Effects of solar ultraviolet radiation on antarctic ...
Li
The roles of ozone and cloud cover. Journal of Applied Meteorology, 30(4), 478-493. Lubin, D., J.E. Frederick, C.R. Booth, T.B. Lucas, and D.A. Neuschuler. 1989. Measurements of enhanced springtime ultraviolet radiation at Palmer Station, Antarctica, Geophysical Research Letters, 16(8), 783-785. Lubin, D., B.G. Mitchell, J.E. Frederick, A.D. Alberts, C.R. Booth, T.B. Lucas, and D.A. Neuschuler. 1992. A contribution toward understanding the biospherical significance of Antarctic ozone depletion. Journal of Geophysical Research, 97(8), 7817-7828. Madronich, S. 1993. UV radiation in the natural and perturbed atomosphere. In M. Tevini (Ed.), UV-B radiation and ozone depletion: Effects on humans, animals, plants, microorganisms, and materi als. Boca Raton, Florida: Lewis Publishers.
McKinlay, A.F., and B.L. Diffey. 1987. A reference action spectrum for ultra-violet induced erythema in human skin. In W.R. Passchler and B.F.M. Bosnajakovic (Eds.), Human exposure to ultraviolet radiation: Risks and regulations. Amsterdam: Elsevier. McPeters, R. 1994 Personal communication. Setlow, R.B. 1974. The wavelengths in sunlight effective in producing skin cancer: A theoretical analysis. Proceedings of the National Academy of Science, 71(9), 3363-3366. Smith, R.C., and K.S. Baker. 1989. Stratospheric ozone, middle ultraviolet radiation and phytoplankton productivity. Oceanography, 2(2),4-10.
Smith, R., K. Baker, D. Menzies, and K. Waters. 1991. Biooptical measurements from the IceColors 90 cruise 5 Oct-21 Nov 1990 (Scripps Institution of Oceanography Reference 91-13). La Jolla, California: Scripps Institution of Oceanography. Smith, R.C., B.B. Prézelin, K.S. Baker, R.R. Bidigare, N.P. Boucher, T. Coley, D. Karentz, S. Maclntyre, H.A. Matlick, D. Menzies, M. Ondrusek, Z. Wan, and K.J. Waters. 1992. Ozone depletion: Ultraviolet radiation and phytoplankton biology in antarctic waters. Science, 256(5047), 952-959. Smith, R.C., Z. Wan, and K.S. Baker. 1992. Ozone depletion in Antarctica: Modeling its effect under clear-sky conditions. Journal of Geophysical Research, 97(C5), 7383-7397. Stamnes, K. 1993. The stratosphere as a modulator of ultraviolet radiation into the biosphere. Surveys in Geophysics, 14, 167-186. Stamnes, K., Z. un, J. Slusser, C.R. Booth, and T.B. Lucas. 1992. Several-fold enhancement of biologically effective ultraviolet radiation levels at McMurdo Station, Antarctica, during the 1990 ozone hole, Geophysical Research Letters, 19(10), 1013-1016. Stamnes, K., J. Slusser, and M. Boden. 1991. Derivation of total ozone abundance and cloud effects from spectral irradiance measurements, Applied Optics, 30(30), 4418-4426. Stamnes, K., J. Slusser, M. Bowen, C.R. Booth, and T.B. Lucas. 1990. Biologically effective ultraviolet radiation, total ozone abundance, and cloud optical depth at McMurdo Station, Antarctica, September 15, 1988 through April 15, 1989. Geophysical Research Letters, 17(12),2181-2184.
