UV effects that come and go: a global comparison of marine benthic community level impacts

Ambient UV radiation has substantially increased during the last decades, but its impact on marine benthic communities is hardly known. The aim of this study was to globally compare and quantify how shallow hard‐bottom communities are affected by UV during early succession. Identical field experiments in 10 different coastal regions of both hemispheres produced a consistent but unexpected pattern: (i) UV radiation affected species diversity and community biomass in a very similar manner, (ii) diversity and biomass were reduced to a larger extent by UVA than UVB radiation, (iii) ambient UV levels did not affect the composition of the communities, and (iv) any UV effects disappeared during species succession after 2–3 months. Thus, current levels of UV radiation seem to have small, predictable, and transient effects on shallow marine hard‐bottom communities.     

Ultraviolet radiation and consumer effects on a field-grown intertidal macroalgal assemblage in Antarctica

Ultraviolet radiation (UVR) research on marine macroalgae has hithero focussed on physiological effects at the organism level, while little is known on the impact of UV radiation on macroalgal assemblages and even less on interactive effects with other community drivers, e.g. consumers. Field experiments on macrobenthos are scarce, particularly in the Antarctic region. Therefore, the effects of UVR and consumers (mainly limpets were excluded) on early successional stages of a hard bottom macroalgal community on King George Island, Antarctica, were studied. In a two‐factorial design experimental units [(1) ambient radiation, 280–700 nm; (2) ambient minus UVB, 320–700 nm and (3) ambient minus UVR, 400–700 nm vs. consumer–no consumer] were installed between November 2004 and March 2005 (n= 4 plus controls). Dry mass, species richness, diversity and composition of macroalgal assemblages developing on ceramic tiles were followed. Consumers significantly suppressed green algal recruits and total algal biomass but increased macroalgal richness and diversity. Both UVA and UVB radiation negatively affected macroalgal succession. UVR decreased the density of Monostroma hariotii germlings in the first 10 weeks of the experiment, whereas the density of red algal recruits was significantly depressed by UVR at the end of the study. After 106 days macroalgal diversity was significantly higher in UV depleted than in UV‐exposed assemblages. Furthermore, species richness was significantly lower in the UV treatments and species composition differed significantly between the UV‐depleted and the UV‐exposed treatment. Marine macroalgae are very important primary producers in coastal ecosystems, serving as food for herbivores and as habitat for many organisms. Both, UVR and consumers significantly shape macroalgal succession in the Antarctic intertidal. Consumers, particularly limpets can mediate negative effects of ambient UVR on richness and diversity till a certain level. UVB radiation in general and an increase of this short wavelength due to stratospheric ozone depletion in particular may have the potential to affect the zonation, composition and diversity of Antarctic intertidal seaweeds altering trophic interactions in this system.     

Differential sensitivity of canola (Brassica napus) seedlings to ultraviolet-B radiation,water stress and abscisic acid

Responses of canola (Brassica napus L.) seedlings to three ultraviolet (UV)-B levels [0 (zero), 5 (ambient) and 10 (enhanced) kJ m^?2 d^?1], two watering regimes (well-watered and water-stressed), and two abscisic acid (ABA) levels (with and without application) were investigated. Overall, enhanced UVB and water stress negatively affected plant growth and physiology, but ABA had very little effect. Enhanced UVB decreased stem height, leaf area, plant dry matter, water use efficiency and wax content, but increased concentrations of chlorophyll a, carotenoids and flavonoids, and ethylene evolution. Water stress reduced stem height and diameter, leaf area, plant dry matter, leaf weight ratio and shoot:root weight ratio under zero and ambient UVB. Water stress also reduced chlorophyll a and carotenoids in plants exposed to enhanced UVB. ABA with watering regime had significant interactive effects only on leaf dry matter and wax content. We found that enhanced UVB and water stress adversely affected B. napus seedlings. Interaction between these two factors affected plant performance. In this interaction, ABA had little significant role. Also, optimum vegetative growth and biomass were achieved under ambient UVB.     

