Ultraviolet LightC E Williamson, Miami University, Oxford, OH, USAP J Neale, Smithsonian Environmental Research Center, Edgewater, MD, USA

ã 2009 Elsevier Inc. All rights reserved.

Introduction

Ultraviolet (UV) is short-wavelength, high-energysolar radiation that plays multiple roles in inlandwaters. Although UV is mutagenic, carcinogenic,and can cause a variety of other damage in livingorganisms, small amounts of UV may be beneficial.For example, a small amount of UV at potentiallydamaging wavelengths, less than 315 nm, is neces-sary for the synthesis of vitamin D, while longerwavelength UV activates DNA repair enzymes andserves as an environmental cue in mate selection andforaging behavior in fish as well as in the verticalmigration of zooplankton.The important role of UV in inland aquatic ecosys-

tems went largely unrecognized for many yearsbecause it was believed that incident solar UV wasrapidly absorbed within the surface waters. Althougha few early studies demonstrated that UV penetratedmany meters into the water column of highly trans-parent lakes, these studies were unappreciated. In-strumentation for measuring underwater UV was notwidely available for many years and few measure-ments were made of UVattenuation in inland waters.This all changed following the discovery of severeozone depletion in Antarctica (the ‘ozone hole’) inthe mid-1980s and subsequent evidence for ozonedepletion and elevated UV exposure in the Arctic andeven temperate latitudes. Submersible UVradiometersbecame more widely available to study this disturbingglobal UV trend and recognition of the importance ofUV in inland waters has increased in recent years.Although ozone is the major UV-absorbing sub-

stance in the atmosphere, colored or chromophoricdissolved organic matter (CDOM) is the primary UV-absorbing substance in most inland waters. Variationsin CDOM concentrations in inland waters contributefar more to the regulation of underwater UV radiationthan variations in atmospheric ozone. Climate warm-ing and other disturbances can in turn alter concen-trations and quality of CDOM in inland waters andcontribute to strong temporal and spatial gradientsin underwater UV exposure. It is these strong anddynamic gradients in underwater UV exposure andthe sensitivity of UV transparency to environmentalchange that drive the need to understand the causesand consequences of UVexposure in inland waters.

Wavelengths and Units of Measurement

UV radiation can be separated according to wave-lengths into UV-C (200–280nm), UV-B (280–320nm),and UV-A (320–400nm). The Commission Interna-tionale de l’Eclairage (CIE; International Commissionon Illumination) uses 280nm to separate UV-Cand UV-B, 315nm to separate UV-B and UV-A, and400nm to separate UV-A and photosynthetically activeradiation (PAR). Some investigators use 290 and320nm as breakpoints. Here, we adopt the 280and 320nm wavelengths to separate out the types ofUV, as these are more commonly used in aquatic stud-ies. Although PAR is most commonly measured inquantum units (moles of photons), UV is more com-monly measured in energetic units. Radiant flux ofUV is measured in Watts (J s�1), radiant exposure(dose or fluence) in J m�2, and irradiance (dose rate,fluence rate, or radiant flux density) in W m�2.

Atmospheric Controls on Incident UV inInland Waters

The amount of solar UV radiation incident on inlandwaters is influenced by sun angle, elevation, cloudcover, and atmospheric aerosol and ozone concentra-tions. Sun angle (usually measured from the zenith,directly overhead) is the most important determinantof UV exposure. The slant path through the atmos-phere is a function of zenith angle. Longer pathsincrease atmospheric absorption and scattering, andat higher angles the incident beam is projected over alarger area. Changes in sun position drive the diurnaland seasonal variation in UV (Figure 1) and most ofthe UV variation with latitude. Aerosols and cloudsare the most important factors affecting the day-to-day variation in UV (Figure 1), primarily by scatteringradiation back to space. Aerosols scatter the shorterUV wavelengths more efficiently than visible, so UVcan be substantially lowered by ‘hazy’ conditionseven under apparently ‘sunny’ conditions. Undermost conditions, aerosol and molecular (Rayleigh)scattering divert so many UV photons from a straightpath (direct beam) that UVexposure is primarily fromsky (diffuse) light. The ratio of diffuse to direct lightincreases with shorter wavelengths.