Effects of solar ultraviolet radiation on antarctic phytoplankton during springtime ozone depletion VIRGINIA E. V1LLAFAIE, E. WALTER HELBLING, and OSMUND HOLM-I-IANSEN, Polar Research Program, Scripps Institution of
Livio
Oceanography, University of California at San Diego, La Jolla, California 92093-0202 SALA, Facultad de Ciencias Naturales, Universidad Nacional de la Patagonia, Chubut, Argentina
n recent years, much attention has been given to the forI mation of the seasonal ozone "hole" over Antarctica, with the concomitant increase in ultraviolet-B [UV-B, 280-320nanometer (nm)] radiation levels (see Weiler and Penhale 1994). The enhanced UV-B radiation can be very damaging to biological systems and has been shown to cause a significant decrease in rates of primary production (Smith et al. 1992; Holm-Hansen, Helbling, and Lubin 1993). In this paper, we are concerned with describing the impact of "normal" ultraviolet radiation (UVR), as well as enhanced UV-B radiation, on natural assemblages of phytoplankton as well as on just the nanoplankton fraction [cells less than 20 micrometers (sm)] and the microplankton fraction (cells >20 tm). Our studies also included estimation of the impact of UVR as influenced by the taxonomic composition of the phytoplankton and the mitigating effect of cellular UV-absorbing compounds. All studies were carried out at Palmer Station (64.7 0 S 64.1 0 W) on Anvers Island from early October to the end of December 1993. This period provided excellent opportunities to document the impact of enhanced UV-B radiation on phytoplankton because the ozone hole was very well developed over Palmer Station in the month of October;
column ozone concentrations ranged from 140 to 220 Dobson units (DU). During the period of study, incident solar radiation was monitored continuously (and recorded every minute) using a spectroradiometer (model PUV-510; Biospherical Instruments, Inc.) with sensors for photosynthetically available radiation (PAR, 400-700 nm), and four UV wavelengths (305, 320, 340, and 380 nm). Phytoplankton samples were taken at a coastal site at 1-meter depth with a 5-liter Go-Flo bottle and were used for chlorophyll-a (chl-a) analysis, determination of absorption spectra (250-750 nm) of the particulate fraction, floristic analysis, and carbon-14 ( 14c) incorporation. Chl-a analyses were performed by fluorometric techniques (Holm-Hansen et al. 1965; Holm-Hansen and Riemann 1978). The chl-a of the nanoplankton fraction was obtained by prefiltering the sample through a nylon mesh fabric (Nitex®) with a mesh opening of 20 rim; the filtrate was treated in the same way as for total chl-a concentrations. For absorption spectra analysis, a variable amount of seawater (between 5 and 9 liters) was filtered through a Whatman GF/F filter (47-millimeter). The pigments were extracted in 10 milliliters of absolute methanol, and the extract was used
ANTARCTIC JOURNAL - REVIEW 1994 259
to run a spectrum (250 to 750 nm) with a Perkin-Elmer Lambda 6 spectrophotometer using 1- or 10-centimeter quartz cuvettes. Water samples for floristic analysis were preserved in buffered formalin, and the analyses were done by inverted microscope techniques. Phytoplankton biomass in the different taxonomic groups (diatoms, dinoflagellates, and flagellates) was calculated from the biovolumes and applying the equations of Strathmann (1967). For measurements of 14C incorporation, replicate samples of total phytoplankton and of the nanoplankton fraction were placed in 50-milliliter tubes under three different spectral treatments: • quartz tubes (cells exposed to PAR+UV-A+UV-B), • pyrex tubes covered with a Mylar sheet (PAR+UV-A), and • pyrex tubes covered with Plexiglas UF-3 (PAR). The samples were inoculated with 5 microcuries of 14C_ bicarbonate (Steemann Nielsen 1952) and exposed to solar radiation for 6-10 hours (centered on local noon) in an outdoor incubator with flowing surface seawater for temperature control. The 14C incorporation by phytoplankton was measured by standard liquid scintillation techniques. Data in figure 1 show the PlC incorporation in the total and the nano- and microplankton fractions of phytoplankton assemblages. For total phytoplankton (figure 1A), a significant difference in the assimilation numbers for the three treatments (quartz, Mylar, and Plexiglas) is evident, with higher assimilation numbers (1.16 milligrams of carbon per milligram of chl-a per hour) in the Plexiglas treatment (which received only PAR). A comparison of both fractions of phytoplankton (figure 1B) shows that the assimilation numbers in microplankton were slightly lower than those in nanoplankton, but they were not significantly different. It should be noted that there was no dif ferential decrease in chl-a concentrations between any of the treatments, so that changes in assimilation numbers reflect changes in rates of carbon dioxide fixation. A significant difference between the two fractions is evident, however, (figure 1 C when the relative enhancement of Mylar and quartz are compared. The microplankton under the Mylar treatment showed a higher relative enhancement of photosynthesis as compared to the nanoplankton, indicating a greater inhibition of photosynthesis in the microplankton when the samples are exposed to UV-B radiation. This result is consistent with previous findings by Helbling et al. (1992). The absorption spectra of methanol extracts of natural assemblages of phytoplankton showed a peak at 330 nm. When the values of the chlorophyll-specific absorption (ap*) at 330 nm are compared to the relative enhancement of photosynthesis in phytoplankton, an inverse relationship between these two variables is evident (figure 2A). Data in figure 2B show the relationship between the relative enhancement of photosynthetic rate and the percentage of biomass that is accounted for by diatoms. When diatoms dominated the phytoplankton samples, inhibition of photosynthesis was less, suggesting that diatoms have greater resistance to damage by UVR than flagellates and dinoflagellates. Such a relationship was previously discussed by Helbling, Villafafle, and Holm-Hansen (1994, pp. 207-227), who showed that diatoms have relatively high cellular concentra-
tion of UVR absorbing compounds. This result suggests that these compounds could act as a "protective" mechanism against UVR-induced damage and is in agreement with previous studies of UV-absorbing compounds by Dunlap, Chalker, and Oliver (1986). During the period from October to December, a sample of near-surface water from Arthur Harbor was taken every 2-3 days to determine the sensitivity of the natural phytoplankton assemblage to incident solar UVR in one of the standard outdoor incubators. The data show that the percentage of enhancement of photosynthesis varies a great deal, both when UV-B was selectively screened off (figure 3A) and when all UV-B and UV-A were screened off (figure 3B; note the change in scale for the ordinate). This variability is -c -C
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Mylar Plexiglas Figure 1. Effects of solar radiation on antarctic phytoplankton when different portions of the spectrum have been screened out using sharp cut-off filters: quartz—all radiation (PAR+UV-A+UV-B) received; Mylar—the UV-B portion screened out; Plexiglas—all UVR screened out. A. Assimilation numbers of total phytoplankton [in milligrams of carbon per milligram of chl-a per hour (mg C mg chi-a-' h- i )] as a function of the different treatments. B. Assimilation numbers (mg C mg chl-a- 1 h-i ) in the nanoplankton and microplankton fractions of the phytoplankton as a function of the different treatments. C. Percentage of relative enhancement as compared to the quartz treatment for nano- and microplankton under Mylar and Plexiglas.
