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.     

Studies on the effects of UV-B radiation on phytoplankton of sub-antarctic lakes and ponds

Summary Experiments were performed to determine the effects of UV-B (ultraviolet, 280–320 nm) radiation on motility and growth of phytoplankton from lakes and ponds in South Georgia. After 4 h of solar radiation and 4h artificial radiation (UV-B_BE 11.6 kJ m^-2 day^-1, UV-B lamps) the swimming velocity of Cryptomonas sp. decreased. The growth rate of Botryococcus, Lyngbya sp. and Stauraslrum sp. did not show any significant variations between the different light conditions. The UV-B component was reduced by filtering solar radiation through glass bottles und cellulose acetate. Cloudy days had only 30% of the radiation of clear days in both the PAR (photosynthetic active radiation) and UV-B regions. The ponds contained large amounts of humic substances, which are responsible for the absorbance in the UV region.     

Different physiological responses of cyanobacteria to ultraviolet‐B radiation under iron‐replete and iron‐deficient conditions: Implications for underestimating the negative effects of UV‐B radiation

Iron deficiency has been considered one of the main limiting factors of phytoplankton productivity in some aquatic systems including oceans and lakes. Concomitantly, solar ultraviolet‐B radiation has been shown to have both deleterious and positive impacts on phytoplankton productivity. However, how iron‐deficient cyanobacteria respond to UV‐B radiation has been largely overlooked in aquatic systems. In this study, physiological responses of four cyanobacterial strains (Microcystis and Synechococcus), which are widely distributed in freshwater or marine systems, were investigated under different UV‐B irradiances and iron conditions. The growth, photosynthetic pigment composition, photosynthetic activity, and nonphotochemical quenching of the different cyanobacterial strains were drastically altered by enhanced UV‐B radiation under iron‐deficient conditions, but were less affected under iron‐replete conditions. Intracellular reactive oxygen species (ROS) and iron content increased and decreased, respectively, with increased UV‐B radiation under iron‐deficient conditions for both Microcystis aeruginosa FACHB 912 and Synechococcus sp. WH8102. On the contrary, intracellular ROS and iron content of these two strains remained constant and increased, respectively, with increased UV‐B radiation under iron‐replete conditions. These results indicate that iron‐deficient cyanobacteria are more susceptible to enhanced UV‐B radiation. Therefore, UV‐B radiation probably plays an important role in influencing primary productivity in iron‐deficient aquatic systems, suggesting that its effects on the phytoplankton productivity may be underestimated in iron‐deficient regions around the world.     

Effects of nutrients and dissolved organic matter on the response of phytoplankton to ultraviolet radiation: experimental comparison in spring versus summer

The effects of nutrients and dissolved organic matter (DOM) on the response of phytoplankton community structure to ultraviolet radiation (UVR) was studied using natural phytoplankton assemblages from Lake Giles (Northeastern Pennsylvania), a temperate, oligotrophic, highly UVR-transparent lake. Microcosm experiments were conducted in 1-l bags in the spring and summer. A factorial design was used, with two UVR treatments (ambient and reduced), two nutrient treatments (control with no nutrients added, and nitrogen and phosphorus addition together), and two DOM treatments (control of 1 mg l⁻¹ and doubled). In April, UVR affected the overall phytoplankton community structure, causing a shift in the dominant species. Significant interactive effects of UVR × nutrients and UVR × DOM were found on total phytoplankton biovolumes. In July, all taxa responded positively to the N + P addition, and were affected differentially by the UVR treatments. The initial communities varied in April and July, but Synura sp. and Chroomonas sp. were present in both seasons. Synura sp. responded positively to the addition of DOM in April and the reduction of UVR in July. Chroomonas sp. responded positively to the reduction of UVR in April and the addition of nutrients in July. The differential sensitivity of these two species suggests that changing environmental factors between spring and summer promoted differences in the relative importance of UVR in changing phytoplankton community structure. Handling editor: Luigi Naselli-Flores     

Filamentous green algae inhibit phytoplankton with enhanced effects when lakes get warmer

