THE EFFECTS OF ULTRAVIOLET‐B RADIATION ON ANTARCTIC SEA‐ICE ALGAE1

The impacts of ultraviolet‐B radiation (UVB) on polar sea‐ice algal communities have not yet been demonstrated. We assess the impacts of UV on these communities using both laboratory experiments on algal isolates and by modification of the in situ spectral distribution of the under‐ice irradiance. In the latter experiment, filters were attached to the upper surface of the ice so that the algae were exposed in situ to treatments of ambient levels of PAR and UV radiation, ambient radiation minus UVB, and ambient radiation minus all UV. After 16 d, significant increases in chl a and cell numbers were recorded for all treatments, but there were no significant differences among the different treatments. Bottom‐ice algae exposed in vitro were considerably less tolerant to UVB than those in situ, but this tolerance improved when algae were retained within a solid block of ice. In addition, algae extracted from brine channels in the upper meter of sea ice and exposed to PAR and UVB in the laboratory were much more tolerant of high UVB doses than were any bottom‐ice isolates. This finding indicates that brine algae may be better adapted to high PAR and UVB than are bottom‐ice algae. The data indicate that the impact of increased levels of UVB resulting from springtime ozone depletion on Antarctic bottom‐ice communities is likely to be minimal. These algae are likely protected by strong UVB attenuation by the overlying ice and snow, by other inorganic and organic substances in the ice matrix, and by algal cells closer to the surface.     

Effects of UV radiation on five marine microphytobenthic Wadden sea diatoms isolated from the Solthörn tidal flat (Lower Saxony,southern North Sea) – Part I: growth and antioxidative defence strategies

The unconsolidated sediment of intertidal mudflats constitutes a highly unstable environment, due to continuously changing water levels and currents as well as temporary exposure to the air. Therefore, diatoms inhabiting marine intertidal areas are subjected to strongly changing surface light and UV intensities due to exposure at low tide. Five marine intertidal diatoms (Achnanthes exigua, Cocconeis peltoides, Diploneis littoralis, Navicula digitoradiata and Amphora exigua) were isolated from the Solthörn tidal flat (Lower Saxony, southern North Sea). Semi-continuous cultures were used to determine the effect of UV radiation (photosynthetically active radiation only [PAR], PAR+UV-B, PAR+UV-A, PAR+UV-B+UV-A) during short- and long-term exposure (6 h or 30 days). Growth rates, chlorophyll a (chl a), antioxidant capacities, accumulation of phenolic compounds (e.g. flavonoids) and DMSP, and activities of antioxidant enzymes (superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase and glutathione reductase) were assessed. UV-A had only minor effects on cells, while growth rate, chl a content and protein content were significantly reduced after long-term UV-B exposure. Achnanthes exigua extracts showed the highest antioxidant capacity. The highest activity of SOD, APX and MDHAR was found under long-term combined UV exposure (PAR+UV-B+UV-A). Overall, the antioxidative defence of the five isolates was stimulated during exposure to UV radiation, as may be found during emersion. Emersion induces oxidative stress and, as a result, growth of the five diatom taxa was inhibited to suit changing environmental conditions. All five taxa tested in the present study showed species-specific acclimatization potentials, providing possible explanations for variability in population, species composition and ecosystem structures in the face of climatic variations.     

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

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

NUTRIENT LIMITATION AND HIGH IRRADIANCE ACCLIMATION REDUCE PAR AND UV‐INDUCED VIABILITY LOSS IN THE ANTARCTIC DIATOM CHAETOCEROS BREVIS (BACILLARIOPHYCEAE)1

