR-only produced high concentrations of porphyra-334, whereas additional UVR caused formation of high concentrations of mycosporine-glycine:valine and lower concentrations of porphyra-334. Krill were fed with either of these two cultures on eight occasions over 63 days. A third group was starved for the duration of the experiment. Animals were analysed after 36 and 63 days for MAA content. Remaining animals from all treatments were starved for a further 35 days and analysed to examine MAA retention characteristics. Our findings are that krill acquired different MAAs from dietary algae depending on the light conditions under which the algae were grown. Specifically, krill fed algae grown under PAR-only had higher concentrations of porphyra-334 than starved krill. Conversely, krill fed algae grown under PAR with additional UVR had high body concentrations of mycosporine-glycine:valine. MAA concentrations in starved krill remained static throughout the experiment. However, long term starvation (35 days) caused levels of certain acquired MAAs to decline. From this we can infer that MAA concentrations in krill are dependent on the MAA content of phytoplankton, and therefore the algae's response to UV exposure. This has implications for transfer of MAAs through marine trophic webs.     

Effects of ultraviolet radiation and nutrients on the structure-function of phytoplankton in a high mountain lake

The combined effect of high solar ultraviolet radiation (UVR) and nutrient supply in a phytoplankton community of a high mountain lake is analyzed in a in situ experiment for 6 days with 2 × 2 factorial design. Interactive UVR × nutrient effects on structural and functional variables (algal biomass, chlorophyll a (chl a), primary production (PP), maximal electron transport rate (ETR(max)), and alkaline phosphatase activity (APA)), as well as stoichiometric ones (sestonic N per cell and N:P ratio) were found. Under non-nutrient enriched conditions, no deleterious effects of UVR on structural variables, PP, photosynthetic efficiency and ETR(max) were observed, whereas only particulate and total APA were affected by UVR. However, percentage excreted organic carbon (%EOC), dissolved APA and sestonic C and P per cell increased under UVR, leading to a decrease in algal C:P and N:P ratios. After nutrient enrichment, chl a, total algal biomass and PP were negatively affected by UVR whereas %EOC, ETR(max) and internal C, P and N content increased. We suggest that the mechanism of algal acclimation to UVR in this high UVR flux ecosystem seems to be related to the increase of internal algal P-content mediated by physiological mechanisms to save P and by a stimulatory UVR effect on dissolved extracellular APA. The mechanism involved in the unmasking effect of UVR after nutrient-enrichment may be the result of a greater sensitivity to UVR-induced cell damage, making the negative UVR effects more evident.     

The Effects of Ultraviolet Radiation and Nutrient Additions on Periphyton Biomass and Composition in a Sub‐Alpine Lake (Castle Lake,USA)

Rising levels of ultraviolet radiation (UVR) striking the Earth's surface have led to numerous studies assessing its inhibitory effects on phytoplankton and periphyton in aquatic systems. Mineral nutrients such as nitrogen (N) and phosphorus (P) have been shown to increase aspects of algal metabolism and compensate for UVR inhibition. An in situ substratum enrichment technique and UV shielding was used to assess the effects of nutrient additions on periphyton exposed to different levels of UVR in Castle Lake, California during July‐August, 1997. UV shielding had no effect on total periphyton biomass, but caused shifts in species composition. The dominant periphyton species, Anabaena circinalis RAB., demonstrated sensitivity to ambient levels of UV radiation possibly due to UV inhibition of N2 ‐fixation. Total diatom biovolume decreased when shielded from UVR. Phosphorus additions continually elicited an increase in periphyton biovolume at all levels of analysis. These results suggest an interaction between nutrient status/availability and UV sensitivity.