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Figure 1 Example plots for diurnal and annual variation in total UV (290–400nm) for summer and winter solstices and equinoxes

(fall shown, spring is similar) at a temperate location (Edgewater, Maryland, USA). The diurnal curves show maximum (clear sky)

irradiance (W m�2) calculated using a radiative transfer model. The annual curve is for total daily UV (kJ m�2) as measured at theSmithsonian Environmental Research Center (Edgewater, Maryland, USA) for 2003. The annual range for total daily UV is a factor of 3,

a similar range of variation occurs day-to-day due to variations in cloud cover, aerosols, and ozone.

706 Light and Heat in Aquatic Ecosystems _ Ultraviolet Light

Clouds are less wavelength selective than aerosols,but can still have different effects on UV than visiblelight. This is another consequence of the higher diffuse/direct ratio in UV: clouds blocking the solar disc, butnot the sky, have only a small effect on UV. Incidentirradiance can actually exceed ‘clear sky’ levels ifhigh clouds away from the sun act as reflectors.Uniform cloudiness, on the other hand, has about thesame effect onUVas visible light, usually>50%overallreduction.Ozone absorbance is the most wavelength depen-

dent of all atmospheric controls on UV, absorbingessentially all UV wavelengths shorter than about290 nm and having little effect for wavelengths longerthan 330 nm. Because of ozone, only UV-A and UV-Bcomponents impinge on inland waters. Total columnozone in the atmosphere varies across the Earth’s sur-face and with season, independent of any of the ozonedepletion events reported in recent decades. Columnozone generally varies from about 240 Dobson units(DU) at the equator to over 440 DU near the NorthPole. The seasonal variation in ozone is minimal at theequator with a range between 240 and 260DU. At theNorth Pole, ozone ranges more widely from 280 DUin September to over 440 DU in March. At the SouthPole, the seasonal variation in the absence of the ozonehole ranges from less than 300DU in the austral fall toover 340 DU in the austral spring.The combination of a sun angle closer to the zenith

and the reduced ozone at the equator leads inlandwaters in tropical regions to be exposed tomuch higherUV levels than temperate, boreal, or polar systems.A major exception to this is in regions with severeozone depletion. For example, theWorld HealthOrga-nization uses an erythemalUV index that ranges from1to18 to reflect potential biological damage (sunburn in

this case). This UV index generally ranges from 4 to 10in Temperate Zones and from 10 to 16 in the tropics.During ozone hole events, total column ozone inAntarctica can be depleted to levels less than 100 DU.Although UV indices around Antarctica are typicallylow at all times due to the southerly location, itincreases from 1–3 to 4–6 in coastal locations underozone-depleted airmasses and values as high as 12 havebeen observed.

The potential for increased exposure to damagingUV as a consequence of ozone depletion inspiredthe Vienna Convention for the Protection of theOzone Layer in 1985 and the Montreal Protocol onSubstances that Deplete the Ozone Layer in 1987.These international agreements were successful inreducing the levels of anthropogenic ozone-depletingsubstances in the atmosphere and subsequently therate of stratospheric ozone depletion. However, as of2006, current concentrations of chlorine and brominein the stratosphere are still adequate to catalyzedepletion of all of the ozone in the critical 14–21 kmaltitude layer of the polar vortex, and the Antarcticozone mass deficit in 2006 was greater than in anypreceding year, exceeding the previous records in2000 and 2003. Stratospheric ozone concentrationsare not expected to recover to the levels observed inthe 1970s until the middle of the 21st century.

Environmental Regulation of UnderwaterUV Exposure and Impacts

Effects of CDOM on UV Transparency

UV transparency can vary greatly among inlandwater bodies, with the 1% attenuation depths

Light and Heat in Aquatic Ecosystems _ Ultraviolet Light 707

ranging from a fraction of a meter to well over 10m(Figure 2). This variation is largely a function of dis-solved organic carbon (DOC), the optically activecomponent of which is CDOM (Figure 3). Environ-mental change can similarly alter UV transparency

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Figure 2 Depth profiles for five selected lakes showing the

wide variation in UV attenuation in inland waters. The intersectionwith the vertical axis shows the depth to which 1% of the

subsurface 320nm UV penetrates, referred to as the 1%

attenuation depth (Z1%). Wavelengths of 320nm are among the

most highly damaging solar radiation when you combine thesolar spectrum with a biological weighting function to get the

biologically effective exposure (see Figure 9).

