The impact of extreme weather events exceeds those due to global-change drivers on coastal phytoplankton assemblages.

Extreme wind and rainfall events have become more frequent phenomena, impacting coastal ecosystems by inducing increased mixing regimes in the upper mixed layers (UML) and reduced transparency (i.e. browning), hence affecting phytoplankton photosynthesis. In this study, five plankton assemblages from the South Atlantic Ocean, from a gradient of environmental variability and anthropogenic exposure, were subjected to simulated extreme weather events under a global change scenario (GCS) of increased temperature and nutrients and decreased pH, and compared to ambient conditions (Control). Using multiple linear regression (MLR) analysis we determined that evenness and the ratio of diatoms/ (flagellates + dinoflagellates) significantly explained the variations (81-91 %) of the photosynthesis efficiency (i.e. Pchla/ETRchla ratio) for each site under static conditions. Mixing speed and the optical depth (i.e. attenuation coefficient * depth, kdz), as single drivers, explained 40-76 % of the variability in the Pchla/ETRchla ratio, while GCS drivers <9 %. Overall, assemblages with high diversity and evenness were less vulnerable to extreme weather events under a GCS. Extreme weather events should be considered in global change studies and conservation/management plans as even at local/regional scales, they can exceed the predicted impacts of mean global climate change on coastal primary productivity.

Browning, nutrient inputs, and fast vertical mixing from simulated extreme rainfall and wind stress alter estuarine phytoplankton productivity.

Browning and nutrient inputs from extreme rainfall, together with increased vertical mixing due to strong winds, are more frequent in coastal ecosystems; however, their interactive effects on phytoplankton are poorly understood. We conducted experiments to quantify how browning, together with different mixing speeds (fluctuating radiation), and a nutrient pulse alter primary productivity and photosynthetic efficiency in estuarine phytoplankton communities. Phytoplankton communities (grazers excluded) were exposed simultaneously to these drivers, and key photosynthetic targets were quantified: oxygen production, electron transport rates (ETRs), and carbon fixation immediately following collection and after a 2-d acclimation/adaptation period. Increasing mixing speeds in a turbid water column (e.g. browning) significantly decreased ETRs and carbon fixation in the short term. Acclimation/adaptation to this condition for 2 d resulted in an increase in nanoplanktonic diatoms and a community that was photosynthetically more efficient; however, this did not revert the decreasing trend in carbon fixation with increased mixing speed. The observed interactive effects (resulting from extreme rainfall and strong winds) may have profound implications in the trophodynamics of highly productive system such as the Southwest Atlantic Ocean due to changes in the size structure of the community and reduced productivity.

Extreme and gradual rainfall effects on winter and summer estuarine phytoplankton communities from Patagonia (Argentina).

Rainfall events bring both, terrigenous materials (including DOM) and nutrients to the aquatic system (e.g., via riverine runoff) having potential effects on the structure and metabolism of the phytoplankton communities. As extreme rainfall events in Patagonia occurred more frequently in the last decade (2010-2019) as compared to the previous ones (1972-2009), we exposed winter and summer phytoplankton communities (using microcosms) to solar radiation, simulating two rainfall conditions - a single extreme vs. intermittent i.e., with gradual inputs, and we assessed their photosynthetic and growth rates responses and taxonomic changes. Rainfall scenarios significantly increased growth of both communities, mainly of small nanoplanktonic species, as compared to the control. Small nanoplanktonic centric diatoms increased and dominated in both rainfall scenarios, as compared to the control, during winter and summer, with significantly smaller cells during summer as compared to winter. Photosynthetic efficiency increased in both rainfall scenarios at the end of the experiment as compared to the control. Overall, the change towards small cells (associated to rainfall events) that can use more effectively solar radiation and nutrients (as compared to large cells) may have a significant impact on the trophic webs of the South West Atlantic Ocean by favoring grazing pressure by microzooplankton, especially during summer.

Multiple interacting environmental drivers reduce the impact of solar UVR on primary productivity in Mediterranean lakes.