ANTARCTIC JOURNAL - REVIEW 1994 260
thought to be due primarily to changes in taxonomic composition of the phytoplankton, to photoadaptive responses related to increasing incident irradiation, and to variations in the ratio of UV-B/UV-A irradiance as influenced by changes in column ozone concentrations. During the period covered by our study, column ozone concentrations ranged from 140 DU in October to 325 DU in December. The data shown in figure 3 are presently being analyzed to gain a better understanding of the relative importance of the factors that influence the sensitivity of antarctic phytoplankton to seasonal changes in the fluence of incident UV-B radiation. Grateful acknowledgment is made to Emilio R. Marguet (Universidad Nacional de la Patagonia, Argentina), Bruce Chalker (Australian Institute of Marine Sciences, Australia), and to Antarctic Support Associates personnel at Palmer Station for their generous help during this field season. This research was supported by National Science Foundation grant OPP 92-20150. V.E. Villafañe and E.W. Heibling were at Palmer Station from 3 October 1993 to 3 January 1994; 0. Holm-Hansen and L. Sala were at the Station from 3 October to 15 November 1993.
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References
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Dunlap, W.C., B.E. Chalker, and J.K. Oliver. 1986. Bathymetric adaptations of reef-building corals at Davies Reef, Great Barrier Reef, Australia. III. UV-B absorbing compounds. Journal of Experimental Marine Biology and Ecology, 104(3), 239-248. Heibling, E.W., V. Villafane, M. Ferrario, and 0. Holm-Hansen. 1992. Impact of natural ultraviolet radiation on rates of photosynthesis and on specific marine phytoplankton species. Marine Ecology Progress Series, 80(1), 89-100. Heibling, E.W., V. Villafane, and 0. Holm-Hansen. 1994. Effects of ultraviolet radiation on antarctic marine phytoplankton photosynthesis with particular attention to the influence of mixing. In S.
10 1 15 30 45 60 75 90 Relative enhancement (%) Figure 2. Relationship between sensitivity to UVR and presence of cellular UV-absorbing compounds. A. Percentage of relative enhancement of photosynthesis for natural assemblages of phytoplankton when all UVR has been screened out (by UF-3 filter) as a function of the specific absorption (ap*) at 330 nm. B. Relationship between the percentage of phytoplankton biomass that is represented by diatoms and the percentage of relative enhancement of the phytoplankton assemblage. I
1
I
Weiler and P. Penhale (Eds.), Ultraviolet radiation in Antarctica: Measurements and biological effects (Antarctic Research Series,
Vol. 62). Washington, D.C.: American Geophysical Union. Holm-Hansen, 0., E.W. Helbling, and D. Lubin. 1993. Ultraviolet radiation in Antarctica: Inhibition of primary production. Photochemistry and Photobiology, 58(4), 567-570. 40 35. E
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Figure 3. Enhancement of photosynthetic rate in experimental samples (as compared to controls in quartz tubes) as a function of the UV-B irradiance incident upon the sample tubes. A. The effect of screening off UV-B by use of a Mylar filter. B. The effect of screening off both UV-13 and UV-A radiation by use of a Plexiglas (UF-3) pre-filter. ANTARCTIC JOURNAL
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Holm-Hansen, 0., C.J. Lorenzen, R.W. Holmes, and J.D.H. Strickland. 1965. Fluorometric determination of chlorophyll. Journal de Conseil pour l'Exploration de laMer, 30(1), 3-15. Holm-Hansen, 0., and B. Riemann. 1978. Chlorophyll a determination: Improvements in methodology. OIKOS, 30(3), 438-447. Smith, R.C., B.B. Prézelin, K.S. Baker, R.R. Bidigare, N.P. Boucher, T. Coley, D. Karentz, S. Maclntyre, H.A. Matlick, D. Menzies, M. Ondrusek, Z. Wan, and K.J. Waters. 1992. Ozone depletion: Ultraviolet radiation and phytoplankton biology in antarctic waters. Science, 255(5047), 952-959.