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Figure 3 Variation in UV attenuation among lakes as a functionof DOC concentration. The depth to which 1% of 320 nm UV

penetrates as estimated from diffuse attenuation coefficients is

shown for a series of glacial lakes from the northern and southernhemispheres. Each data point represents one lake. Note the

strong inflection point below about 100 mM DOC, indicating the

high sensitivity of these low DOC lakes to small changes in DOC

inputs. Modified from Figure 1 in Williamson CE, Stemberger RS,Morris DP, Frost TM, and Paulsen SG (1996) Ultraviolet radiation

in North American Lakes: Attenuation estimates from DOC

measurements and implications for plankton communities.

Limnology and Oceanography 41: 1024–1034, with permissionfrom the American Society of Limnology and Oceanography.

within a single water body, leading to strong interan-nual trends of change in UV transparency (Figure 4).

A wide variety of dissolved and particulate com-pounds contribute to the UV transparency and henceunderwater UV exposure levels in inland waters. Themost important of these is CDOM. Similar to ozone,CDOM selectively absorbs shorter wavelengths ofsolar radiation, leading to more rapid attenuation ofshorter wavelengths in the water column (Figure 5).

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Figure 4 Trends of changing UV transparency in an easternPennsylvania, USA lake over time. The changes in the 1% 320nm

UV attenuation depth (estimated from epilimnetic diffuse

attenuation coefficient) in Lake Giles have been plotted overthe past 14 years. Understanding the ecological drivers behind

these types of changes in UV transparency over time is a major

area for future research.

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Figure 5 Attenuation of 305, 320, and 380nm UV and PAR in

Lake Giles, northeastern Pennsylvania, USA showing the

wavelength-dependent attenuation with depth in the watercolumn that is characteristic of most lakes. In lakes with

extremely low concentrations of CDOM, such as Lake Tahoe, the

380 nm UV may actually attenuate less rapidly than longer

wavelength PAR due to the peak transparency of pure water inthe short-wavelength blue which is close in wavelength to UV-A.

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Figure 6 Seasonal variation in UV transparency in Lake Giles, a

low DOC lake in northeastern Pennsylvania, USA. These

seasonal changes are largely driven by photobleaching, withminimal change in DOC concentrations. Values are expressed as

the 1% attenuation depth for 320 nm UV from epilimnetic diffuse

attenuation coefficients. Note the change in the 1% attenuation

depth from 3m in April to over 15m in mid-July. Actual depths towhich UV penetratesmay be somewhat less when UV penetrates

into the hypolimnion where UV transparency may be lower.

Modified from Figure 4 in Williamson CE, Hargreaves BR, Orr PS,and Lovera PA (1999) Does UV play a role in changes in predation

and zooplankton community structure in acidified lakes?

Limnology and Oceanography 44: 774–783, with permission from

the American Society of Limnology and Oceanography.

708 Light and Heat in Aquatic Ecosystems _ Ultraviolet Light

CDOM also plays a major role in determining thedepth of the surface mixed layer in small lakes andponds which will in turn influence UV exposure ofnonmotile planktonic organisms in this stratum. Dur-ing winter ice formation, CDOM is excluded suchthat ice with few air bubbles is more UV transparentthan unfrozen water. Air bubbles or snow cover dra-matically decrease transmission of UV by ice. In lakeswith anoxic hypolimnia, the oxidation of ferrous ironto ferric iron may cause substantial increases in UVabsorbance. Dissolved organic matter (DOM) cancomplex with the iron and increase its retention inthe surface waters, thus decreasing UV transparency.CDOM in inland waters may come from autochtho-

