Potential Effects of Increased Ultraviolet Radiation on the Productivity

Chapter 9

Potential Effects of Increased Ultraviolet Radiation on the Productivity of the Southern Ocean 1

2

Sayed Z. El-Sayed and F. Carol Stephens 1

Department of Oceanography, Texas A&M University, College Station, TX 77843 Naval Oceanographic and Atmosphere Research Laboratory, Stennis Space Center, Bay St. Louis, MS 39529

2

A month-long study of the effects of ultraviolet radiation (UV) on phytoplankton and ice-algae collected from Arthur Harbor, Anvers Island, Antarctica, was carried out during November-December 1987. The parameters studied included: primary production rates, photosynthetic pigments and the photosynthesis-irradiance (P vs I) relation-ship. The results showed an enhancement of the photosynthetic rates when U V was excluded; conversely, production rates were lower under enhanced UV conditions. Significant changes in phytoplankton pigmentation also occurred as a result of changes in the UV levels. The implications of these findings to our understanding of the trophodynamics of the Southern Ocean and the bearing these have on global marine productivity are discussed. Composition

of

Solar

Ultraviolet

Radiation

Although solar ultraviolet (UV) radiation comprises about 8 percent of the total amount of electromagnetic radiation emitted by the sun, this relatively small amount of radiation plays a significant role in the survival of biological systems. U V radiation is generally defined as radiation of wavelengths between 190 and 400 nanometers (nm). Within this waveband three spectral regions are recognized: UV-A (320-400 nm); UV-B (280-320 nm); and U V - C (200-280 nm). U V - A , which is of lower energy, is not appreciably affected by ozone in the stratosphere and U V - C is completely absorbed before reaching the earth's surface; hence U V - B is the most biologically injurious component of sunlight reaching the earth. Electromagnetic radiation in this wavelength band can cause skin cancer in humans, has been linked to cataracts, and can suppress the immune system. Studies have also shown that U V - B is harmful to many other forms of life, from bacteria (1) to higher plants and crops (2). And recent investigations suggest that aquatic ecosystems may be especially vulnerable (3, 4). 0097-6156/92/0483-0188$06.00/0 © 1992 American Chemical Society

9. EL-SAYED & STEPHENS

UV Radiation Effects on Oceanic Productivity 189

Ozone and Ozone Depletion. Ozone, O 3 , a triatomic form of oxygen, is rare in the lower atmosphere. Small but crucial amounts in the stratosphere (altitude between 10 and 50 kilometers above the earth's surface) shield the earth from most solar ultraviolet radiation. Anthropogenic releases of chlorofluorocarbons (CFCs), nitrous oxide, bromine-containing halons, and solvents such as methyl chloroform and carbon tetrachloride are contributing to the decrease in stratospheric ozone (5, 6), resulting in an increase in the level of U V - B radiation reaching the earth's surface (7). Stratospheric ozone concentration can be viewed as a column of gas 0.32 cm high when standard temperature and pressure are applied. Recent measurements of the total ozone column from 1979/80 to 1987/88 provide evidence of ozone change over this 8-year period (8), as much as 30-40% decrease over the Antarctic in springtime. Although the drastic thinning of the ozone layer apparently began about 1976, it was not until May 1985 that scientists with the British Antarctic Survey published the results of the observations they had been making over Hallet Station, Antarctica, since 1957. It was found that the ozone level over Antarctica decreased drastically each September and October and then gradually replenished itself by the end of November (9). Scientists monitoring the annual thinning of the ozone layer over Antarctica reported that in October 1987 the ozone reached one of its lowest recorded levels ever, and the decline lasted into December, the longest period ever. In 1990, the ozone level was even lower than in 1987. At its maximum, the "hole" was the size of the United States. More alarmingly, recent data indicate that ozone depletion is occurring on a global scale, although nowhere as extensively as over Antarctica.

Effects

of

U V wavelengths

on

biological

systems.