1. Filamentous green algae (FGA) may represent an alternative state in high‐nutrient shallow temperate lakes. Furthermore, a clear water state is sometimes associated with the dominance of FGA; however, the mechanisms involved remain uncertain. 2. We hypothesised that FGA may promote a clear water state by directly suppressing phytoplankton growth, mostly via the release of allelochemicals, and that this interaction may be affected by temperature. 3. We examined the relationships between FGA, phytoplanktonic chlorophyll a concentrations and zooplankton in a series of mesocosms (2.8 m³) mimicking enriched shallow ponds now and in a future warmer climate (0 and c. 5 °C above ambient temperatures). We then tested the potential allelopathic effects of FGA (Cladophora sp. and Spirogyra sp.) on phytoplankton using several short‐term microcosms and laboratory experiments. 4. Mesocosms with FGA evidenced lower phytoplanktonic chlorophyll a concentrations than those without. Zooplankton and zooplankton : phytoplankton biomass ratios did not differ between mesocosms with and without FGA, suggesting that grazing was not responsible for the negative effects on phytoplanktonic biomass (chlorophyll a). 5. Our field microcosm experiments demonstrated that FGA strongly suppressed the growth of natural phytoplankton at non‐limiting nutrient conditions and regardless of phytoplankton initial concentrations or micronutrients addition. Furthermore, we found that the negative effect of FGA on phytoplankton growth increased up to 49% under high incubation temperatures. The experiment performed using FGA filtrates confirmed that the inhibitory effect of FGA on phytoplankton may be attributed to allelochemicals. 6. Our results suggest that FGA control of phytoplankton growth may be an important mechanism for stabilising clear water in shallow temperate lakes dominated by FGA and that FGA may play a larger role when lakes get warmer.     

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.     

Comparing microscopic counts and pigment analyses in 46 phytoplankton communities from lakes of different trophic state

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TWO NEW SPECIES OF PSEUDOKEPHYRION (CHRYSOPHYCEAE) IN THE PLANKTON OF A LAKE1

Two new species of Pseudokephyrion are described from Little Aurora Lake, Ontario, Canada. The distribution of both P. taeniatum sp. nov. and P. auroreum sp. nov. is restricted, for reasons which are not clear; neither species was found in samples from lakes adjacent to Little Aurora Lake. All lakes in the vicinity are very dilute, oligotrophic and acidic.     

Community Structure of Tintinnid Ciliates of the Microzooplankton in the South West Pacific Ocean: Comparison of a High Primary Productivity with a Typical Oligotrophic Site

Transient ‘hot spots’ of phytoplankton productivity occur in the generally oligotrophic Southern Pacific Ocean and we hypothesized that the population structure of tintinnid ciliates, planktonic grazers, would differ from that of a typical oligotrophic sites. Samples were collected over a 1‐wk period at each of two sites between Fiji and Tahiti: one of elevated chlorophyll a concentrations and primary productivity with an abundance of N‐fixing cyanobacteria Trichodesmium, and a distant oligotrophic site. Tintinnid abundance differed between the sites by a factor of 2. A single species (Favella sp.), absent from the oligotrophic site, highly dominated the ‘hot spot’ site. However, total species richness was identical (71 spp.) as well as short‐term temporal variability (2–4 d). At both sites, species abundance distributions most closely fit a log‐series or log‐normal distribution and the abundance distributions of ecological types, forms of distinct lorica oral diameter, were the typical geometric. Morphological diversity was only slightly lower at the high productivity site. We found that communities of these plankton grazers in ‘hot spots’ of phytoplankton productivity in oligotrophic systems, although harboring different species, differ little from surrounding oligotrophic areas in community structure.     

Hydrology and Diatom Phytoplankton of High Arctic Lakes and Ponds on Store Koldewey,Northeast Greenland

We document hydrological and phytoplankton characteristics of nine lakes and two ponds on Store Koldewey, a culturally undisturbed island off Northeast Greenland. The limnological survey included the recording of temperature, conductivity, oxygen concentration and saturation, pH, ionic composition, transparency, and the diatom phytoplankton community. In summer 2003, the lakes were cold, monomictic, thermally unstratified, alkaline and likely oligotrophic water bodies. Diatom phytoplankton was present in six lakes and consisted of four dominant species (Aulacoseira tethera, Cyclotella pseudostelligera, C. rossii, and Fragilaria tenera). The concentration of planktonic diatoms varied distinctly between the lakes. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Changes in phytoplankton,benthic invertebrate and fish assemblages of boreal lakes following invasion by Gonyostomum semen