Does ambient substrate composition influence consumer diversity effects on algal removal?

Benthic substrates constitute an important habitat template for aquatic communities and may affect the contributions of benthic organisms to ecological processes. To test the effects of ambient substrate composition on the process of algae accrual and removal, we conducted an experiment to examine how substrate type influenced consumer richness effects. We hypothesized that algal removal from focal substrates (ceramic tiles) would be influenced by the surrounding ambient substrate through its effect on nutrient cycling and subsequent algal growth. We manipulated consumer richness in mesocosms at one or three species while holding consumer biomass constant. Aquatic consumers were an amphipod, a snail, and a water boatman, and ambient substrates were either sand or gravel. After 21 days, ambient substrate influenced epilithic algal accrual on tiles, affected physio-chemical parameters within mesocosms, and modified consumer behavior. Chlorophyll a was approximately 2× greater on control tiles surrounded by sand, and FPOM and turbidity were greater on sand than gravel when consumers were present. Substrate modified consumer behavior such that consumers congregated around focal substrates in sand, but dispersed around them in gravel. Consumers also had substrate-specific influences on epilithic chlorophyll, causing a decrease in sand and an increase in gravel. Algal assemblages on focal tiles were dominated by diatoms, and their composition responded to consumer richness and identity, but not substrate. Our data suggest that direct effects (e.g., consumptive removal of epilithon from focal tiles) were more pronounced in sand, whereas indirect effects (e.g., bioturbation and enhanced mixing) promoted algal accrual in gravel. These results show that algae production on exposed surfaces may change as underlying substrate composition changes, and that substrate type can alter consumer diversity effects on algal removal.     

Mycosporine-like amino acids in Antarctic sea ice algae,and their response to UVB radiation

Mycosporine like amino acids (MAAs) were detected in low concentration in sea ice algae growing in situ at Cape Evans, Antarctica. Four areas of sea ice were covered with plastics of different UV absorption exposing the bottom- ice algal community to a range of UV doses for a period of 15 days. Algae were exposed to visible radiation only; visible + UV radiation; and visible + enhanced UV radiation. MAA content per cell at the start of the experiment was low in snow-covered plots but higher in samples from ice with no snow cover. During the study period, the MAA content per cell reduced in all treatments, but the rate of this decline was less under both ambient UV and visible radiation than under snow covered plots. While low doses of UVB radiation may have stimulated some MAA production (or at least slowed its loss), relatively high doses of UVB radiation resulted in almost complete loss of MAAs from ice algal cells. Despite this reduction in MAA content per cell, the diatoms in all samples grew well, and there was no discernible effect on viability. This suggests that MAAs may play a minor role as photoprotectants in sea ice algae. The unique structure of the bottom ice algal community may provide a self-shading effect such that algal cells closest to the surface of the ice contain more MAAs than those below them and confer a degree of protection on the community as a whole.     

Plant community responses to increased precipitation and belowground litter addition: Evidence from a 5‐year semiarid grassland experiment

Global climate change is predicted to stimulate primary production and consequently increases litter inputs. Changing precipitation regimes together with enhanced litter inputs may affect plant community composition and structure, with consequent influence on diversity and ecosystem functioning. Responses of plant community to increased precipitation and belowground litter addition were examined lasting 5 years in a semiarid temperate grassland of northeastern China. Increased precipitation enhanced community species richness and abundance of annuals by 16.8% and 44%, but litter addition suppressed them by 25% and 54.5% after 5 years, respectively. During the study period, perennial rhizome grasses and forbs had consistent negative relationship under ambient plots, whereas positive relationship between the two functional groups was found under litter addition plots after 5 years. In addition, increased precipitation and litter addition showed significant interaction on community composition, because litter addition significantly increased biomass and abundance of rhizome grasses under increased precipitation plots but had no effect under ambient precipitation levels. Our findings emphasize the importance of water availability in modulating the responses of plants community to potentially enhanced litter inputs in the semiarid temperate grassland.     