Since

sunlight in the U V range is absorbed by proteins, damage to several basic cellular mechanisms may occur. The diversity of damaged pathways in plant cells includes damage to the Hill reaction carrier proteins, membrane damage, and damage to D N A . Although the pathways are different, all have the common property of greater damaging effect as wavelengths decrease. Consequences of Ozone Depletion. Ozone depletion over Antarctica is causing renewed concern about the consequences of increased levels of UV reaching the earth's biosphere. One area of concern involves the free-floating microscopic plants, known collectively as phytoplankton (the grass of the sea), which through the process of photosynthesis, fix carbon dioxide into living organic matter. Phytoplankton forms the basis of the marine food chain on which zooplankton (animal plankton) and all other components of the ecosystem depend for their sustenance. Numerous studies have shown that increased levels of U V affect photosynthetic activity (10-23), growth rate (24), nitrogen metabolism (25), and locomotion (26) of phytoplankton. Additionally, increases in UV-B are likely to alter community diversity as well as phytoplankton species composition. Thus, by weakening the base of the food web and altering trophodynamic relationships, UV-induced changes could potentially have far-reaching effects on the entire ecosystem.

190

THE SCIENCE OF GLOBAL CHANGE

Effects of U V Radiation on Antarctic Phytoplankton. Because ozone depletion and U V - B increases are greatest over the Antarctic, biological effects of U V on plankton in that region are particularly important. Although the effects of ultraviolet radiation on marine phytoplankton in temperate and subtropical regions had been studied by several investigators, no such studies had been carried out on Antarctic phytoplankton. In this chapter we will focus on the effects of ultraviolet radiation on Antarctic phytoplankton. We will discuss the results of an investigation which we undertook in late austral spring/early summer 1987 to study the effects of U V on these algal cells. We will then discuss the potential threat to the Antarctic ecosystem as a result of increased UV-B radiation. Although our original plans were to conduct the field experiments in the open waters of Arthur Harbor, Anvers Island, Antarctica, they were thwarted by unusually heavy sea ice, which covered almost all of the harbor during the entire period of our investigation. It was necessary, therefore, to modify our sampling strategy and to depend, for the most part, on the water pumped from a hole (cut through sea-ice four to five feet thick) to the experimental site. We also experimented with ice-algae collected in the vicinity of Palmer Station's ship dock, and with phytoplankton collected from the open waters at Bonaparte Point (Fig. 1). The

Experimental

Set-up

Our investigation was designed to examine the effects of ultraviolet radiation on the rates of carbon fixation as well as on the chlorophyll and carotenoid pigments of the algal cells. Responses of natural phytoplankton and ice-algae communities to ultraviolet radiation were assessed by measuring changes in the photosynthetic rates and pigmentation following incubation in an array of tanks and chambers. Seawater containing natural population of phytoplankton was pumped from the ice-hole into a mixing tank, from which it flowed (by gravity) into a series of 18-liter flow-through chambers. The chambers were situated within the larger treatment tanks (3 per tank), through which seawater was pumped in order to maintain a constant temperature (±0.5°C), (Fig. 2). Mixing within the chambers was accomplished with air bubbles. Samples collected from these tanks, from Bonaparte Point, and from the pack-ice were included in plastic bags placed in these chambers for measurements of l ^ C uptake. These tanks were kept outdoors under natural sunlight conditions and provided the following irradiation conditions: Treatment 1 : Treatment 2: Treatment 3: Treatment 4:

Near ambient light, including U V - A and UV-B Near ambient visible light, excluding both UV-A and UV-B (99.7% U V removed) Near ambient visible and U V - A light, but with reduced levels of solar UV-B (35% UV-B removed) Near ambient light including U V - A and U V - B plus artificially enhanced U V - A and U V - B (approximately 3% enhancement).

9. EI^SAYED & STEPHENS


Materials and Techniques. The tanks and chambers for treatments 1, 3 and 4 were constructed of 3/16-inch-thick OP-4 Plexiglas; those for treatment 2 were constructed of 1/4-inch-thick OP-2 Plexiglas. Mylar D (thickness 4 mils) was placed over the top of treatment tank 3 to reduce levels of ambient U V - B . Transmission spectra of these materials are shown in Fig. 3. Enhancement of U V - A and U V - B was achieved by placing four FS-20 Westinghouse fluorescent sunlamps underneath treatment tank 4. In order to exclude most of the radiation less than 290 nm wavelength emitted by the lamps, a sheet of 4 mils thick Kodacel (TA 401), which had been preconditioned by exposure to a sunlamp for approximately 100 hours, was placed between the sunlamps and the bottom of the enhanced UV tank. The effective biological dose (Εε; 18) at local apparent noon for each chamber was estimated by multiplying the relative biological efficiency for photoinhibition [ ε ρ ι ( λ ) ] by the available spectral irradiance [Ε(λ)], and integrating with respect to wavelength (λ, 290400 nm): Ee=J(X)«epi(X)*dX. The downward spectral irradiance data [Ed(λ), the ambient U V radiation] were generated with the atmospheric model of Frederick and Lubin (27) and ozone levels measured during the course of this study (J.E. Frederick, pers. commun.). The relative degree of U V enhancement (%ENH) for each chamber was calculated as follows: £