1. During recent decades, Gonyostomum semen populations have spread in northern temperate regions forming dense blooms that may dominate the phytoplankton assemblage for extended periods. In this study, we investigate the effects of G. semen blooms in boreal brown water lakes with special emphasis on phytoplankton, fish and benthic invertebrate assemblages using data from 10 boreal lakes sampled annually over a 10‐year period. 2. Significant differences in phytoplankton and benthic invertebrate assemblages were found between lakes with high (3.01 mm³ L^?1; >80% phytoplankton biomass) and lakes with low G. semen biomass (0.03 mm³ L^?1; <5% phytoplankton biomass). In particular, high G. semen lakes had lower biomass of smaller, edible phytoplankton and a higher abundance and biomass of benthic invertebrates, especially Chaoborus flavicans, and perch than low G. semen lakes. 3. The length distribution of fish also suggested a tendency towards large and older fish and a lower recruitment success in high G. semen lakes, as denoted by lower abundances of fish shorter than 10 cm and higher biomass and abundance of fish longer than 15 cm in high G. semen lakes. 4. This study shows that high G. semen lakes are characterised by less edible phytoplankton, dominance by a few species and enhanced benthic secondary and fish production. Hence, the conjecture that high biomasses of G. semen create a bottleneck in the energy transfer to higher trophic levels seems less likely in boreal lakes.     

Winter phytoplankton community structure in three shallow temperate lakes during ice cover

The general model of seasonal phytoplankton succession in temperate lakes suggests that winter phytoplankton growth is minimal under ice-cover. However, some studies have found diverse phytoplankton communities during winter. The primary objectives of this study were to determine the species composition and the changes in the winter phytoplankton community structure under the ice. For 2 consecutive winters, phytoplankton samples were collected under ice-cover at 4 sites on 3 lakes in Arrowwood National Wildlife Refuge (ANWR), near Pingree, North Dakota. Ninety taxa were identified and enumerated. Densities of several of these taxa frequently exceeded 10⁶ cells l^–1. The winter phytoplankton communities in these lakes were dominated by flagellates, principally cryptomonads, a synurophyte (Synura uvella), small chrysophytes (Chrysococcus spp., Kephyrion spp.) and a dinoflagellate (Peridinium aciculiferum), as well as non-flagellate microchlorophytes (Monoraphidium spp., Ankistrodesmus spp., and Pseudodictyosphaerium sp.), a cyanobacterium (Gloeocapsa aeruginosa) and centric diatoms (Stephanodiscus minutulus, S. parvus and Cyclotella meneghiniana).     

The microbial plankton of freshwater lakes in the Vestfold Hills,Antarctica

Summary The plankton of twelve freshwater and slightly saline lakes in the Vestfold Hills, Antarctica was sampled in February 1991. All of the lakes are oligotrophic. The chlorophyll a concentrations in the lakes ranged from 0.10–2.69 g · 1^–1. The majority of the phytoplankton were flagellates or picoplanktonic cyanobacteria with the species composition varying between the lakes. Cyanobacteria were found in five of the lakes. Five to 6 species of ciliated protozoa occurred, among them oligotrichs, including the mixotrophic species Strombidium viride. The concentrations of protists and bacteria were an order to several orders of magnitude lower than reported from lower latitude oligotrophic lakes. Low species diversity and low numbers in the plankton characterise these eastern Antarctica lakes which reflects their low nutrient status and isolation.     

Late fall phytoplankton dynamics in three lakes, Rocky Mountain National Park

We studied phytoplankton population dynamics during the month preceding formation of ice cover in three small subalpine lakes in Rocky Mountain National Park, Colorado, U.S.A. The outflow from Emerald Lake, which is surrounded by talus, flows into Dream Lake, which is surrounded by sub-alpine forest. Nymph Lake is a lower seepage lake with abundant macrophytes in summer. The major ion concentrations in the three lakes were similar during the study, although Emerald and Dream Lakes had higher concentrations of nitrate and silica than Nymph Lake. A principal component analysis (PCA) showed that the phytoplankton in Emerald and Dream Lakes were distinct from the phytoplankton in Nymph Lake. The species composition changed in each lake during the late fall. The patterns of change in Emerald and Dream Lakes were similar on the PCA diagram despite the greater abundance of diatoms in Dream Lake and the decreasing flow from Emerald Lake into Dream Lake during the fall. In Nymph Lake, a progressive shift in species distribution occurred with a decrease in the most abundant chlorophyte, Chlamydomonas sp., and increases in several species, including two chrysophytes and the diatom Eunotia sp. The marked change in species composition in all three lakes suggests that phytoplankton populations are influenced by changes in water temperatures and incident solar radiation that occur during the late fall. We also compared these data with phytoplankton data for two fall periods from two other hydrologically connected Rocky Mountain lakes. PCA analysis showed that the difference between years was greater than the change during the fall and that the fall species composition in these two lakes was distinct from that in Emerald and Dream Lakes or in Nymph Lake. Studying phytoplankton dynamics in alpine and sub-alpine lakes may offer clues as to how these ecosystems may respond to projected climate changes in the Rocky Mountain region, such as warmer temperatures and later formation of ice-cover.     