Steemann Nielsen, E. 1952. The use of radiocarbon ( 14 C) for measuring organic production in the sea. Journal du Conseil International pour l'Exploration de laMer, 18(2), 117-140. Strathmann, R.R. 1967. Estimating the organic carbon content of phytoplankton from cell volume of plasma volume. Limnology and Oceanography, 12(3),411-418. Weiler, S., and P. Penhale (Eds.). 1994. Ultraviolet radiation in Antarctica: Measurements and biological effects (Antarctic Research Series, Vol. 62). Washington, D.C.: American Geophysical Union.
In situ inhibition of primary production due to ultraviolet radiation in Antarctica E. WALTER HELBLING, VIRGINIA E. VILLAFANE, and OSMUND HOLM-HANSEN, Polar Research Program, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093-0202
nhibition of photosynthesis due to ultraviolet radiation I (UVR) in antarctic phytoplankton has been documented by many authors (see Weiler and Penhale 1994). Most of these studies have used temperature-controlled incubators in which phytoplankton are exposed to either solar radiation or to UVvisible radiation provided by lamps. Although such experiments are invaluable for determining the effects of solar radiation on the metabolic activity of phytoplankton, they suffer from the fact that the cells will not be exposed to the same spectral irradiance that they would experience at various depths in the water column. The use of in situ incubations of natural phytoplankton assemblages provides the most direct and most realistic procedure to determine the effect of solar UVR on rates of primary production. In this paper, we report preliminary data obtained from such in situ incubations carried out from October through December 1993 at Palmer Station (64.7°S 64.1°W) on Anvers Island, Antarctica. Radiation measurements of surface solar irradiance were done using a PIJV-510 spectroradiometer (Biospherical Instruments, Inc.) and of attenuation of solar radiation in the water column [surface to 100 meters (m)] using a PUV-500 spectroradiometer. Both of these instruments have four channels for UVR [305, 320, 340, and 380 nanometers (nm)] and one channel for photosynthetic available radiation (PAR; 400 to 700 nm). The underwater unit also has sensors for depth and temperature, in addition to a sensor for 683-nm upwelling radiation. This combination of sensors allows estimation of the instantaneous rate of photosynthesis (Chamberlin et al. 1990). Data from each channel in the surface unit were recorded every minute on a 386 computer, whereas data from all chan nels in the underwater unit were displayed in real time and stored (once every second) on a laptop computer. Water samples were obtained at different depths at one station in Arthur Harbor using a 5-liter Go-Flo bottle. From each of the eight depths, replicate samples were placed in 50milliliter tubes for three different spectral treatments: quartz tubes (PAR4JV-A+UV-B),
•Pyrex tubes covered with a Mylar sheet (PAR+UV-A), and • Pyrex tubes covered with Plexiglas UF-3 (PAR). The samples were inoculated with 5 microcuries of carbon-14 bicarbonate (Steemann Nielsen 1952) and were placed on trays that were deployed at the same depths from which the samples had been obtained. The incubations lasted for 6-8 hours (centered on local noon), and the carbon-14 incorporation into the particulate fraction was measured by standard liquid scintillation techniques. Samples were also taken for chlorophyll-a (chi-a) analysis, which was done by fluorometric methods (Holm-Hansen et al. 1965; Holm-Hansen and Riemann 1978). In addition, samples were taken and fixed in buffered formalin for determination and enumeration of phytoplankton species using an inverted microscope. Representative profiles of the optical characteristics of the upper water column are given in figure 1A. The attenuation coefficients of the four UVR wavelengths were 0.25, 0.21, 0. 18, and 0.12 per meter for 305, 320, 340, and 380 rim, respectively. The 1 percent irradiance level for PAR was at about 46 m; an increase in the attenuation coefficients for both PAR and UVR at 380 nm is noticeable between 5 m and 10 m. This change in the attenuation coefficient for PAR between 5 m and 10 m is related to the increase in chl-a concentrations between the surface (approximately 0.4 milligrams of chl-a per cubic meter) and at 10 m depth (more than 3 milligrams of chl-a per cubic meter). The instantaneous rate of photosynthesis (data from the PUV-500) also showed a peak at about 10 m, with values of more than 800 nanomoles carbon per cubic meter per second (figure 1B). Below this depth, both chl-a and rates of photosynthesis decreased rapidly. The phytoplankton crop at all depths was dominated by cryptophytes. Data in figure 2 show the results from the in situ incubations for the same station mentioned in figure 1. The change of rate of carbon fixation with depth resembled the change shown by instantaneous production (determined from 683nm upwelling radiation), with maximal values at about 10 m
ANTARCTIC JOURNAL - REVIEW 1994 262
The roles of ozone and cloud cover. Journal of Applied Meteorology, 30(4), 478-493. Lubin, D., J.E. Frederick, C.R. Booth, T.B. Lucas, and D.A. Neuschuler. 1989. Measurements of enhanced springtime ultraviolet radiation at Palmer Station, Antarctica, Geophysical Research Letters, 16(8), 783-785. Lubin, D., B.G. Mitchell, J.E. Frederick, A.D. Alberts, C.R. Booth, T.B. Lucas, and D.A. Neuschuler. 1992. A contribution toward understanding the biospherical significance of Antarctic ozone depletion. Journal of Geophysical Research, 97(8), 7817-7828. Madronich, S. 1993. UV radiation in the natural and perturbed atomosphere. In M. Tevini (Ed.), UV-B radiation and ozone depletion: Effects on humans, animals, plants, microorganisms, and materi als. Boca Raton, Florida: Lewis Publishers.