The effects of high PAR (400–700 nm), UVA (315–400 nm), and UVB (280–315 nm) radiation on viability and photosynthesis were investigated for Chaetoceros brevis Schütt. This Antarctic marine diatom was cultivated under low, medium, and high irradiance and nitrate, phosphate, silicate, and iron limitation before exposure to a simulated surface irradiance (SSI) treatment, with and without UVB radiation. Light‐harvesting and protective pigment composition and PSII parameters were determined before SSI exposure, whereas viability was measured by flow cytometry in combination with a viability stain after the treatment. Recovery of PSII efficiency was measured after 20 h in dim light in a separate experiment. In addition, low and high irradiance acclimated cells were exposed outdoors for 4 h to assess the effects of natural PAR, UVA, and UVB on viability. Low irradiance acclimated cells were particularly sensitive to photo induced viability loss, whereas no viability loss was found after acclimation to high irradiance. Furthermore, nutrient limitation reduced sensitivity to photo induced viability loss, relative to nutrient replete conditions. No additional viability loss was found after UVB exposure. Sunlight exposed cells showed no additional UVB effect on viability, whereas UVA and PAR significantly reduced the viability of low irradiance acclimated cells. Recovery of PSII function was nearly complete in cultures that survived the light treatments. Increased resistance to high irradiance coincided with an increased ratio between protective‐ and light‐harvesting pigments before the SSI treatment, demonstrating the importance of nonphotochemical quenching by diatoxanthin for survival of near‐surface irradiance. We conclude that a sudden transfer to high irradiance can be fatal for low irradiance acclimated C. brevis.     

DENSITY-DEPENDENT GROWTH,ECOLOGICAL STRATEGIES,AND EFFECTS OF NUTRIENTS AND SHADING ON BENTHIC DIATOM SUCCESSION IN STREAMS1

The importance of immigration, growth, and competition for nutrients and light in benthic diatom succession was studied in experimental channels in a low-nutrient stream. Diatom accumulation was greater in channels enriched with nitrate and phosphate (NP) than in control channels, reaching about 5 × 10⁶ and 2 × 10⁶ cells-cm^?2, respectively, after 30 d. Shading during late stages of community development reduced algal standing crop. Synedra ulna (Nitz.) Ehr. and Achnanthes minutissima Kütz. were codominant during early stages of community development in both habitats, but succession to an A. minutissima-dominated community was much faster in NP-enriched than in control conditions. Species dominating early stages tended to immigrate quickly, whereas species that increased in relative abundance during community development had either fast growth rates or fast immigration and average growth rates. Decreases in growth rates indicated resource supply became limiting during community development in control and enriched channels. Density-dependent competition was indicated because nutrient concentrations in the water column and light did not decrease during the 30-d study. Species autecologies were defined by effects of nutrient enrichment, shading, and community development on species growth rates. Differing autecologies of early and late succession species indicated that competition for nutrients was more important than competition for light. Species autecologies also indicated ecological strategies. The species most stimulated by nutrient enrichment were least able to maintain growth rates as algal abundances on substrata increased. In addition, these species that best sustained their growth rates during succession tended to have the highest immigration rates, indicating that drift and immigration may have been an important mechanism of persistence for some populations when resources become limiting within thick benthic mats.     

Effects of UV radiation on five marine microphytobenthic Wadden sea diatoms,isolated from the Solthörn tidal flat (Lower Saxony,southern North Sea) – Part II: changes in carbohydrate,amino acid and fatty acid composition

Benthic diatoms inhabiting intertidal flats face highly variable environmental conditions, due to changing water levels and exposure during low tide. The present study is the second part of a more extensive study of the adaptive potential of these species in response to varying UV radiations in the Solthörn tidal flat (Lower Saxony, southern North Sea). Five isolates (Achnanthes exigua, Amphora exigua, Cocconeis peltoides, Diploneis littoralis and Navicula digitoradiata), which were found in this area in high cell numbers in summer 2008, were used in semi-continuous cultures to study the physiological effects of UV-radiation (PAR [photosynthetically active radiation], PAR+UV-A, PAR+UV-B, PAR+UV-B+UV-A). For short- and long-term exposures (6 h, 30 days), the composition of intercellular carbohydrates, amino and fatty acids were analysed in exponential-phase cultures grown at a salinity of 30 in a 12?:?12 h light?:?dark cycle at 20?°C. Although all tested species showed distinct differences in their initial carbohydrate, amino and fatty acid compositions and in their responses to the different UV treatments, general response patterns could be identified. Overall physiological responses to short- and long-term UV treatments included the accumulation of proline as well as an increase in total carbohydrates and lipids, whereas significant differences in the composition of carbohydrates, amino and fatty acids occurred after long-term exposure to the UV treatments (P < 0.05). While UV-A exposure led to higher accumulations of phenylalanine, aspartic acid and saturated fatty acids, the response to UV-B long-term exposure included increases of galactose, mannose and unsaturated fatty acids in the cells. In both UV experiments there was a noteworthy accumulation of the amino acid tryptophan in most species. The combined UV-A+UV-B experiment showed a significant (P < 0.05) increase of aspartic acid, phenylalanine, galactose and saturated fatty acids in a majority of species. Overall, the results indicated significant differences in the physiological responses of the five diatom taxa during UV exposure, which suggests species-specific acclimation strategies that may explain the growth insensitivity towards at least short-term UV.     