Multiple Strategies of Bloom-Forming Microcystis to Minimize Damage by Solar Ultraviolet Radiation in Surface Waters

The occurrence of bloom-forming cyanobacteria is one of the most obvious sign of eutrophication in freshwaters. Although in eutrophic lakes water transparency in the ultraviolet (UV) region is strongly reduced, bloom-forming cyanobacteria are exposed to high solar UV radiation at the surface. Here, we show that, in a natural phytoplankton community from a very eutrophic lake, Microcystis synthesizes UV sunscreen compounds identified as mycosporine-like amino acids (MAAs). The biomass-specific MAA concentration was significantly correlated with the occurrence of Microcystis but not with other algal groups, even though they were dominant in terms of biomass. Based on a photo-optical model, we estimated that the maximum MAA concentration per cell observed (2.5% dry weight) will confer only ~40% of internal screening to a single layer of Microcystis cells. Thus, the formation of a colony with several layers of cells is important to afford an efficient UV screening by internal self-shading. Overall, we propose that Microcystis uses a combination of photoprotective strategies (MAAs, carotenoids) to cope with high solar UV radiation at the water surface. These strategies include also the screening of UV radiation by d-galacturonic acid, one of the main chemical components of the slime layer in Microcystis.     

Transplant experiments. Growth-limiting factors for diatoms and bluegreen algae in Norwegian lakes

Population densities and total phosphorus concentrations in samples from different lakes of south-eastern Norway were determined. In addition some transplant experiments with dilute phytoplankton populations were carried out. A laboratory batch culture method was used.The diatoms studied may be divided into three ecological groups based on their cell densities and total phosphorus concentrations in the samples. This classification was supported by the experimental results. Cyclotella spp., Asterionella formosa and Tabellaria fenestrata did not grow or had low growth rates above pH 9. Synedra cf. acus and Fragilaria crontonensis had often high growth rates within the pH 9–10 range, but were not able to grow at pH values above 10. High pH-values had no effect on the growth rate of Oscillatoria. Oscillatoria, Synedra and Stephanodiscus were severely growth-limited in filtered water from oligotrophic lakes. Maximum growth rates of all the populations studied were often obtained after addition of phosphate and chelated iron (FeEDTA) in combination to filtered water samples from oligotrophic/mesotrophic lakes.     

The Seasonal Dynamics and Composition of Photosynthetic Picoplankton Communities in Temperate Lakes in Ontario,Canada