McKinlay, A.F., and B.L. Diffey. 1987. A reference action spectrum for ultra-violet induced erythema in human skin. In W.R. Passchler and B.F.M. Bosnajakovic (Eds.), Human exposure to ultraviolet radiation: Risks and regulations. Amsterdam: Elsevier. McPeters, R. 1994 Personal communication. Setlow, R.B. 1974. The wavelengths in sunlight effective in producing skin cancer: A theoretical analysis. Proceedings of the National Academy of Science, 71(9), 3363-3366. Smith, R.C., and K.S. Baker. 1989. Stratospheric ozone, middle ultraviolet radiation and phytoplankton productivity. Oceanography, 2(2),4-10.
Smith, R., K. Baker, D. Menzies, and K. Waters. 1991. Biooptical measurements from the IceColors 90 cruise 5 Oct-21 Nov 1990 (Scripps Institution of Oceanography Reference 91-13). La Jolla, California: Scripps Institution of Oceanography. Smith, R.C., B.B. Prézelin, K.S. Baker, R.R. Bidigare, N.P. Boucher, T. Coley, D. Karentz, S. Maclntyre, H.A. Matlick, D. Menzies, M. Ondrusek, Z. Wan, and K.J. Waters. 1992. Ozone depletion: Ultraviolet radiation and phytoplankton biology in antarctic waters. Science, 256(5047), 952-959. Smith, R.C., Z. Wan, and K.S. Baker. 1992. Ozone depletion in Antarctica: Modeling its effect under clear-sky conditions. Journal of Geophysical Research, 97(C5), 7383-7397. Stamnes, K. 1993. The stratosphere as a modulator of ultraviolet radiation into the biosphere. Surveys in Geophysics, 14, 167-186. Stamnes, K., Z. un, J. Slusser, C.R. Booth, and T.B. Lucas. 1992. Several-fold enhancement of biologically effective ultraviolet radiation levels at McMurdo Station, Antarctica, during the 1990 ozone hole, Geophysical Research Letters, 19(10), 1013-1016. Stamnes, K., J. Slusser, and M. Boden. 1991. Derivation of total ozone abundance and cloud effects from spectral irradiance measurements, Applied Optics, 30(30), 4418-4426. Stamnes, K., J. Slusser, M. Bowen, C.R. Booth, and T.B. Lucas. 1990. Biologically effective ultraviolet radiation, total ozone abundance, and cloud optical depth at McMurdo Station, Antarctica, September 15, 1988 through April 15, 1989. Geophysical Research Letters, 17(12),2181-2184.