UVB effects on early developmental stages of commercially important macroalgae in southern Chile

High levels of ultraviolet-B radiation (UVB) could represent a danger to seaweeds by affecting their physiological processes and development. The aim of this work was to study the effects of UVB radiation on early developmental stages of commercially and ecologically important marine algal species in southern Chile, considering spores survival and embryos growth. Spores of Mazzaella laminarioides, Gigartina skottsbergii, Sarcothalia crispata and embryos of G. skottsbergii and Macrocystis pyrifera were submitted to treatments of a) photosynthetically active radiation (PAR: Control), b) PAR+UVA (PA) and c) PAR+UVA+UVB (PAB). UV radiation did not affect spore survival of M. laminarioides S. crispata and G. skottsbergii (P = 0.55, P = 0.6 & P = 0.25 respectively), but did provoke differences in the growth rate of G. skottsbergii embryos (P = 0.00). Differences in survival and growth of M. pyrifera embryos were also observed (P = 0.001 & P = 0.007, respectively). Differences in growth of M. pyrifera embryos were observed only in the first five days, whereas changes in survival persisted until the end of the experiment. Additionally, UVB provoked morphological alteration in M. pyrifera embryos, as evidenced by progressive curling. These results suggest that the initial stages of the subtidal algae species G. skottsbergii and M. pyrifera cultivated in laboratory conditions were sensitive to UVA and UVB radiation, and their recruitment and development could be affected as well in natural conditions found in southern South America, where the ozone layer has thinned more than in other parts of the planet.     

UV‐radiation effects on photosynthesis and photoprotection in gametophytic and sporophytic stages of the freshwater red alga Kumanoa ambigua (Rhodophyta,Batrachospermales)

The aim of this study was to analyze the photosynthetic performance of gametophytic and sporophytic (‘Chantransia’) stages of Kumanoa ambigua in culture under UV radiation. We hypothesized that both life history stages of K. ambigua would exhibit different photosynthetic responses to UVR exposure. Experiments were performed under three conditions: (i) photosynthetically active radiation (PAR) only (400–700 nm), P control; (ii) PAR + UVA (320–700 nm), PA treatment; and (iii) PAR + UVA + UVB (280–700 nm), PAB treatment. The photosynthetic parameters were measured as in vivo chlorophyll a fluorescence. Differences were found between life stages, observing higher values of NPQ and effective quantum yields (ΔF/F_m′) under UVA and PAR in gametophytes compared to sporophytes. One type of mycosporine‐like amino acid (MAA) was detected in the gametophyte in all treatments, but not in the ‘Chantransia’ stage. The increased photosynthetic performance for some parameters and the presence of MAA in gametophyte suggest that it is less sensitive to UV radiation, particularly UVA, in comparison to sporophyte under culture conditions. This approach is relevant for a better understanding of the adaptation and physiological acclimation of freshwater Rhodophyta to varying light climates in terms of global changes.     