nous sources such as algae or macrophytes, but theprimary source is generally allochthonous sourcesderived from the leaching and decomposition of terres-trial vegetation within the surrounding watershed.Allochthonous CDOM is generally composed of morerefractory, higher molecular weight compounds thatare more highly UV-absorbing and thus more photo-labile. Autochthonous CDOM on the other hand iscomposed of smaller molecular weight compoundsthat are less UV absorbing and correspondingly lessphotolabile but more biolabile. When exposed tosolar UV radiation, CDOM undergoes important pho-tochemical changes that involve photolysis and photo-bleaching (seeColor of Aquatic Ecosystems), renderingit less UV absorbing and more aliphatic. Photobleach-ing of CDOM in combination with other seasonalchanges in hydrologic inputs and biota can substan-tially increase the UV transparency of inland watersduring the summer months (Figure 6). In endorheichypersaline lakes, the long residence time of CDOMleads it to be strongly photobleached andmuch less UVabsorbing.

Effects of Elevation, Plankton, and Particulates

on UV Transparency

Living organisms in alpine waters are exposed to par-ticularly high levels of UV. Shorter wavelength UV-B(300 nm)may increase by asmuch as 24%per 1000min elevation due to the shorter pathlength through theatmosphere. The lowdensities of terrestrial vegetationin alpine ecosystems also reduce the inputs of UV-absorbing CDOM into alpine waters (Figure 7). Thetiming of ice-out in alpine environments is often closeto summer solstice, thus exposing aquatic organismsto maximum UV levels at very low temperatures thatslow down DNA repair enzymes.In extremely low DOC alpine systems and in higher

DOC hypereutrophic, algal-rich systems phytoplank-ton may play a role in regulating UV transparency.Although zooplankton are known to increase visible

transparency in lakes during seasonal ‘clear-waterphases’ through their grazing on phytoplankton, thecontribution of zooplankton to seasonal increases inUV transparency is small relative to photobleaching.In highly turbid systems, suspended particulates mayreduce UV transparency but this effect is generallyless than the effects of dissolved substances.

Mechanisms of UV Damage in AquaticOrganisms

UV can cause substantial damage to a wide variety ofaquatic organisms ranging from viruses, bacteria, andphytoplankton to zooplankton, fish, and amphibians.Direct and indirect mechanisms of UV damage influ-ence critical cellular processes in aquatic organismsranging from DNA replication and transcription tocell survival, growth, motility, photosynthesis, orienta-tion, and nutrient uptake. The relative importance ofoverall exposure versus irradiance (often referred toimprecisely as ‘dose’ and ‘dose rate,’ respectively, seeglossary) in causing UV damage is a major concern.The reciprocity principal argues that damage is a func-tion of total dose and independent of dose rate. How-ever, the presence of active repair processes generallyinvalidates the reciprocity principle because repair per-mits high survival rates over long periods of low expo-sure. Short-term periods of exposure to high dose rates

Figure 7 Watersheds of alpine Lake Alta in New Zealand with very little vegetation (left) and temperate lowland Lake Lacawac innortheastern USA with abundant vegetation (right). The amount of vegetation in the surrounding watershed is a key regulator of

variations in UV transparency among inland waters.

Light and Heat in Aquatic Ecosystems _ Ultraviolet Light 709

of UVoften have a much more severe effect on organ-ism survival (‘one bad day’ hypothesis).

Primary Mechanisms and Targets of UV Damage

There are two primary mechanisms of UV damage toliving organisms at the subcellular level: damage bydirect absorption of UV by a molecule or complex,and damage through the production of reactive oxy-gen species (ROS). Biomolecules that absorb theshorter wavelength, higher energy UV are most sus-ceptible to direct damage. DNA and RNA are the twomost highly UV-absorbing biomolecules, and due toits lower turnover rate in the cell, DNA is consideredthe primary target of UV damage. The primacy ofDNA damage has given rise to the use of DNA dosi-meters to assess DNA damage in the environment.These are small quartz tubes with DNA solutions thatprovide an integrated assessment of potential DNAdamage to aquatic organisms by examining the UV-induced photoproducts in the absence of repair pro-cesses. The primary photoproduct in UV-irradiatedDNA is the cyclobutane pyrimidine dimer (CPD),followed by the pyrimidine(6–4)pyrimidone dimer[(6–4)PD]. Although CPDs are the most abundantphotoproduct (up to 80–90% of total photoprod-ucts), the (6–4)PD photoproducts may be as muchas 300 times more cytotoxic than the CPDs due totheir effectiveness in blocking DNA polymerase.The second major type of UV damage results from

the production of ROS including hydrogen peroxide,singlet oxygen, and hydroxyl and superoxide radi-cals. The lower energy, longer wavelengths of UVare largely responsible for ROS production both inorganisms and the surrounding water. Damage from