Effects of solar ultraviolet radiation on antarctic phytoplankton during springtime ozone depletion VIRGINIA E. V1LLAFAIE, E. WALTER HELBLING, and OSMUND HOLM-I-IANSEN, Polar Research Program, Scripps Institution of
Livio
Oceanography, University of California at San Diego, La Jolla, California 92093-0202 SALA, Facultad de Ciencias Naturales, Universidad Nacional de la Patagonia, Chubut, Argentina
n recent years, much attention has been given to the forI mation of the seasonal ozone "hole" over Antarctica, with the concomitant increase in ultraviolet-B [UV-B, 280-320nanometer (nm)] radiation levels (see Weiler and Penhale 1994). The enhanced UV-B radiation can be very damaging to biological systems and has been shown to cause a significant decrease in rates of primary production (Smith et al. 1992; Holm-Hansen, Helbling, and Lubin 1993). In this paper, we are concerned with describing the impact of "normal" ultraviolet radiation (UVR), as well as enhanced UV-B radiation, on natural assemblages of phytoplankton as well as on just the nanoplankton fraction [cells less than 20 micrometers (sm)] and the microplankton fraction (cells >20 tm). Our studies also included estimation of the impact of UVR as influenced by the taxonomic composition of the phytoplankton and the mitigating effect of cellular UV-absorbing compounds. All studies were carried out at Palmer Station (64.7 0 S 64.1 0 W) on Anvers Island from early October to the end of December 1993. This period provided excellent opportunities to document the impact of enhanced UV-B radiation on phytoplankton because the ozone hole was very well developed over Palmer Station in the month of October;
column ozone concentrations ranged from 140 to 220 Dobson units (DU). During the period of study, incident solar radiation was monitored continuously (and recorded every minute) using a spectroradiometer (model PUV-510; Biospherical Instruments, Inc.) with sensors for photosynthetically available radiation (PAR, 400-700 nm), and four UV wavelengths (305, 320, 340, and 380 nm). Phytoplankton samples were taken at a coastal site at 1-meter depth with a 5-liter Go-Flo bottle and were used for chlorophyll-a (chl-a) analysis, determination of absorption spectra (250-750 nm) of the particulate fraction, floristic analysis, and carbon-14 ( 14c) incorporation. Chl-a analyses were performed by fluorometric techniques (Holm-Hansen et al. 1965; Holm-Hansen and Riemann 1978). The chl-a of the nanoplankton fraction was obtained by prefiltering the sample through a nylon mesh fabric (Nitex®) with a mesh opening of 20 rim; the filtrate was treated in the same way as for total chl-a concentrations. For absorption spectra analysis, a variable amount of seawater (between 5 and 9 liters) was filtered through a Whatman GF/F filter (47-millimeter). The pigments were extracted in 10 milliliters of absolute methanol, and the extract was used
ANTARCTIC JOURNAL - REVIEW 1994 259
to run a spectrum (250 to 750 nm) with a Perkin-Elmer Lambda 6 spectrophotometer using 1- or 10-centimeter quartz cuvettes. Water samples for floristic analysis were preserved in buffered formalin, and the analyses were done by inverted microscope techniques. Phytoplankton biomass in the different taxonomic groups (diatoms, dinoflagellates, and flagellates) was calculated from the biovolumes and applying the equations of Strathmann (1967). For measurements of 14C incorporation, replicate samples of total phytoplankton and of the nanoplankton fraction were placed in 50-milliliter tubes under three different spectral treatments: • quartz tubes (cells exposed to PAR+UV-A+UV-B), • pyrex tubes covered with a Mylar sheet (PAR+UV-A), and • pyrex tubes covered with Plexiglas UF-3 (PAR). The samples were inoculated with 5 microcuries of 14C_ bicarbonate (Steemann Nielsen 1952) and exposed to solar radiation for 6-10 hours (centered on local noon) in an outdoor incubator with flowing surface seawater for temperature control. The 14C incorporation by phytoplankton was measured by standard liquid scintillation techniques. Data in figure 1 show the PlC incorporation in the total and the nano- and microplankton fractions of phytoplankton assemblages. For total phytoplankton (figure 1A), a significant difference in the assimilation numbers for the three treatments (quartz, Mylar, and Plexiglas) is evident, with higher assimilation numbers (1.16 milligrams of carbon per milligram of chl-a per hour) in the Plexiglas treatment (which received only PAR). A comparison of both fractions of phytoplankton (figure 1B) shows that the assimilation numbers in microplankton were slightly lower than those in nanoplankton, but they were not significantly different. It should be noted that there was no dif ferential decrease in chl-a concentrations between any of the treatments, so that changes in assimilation numbers reflect changes in rates of carbon dioxide fixation. A significant difference between the two fractions is evident, however, (figure 1 C when the relative enhancement of Mylar and quartz are compared. The microplankton under the Mylar treatment showed a higher relative enhancement of photosynthesis as compared to the nanoplankton, indicating a greater inhibition of photosynthesis in the microplankton when the samples are exposed to UV-B radiation. This result is consistent with previous findings by Helbling et al. (1992). The absorption spectra of methanol extracts of natural assemblages of phytoplankton showed a peak at 330 nm. When the values of the chlorophyll-specific absorption (ap*) at 330 nm are compared to the relative enhancement of photosynthesis in phytoplankton, an inverse relationship between these two variables is evident (figure 2A). Data in figure 2B show the relationship between the relative enhancement of photosynthetic rate and the percentage of biomass that is accounted for by diatoms. When diatoms dominated the phytoplankton samples, inhibition of photosynthesis was less, suggesting that diatoms have greater resistance to damage by UVR than flagellates and dinoflagellates. Such a relationship was previously discussed by Helbling, Villafafle, and Holm-Hansen (1994, pp. 207-227), who showed that diatoms have relatively high cellular concentra-
tion of UVR absorbing compounds. This result suggests that these compounds could act as a "protective" mechanism against UVR-induced damage and is in agreement with previous studies of UV-absorbing compounds by Dunlap, Chalker, and Oliver (1986). During the period from October to December, a sample of near-surface water from Arthur Harbor was taken every 2-3 days to determine the sensitivity of the natural phytoplankton assemblage to incident solar UVR in one of the standard outdoor incubators. The data show that the percentage of enhancement of photosynthesis varies a great deal, both when UV-B was selectively screened off (figure 3A) and when all UV-B and UV-A were screened off (figure 3B; note the change in scale for the ordinate). This variability is -c -C
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Mylar Plexiglas Figure 1. Effects of solar radiation on antarctic phytoplankton when different portions of the spectrum have been screened out using sharp cut-off filters: quartz—all radiation (PAR+UV-A+UV-B) received; Mylar—the UV-B portion screened out; Plexiglas—all UVR screened out. A. Assimilation numbers of total phytoplankton [in milligrams of carbon per milligram of chl-a per hour (mg C mg chi-a-' h- i )] as a function of the different treatments. B. Assimilation numbers (mg C mg chl-a- 1 h-i ) in the nanoplankton and microplankton fractions of the phytoplankton as a function of the different treatments. C. Percentage of relative enhancement as compared to the quartz treatment for nano- and microplankton under Mylar and Plexiglas.