Increases in rainfall, continental runoff, and atmospheric dust deposition are reducing water transparency in lakes worldwide (i.e. higher attenuation Kd). Also, ongoing alterations in multiple environmental drivers due to global change are unpredictably impacting phytoplankton responses and lakes functioning. Although both issues demand urgent research, it remains untested how the interplay between Kd and multiple interacting drivers affect primary productivity (Pc). We manipulated four environmental drivers in an in situ experiment-quality of solar ultraviolet radiation (UVR), nutrient concentration (Nut), CO2 partial pressure (CO2), and light regime (Mix)-to determine how the Pc of nine freshwater phytoplankton communities, found along a Kd gradient in Mediterranean ecosystems, changed as the number of interacting drivers increased. Our findings indicated that UVR was the dominant driver, its effect being between 3-60 times stronger, on average, than that of any other driver tested. Also, UVR had the largest difference in driver magnitude of all the treatments tested. A future UVR × CO2 × Mix × Nut scenario exerted a more inhibitory effect on Pc as the water column became darker. However, the magnitude of this synergistic effect was 40-60% lower than that exerted by double and triple interactions and by UVR acting independently. These results illustrate that although future global-change conditions could reduce Pc in Mediterranean lakes, multiple interacting drivers can temper the impact of a severely detrimental driver (i.e. UVR), particularly as the water column darkens.

Anthropogenic pollution of aquatic ecosystems: Emerging problems with global implications.

Aquatic ecosystems cover over two thirds of our planet and play a pivotal role in stabilizing the global climate as well as providing a large array of services for a fast-growing human population. However, anthropogenic activities increasingly provoke deleterious impacts in aquatic ecosystems. In this paper we discuss five sources of anthropogenic pollution that affect marine and freshwater ecosystems: sewage, nutrients and terrigenous materials, crude oil, heavy metals and plastics. Using specific locations as examples, we show that land-based anthropogenic activities have repercussions in freshwater and marine environments, and we detail the direct and indirect effects that these pollutants have on a range of aquatic organisms, even when the pollutant source is distant from the sink. While the issues covered here do focus on specific locations, they exemplify emerging problems that are increasingly common around the world. All these issues are in dire need of stricter environmental policies and legislations particularly for pollution at industrial levels, as well as solutions to mitigate the effects of anthropogenic pollutants and restore the important services provided by aquatic ecosystems for future generations.

Dual role of DOM in a scenario of global change on photosynthesis and structure of coastal phytoplankton from the South Atlantic Ocean.

We evaluated the dual role of DOM (i.e., as a source of inorganic nutrients and as an absorber of solar radiation) on a phytoplankton community of the western South Atlantic Ocean. Using a combination of microcosms and a cluster approach, we simulated the future conditions of some variables that are highly influenced by global change in the region. We increased nutrients (i.e., anthropogenic input) and dissolved organic matter (DOM), and we decreased the pH, to assess their combined impact on growth rates (µ), species composition/abundance and size structure, and photosynthesis (considering in this later also the effects of light quality i.e., with and without ultraviolet radiation). We simulated two Future conditions (Fut) where nutrients and pH were similarly manipulated, but in one the physical role of DOM (Futout) was assessed whereas in the other (Futin) the physico-chemical role was evaluated; these conditions were compared with a control (Present condition, Pres). The µ significantly increased in both Fut conditions as compared to the Pres, probably due to the nutrient addition and acidification in the former. The highest µ were observed in the Futout, due to the growth of nanoplanktonic flagellates and diatoms. Cells in the Futin were photosynthetically less efficient as compared to those of the Futout and Pres, but these physiological differences, also between samples with or without solar UVR observed at the beginning of the experiment, decreased with time hinting for an acclimation process. The knowledge of the relative importance of both roles of DOM is especially important for coastal areas that are expected to receive higher inputs and will be more acidified in the future.

Abiotic control of phytoplankton blooms in temperate coastal marine ecosystems: A case study in the South Atlantic Ocean.