SYNTHESIS OF MYCOSPORINE-LIKE AMINO ACIDS IN CHONDRUS CRISPUS (FLORIDEOPHYCEAE) AND THE CONSEQUENCES FOR SENSITIVITY TO ULTRAVIOLET B RADIATION

The induction and protective role of the UV-absorbing compounds known as mycosporine-like amino acids (MAAs) were examined in sublittoral Chondrus crispus Stackh. transplanted for 2 weeks in the spring and summer to shallow water under three irradiance conditions: PAR (photosynthetically active radiation; 400–700 nm), PAR + UVA (PAR + 320– 400 nm), PAR + UVA + UVB (PAR + UVA + 280– 320 nm). Sublittoral thalli collected around Helgoland, North Sea, Germany, from 6 m below the mean low water of spring tides contained less than 0.1 mg·g⁻¹ dry weight (DW) total MAAs, whereas eulittoral samples contained over 1 mg·g⁻¹ DW. Transplantation to shallow water led to the immediate synthesis of three MAAs in the following temporal order: shinorine (λ_max 334 nm), asterina (λ_max 330 nm), and palythine (λ_max 320 nm), with the shinorine content peaking and then declining after 2 days (exposure to 100 mol photons·m⁻²). Maximum total MAA content (2 mg·g⁻¹ DW) also occurred after 2 days of induction, exceeding the content normally found in eulittoral samples. Furthermore, the relative proportion of the different MAAs at this time was different than that in eulittoral samples. After 2 days the total content declined to the eulittoral value, with palythine as the principal MAA. Similar data were obtained for all treatments, indicating that MAA synthesis in C. crispus was induced by PAR and not especially stimulated by UV radiation. The ability of photosystem II (PSII) to resist damage by UVB was tested periodically during the acclimation period by exposing samples to a defined UVB dose in the lab. Changes in chlorophyll fluorescence (F_v/F_m and effective quantum yield, φ_II) indicated that PSII function was inhibited during the initial stage of acclimation but gradually improved with time. No difference among screening treatments was detected except in spring for the samples acclimating to PAR + UVA + UVB. In this treatment F_v/F_m and φ_II were significantly lower than in the other treatments. During the first week of each experiment, growth rates were also significantly reduced by UVB. The reductions occurred despite maximum MAA content, indicating an incomplete protection of photosynthetic and growth-related processes.     

Comparison of the impacts of consumers, ambient UV, and future UVB irradiance on mid-latitudinal macroepibenthic assemblages

Future levels in ultraviolet-B (UVB) radiation are expected to increase directly due to stratospheric ozone depletion and under water indirectly by, for example, global warming effects on DOC concentrations, altered trophic interactions in the plankton, or reduced eutrophication. While detrimental UV effects have been reported at the cellular level, little to nothing is known about community-wide effects of ambient and future UVB radiation. In a 4-month field experiment, the ambient UV regime was (i) reduced by cut-off filters which removed either UVB or total UV from the solar spectrum or (ii) increased to predicted future levels by UVB lamps. To allow relating the effects of present and future UV regimes to another important ecological control of community structure and diversity in subtidal marine habitats, consumer effects were quantified by an exclusion treatment under ambient light regimes. Ambient UV regimes did not affect community structure, biomass accrual, and diversity. In contrast, under enhanced UVB levels, the dominance of the competitively superior blue mussels increased and species richness and biomass accrual decreased. Species composition of the assemblages differed between the two UV regimes. Effects of enhanced UVB radiation and of consumption on biomass accrual, diversity, and structure of the community were comparable in magnitude and timing, but of opposite direction. In contrast, the effects of enhanced UVB radiation on growth and abundance of mussels were in the same direction, but shorter and weaker than consumer effects. Most UV effects were transitory and vanished within the first 2 months of succession. Our results indicate that present and future UVB levels may be of limited importance and not stronger in effect size than other ecological controls in structuring the shallow-water low-diversity macrobenthic communities in temperate regions.     