The influence of ambient ultraviolet light on sperm quality and sexual ornamentation in three-spined sticklebacks (Gasterosteus aculeatus)

Exposure to enhanced levels of ambient ultraviolet (UV) radiation (UVR) can have adverse effects on aquatic organisms including damage at the cellular and molecular level and impairment of development, fecundity and survival. Much research has been conducted on the role of the harmful UVB radiation. However, due to its greater penetration in water the more abundant UVA radiation can also act as an environmental stressor. Little is known about UVR effects on sperm characteristics although sperm cells should be especially prone to UV-induced oxidative stress. Moreover, UV-related changes in oxidative status may affect the phenotypic expression of energetically costly sexual ornaments. We investigated the effects of long-term exposure to ecologically relevant levels of simulated UVA radiation on sperm quality and sexual ornamentation in three-spined sticklebacks (Gasterosteus aculeatus). Males were assigned to three spectral exposure treatments differing in the UV spectral part so that they received either enhanced, moderate or no UVA radiation. The results reveal that exposure to enhanced ambient UVA levels had detrimental effects on both male breeding coloration and sperm velocity providing evidence that UVR affects traits targeted by pre- and post-copulatory sexual selection. By highlighting the role of UVA as a factor influencing fitness-relevant traits, our findings may contribute to a better understanding of the consequences of current and future levels of solar UVR for mating systems and life history.     

INFLUENCE OF NATURAL ULTRAVIOLET RADIATION ON LOTIC PERIPHYTIC DIATOM COMMUNITY GROWTH,BIOMASS ACCRUAL,AND SPECIES COMPOSITION: SHORT-TERM VERSUS LONG-TERM EFFECTS1, 2

Growth rates, accumulation dynamics, and species succession of periphytic diatom communities were examined in the presence and absence of natural ultraviolet (UV) radiation using a series of outdoor, continuous-flow experimental flumes located on the South Thompson River, British Columbia. In a short-term experiment (2–3 wk), log-phase growth rates of naturally seeded diatom communities comprised of Tabellaria fenestrata (Lyngb.) Kütz., T. flocculosa (Roth) Kütz., Fragilaria crotonesis Kitton, and F. vaucheriae (Ehr.) Peter. exposed to 90% ambient photosynthetically active radiation (PAR) + UV were 30–40% lower than growth rates under 90% PAR alone. UV inhibition of growth rate was independent of the degree of P limitation within the range of relative specific growth rates (μ:μ_max-P) of 0.5–1.0. In a long-term trial, inhibition of attached diatom accumulation under 90% PAR + UV during the first 2–3 wk was corroborated. Reduction of full sunlight to 50% PAR + UV prevented the initial inhibition phase. The initial inihibitory effect of 90% PAR + UV on algal accumulation was reversed after 3–4 wk, and by 5 wk total diatom abundance (chlorophyll a, cell numbers and cell biovolumes) in communities exposed to PAR + UV were 2–4-old greater than in communities protected from UV. Under 90% PAR + UV and 50% PAR + UV, a succession to stalked diatom genera (Cymbella and Gomphoneis) occurred. Species succession under UV radiation doubled the mean cell size of the diatom communities. The shift from inhibition to a long-term increase in the autotrophic community under PAR + UV compared to PAR alone provides further evidence against the use of short-term incubation experiments to define the long-term implications of increases in UVB. These results suggest that the ecological effects of present-day levels of UVB and UVB:UVA ratios on autotrophic communities are not well understood and might be mediated through complex trophic level interactions.     