Coastal waters of the South Atlantic Ocean (SAO) sustain one of the highest levels of production of the World's ocean, maintained by dense phytoplankton winter blooms that are dominated by large diatoms. These blooms have been associated to calm weather conditions that allow the formation of a shallow and well illuminated upper mixed layer. In Bahía Engaño, a coastal site in Patagonia, Argentina (chosen as a model coastal ecosystem) winter blooms recurrently peaked on June and they were dominated almost entirely by the microplanktonic diatom Odontella aurita. However, during the year 2015, a new wind pattern was observed - with many days of northerly high-speed winds, deviating from the calm winter days observed during a reference period (2001-2014) used for comparison. We determined that this new wind pattern was the most important factor that affected the phytoplankton dynamics, precluding the initiation of a June bloom during 2015 that instead occurred during late winter (August). Furthermore, the 2015 bloom had a higher proportion of nanoplanktonic cells (as compared to the reference period) and it was co-dominated by O. aurita and Thalassiossira spp. Other variables such as nutrient supply and incident solar radiation did not have an important role in limiting and/or initiating the June 2015 bloom, but temperature might have benefited the growth of small cells during August 2015. If these changes in the timing and/or the taxonomic composition of the bloom persist, they may have important consequences for the secondary production and economic services of the coastal SAO.

Differential impacts of global change variables on coastal South Atlantic phytoplankton: Role of seasonal variations.

Global change is associated to the increase in temperature (T), nutrient inputs (Nut) and solar radiation in the water column. To address their joint impact on the net community production [NCP], respiration [CR] and PSII performance (ΦPSII) of coastal phytoplankton communities from the South Atlantic Ocean over a seasonal succession, we performed a factorial design. For this, we used a 2 × 2 × 2 matrix set-up, with and without UVR, ambient and enriched nutrients, and in situ T and in situ T + 3 °C. The future scenario of global change exerted a dual impact, from an enhancement of NCP and ΦPSII during the pre-bloom to an inhibition of both processes towards the bloom period, when the in situ T and irradiances were lower and the community was dominated by diatoms. The increased inhibition of NCP and ΦPSII during the most productive stage of the annual succession could produce significant alterations of the CO2-sink capacity of coastal areas in the future.

Sex-dependent effects of ultraviolet radiation on the marine amphipod Ampithoe valida (Ampithoidae).

The combined effects of solar radiation and diet on the marine amphipod Ampithoe valida were investigated exposing individuals to two solar radiation treatments: PAB (>280 nm, PAR+UV-A+UV-B) and P (>400 nm, only PAR), and three diets: poor (Ulva rigida) and rich (Porphyra columbina) in UV-absorbing compounds (UVAC), and mixed diet: (U. rigida+P. columbina). Females of A. valida showed higher food consumption rates when diets contained P. columbina, and preferred this macroalgae rather than U. rigida, resulting in a higher content of UVAC in their bodies. Moreover, the content of UVAC increased in the PAB treatment, thus suggesting the existence of a mechanism to accumulate these compounds under UVR. Although UVR affected the survival, the highest mortality rates were found in those females fed with poor-UVAC diets, which evidence that UVAC provided partial protection against UVR. Males preferred mixed diet, and did not show preference for any particular macroalgae. No differences in mortality were observed between radiation treatments, indicating that UVR did not affect the survival of males, independently if they accumulated UVAC or not. The vulnerability of females to UVR would be partially determined by the type of food consumed, which in turn would be closely related to the macroalgae composition of the intertidal they inhabiting. These effects could be even more pronounced under a global change scenario.

Effects of UV radiation on aquatic ecosystems and interactions with other environmental factors.