COMMUNITY STRUCTURE DYNAMICS OF EPILITHIC AND EPIPHYTIC DIATOMS IN A MISSISSIPPI STREAM1

Epilithic and epiphytic diatoms were collected monthly from 18 June 1979 through 19 May 1980 from both shaded and unshaded sites of Sessums Creek, a shallow, oligotrophic and strongly alkaline stream in northeastern Mississippi. The dominant macroalgae were filamentous green algae belonging to the non-mucilage producing genera Cladophora and Rhizoclonium. Ninety taxa were identified with Navicula minima Grun. and Nitzschia dissipata (Kütz.) Grun. accounting for 50% of all valves counted. Other important taxa were Navicula menisculus var. upsaliensis (Grun.) Grun., N. cryptocephala var. veneta (Kütz.) Rabh., Achnanthes minutissima Kütz. and Cymbella turgida (Greg.) Cl. With few exceptions, the more dominant taxa were equally abundant in the shaded and unshaded sites and also in the epilithon and epiphyton. Species diversity (H') and the number of taxa in a sample in all four habitats showed an identical pattern, being lowest in winter and highest in fall. No one pair of habitats was more structurally similar than any-other pair when compared by a similarity index (SIMI). Apparently, differences in light intensity and substrate type had little effect on diatom distribution in Sessums Creek and it was concluded that the locality sampled supported a single, nearly homogeneous diatom community.     

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.     

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.     

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

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

CHRONIC EFFECTS OF ULTRAVIOLET-B RADIATION ON GROWTH AND CELL VOLUME OF PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

Cultures of the temperate estuarine diatom, Phaeodactylum tricornutum Bohlin (NEPCC Clone 31), were grown under ambient intensities of ultraviolet-A radiation (UVAR), photosynthetically active radiation (PAR), and various intensities of ultraviolet-B radiation (UVBR; 290–320 nm). Growth rates and cell volumes were monitored for 36 d. UVBR decreased growth rates and increased cell volumes. Sensitivity of growth to UVBR increased with time. Growth rates of P. tricornutum decreased with increasing ratios of UVBR:UVAR + PAR.     

Ultraviolet-B Wavelengths Regulate Changes in UV Absorption of Cleaner Fish Labroides dimidiatus Mucus

High-energy wavelengths in the ultraviolet-B (UVB, 280-315 nm) and the UVA (315-400-nm) portion of the spectrum are harmful to terrestrial and aquatic organisms. Interestingly, UVA is also involved in the repair of UV induced damage. Organisms living in shallow coral reef environments possess UV absorbing compounds, such as mycosporine-like amino acids, to protect them from UV radiation. While it has been demonstrated that exposure to UV (280-400 nm) affects the UV absorbance of fish mucus, whether the effects of UV exposure vary between UVB and UVA wavelengths is not known. Therefore, we investigated whether the UVB, UVA, or photosynthetically active radiation (PAR, 400-700 nm) portions of the spectrum affected the UV absorbance of epithelial mucus and Fulton’s body condition index of the cleaner fish Labroides dimidiatus. We also compared field-measured UV absorbance with laboratory based high-performance liquid chromatography measurements of mycosporine-like amino acid concentrations. After 1 week, we found that the UV absorbance of epithelial mucus was higher in the UVB+UVA+PAR treatment compared with the UVA+PAR and PAR only treatments; after 2 and 3 weeks, however, differences between treatments were not detected. After 3 weeks, Fulton’s body condition index was lower for fish in the UVB+UVA+PAR compared with PAR and UVA+PAR treatments; furthermore, all experimentally treated fish had a lower Fulton’s body condition index than did freshly caught fish. Finally, we found a decrease with depth in the UV absorbance of mucus of wild-caught fish. This study suggests that the increase in UV absorbance of fish mucus in response to increased overall UV levels is a function of the UVB portion of the spectrum. This has important implications for the ability of cleaner fish and other fishes to adjust their mucus UV protection in response to variations in environmental UV exposure.     

COMPARISON OF ATTACHED DIATOM COMMUNITIES ON NATURAL AND ARTIFICIAL SUBSTRATES1

A study of attached diatom communities on artificial and natural substrates was conducted in Wheelwright Pond, New Hampshire, during 1975. There were differences in the species composition growing on artificial glass slides and natural substrates. The slides favored the accumulation of Achnanthes minutissima Kütz. and decreased the relative abundance of Eunotia incisa W. Sm. ex. Greg. and Cocconeis placentula v. euglypta (Ehr.) Cleve. Large growths of planktonic species were also noted on the slides. Compositional differences between slides positioned at 15–30 cm (upper) and 1 m (lower) from the surface of the water were minimal. A similar community composition of periphytic diatoms was found on five aquatic macrophytes.     