%ENH=( e)

CX

PfrMfrïXlOO. (Εε) amb

Photosynthetic rates were measured using the ^C-uptake method (28). For pigment analyses, the chlorophylls and carotenoids were analyzed by high performance liquid chromatography (HPLC) according to the methods outlined by Bidigare e l a l . (29). Further details of the experimental set-up and methodology used are given in (23).

Effects of U V on Phytoplankton Pigments Short-term (i.e., 4 hr) exposure of phytoplankton and ice-algae to U V radiation produced no significant changes in the concentrations of chlorophyll a, chlorophyll c and fucoxanthin. On the other hand, the 24-48 hr exposure of phytoplankton to U V radiation under ambient visible light conditions produced drastic alterations in phytoplankton pigmentation. For seawater samples collected off Bonaparte Point, there was a selective loss of chlorophyll a with increasing U V dose; however, concentrations of the other pigments showed little or no change. In another experiment, seawater was pumped from a sea-ice hole, incubated for 24 and 48 hours, and analyzed for pigment content. Significant changes in pigment concentrations were observed for all the major photosynthetic pigments including, chlorophyll a, chlorophyll c and fucoxanthin. After a 48-hour incubation period, the highest and lowest pigment concentrations were measured for

Figure 1A. Map of Arthur Harbor showing location of Palmer Station. (Reproduced with permissionfromreference 46. Copyright 1991 Cambridge.)

9.

EL·SAYED & STEPHENS

UV Radiation Effects on Oceanic Productivity

Figure IB. Map of Arthur Harbor showing location of the four sampling sites (A, B, C, and D). (Reproduced with permission from reference 46. Copyright 1991 Cambridge.)

Ambient U V Level Tank

Excluded U V - A and U V - B Level Tank

Reduced U V - B Level Tank

Enhanced U V Level Tank

per

Figure 2. Schematic drawing of the four experimental tanks and chambers used to investigate the effects of UV radiation of Antarctic phytoplankton and ice-algae. (Reproduced with permissionfromreference 23. Copyright 1990 Springer-Verlag, Berlin.)

193

194


Figure 3. Light transmission properties of (A) OP-4 plexiglas and (Β) OP-2 plexiglas. (Reproduced with permissionfromreference 23. Copyright 1990 Springer-Verlag, Berlin.)


Ol 250


1

1

350

'

'

450

>

'

«

550

1

650

1

1

750

Wavelength (nm) Figure 3. Light transmission properties of (C) OP-4 plexiglas plus Mylar D and (D) the plastic bags (Nasco Company) used in this study for the incubation of Antarctic phytoplankton. (Reproduced with permission from reference 23. Copyright 1990 Springer-Verlag, Berlin.)

196


treatment 2 (i.e., U V excluded) and treatment 4 (i.e., U V enhanced), respectively. Intermediate pigment concentrations were observed for treatment 1 (ambient UV) and treatment 3 (reduced UV). Effects

of

U V on

Photosynthetic

Rates

The average photosynthetic rates for each of the three experiments performed, increased with U V exclusion and decreased with increased UV exposure (Table I). However, primary production rates of phytoplankton (collected on 17 November and 3 December 1987) measured under near-ambient light conditions were low, but not statistically different from those measured under either UV-reduced or UV-enhanced conditions. Small but statistically significant increases in primary production rates were measured when U V radiation was excluded. Response by Ice-Algae. In contrast, primary production rates of ice algae (samples collected on 17 November and 3 December 1987) were greater when UV-B radiation was reduced as well as when all U V was excluded. As with phytoplankton, however, production rates of icealgae were not statistically different between samples incubated under ambient and enhanced UV levels. These results suggested that early in the austral springtime, phytoplankton sampled from beneath the ice or near the ice-edge was susceptible to photo-inhibition by exposure to ambient solar Photosynthetically Active Radiation (PAR). On the other hand, ice algae, which had been exposed to ambient PAR for several days prior to measurement of productivity, may have adapted to solar radiation in such a way that productivity rates were affected more by UV than by visible radiation; in other words, they had become adapted to higher levels of PAR, but not to UV-B. Ρ vs I Relationships. In our investigation we em-ployed a technique widely used by plant physiologists, namely, the study of rate of photosynthesis (P) as a function of irradiance (I). The results of one experiment in which phytoplankton were kept under the four experimental treatment conditions for 24 hours are shown in Fig. 4. It is clear from this figure that phytoplankton exposed to visible light alone (i.e., when U V was excluded) had the highest photosynthetic rates, and these rates were maintained over a higher range of irradiance levels. On the other hand, the lowest carbon fixation rates occurred when the phytoplankton were exposed to ambient and enhanced levels of UV. Discussion Potential Effects of U V on Primary Production. The results of the study carried out at Palmer Station during November-December 1987 provided insight into the potential deleterious effects of enhanced U V radiation. These results showed an enhancement of the photosynthetic rates in the tanks where U V - A and U V - B were excluded. Conversely, rates of production were much lower under ambient and enhanced U V