ROS may be at sites within the organism that areremote from the site of UV exposure. The short half-life of most UV-induced ROS prevents them fromaccumulating in inland waters. The exception ishydrogen peroxide with a half-life on the order of afew hours to a day.

UV Damage to Proteins and Lipids

UV can influence protein metabolism through inhibi-tion of nitrogen uptake and thus the rate of proteinsynthesis. The absorption of UV by aromatic aminoacids makes many proteins susceptible to UV damageincluding nitrogenase, the enzyme responsible fornitrogen fixation. Structural proteins can be damagedby UV due to the disruption of covalent sulfur bondsthat are important in tertiary protein structure. UVdamage of proteins in the lens of the eye can lead tocataracts in aquatic organisms. Damage to proteins isalso thought to be the major reason for inhibition ofphotosynthesis by UV through largely UV-A damageto photosystem II and the RUBISCO pathway. Thedouble bonds in many lipids also absorb UV andcombined with lipid peroxidation can damage lipid-rich membranes.

Importance of Spectral Composition

The division of UV into the three bands gives arough indication of biological threat – increasingfrom A to C. The biological effect varies almost con-tinuously with wavelength for several reasons. Theenergy of a photon is inversely proportional to itswavelength (Planck’s Law). Thus, UV-C has the high-est energy and the greatest effect per unit exposure

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710 Light and Heat in Aquatic Ecosystems _ Ultraviolet Light

whereas UV-A has the least. The effectiveness ofshort-wavelength UV, however, increases more rapidlywith decreasing wavelength than the increase in pho-ton energy because biomolecules have higher absor-bance at shorter wavelengths. Thus, effectivenesstypically rises exponentially with decreasing wave-length, with the slope and shape varying for differenteffects (Figure 8). Processes driven by DNA damagehave a characteristically different shape than whendamage is (also) induced by production of ROS. Effec-tiveness curves are termed biological weighting func-tions (BWFs) and must be taken into account whencomparing responses to exposures that differ in spec-tral composition, such as experiments with artificiallamps versus natural solar exposures (Figure 9).

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Daphnia mortality

Figure 8 Biological weighting functions (BWFs) for the effect ofUV on: inhibition of photosynthesis, average for natural

populations of phytoplankton; DNA damage in alfalfa seedlings;

mortality of Daphnia; and photochemical weighting function for

production of hydrogen peroxide (ROS that contribute to CDOMphotobleaching). Note the shape is different when DNA damage

is important versus when ROS are important. The weights have

been normalized to 1 at 300 nm. Data sources are Neale PJ and

DJ Kieber (2000) Assessing biological and chemical effects of UVin the marine environment: Spectral weighting functions. In:

Hester RE and Harrison RM (eds.) Causes and Environmental

Implications of Increased U.V.-B. Radiation. Issues inEnvironmental Science and Technology, pp. 61–83. Royal

Society of Chemistry; Quaite FE, Sutherland BM, and Sutherland

JC (1992) Action spectrum for DNA damage in alfalfa lowers

predicted impact of ozone depletion. Nature 358: 576–578,weights rescaled as effect per unit energy; Williamson CE, Neale

PJ, Grad G, De Lange HJ, and Hargreaves BR (2001) Beneficial

and detrimental effects of UV radiation: Implications of variation

in the spectral composition of environmental radiation for aquaticorganisms. Ecological Applications 11: 1843–1857.