SUBSTRATUM CONDITIONING AND DIATOM COLONIZATION IN DIFFERENT CURRENT REGIMES1

Short-term (3 day) diatom colonization of clean ceramic tiles and tiles conditioned with a thin, nonalgal biofilm was examined in fast- and slow-current outdoor experimental stream channels to assess effects of organic conditioning and current regime on diatom colonization. Colonization rates onto unconditioned tiles were 10 times lower in fast current that in slow. Substratum conditioning reduced the effects of current because organic conditioning of tiles significantly enhanced colonization in fast current, but not in slow current. Two diatom taxa (Nitzschia acicularis W. Sm. and Synedra radians Kütz) colonized unconditioned tiles more rapidly than conditioned tiles in slow current, suggesting the existence of negative interactions between these diatoms and bacteria and/or organics in conditioning films.     

EVIDENCE OF DARK AVOIDANCE BY PHOTOTROPHIC PERIPHYTIC DIATOMS IN LOTIC SYSTEMS1

Short-term (24–48 h) colonization dynamics of periphytic diatoms on artificial (styrofoam) substrata were examined using fast-flushing, continuous-flow troughs located on the North Thompson River, British Columbia. Two parallel troughs, one exposed to natural light and the other completely darkened, showed significant differences in periphyton biomass, chlorophyll a, and algal taxonomic composition with 24 h. Experiments which commenced at the onset of natural darkness demonstrated that rates of algal immigration during the night were the same in both troughs. Within 2–3 h of sunrise, however, certain diatom species (most notably Hannaea arcus (Ehr.) Pair, and Diatoma tenue Ag.) selectively emigrated from the artificially darkened trough but remained in the trough exposed to natural light. More closely adhering species such as Achnanthes minutissima Kütz, also showed significant emigration from the darkened trough after light deprivation for two photoperiods. Data from adhesion, emigration, and sinking rate experiments indicate that differential egress of cells from the darkened versus the lighted environments is the result of cellular regulation of buoyancy or form resistance.     

Effects of solar UV stress on chlorophyll fluorescence kinetics of intertidal macroalgae from southern Spain: a case study in Gelidium species

Photoinhibition and recovery kinetics after short exposure to solar radiation following three different irradiance treatments of irradiances (PAR, PAR+UVA and PAR+UVA+UVB) was assessed in two intertidal species of the genus Gelidium, Gelidium sesquipedale and G. latifolium, collected from Tarifa (southern Spain) using in vivo chlorophyll fluorescence (PAM fluorometry). After 3 h UV radiation exposure, optimal quantum efficiency (Fv/Fm) in G. sesquipedale decreased between 25 and 35% relative to the control. Under PAR alone, values decreased to 60%. In G. latifolium, photoinhibition did not exceed 40%. Similar results were found for the effective quantum yield (ΔF/Fm′), however, no marked differences in relation to light treatments were seen. When plants were shaded for recovery from stress, only in G. latifolium a significant increase in photosynthesis was observed (between 80 and 100% of control). In contrast, photosynthesis of G. sesquipedale suffered a chronic photoinhibition or photodamage under the three light irradiances. Full solar radiation (PAR+UVA+UVB) affected also the electron transport rate in both species. Here, initial slopes of electron transport vs. irradiance curves decreased up to 60% of controls. Although the recovery kinetic under PAR+UVA+UVB conditions was delayed in G. latifolium, after 24 h recovery this species reached significantly higher than G. sesquipedale. PAR impaired electron trasport only in G. sesquipedale. Overall, both species are characterized by different capacity to tolerate enhanced solar radiation. G. latifolium is a sun adapted plant, well suited to intertidal light conditions, whereas G. sesquipedale, growing at shaded sites in the intertidal zone, is more vulnerable to enhanced UV radiation. This revised version was published online in June 2006 with corrections to the Cover Date.