9.

ESSAYED & STEPHENS

Table I.

Habitat


Photosynthetic rates (mean ± standard deviation; n=3) for samples taken from three different habitats and incubated for 4 hours under the four experimental conditions described in the text (Amb = ambient U V ; Exc = U V excluded; Red = reduced U V ; Enh = enhanced U V ; and N M = not measured).

Description

Solar

Irradiance - 2

(PAR) 1

Rate

3

Treatment

1

(μΕίη m

Phytoplankton (Open-water from Bonaparte Point; 17 November 1987)

1318 1221 1260 1318

0.08 0.40 0.11 0.05

± ± ± ±

0.01 0.10 0.01 0.04

1 2 3 4

(Amb) (Exc) (Red) (Enh)

Pre-adapted Ice Algae (21 November

NM NM NM NM

7.40 42.08 17.38 4.98

± ± ± ±

1.56 0.69 4.05 1.18

1 2 3 4


1536 1423 1468 1536

0.04 0.29 0.11 0.01

± ± ± ±

0.02 0.06 0.05 0.01

1 2 3 4


1987)

Phytoplankton (Water Pumped from Sea-ice Hole; 3 December 1987)

s" )

Production

and Collection Date

(mgÇ m" h' )

Number


198

0

500

Irradiance

0

χ: °-

500

Irradiance

1000

1500

(μΕ·πι- ·8- ) 2

1

1000

1500

(μΕ·Γη- ·8- ) 2

1

Figure 4. (a) Volume-based and (B) chlorophyll-based photosynthesis-irradiance data for phytoplankton sampled on December 6, 1987 from the experimental chambers (after flow had been stopped for 24 h). Samples were incubated for 4 h under the four treatment conditions described in the text. (Reproduced with permission from reference 23. Copyright 1990 Springer-Verlag, Berlin.)



conditions. This was the case in all our experiments, whether we used the phytoplankton from under the sea-ice, ice-algae, or phytoplankton found in the open waters off Bonaparte Point. Effects on Pigmentation. As to the phytoplankton pigmentation and how it changed with exposure to varying levels of U V , significant decreases in photosynthetic pigmentation were observed for those phytoplankton samples exposed to enhanced UV radiation. As one might have expected, the magnitude of change varied in relation to the amount and duration of exposure. Reductions in rates of photosynthesis could, in part, be explained by the loss of chlorophyll a and accessory pigments. Effects on Species Composition. Other similar studies have demonstrated that the impact of increased U V radiation on algal cells is not limited to a decrease in rate of photosynthesis, change in pigment concentrations, and production of protective pigments, but also affects the species composition of the algal community. Evidence from laboratory studies by other investigators indicates a differential sensitivity of algal cells to enhanced UV levels (11, 30, 31, 32). Some algal species have been shown to be susceptible even to current levels of U V - B radiation, whereas other species showed little apparent effect even at levels several times the current dose. The possible shift in the composition of the algal population was also accompanied by change in relative proportions of large and small species. Most easily harmed by enhanced U V were the nanoplankton (organisms less than 20 μ m in diameter). Some researchers hypothesize that protozooplankton feeding on Antarctic nanoplankton and picoplankton (organisms less than 2 μ m in diameter) form a major link between the primary producers (i.e., phytoplankton) and the herbivorous zooplankton and krill (33) (Fig. 5). Recent studies carried out by the author and others have shown that the nanoplankton and picoplankton make up a substantial proportion (up to 95 percent or more) of the total biomass of phytoplankton and primary production in Antarctic waters (see 34 for review). This finding, plus the possibility that these organisms are especially sensitive to U V - B could have serious implications for the overall community structure and trophic relationships in the Antarctic marine ecosystem.