Effects of regular and irregular temporal patterns of disturbance on biomass accrual and species composition of a subtidal hard-bottom assemblage

Assessing patterns of species distribution and abundance is important to understand the driving processes of, and predict future changes in, biodiversity. To this date, ecological studies have been mainly designed to investigate the effects of the mean magnitude of predictor variables, although ecological factors naturally vary in space and time. In a nine month long field experiment, we tested the effects of different temporal patterns (regular, lowly and highly irregular) in biomass removal (=disturbance event) on the diversity, species composition, and biomass accrual of macrobenthic assemblages grown on 15 × 15 cm² PVC-panels. For each pattern of disturbance, disturbance events were timed at three sequences to control for possible confounding effects with recruitment patterns. Disturbance intensity was kept identical among treatments. Assemblages developed in the absence of disturbance for 3 months prior to a 150-day manipulation period, during which the biomass from 20% of the panel area was removed at each of ten disturbance events. Additional undisturbed settlement panels were deployed in the field to assess monthly recruitment rates and species succession over a one year period. Disturbance (i) reduced biomass and total species cover, (ii) changed species composition during the first half of the manipulation period significantly, and (iii) was without effect on species richness and evenness. Irregular disturbance regimes enhanced the abundance of the ascidian Ciona intestinalis, biomass accrual, and total species cover of assemblages relative to the regular disturbance regime, but had either no or only transient effects on diversity and species composition, respectively. Neither the degree of irregularity in disturbance nor the sequence of disturbance events affected any of the response variables significantly. Recruitment of species was strongly seasonal with almost only diatoms recruiting during winter, while recruitment was most intense during summer. Our results suggest that the temporal patterns of predictor variables might be of low explanatory power for the variance of responses in communities with seasonal recruitment patterns that are exposed to a high level of disturbance. Thus the need to include temporal patterns of predictor variables in experimental designs may depend on community dynamics and the characteristics of the process under investigation. S. Wollgast and M. Molis contributed equally to this work.     

Seasonal changes in the deleterious effects of solar ultraviolet-B radiation on eggs of the twospotted spider mite, <Emphasis Type="Italic">Tetranychus urticae</Emphasis> (Acari: Tetranychidae)

Solar ultraviolet-B (UVB) radiation has deleterious effects on plant-dwelling mites. We assessed the biological effects of UVB radiation on the eggs of the twospotted spider mite, Tetranychus urticae Koch, under both near ambient (UV+) and UV-attenuated (UV−) conditions from spring to autumn and compared them to the effects of temperature and humidity. The ambient daily UVB irradiance increased from January to August and then decreased rapidly until December, whereas egg hatchability under UV+ was lowest in April (10.7%) and increased almost linearly until October (74.9–92.3%). In contrast, hatchability under UV− was consistently high (96.2–99.8%) through all seasons. For UV+, the stepwise multiple linear regression analysis supported the negative correlation of hatchability with cumulative UVB irradiance during egg periods (cumulative dose), but did not support that with the mean daily UVB irradiance (dose rate), suggesting that UVB-induced mortality in T. urticae eggs is cumulative dose dependent rather than dose rate dependent. The high mortality in April may have reflected the slower development caused by the relatively lower temperature and higher UVB radiation, increasing the cumulative dose, while the low mortality in October may have reflected the faster development caused by the relatively higher temperature and lower UVB radiation, decreasing the cumulative dose.     

Bottom–up and top–down forces structuring consumer communities in an experimental grassland

Synthesis The interplay between bottom‐up and top‐down effects is certainly a general manifestation of any changes in both species abundances and diversity. Summary variables, such as species numbers, diversity indices or lumped species abundances provide too limited information about highly complex ecosystems. In contrast, species by species analyses of ecological communities comprising hundreds of species are inevitably only snapshot‐like and lack generality in explaining processes within communities. Our synthesis, based on species matrices of functional groups of all trophic levels, simplifies community complexity to a manageable degree while retaining full species‐specific information. Taking into account plant species richness, plant biomass, soil properties and relevant spatial scales, we decompose variance of abundance in consumer functional groups to determine the direction and the magnitude of community controlling processes. After decades of intensive research, the relative importance of top–down and bottom–up control for structuring ecological communities is still a particularly disputed issue among ecologists. In our study, we determine the relative role of bottom–up and top–down forces in structuring the composition of 13 arthropod functional groups (FG) comprising different trophic consumer levels. Based on species‐specific plant biomass and arthropod abundance data from 50 plots of a grassland biodiversity experiment, we quantified the proportions of bottom–up and top–down forces on consumer FG composition while taking into account direct and indirect effects of plant diversity, functional diversity, community biomass, soil properties and spatial arrangement of these plots. Variance partitioning using partial redundancy analysis explained 21–44% of total variation in arthropod functional group composition. Plant‐mediated bottom–up forces accounted for the major part of the explainable variation within the composition of all FGs. Predator‐mediated top–down forces, however, were much weaker, yet influenced the majority of consumer FGs. Plant functional group composition, notably legume composition, had the most important impact on virtually all consumer FGs. Compared to plant species richness and plant functional group richness, plant community biomass explained a much higher proportion of variation in consumer community composition.     