Interactions between climate change and UV radiation are having strong effects on aquatic ecosystems due to feedback between temperature, UV radiation, and greenhouse gas concentration. Higher air temperatures and incoming solar radiation are increasing the surface water temperatures of lakes and oceans, with many large lakes warming at twice the rate of regional air temperatures. Warmer oceans are changing habitats and the species composition of many marine ecosystems. For some, such as corals, the temperatures may become too high. Temperature differences between surface and deep waters are becoming greater. This increase in thermal stratification makes the surface layers shallower and leads to stronger barriers to upward mixing of nutrients necessary for photosynthesis. This also results in exposure to higher levels of UV radiation of surface-dwelling organisms. In polar and alpine regions decreases in the duration and amount of snow and ice cover on lakes and oceans are also increasing exposure to UV radiation. In contrast, in lakes and coastal oceans the concentration and colour of UV-absorbing dissolved organic matter (DOM) from terrestrial ecosystems is increasing with greater runoff from higher precipitation and more frequent extreme storms. DOM thus creates a refuge from UV radiation that can enable UV-sensitive species to become established. At the same time, decreased UV radiation in such surface waters reduces the capacity of solar UV radiation to inactivate viruses and other pathogens and parasites, and increases the difficulty and price of purifying drinking water for municipal supplies. Solar UV radiation breaks down the DOM, making it more available for microbial processing, resulting in the release of greenhouse gases into the atmosphere. In addition to screening solar irradiance, DOM, when sunlit in surface water, can lead to the formation of reactive oxygen species (ROS). Increases in carbon dioxide are in turn acidifying the oceans and inhibiting the ability of many marine organisms to form UV-absorbing exoskeletons. Many aquatic organisms use adaptive strategies to mitigate the effects of solar UV-B radiation (280-315 nm), including vertical migration, crust formation, synthesis of UV-absorbing substances, and enzymatic and non-enzymatic quenching of ROS. Whether or not genetic adaptation to changes in the abiotic factors plays a role in mitigating stress and damage has not been determined. This assessment addresses how our knowledge of the interactive effects of UV radiation and climate change factors on aquatic ecosystems has advanced in the past four years.

Productivity of aquatic primary producers under global climate change.

The productivity of aquatic primary producers depends on a number of biotic and abiotic factors, such as pH, CO2 concentration, temperature, nutrient availability, solar UV and PAR irradiances, mixing frequency as well as herbivore pressure and the presence of viruses, among others. The effects of these factors, within a climate change context, may be additive, synergistic or antagonistic. Since some of them, e.g. solar radiation and temperature, vary along a latitudinal gradient, this perspective about the effects of global climate change on primary producers will consider ecosystems individually, separated into polar (Arctic and Antarctic), temperate and tropical waters. As coastal waters are characterized by lower light penetration and higher DOM and nutrient concentrations, they are considered in a separate section. Freshwater systems are also governed by different conditions and therefore also treated in their own section. Overall, we show that although there are general common trends of changes in variables associated with global change (e.g. the impact of UVR on photosynthesis tends to decrease with increasing temperature and nutrient input), the responses of aquatic primary producers have great variability in the different ecosystems across latitudes. This is mainly due to direct or indirect effects associated with physico-chemical changes that occur within water bodies. Therefore we stress the need for regional predictions on the responses of primary producers to climate change as it is not warranted to extrapolate from one system to another.

Influence of light history on the photosynthetic and motility responses of Gymnodinium chlorophorum exposed to UVR and different temperatures.

In the wake of global climate change, phytoplankton productivity and species composition is expected to change due to altered external conditions such as temperature, nutrient accessibility, pH and exposure to solar visible (PAR) and ultraviolet radiation (UVR). The previous light history is also of importance for the performance of phytoplankton cells. In order to assess the combined impacts of UVR and temperature on the dinoflagellate Gymnodinium chlorophorum we analyzed the effective photochemical quantum yield (Y), relative electron transport rate vs. irradiance curves (rETR vs. I), percentage of motile cells and swimming velocity. Cells were grown at three different temperatures (15, 20 and 25 °C) and two PAR intensities: low light (LL, 100 µmol photons m(-2) s(-1)) and high light (HL, 250 µmol photons m(-2) s(-1)). Pre-acclimated cells were then exposed to either PAR only (P), PAR+UV-A (PA) or PAR+UV-A+UV-B (PAB) radiation at two different irradiances, followed by a recovery period in darkness. The Y decreased during exposure, being least inhibited in P and most in PAB treatments. Inhibition was higher and recovery slower in LL-grown cells than in HL-grown cells at 15° and 20 °C, but the opposite occurred at 25 °C, when exposed to high irradiances. Maximal values of rETR were determined at t0 as compared to the different (before and after exposure) radiation treatments. The effects of temperature and UVR on rETR were antagonistic in LL-grown cells (i.e., less UVR inhibition at higher temperature), while it was synergistic in HL cells. Swimming velocity and percentage of motile cells were not affected at all tested temperatures and exposure regimes, independent of the light history. Our results indicate that, depending on the previous light history, increased temperature and UVR as predicted under climate change conditions, can have different interactions thus conditioning the photosynthetic response of G. chlorophorum.