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(b) 300 320 340 360 380 400Wavelength (nm)

300 320 340 360 380 400Wavelength (nm)

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Figure 9 Spectral distribution of UV (280–400nm) in a UV

incubator (Phototron) using fluorescent lamps (combination of

Spectronics XX15-B and UVA-340, Q-Panel) and the average

spectral irradiance in a shallow water column (1.5 m deep)(Rhode River, MD, USA, near summer solstice, noon, 40� N). (a)unweighted radiation; (b) radiation weighted by the BWF for

inhibition of photosynthesis in natural phytoplankton

populations; and (c) radiation weighted by the BWF for DNAdamage in alfalfa. The BWFs are shown in Figure 8. Total(280–400nm) UV is the same for the unweighted spectra, but the

lamps emit a greater proportion of more damaging UV-B, so thatthe weighted irradiance in the incubator is >3 times higher for

inhibition of photosynthesis and more >11 times higher for DNA

damage compared to water column irradiance. Source for

spectra: Rhode River, Litchman E, and Neale PJ (2005) UVEffects on photosynthesis, growth and acclimation of an

estuarine diatom and cryptomonad. Marine Ecology Progress

Series 300: 53–62. Phototron: Williamson CE, Neale PJ, Grad G,

De Lange HJ, and Hargreaves BR (2001) Beneficial anddetrimental effects of UV radiation: Implications of variation in the

spectral composition of environmental radiation for aquatic

organisms. Ecological Applications 11: 1843–1857.

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Light and Heat in Aquatic Ecosystems _ Ultraviolet Light 711

Mechanisms to Reduce UV Damage

There are three possible levels of response of aquaticorganisms to reduce UV damage in nature: behavioralavoidance, photoprotection by sunscreens or antiox-idants, and molecular repair following the damage.

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Figure 10 Spawning depth of yellow perch (Perca flavescens)

in two lakes in the same geographic area, one of high (Giles) and

one of low (Lacawac) UV transparency. Note that all eggs in LakeGiles were spawned deeper than 2m, while in Lake Lacawac all

eggs were spawned at depths shallower than 2m. It is not known

whether the deeper spawning in the high UV lake is a result of UVavoidance behavior or differential survival coupled with strong

fidelity to spawn at natal sites. Deposition of eggs in the deeper,

colder waters may slow development and hatching and

consequently reduce the time available for perch to reach thecritical size needed to survive the following winter. Modified from

Figure 3 in Huff DD, Grad GG, and Williamson CE (2004)

Environmental constraints on spawning depth of yellow perch:

The roles of low temperature and high solar ultraviolet radiation.Transactions of the American Fisheries Society 133: 718–726,

with permission from the American Fisheries Society.

Behavioral Avoidance of UV Damage

Behavioral avoidance of UV requires the ability todetect UV (photoreceptors sensitive to UV) as well asthe ability to avoid UV (motility). A wide range ofaquatic organisms do have UV photoreceptors includ-ing some bacteria, phytoplankton, protozoa, annelids,mollusks, crustaceans, and fish.Negative phototaxis tolonger wavelength PAR may also be effective in UVavoidance. However, the selective absorption of theshort-wavelength damaging UV-B irradiation byozone and DOM create wide variations in UV:PARratios. The disproportionate increase in damagingUV-B with decreasing wavelength may lead to UVdamage under scenarios of ozone depletion or reducedDOM if organisms cannot detect and avoid UV (‘solarambush’ hypothesis). Exposure to UV-B can also dam-age both motility and orientation behavior, renderingany potential phototactic avoidance less effective.Several species of zooplankton and fish can detect

and avoid UV.Daphnia have UV photoreceptors withpeak absorption at 348 nm and can avoid damagingUV in situ by downward migration. Some species oflarval fish have photoreceptors with absorption peaksin the UV-A range. Laboratory studies have demon-strated that these UV-A photoreceptors may enhancefeeding in larval fish, and field experiments havedemonstrated the ability of juvenile Coho salmon(Oncorhynchus kisutch) to avoid solar UV in shallowstreams. Yellow perch (Perca flavescens) spawndeeper in more UV transparent lakes, but it is notclear that this is a behavioral response (Figure 10).