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thought to be due primarily to changes in taxonomic composition of the phytoplankton, to photoadaptive responses related to increasing incident irradiation, and to variations in the ratio of UV-B/UV-A irradiance as influenced by changes in column ozone concentrations. During the period covered by our study, column ozone concentrations ranged from 140 DU in October to 325 DU in December. The data shown in figure 3 are presently being analyzed to gain a better understanding of the relative importance of the factors that influence the sensitivity of antarctic phytoplankton to seasonal changes in the fluence of incident UV-B radiation. Grateful acknowledgment is made to Emilio R. Marguet (Universidad Nacional de la Patagonia, Argentina), Bruce Chalker (Australian Institute of Marine Sciences, Australia), and to Antarctic Support Associates personnel at Palmer Station for their generous help during this field season. This research was supported by National Science Foundation grant OPP 92-20150. V.E. Villafañe and E.W. Heibling were at Palmer Station from 3 October 1993 to 3 January 1994; 0. Holm-Hansen and L. Sala were at the Station from 3 October to 15 November 1993.
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References
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Dunlap, W.C., B.E. Chalker, and J.K. Oliver. 1986. Bathymetric adaptations of reef-building corals at Davies Reef, Great Barrier Reef, Australia. III. UV-B absorbing compounds. Journal of Experimental Marine Biology and Ecology, 104(3), 239-248. Heibling, E.W., V. Villafane, M. Ferrario, and 0. Holm-Hansen. 1992. Impact of natural ultraviolet radiation on rates of photosynthesis and on specific marine phytoplankton species. Marine Ecology Progress Series, 80(1), 89-100. Heibling, E.W., V. Villafane, and 0. Holm-Hansen. 1994. Effects of ultraviolet radiation on antarctic marine phytoplankton photosynthesis with particular attention to the influence of mixing. In S.
10 1 15 30 45 60 75 90 Relative enhancement (%) Figure 2. Relationship between sensitivity to UVR and presence of cellular UV-absorbing compounds. A. Percentage of relative enhancement of photosynthesis for natural assemblages of phytoplankton when all UVR has been screened out (by UF-3 filter) as a function of the specific absorption (ap*) at 330 nm. B. Relationship between the percentage of phytoplankton biomass that is represented by diatoms and the percentage of relative enhancement of the phytoplankton assemblage. I
1
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Weiler and P. Penhale (Eds.), Ultraviolet radiation in Antarctica: Measurements and biological effects (Antarctic Research Series,
Vol. 62). Washington, D.C.: American Geophysical Union. Holm-Hansen, 0., E.W. Helbling, and D. Lubin. 1993. Ultraviolet radiation in Antarctica: Inhibition of primary production. Photochemistry and Photobiology, 58(4), 567-570. 40 35. E
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Figure 3. Enhancement of photosynthetic rate in experimental samples (as compared to controls in quartz tubes) as a function of the UV-B irradiance incident upon the sample tubes. A. The effect of screening off UV-B by use of a Mylar filter. B. The effect of screening off both UV-13 and UV-A radiation by use of a Plexiglas (UF-3) pre-filter. ANTARCTIC JOURNAL
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Holm-Hansen, 0., C.J. Lorenzen, R.W. Holmes, and J.D.H. Strickland. 1965. Fluorometric determination of chlorophyll. Journal de Conseil pour l'Exploration de laMer, 30(1), 3-15. Holm-Hansen, 0., and B. Riemann. 1978. Chlorophyll a determination: Improvements in methodology. OIKOS, 30(3), 438-447. Smith, R.C., B.B. Prézelin, K.S. Baker, R.R. Bidigare, N.P. Boucher, T. Coley, D. Karentz, S. Maclntyre, H.A. Matlick, D. Menzies, M. Ondrusek, Z. Wan, and K.J. Waters. 1992. Ozone depletion: Ultraviolet radiation and phytoplankton biology in antarctic waters. Science, 255(5047), 952-959.