Ocean acidification alters the photosynthetic responses of a coccolithophorid to fluctuating ultraviolet and visible radiation.

Mixing of seawater subjects phytoplankton to fluctuations in photosynthetically active radiation (400-700 nm) and ultraviolet radiation (UVR; 280-400 nm). These irradiance fluctuations are now superimposed upon ocean acidification and thinning of the upper mixing layer through stratification, which alters mixing regimes. Therefore, we examined the photosynthetic carbon fixation and photochemical performance of a coccolithophore, Gephyrocapsa oceanica, grown under high, future (1,000 µatm) and low, current (390 µatm) CO2 levels, under regimes of fluctuating irradiances with or without UVR. Under both CO2 levels, fluctuating irradiances, as compared with constant irradiance, led to lower nonphotochemical quenching and less UVR-induced inhibition of carbon fixation and photosystem II electron transport. The cells grown under high CO2 showed a lower photosynthetic carbon fixation rate but lower nonphotochemical quenching and less ultraviolet B (280-315 nm)-induced inhibition. Ultraviolet A (315-400 nm) led to less enhancement of the photosynthetic carbon fixation in the high-CO2-grown cells under fluctuating irradiance. Our data suggest that ocean acidification and fast mixing or fluctuation of solar radiation will act synergistically to lower carbon fixation by G. oceanica, although ocean acidification may decrease ultraviolet B-related photochemical inhibition.

Influence of temperature and UVR on photosynthesis and morphology of four species of cyanobacteria.

During the late austral spring of 2009 we carried out experiments (4days of duration) with four cyanobacteria species, Anabaena sp., Nostoc sp., Arthrospira platensis and Microcystis sp., to assess the combined effects of temperature and solar radiation on photosynthesis performance and morphology. Two experimental temperatures (18°C and 23°C, simulating a 5°C increase under a scenario of climate change) and three radiation treatments (by using different filters/materials) were implemented: (i) P (PAR, 400-700nm), (ii) PA (PAR+UV-A, 320-700nm) and, (iii) PAB (PAR+UV-A+UV-B, 280-700nm). In general, samples under the P treatment had less decrease/higher recovery rates of effective photochemical quantum yield (Y) than those receiving UV-A or UV-A+UV-B. The effects of increased temperature were species-specific: At the end of the experiments, it was seen that increased temperature benefited photosynthetic performance of Anabaena sp. and Nostoc sp. but not of Microcystis sp. and A. platensis. Higher temperature was also associated to an increase in the chain area of Anabaena sp., and to bigger trichomes in A. platensis; however, no morphological effects were observed in Microcystis sp. In addition, in Nostoc sp. the increase in temperature counteracted the UVR impact on the reduction of the chain area. How these effects and mechanisms will affect the trophodynamics and production of aquatic ecosystems is still uncertain, but the specificity of the responses suggests that not all cyanobacteria would be equally benefited by temperature increases therefore affecting the balance and interaction among species in the water column.

Combined effects of UVR and temperature on the survival of crab larvae (Zoea I) from Patagonia: the role of uv-absorbing compounds.