Future increase in temperature more than decrease in litter quality can affect microbial litter decomposition in streams

The predicted increase in atmospheric CO(2) concentration for this century is expected to lead to increases in temperature and changes in litter quality that can affect small woodland streams, where water temperature is usually low and allochthonous organic matter constitutes the basis of the food web. We have assessed the individual and interactive effect of water temperature (5 and 10°C) and alder litter quality produced under ambient CO(2) levels (ambient litter) or under CO(2) concentrations predicted for 2050 (elevated litter) on litter decomposition and on fungal activity and assemblage structure. Litter decomposition rates and fungal respiration rates were significantly faster at 10 than at 5°C, but they were not affected by litter quality. Litter quality affected mycelial biomass accrual at 5 but not at 10°C, while increases in temperature stimulated biomass accrual on ambient but not on elevated litter. A similar pattern was observed for conidial production. All variables were stimulated on elevated litter at 10°C (future scenario) compared with ambient litter at 5°C (present scenario), but interactions between temperature and litter quality were additive. Temperature was the factor that most strongly affected the structure of aquatic hyphomycete assemblages. Our results indicate that if future increases in atmospheric CO(2) lead to only slight modifications in litter quality, the litter decomposition and fungal activities and community structure will be strongly controlled by increased water temperature. This may have serious consequences for aquatic systems as faster litter decomposition may lead to food depletion for higher trophic levels.     

Effects of experimentally-enhanced precipitation and nitrogen on resistance,recovery and resilience of a semi-arid grassland after drought

Resistance, recovery and resilience are three important properties of ecological stability, but they have rarely been studied in semi-arid grasslands under global change. We analyzed data from a field experiment conducted in a native grassland in northern China to explore the effects of experimentally enhanced precipitation and N deposition on both absolute and relative measures of community resistance, recovery and resilience—calculated in terms of community cover—after a natural drought. For both absolute and relative measures, communities with precipitation enhancement showed higher resistance and lower recovery, but no change in resilience compared to communities with ambient precipitation in the semi-arid grassland. The manipulated increase in N deposition had little effect on these community stability metrics except for decreased community resistance. The response patterns of these stability metrics to alterations in precipitation and N are generally consistent at community, functional group and species levels. Contrary to our expectations, structural equation modeling revealed that water-driven community resistance and recovery result mainly from changes in community species asynchrony rather than species diversity in the semi-arid grassland. These findings suggest that changes in precipitation regimes may have significant impacts on the response of water-limited ecosystems to drought stress under global change scenarios.     

Combination treatment of elevated UVB radiation,CO2 and temperature has little effect on silver birch (Betula pendula) growth and phytochemistry