Steemann Nielsen, E. 1952. The use of radiocarbon ( 14 C) for measuring organic production in the sea. Journal du Conseil International pour l'Exploration de laMer, 18(2), 117-140. Strathmann, R.R. 1967. Estimating the organic carbon content of phytoplankton from cell volume of plasma volume. Limnology and Oceanography, 12(3),411-418. Weiler, S., and P. Penhale (Eds.). 1994. Ultraviolet radiation in Antarctica: Measurements and biological effects (Antarctic Research Series, Vol. 62). Washington, D.C.: American Geophysical Union.
In situ inhibition of primary production due to ultraviolet radiation in Antarctica E. WALTER HELBLING, VIRGINIA E. VILLAFANE, and OSMUND HOLM-HANSEN, Polar Research Program, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093-0202
nhibition of photosynthesis due to ultraviolet radiation I (UVR) in antarctic phytoplankton has been documented by many authors (see Weiler and Penhale 1994). Most of these studies have used temperature-controlled incubators in which phytoplankton are exposed to either solar radiation or to UVvisible radiation provided by lamps. Although such experiments are invaluable for determining the effects of solar radiation on the metabolic activity of phytoplankton, they suffer from the fact that the cells will not be exposed to the same spectral irradiance that they would experience at various depths in the water column. The use of in situ incubations of natural phytoplankton assemblages provides the most direct and most realistic procedure to determine the effect of solar UVR on rates of primary production. In this paper, we report preliminary data obtained from such in situ incubations carried out from October through December 1993 at Palmer Station (64.7°S 64.1°W) on Anvers Island, Antarctica. Radiation measurements of surface solar irradiance were done using a PIJV-510 spectroradiometer (Biospherical Instruments, Inc.) and of attenuation of solar radiation in the water column [surface to 100 meters (m)] using a PUV-500 spectroradiometer. Both of these instruments have four channels for UVR [305, 320, 340, and 380 nanometers (nm)] and one channel for photosynthetic available radiation (PAR; 400 to 700 nm). The underwater unit also has sensors for depth and temperature, in addition to a sensor for 683-nm upwelling radiation. This combination of sensors allows estimation of the instantaneous rate of photosynthesis (Chamberlin et al. 1990). Data from each channel in the surface unit were recorded every minute on a 386 computer, whereas data from all chan nels in the underwater unit were displayed in real time and stored (once every second) on a laptop computer. Water samples were obtained at different depths at one station in Arthur Harbor using a 5-liter Go-Flo bottle. From each of the eight depths, replicate samples were placed in 50milliliter tubes for three different spectral treatments: quartz tubes (PAR4JV-A+UV-B),
•Pyrex tubes covered with a Mylar sheet (PAR+UV-A), and • Pyrex tubes covered with Plexiglas UF-3 (PAR). The samples were inoculated with 5 microcuries of carbon-14 bicarbonate (Steemann Nielsen 1952) and were placed on trays that were deployed at the same depths from which the samples had been obtained. The incubations lasted for 6-8 hours (centered on local noon), and the carbon-14 incorporation into the particulate fraction was measured by standard liquid scintillation techniques. Samples were also taken for chlorophyll-a (chi-a) analysis, which was done by fluorometric methods (Holm-Hansen et al. 1965; Holm-Hansen and Riemann 1978). In addition, samples were taken and fixed in buffered formalin for determination and enumeration of phytoplankton species using an inverted microscope. Representative profiles of the optical characteristics of the upper water column are given in figure 1A. The attenuation coefficients of the four UVR wavelengths were 0.25, 0.21, 0. 18, and 0.12 per meter for 305, 320, 340, and 380 rim, respectively. The 1 percent irradiance level for PAR was at about 46 m; an increase in the attenuation coefficients for both PAR and UVR at 380 nm is noticeable between 5 m and 10 m. This change in the attenuation coefficient for PAR between 5 m and 10 m is related to the increase in chl-a concentrations between the surface (approximately 0.4 milligrams of chl-a per cubic meter) and at 10 m depth (more than 3 milligrams of chl-a per cubic meter). The instantaneous rate of photosynthesis (data from the PUV-500) also showed a peak at about 10 m, with values of more than 800 nanomoles carbon per cubic meter per second (figure 1B). Below this depth, both chl-a and rates of photosynthesis decreased rapidly. The phytoplankton crop at all depths was dominated by cryptophytes. Data in figure 2 show the results from the in situ incubations for the same station mentioned in figure 1. The change of rate of carbon fixation with depth resembled the change shown by instantaneous production (determined from 683nm upwelling radiation), with maximal values at about 10 m
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