The aim of our study was to assess the combined impact of UVR (280-400 nm) and temperature on the first larval stage (Zoea I) of three crab species from the Patagonian coast: Cyrtograpsus altimanus, C. angulatus, and Leucippa pentagona. We determined the survival response of newly hatched Zoea I after being exposed for 8-10 h under a solar simulator (Hönle SOL 1200) at 15 and 20 degrees C. There was no mortality due to Photosynthetic Active Radiation (PAR, 400-700 nm) or ultraviolet-A radiation (UV-A, 315-400 nm), and all the observed mortality was due to ultraviolet-B radiation (UV-B, 280-315 nm). The data of larval mortality relative to exposure time was best fit using a sigmoid curve. Based on this curve, a threshold (Th) and the lethal dose for 50% mortality (LD(50)) were determined for each species. Based on the Th and LD(50), C. altimanus was found to be the most resistant species, while L. pentagona was found to be the most sensitive to UV-B. For both species of Cyrtograpsus, mortality was significantly lower at 20 degrees C than at 15 degrees C; however, no significant differences between the two temperature treatments were found in L. pentagona. Bioaccumulation of UV-absorbing compounds in the gonads and larvae of C. altimanus, and to a lesser extent in C. angulatus, might have contributed for counteracting the impact of UV-B. However, most of the resilience to UV-B observed with the increase in temperature might be due to an increase in metabolic activity caused by a repair mechanism mediated by enzymes.

Antioxidative responses of two marine microalgae during acclimation to static and fluctuating natural UV radiation.

Photoacclimation properties were investigated in two marine microalgae exposed to four ambient irradiance conditions: static photosynthetically active radiation (PAR: 400-700 nm), static PAR + UVR (280-700 nm), dynamic PAR and dynamic PAR + UVR. High light acclimated cultures of Thalassiosira weissflogii and Dunaliella tertiolecta were exposed outdoors for a maximum of 7 days. Dynamic irradiance was established by computer controlled vertical movement of 2 L bottles in a water filled basin. Immediate (<24 h), short-term (1-3 days) and long-term (4-7 days) photoacclimation was followed for antioxidants (superoxide dismutase, ascorbate peroxidase and glutathione cycling), growth and pigment pools. Changes in UVR sensitivity during photoacclimation were monitored by measuring UVR-induced inhibition of carbon assimilation under standardized UV conditions using an indoor solar simulator. Both species showed immediate antioxidant responses due to their transfer to the outdoor conditions. Furthermore, upon outdoor exposure, carbon assimilation and growth rates were reduced in both species compared with initial conditions; however, these effects were most pronounced in D. tertiolecta. Outdoor UV exposure did not alter antioxidant levels when compared with PAR-only controls in both species. In contrast, growth was significantly affected in the static UVR cultures, concurrent with significantly enhanced UVR resistance. We conclude that antioxidants play a minor role in the reinforcement of natural UVR resistance in T. weissflogii and D. tertiolecta.

Combined effects of solar ultraviolet radiation and nutrients addition on growth, biomass and taxonomic composition of coastal marine phytoplankton communities of Patagonia.

Experiments (6-8 days) were conducted during late summer, late fall and late winter, 2003 with waters collected off Bahía Nueva, Chubut, Argentina (42.7 degrees S, 65 degrees W) to determine the combined effects of solar ultraviolet radiation (UVR, 280-400 nm) and nutrient addition on phytoplankton communities. Samples were put in UVR-transparent containers and incubated under two radiation treatments: (a) Samples exposed to full solar radiation (PAB treatment, 280-400 nm) and (b) Samples exposed only to PAR (PAR treatment, 400-700 nm). At the beginning of the experiments, nutrients (i.e., NaPO(4)H(2) and NaNO(3)) were added to one set of samples from each radiation treatment (N cultures) whereas in the other set, nutrients remained at the concentration of the seawater. Chlorophyll a, biomass, UV-absorbing compounds and taxonomic composition were recorded throughout the experiments. N cultures always had significantly higher growth rates (P<0.05) than that in non-enriched cultures. At the beginning of experiments, phytoplankton communities were generally dominated by monads and flagellates but by the end, diatoms comprised the bulk of biomass, with only one to four taxa dominating, suggesting a selection towards more tolerant/less sensitive species. Over the experimental time frame, the observed taxonomic changes were mostly due to nutrient availability, and to a lesser extent to solar UVR exposure. Overall, the results indicate that environmental conditions (i.e., light history, nutrient concentration) together with the physiological status of the cells play a very important role at the time to assess the combined effect of nutrient addition and solar radiation on coastal phytoplankton assemblages from Patagonia.

ELDONET--a decade of monitoring solar radiation on five continents.