The seasonal abundance and composition of photosynthetic picoplankton (0.2-2 μm) was compared among five oligotrophic to mesotrophic lakes in Ontario. Epilimnetic picocyanobacteria abundance followed a similar pattern in all lakes; maximum abundance (2-4 × 10⁵ cells · ml⁻¹) occurred in late summer following a period of rapid, often exponential increase after epilimnetic temperatures reached 20 °C. In half of the lakes picocyanobacteria abundance was significantly correlated with temperature, while in other lakes the presence of a small spring peak resulted in a poor correlation with temperature. In all lakes there was a significant correlation between epilimnetic abundance and day of the year. Correlations with water chemistry parameters (soluble reactive phosphorus, total phosphorus, particulate C: P and C: N) were generally weaker or insignificant. However, in the three lakes with the highest spring nitrate concentrations, a significant negative correlation with nitrate was observed. During summer stratification, picocyanobacteria abundance reached a maximum within the metalimnion and at or above the euphotic zone (1% of incident light) in all lakes. These peaks were not related to nutrient gradients. The average total phytoplankton biomass ranged from 0.5 g m⁻³ (wet weight) in the most oligotrophic lake to 1.4 g m⁻³ for the most mesotrophic with picoplankton biomass ranging from 0.01 g m⁻³ to 0.3 g m⁻³. Picocyanobacteria biomass comprised 1 to 9 % of total phytoplankton biomass in late summer, but in one year for one lake represented a maximum of 56%. Other photosynthetic picoplankton (unidentified eukaryotes, Chlorella spp. Nannochloris spp.), although less abundant (10³ cells · ml⁻¹) than picocyanobacteria, represented biomass equal or greater than that of the picocyanobacteria in spring and early summer. On average, half of the photosynthetic picoplankton biomass was eukaryotic in the more coloured lakes, while in the clear lakes less than 20% was eukaryotic. Among the lakes there was a significant positive correlation between the average light extinction coefficient and the proportion of eukaryotic biomass of the picoplankton. In mesotrophic Jack's Lake, the contribution of picoplankton to the maximum photosynthetic rate ranged from 10 to 47% with the highest values in the spring (47%) and late summer (33%), as a result of eukaryotic picoplankton and picocyanobacteria respectively. Picocyanobacteria cell specific growth rates were high during July (0.6-0.8 day⁻¹) and losses were close to 80% of the growth rate. Thus, despite low biomass, photosynthetic picoplankton populations appeared to turn over rapidly and potentially contributed significantly to planktonic food webs in early spring and late summer.     

Zooplankton biomass dynamics in oligotrophic versus eutrophic conditions: a test of the PEG model

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Effects of Asian dust input on eukaryotic phytoplankton community structure in the open areas in the Northwestern Pacific Ocean

To study the effects of aerosol particulates originated from Asian dust on the growth of eukaryotic phytoplankton in the oligotrophic open ocean, we conducted deck-board incubation experiments in the oligotrophic region of Northwestern Pacific Ocean (NWPO). Our results showed that when dust was added at a concentration of 2?mg/L (Dust-2), the NO₃^–N concentration increased by 3.2 fold, and chlorophyll a (chl a) concentration of nano- and micro-eukaryotic phytoplankton increased, while that of pico-eukaryotic phytoplankton did not change significantly. The microphytoplankton abundance increased but the species number decreased in Dust-2. Community structure of eukaryotic microphytoplankton also changed after dust addition. The abundance of diatoms in Dust-2 (23,072 cells/L) was 4.0 fold of that in the control (5750 cells/L), and 2.4 fold of that in Dust-1 (0.2?mg/L of dust addition) (9425 cells/L) at the 7th day of incubation. Abundance of dinoflagellates decreased in Dust-2, being 42.2% that of the control at the 7th day of the incubation. Effects of dust addition on the growth of phytoplankton differed among the dominant genera: growth of Pseudo-nitzschia and Chaetoceros were promoted while that of Prorocentrum was inhibited, and growth of Thalassiosira, Heterocapsa and Scrippsiella was not influenced significantly. The growth of nano- and pico-eukaryotic phytoplankton was promoted, with the cell abundance in Dust-2 2.4 fold of that in control. Our results indicated that Asian dust-originated aerosol particulates could provide nutrients to the oligotrophic NWPO, increase the marine productivity in the area, and alter the eukaryotic phytoplankton community structure.     

Deep chlorophyll maxima and UVR acclimation by epilimnetic phytoplankton

1. It is well established that ultraviolet radiation (UVR) has many harmful effects on phytoplankton, but the factors controlling algal sensitivity to UVR are not fully understood. 2. We exposed phytoplankton communities from the epilimnia and deep chlorophyll maxima (DCM) of 2 Canadian lakes to 14 irradiance treatments of various spectral quality and monitored changes in the maximum quantum efficiency of Photosystem II photochemistry (F_v/F_m) using a pulse amplitude modulation fluorometer. 3. Phytoplankton from DCM did not show marked differences from epilimnetic communities in taxonomy or nutrient status, but exhibited substantially higher photosynthetic impairment under UVR exposure. 4. Our results suggest that epilimnetic phytoplankton acclimate to in situ light conditions in a spectrally‐specific manner, and that ultraviolet‐A radiation is a stronger stressor than ultraviolet‐B or photosynthetically active radiation in the mixed layers of our study lakes. Model estimates of damage and recovery rate constants revealed that the phytoplankton of the two lakes relied upon different strategies of UVR‐acclimation, in one lake minimising susceptibility to photodamage and in the other maximising recovery efficiency.