Effect

of

U V on

Water

Column

Phytoplankton.

So far we

discussed only the possible effects of U V - B radiation on surface and near-surface phytoplankton and on ice algae. But what about the phytoplankton that live well below the surface? Photosynthetic organisms are found throughout the water column, down to (and even below) the so-called euphotic depth, i.e., the depth at which only 1 percent of surface light penetrates. Although turbidity of the water may limit the penetration of light into coastal waters, in calm waters of the open ocean, 1 percent of surface UV light can penetrate to greater depths. Thus, U V may still have a damaging effect on a significant portion of the euphotic layer and on the species that move up and down in the water column. In periods of very low mixing (e.g., in calm weather or when the ice melts during the austral summer), the low salinity of the meltwater contributes to the stability of the water column, thus helping to retain phytoplankton in near-surface waters

200


TERTIARY PRODUCTION

Figure 5. Paradigm of energy flow within the Antarctic marine ecosystem. (Reproduced with permission from reference 33 by the Scientific Committee on Antarctic Research of the International Council of Scientific Unions. Copyright 1987.)



resulting in increased productivity. On the other hand, the low stability of the water prevents the phytoplankton from remaining in the optimal light zone long enough for extensive production, thus contributing to low productivity. Accordingly, the residence time of deeper water organisms in the near-surface layer (and therefore their exposure to UV-B radiation) must be considered when assessing the total effect of UV on plankton production.

Comparison

with

other

Studies.

How do the results of our

investigation compare with similar studies? Our results corroborate the data provided in a similar study of the effect of U V - B on primary productivity in the southeastern Pacific Ocean (35). In the latter study, it was noted that enhanced UV-B radiation caused significant decreases in the productivity of surface and deep samples. Compared to ambient, primary productivity decreased with increasing doses of U V - B . In another study in which in situ experiments using natural Antarctic phytoplankton populations, it was noted that incident solar radiation significantly depressed photosynthetic rates in the upper 10-15 meters of the water column (36). It was also found that the spectral region between 305 and 350 nm was responsible for approximately 75 percent of the overall inhibitory effect.

How

do

Phytoplankton

Cope

with

Enhanced

UV?

Several

investigators have reported the existence in Antarctic algae of U V absorbing mycosporine amino acids identical to those of tropical and temperate marine species (37). These compounds absorb in the U V - B region of the spectrum and may act as sunscreens which may provide some measure of protection from damaging U V - B . Plants (as well as animals), in general, have several mechanisms to repair the genetic damage caused by exposure to UV radiation. It is well known that direct mutagenic and lethal effects of U V exposure occur from damage to DNA. According to some investigators (Karentz, D. Antarctic J., in press) when DNA molecules are exposed to UV radiation, they change their structure. If the DNA molecules are not repaired, mutations in impaired cells may affect the genetic make-up of future generations. There are three known cellular repair mechanisms: photoreactivation, excision repair and recom-bination repair. Of these the former is the primary means of correcting UV-induced DNA damage in bacteria and phytoplankton (Karentz, ibid.). While some organisms have only one DNA repair mechanism, others may have two or three. The question now is, how effective are these pathways in Antarctic organisms? Only future research will tell. However, laboratory experiments on one phytoplankton species suggest an inability to adapt to increasing U V - B radiation (Behrenfeld and Hardy, in review).

Effects

of

U V on

Phytoplankton-A

Cautionary

Note.

Despite

the preliminary nature of the results of the Palmer Station experiments, there seems to be sufficient evidence to suggest that elevated U V - B radiation can decrease the photosynthetic rates of Antarctic phytoplankton and ice-algae, and bring about a pigmentation change in algal cells (38, 39). However, we do not yet know whether these findings can be applied to the phytoplankton and ice-algae of the whole Southern Ocean. Considerable caution must be exercised in extrapolating the results of these short-term laboratory/field

202


experiments to the real world. In the latter one has to contend with a host of complex, interrelated factors, among which are: the depth at which U V is attenuated; the depth of the water column mixed by the wind; the residence time of the algal cells in the euphotic zone; seasonality of U V exposure; behavioral response of the targeted organisms to solar U V and their ability to repair damaged D N A molecules. As to whether the effect of U V - B on algal populations is short-lived, long-lasting, or reversible, our limited data do not provide a final answer. Here again, only future research will provide the data needed to answer this question.