The European light dosimeter network (ELDONET) comprises more than 40 stations in 24 countries on 5 continents. The present report compares solar radiation data in the photosynthetic active radiation, UV-A (315-400 nm) and UV-B (280-315 nm) wavelength ranges for 17 stations at different latitudes on the northern and southern hemispheres for up to 10 years of monitoring. While the maximal irradiances on clear days follow a latitudinal gradient due to the cosine dependence on the solar angle, the total doses strongly depend on the local climate and atmospheric conditions as well as the day-length distribution over the year. UV-B irradiances and doses are strongly influenced by the total column ozone, which is recorded for all covered stations.

Interactive effects of ultraviolet radiation and nutrient addition on growth and photosynthesis performance of four species of marine phytoplankton.

Experiments (6-8 days) were carried out during the austral summer of 2005 in Chubut, Argentina (43 degrees S, 65 degrees W) to determine the interactive effects of solar UVR (280-400 nm) and nutrient addition on growth and chlorophyll fluorescence of four species of marine phytoplankton--the diatoms Thalassiosira fluviatilis Hustedt and Chaetoceros gracilis Schütt, and the dinoflagellates Heterocapsa triquetra (Ehrenberg) Stein and Prorocentrum micans (Ehrenberg). Samples were incubated under three radiation treatments (two sets of each radiation treatment): (a) samples exposed to full solar radiation (PAR+UVR, PAB treatment, 280-700 nm); (b) samples exposed to PAR and UV-A (PA treatment, 320-700 nm) and (c) samples exposed only to PAR (P treatment, 400-700 nm). At the beginning of the experiments, nutrients (i.e., NaPO(4)H(2) and NaNO(3)) were added to one set of samples from each radiation treatment ("N" cultures) whereas in the other, the nutrients concentration was that of the culture medium. At all times, the lowest growth rates (mu) were determined in the PAB treatments, where enriched cultures had significantly higher mu (P<0.05) than non-enriched cultures. Daily cycles of photochemical quantum yield (Y) displayed a pattern of relatively high values early in the morning with a sharp decrease at noon; recovery was observed late in the afternoon. In general, higher Y values were determined in enriched cultures than in non-enriched cultures. As the experiments progressed, acclimation (estimated as the difference between Y at noon and that at time zero) was observed in all species although in variable degree. All species displayed some degree of UVR-induced decrease in the photochemical quantum yield, although it was variable among treatments and species. However, this effect decreased with time, and this pattern was more evident in the dinoflagellates, as the concentration of UV-absorbing compounds increased. Thus, under conditions of nutrient enrichment as may occur by river input or by re-suspension by mixing, dinoflagellates outcompete with diatoms because they may have a higher fitness under UVR stress.

Vertical migration and motility responses in three marine phytoplankton species exposed to solar radiation.

Diurnal vertical migration in the water column and the impact of solar radiation on motility were investigated in three marine phytoplankton species: Tetraselmis suecica, Dunaliella salina and Gymnodinium chlorophorum. Cells were exposed to solar radiation either in ultraviolet radiation (UVR, 280-400 nm) transparent Plexiglas tubes (45 cm length, 10 cm diameter) or in quartz tubes under three radiation treatments: PAB (280-700 nm), PA (320-700 nm) and P (400-700 nm). The three species displayed different behavior after exposure to solar radiation. Tetraselmis suecica was insensitive to UVR and under high solar radiation levels, cells accumulated preferentially near the surface. Exposure experiments did not indicate any significant changes in swimming speed nor in the percentage of motile cells after 5 h of exposure. On the other hand, D. salina was sensitive to UV-B displaying a significant decrease in swimming speed and percentage of motile cells after 2-3 h of exposure. Moreover, D. salina cells migrated deep in the water column when irradiance was high. The response of G. chlorophorum was in between that of the other two species tested, with a slight (but significant) decrease in swimming speed and percentage of motile cells in all radiation treatments after 5 h of exposure. While G. chlorophorum cells were more or less homogenously distributed in the water column, a slight (but significant) avoidance response to high radiation was observed at local noon, with cells migrating deep in the water column. Our data clearly indicate that these sub-lethal effects of solar radiation are species-specific and they might have important implications for the aquatic ecosystem.