Elevations of carbon dioxide, temperature and ultraviolet‐B (UBV) radiation in the growth environment may have a high impact on the accumulation of carbon in plants, and the different factors may work in opposite directions or induce additive effects. To detect the changes in the growth and phytochemistry of silver birch (Betula pendula) seedlings, six genotypes were exposed to combinations of ambient or elevated levels of CO₂, temperature and UVB radiation in top‐closed chambers for 7 weeks. The genotypes were relatively similar in their responses, and no significant interactive effects of three‐level climate factors on the measured parameters were observed. Elevated UVB had no effect on growth, nor did it alter plant responses to CO₂ and/or temperature in combined treatments. Growth in all plant parts increased under elevated CO₂, and height and stem biomass increased under elevated temperature. Increased carbon distribution to biomass did not reduce its allocation to phytochemicals: condensed tannins, most flavonols and phenolic acids accumulated under elevated CO₂ and elevated UVB, but this effect disappeared under elevated temperature. Leaf nitrogen content decreased under elevated CO₂. We conclude that, as a result of high genetic variability in phytochemicals, B. pendula seedlings have potential to adapt to the tested environmental changes. The induction in protective flavonoids under UVB radiation together with the positive impact of elevated CO₂ and temperature mitigates possible UVB stress effects, and thus atmospheric CO₂ concentration and temperature are the climate change factors that will dictate the establishment and success of birch at higher altitudes in the future.     

Effects of UVB radiation on marine phytoplankton communities

The impact of enhanced and reduced UVB radiation (UVBR) on pelagic ecosystems was studied during two mesocosm experiments in May and June/July 1994. The ambient UVBR exposure was either reduced with mylar foil or artificially enhanced with UVB fluorescent tubes. Developments in the phytoplankton communities were followed during 11 and 8 day periods using several structural and functional parameters. In the May experiment, enhanced UVBR significantly stimulated carbon dioxide fixation, activity of alkaline phosphatase and content of fatty acids. In the June-July experiment, the effects induced by changed UVBR were smaller with some indications of decreased algal biomass with enhanced UVBR. Several of the measured parameters indicated that the two experiments represented different stages in the plankton community development. In the May experiment, the community was in a development stage, moving from nutrient-replete to nutrient-depleted conditions, while the community in June/July was depleted of nutrients from the start. The stimulating effects of UVBR in May are suggested to be the secondary effects of a photochemically induced breakdown of dissolved organic matter, resulting in an increase in available nutrients.      

Experimental examination of the effects of ultraviolet-B radiation in combination with other stressors on frog larvae

Ultraviolet-B radiation (UVB) is a ubiquitous stressor with negative effects on many aquatic organisms. In amphibians, ambient levels of UVB can result in impaired growth, slowed development, malformations, altered behavior and mortality. UVB can also interact with other environmental stressors to amplify these negative effects on individuals. In outdoor mesocosm and laboratory experiments we studied potential synergistic effects of UVB, a pathogenic fungus, Batrachochytrium dendrobatidis (Bd), and varying temperatures on larval Cascades frogs (Rana cascadae). First, we compared survivorship, growth and development in two mesocosm experiments with UVB- and Bd-exposure treatments. We then investigated the effects of UVB on larvae in the laboratory under two temperature regimes, monitoring survival and behavior. We found reduced survival of R. cascadae larvae with exposure to UVB radiation in all experiments. In the mesocosm experiments, growth and development were not affected in either treatment, and no effect of Bd was found. In the laboratory experiment, larvae exposed to UVB demonstrated decreased activity levels. We also found a trend towards reduced survival when UVB and cold temperatures were combined. Our results show that amphibian larvae can suffer both lethal and sublethal effects when exposed to UVB radiation.     

Interactive effects of UV radiation and water availability on seedlings of six woody Mediterranean species

To assess the effects of UV radiation and its interaction with water availability on Mediterranean plants, we performed an experiment with seedlings of six Mediterranean species (three mesophytes vs three xerophytes) grown in a glasshouse from May to October under three UV conditions (without UV, with UVA and with UVA+UVB) and two irrigation levels (watered to saturation and low watered). Morphological, physiological and biochemical measures were taken. Exposure to UVA+UVB increased the overall leaf mass per area (LMA) and the leaf carotenoids/chlorophyll a + b ratio of plants in relation to plants grown without UV or with UVA, respectively. In contrast, we did not find a general effect of UV on the leaf content of phenols or UVB‐absorbing compounds of the studied species. Regarding plant growth, UV inhibited the above‐ground biomass production of well‐watered plants of Pistacia lentiscus. Conversely, under low irrigation, UVA tended to abolish the reduction in growth experienced by P. lentiscus plants growing in a UV‐free environment, in accordance with UVA‐enhanced apparent electron transport rate (ETR) values under drought in this species. UVA also induced an overall increase in root biomass when plants of the studied species were grown under a low water supply. In conclusion, while plant exposition to UVA favored root growth under water shortage, UVB addition only gave rise to photoprotective responses, such as the increase in LMA or in the leaf carotenoids/chlorophyll a + b ratio of plants. Species‐specific responses to UV were not related with the xerophytic or mesophytic character of the studied species.     