Effect of U V on Productivity of the Southern Ocean. Has ozone depletion over Antarctica affected the productivity of the Southern Ocean? There is no easy answer. First, one has to take into account the fact that the drastic decrease of ozone over Antarctica has been reported as recently as 1976, a relatively short time in the evolution of the organisms to develop mechanisms to cope with elevated UV. One of the most vexing problems in studying the effects of U V radiation on productivity, is a dearth of historical data on the level of UV. Without these baselines, normal fluctuations could easily be interpreted as decline in productivity. Second, there is a host of biotic and abiotic factors that play significant roles in governing the productivity of the Southern Ocean (40). Ultraviolet radiation is but one more complicating factor to be considered in an already stressful environment. Effect of U V on Global Productivity. As to the long-term effects of enhanced U V radiation on the Antarctic marine ecosystem, based on the Arthur Harbor findings and given the long residence time of anthropogenic atmospheric pollutants; e.g. chlorofluorocarbons (estimated to be between 20 and 380 years!), the impact of elevated UV-B could be long-lasting and potentially damaging. Since phytoplankton are the basic primary producers in the Antarctic Ocean on which all other components of the ecosystem (zooplankton, krill, fish, squid, winged birds, penguins, seals, and whales) depend for their livelihood (Fig. 6), any substantial decrease in the productivity of these waters, or any change in their community structure, could have far-reaching ecological implications. Although in this chapter we have focused on the potential effects of increased U V - B radiation on the Antarctic marine ecosystem, our results also have bearing on efforts to describe the effects of U V radiation on global marine productivity. However, here again, considerable uncertainties still remain in assessing the effects of ozone depletion on global production. Several authors have predicted a significant decrease in primary production over a wide latitudinal area in response to predicted increase in U V - B reaching the earth's surface. This decrease would likely affect higher trophic levels and would very likely decrease fisheries yield (41, 42). For instance, it has been estimated that a 5 percent reduction in primary productivity would lead to a 6 to 9 percent reduction in fisheries catch (43). Another farreaching ecological implication is that any substantial decrease in marine primary production would decrease the ocean's capacity as a sink for anthropogenic C 0 2 (44), thus exacerbating the global greenhouse warming.



KRILL

BLUE WHALE

CRABEATER SEAL

WINGED BIRDS ADÉLIE PENGUIN

SMALL FISHES SQUID

SKUA

A

EMPEROR PENGUIN

LARGE FISHES

WEDDELL SEAL

ROSS SEAL

Figure 6. Food web in the Antarctic marine ecosystem showing the key position occupied by krill. (Reproduced with permission from reference 45. Copyright 1962 W. H. Freeman and Company.)

204


Concluding

Remarks

•

The Antarctic ozone hole is the result of anthropogenic release of trace gases into the atmosphere (CFCs in particular), causing a decrease in stratospheric ozone and a subsequent increase in solar ultraviolet radiation reaching the earth's surface.

•

The ozone hole has been recurring now for over a decade and we can expect this phenomenon to continue in varying magnitude as an annual event.

•

Increased U V - B radiation decreased the and changed pigment concentration.

•

Evidence exists of enhanced U V levels.

differential

•

Considerable caution results of the Palmer the Southern Ocean), ocean. The results treated as preliminary

must be exercised in extrapolating the Station experiments to the real world (i.e., as tanks and chambers do not mimic the of our investigation, therefore, should be and suggestive only.

•

Further research is needed to address questions/issues related to (a) latitudinal effects of U V - B radiation, i.e., will the damage be largely confined to the polar regions?, (b) effects of U V - B on the chemical concentration of vital nutrients such as vitamins and trace metals, (c) what are the effects of lethal and sublethal responses to U V - B and the reasons for differences in species response, (d) quantification U V exposure, and (e) UVphotobiology of individual species.

rate

sensitivity

of

of

photosynthesis

algal

cells

to

Acknowledgments The contributions of Dr. R. R. Bidigare and M . E . Ondrusek to all aspects of the project, especially pigment analyses and calculations of U V doses, are gratefully acknowledged. This work was supported by the U.S. National Science Foundation, Division of Polar Programs (DPP-8812972).

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Potential Effects of Increased Ultraviolet Radiation on the Productivity

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