Temperature indirectly affects benthic microalgal diversity by altering effects of top–down but not bottom–up control

Understanding the ecological mechanisms that underlie species diversity decline in response to environmental change has become an urgent objective in current ecological research. Not only direct (lethal) effects on single species but also indirect effects altering biotic interactions between species within and across trophic levels comprise the driving force of ecosystem change. In an experimental marine benthic microalgae–grazer system we tested for indirect effects of moderate temperature change on algal diversity by manipulation of temperature, nutrient supply and grazer density. In our model system warming did not exert indirect effects on microalgal diversity via effects on resource competition. However, moderate warming strengthened consumer control and thereby indirectly affected algal community structure which ultimately resulted in decreased diversity. Only in low temperature and low nutrient regimes did the antagonizing mechanisms of bottom–up and top–down regulation establish a balancing effect on algal diversity within 29 days (corresponding to 15–29 algae generations). Effects of thermal habitat change did not appear before 9–18 algae generations, which points to the relevance of longer‐term experiments and ecological monitoring in order to separate transient biotic responses and subtle changes of community dynamics in consequence to global change.     

A meta-analysis of the interactive effects of UV and drought on plants

Interactions between climate change and UV penetration in the biosphere are resulting in the exposure of plants to new combinations of UV radiation and drought. In theory, the impacts of combinations of UV and drought may be additive, synergistic or antagonistic. Lack of understanding of the impacts of combined treatments creates substantial uncertainties that hamper predictions of future ecological change. Here, we compiled information from 52 publications and analysed the relative impacts of UV and/or drought. Both UV and drought have substantial negative effects on biomass accumulation, plant height, photosynthesis, leaf area and stomatal conductance and transpiration, while increasing stress-associated symptoms such as MDA accumulation and reactive-oxygen-species content. Contents of proline, flavonoids, antioxidants and anthocyanins, associated with plant acclimation, are upregulated both under enhanced UV and drought. In plants exposed to both UV and drought, increases in plant defense responses are less-than-additive, and so are the damage and growth retardation. Less-than-additive effects were observed across field, glasshouse and growth-chamber studies, indicating similar physiological response mechanisms. Induction of a degree of cross-resistance seems the most likely interpretation of the observed less-than-additive responses. The data show that in future climates, the impacts of increases in drought exposure may be lessened by naturally high UV regimes.     

Effects of solar UV radiation on aquatic ecosystems and interactions with climate change.

Recent results continue to show the general consensus that ozone-related increases in UV-B radiation can negatively influence many aquatic species and aquatic ecosystems (e.g., lakes, rivers, marshes, oceans). Solar UV radiation penetrates to ecological significant depths in aquatic systems and can affect both marine and freshwater systems from major biomass producers (phytoplankton) to consumers (e.g., zooplankton, fish, etc.) higher in the food web. Many factors influence the depth of penetration of radiation into natural waters including dissolved organic compounds whose concentration and chemical composition are likely to be influenced by future climate and UV radiation variability. There is also considerable evidence that aquatic species utilize many mechanisms for photoprotection against excessive radiation. Often, these protective mechanisms pose conflicting selection pressures on species making UV radiation an additional stressor on the organism. It is at the ecosystem level where assessments of anthropogenic climate change and UV-related effects are interrelated and where much recent research has been directed. Several studies suggest that the influence of UV-B at the ecosystem level may be more pronounced on community and trophic level structure, and hence on subsequent biogeochemical cycles, than on biomass levels per se.