Protection against UV damage: Sunscreens and

defense compounds

There are two major types of UV photoprotectivecompounds: sunscreens that absorb UV radiationdirectly and antioxidants that reduce damage fromROS. Some compounds serve both functions. Sun-screens are less effective in very small organisms,such as bacteria, where the pathlength of a UV photonis too short to be effectively absorbed before causingdamage. Photoprotective compounds present inaquatic organisms include carotenoids, mycosporine-like amino acids (MAAs), and melanin. Carotenoidshave absorption peaks in the short-wavelength blue

range but this absorption extends into the UV, thusallowing absorption and dissipation of UV energy asheat. Carotenoids are also important quenchingagents that react with ROS to reduce levels of cellulardamage. Environmental concentrations of carote-noids are not consistently correlated with UV expo-sure, and plant-derived carotenoids in zooplanktonmay actually reach their highest concentrations incolder waters under nonpeak UV conditions.

MAAs are photoprotective compoundswith absorp-tion peaks generally in the 310–360nm range; withsome also functioning as antioxidants. The MAAsderive their name from the fungi in which they wereinitially discovered to be abundant during sporulation.Fungi, bacteria, and algae are the only groups that cansynthesize MAAs because they have the shikimatepathway, a biochemical pathway related to the path-way used to produce photoprotective flavonoidcompounds in higher plants. Animals must acquireMAAs through their diet. The production of MAAsis photoinducible and MAA concentrations in bothphytoplankton and zooplankton are known to becorrelated with environmental UV exposure.

712 Light and Heat in Aquatic Ecosystems _ Ultraviolet Light

Among the crustacean zooplankton, cladoceransgenerally have low concentrations of carotenoidsand MAAs but a few species do utilize melanin.Copepods have little melanin but depend heavily oncarotenoids and MAAs. Melanin is often lacking inepidermal layers in fish, but is common in the epider-mis of many amphibians, particularly in early devel-opmental stages, such as eggs and larvae. Some fishsecrete UV-absorbing compounds in their mucuswhile others have UV-reflective guanine-based com-pounds in their scales.Several common antioxidant enzymes and vitamins

in marine algae are capable of reducing oxidativedamage from ROS produced during UV exposure,but little is known about how widespread this processis in other aquatic organisms. These compoundsinclude superoxide dismutase (SOD), glutathione per-oxidase, ascorbate peroxidase, catalase, vitamin E(a-tocopherol), and vitamin C (ascorbic acid).

Molecular Repair of UV Damage

UV-induced DNA lesions can be repaired by a varietyof molecular processes including photoenzymaticrepair (PER, also referred to as photorepair or photo-reactivation), nucleotide excision repair (NER), andbase excision repair (BER). PER is mediated by asingle enzyme, photolyase, which requires UV-A orvisible light for activation. Thus, PER is often referredto as ‘light repair.’ PER is specific to damage inducedby UVand is present in a wide range of organisms butnot all species. NER and BER on the other hand arenot specific to UV damage and are universal amongall organisms. NER is often referred to as ‘darkrepair’ because it does not require UV-A or visiblelight but rather depends on the metabolic energy ofATP. In bacteria, the recA gene codes for a proteinthat promotes DNA recombinational repair. Expo-sure to UV transforms the recA-derived protein intoa proteinase that destroys a repressor and subse-quently enables the expression of a suite of DNArepair genes. Because these DNA repair processesare enzyme dependent, they are often less effectiveat lower environmental temperatures.

Interactive and Indirect Effects of UV inAquatic Ecosystems

Perhaps the most compelling role of UV in inlandwaters is not the potential for direct damage, buthow it interacts with other important limnologicaland ecological stressors across gradients of UV

transparency. Organisms that respond to UV bybehavioral avoidance may be forced into habitatswhere temperature, food, or potential predators aresuboptimal. For example, migration by zooplanktonor egg deposition by amphibians and fish into deeper,cooler waters may reduce growth and reproduction.

Variation in UV tolerance among organisms at dif-ferent trophic levels may alter aquatic food webs. Forexample, UV exposure can inhibit uptake of criticalnutrients such as N and P as well as alter the concen-trations and quality of pigments, proteins, lipids, andfatty acids in phytoplankton with important conse-quences for their nutritional quality to zooplanktongrazers. Zooplankton that respond to UV by increas-ing their concentrations of photoprotective com-pounds may increase their susceptibility to visualpredators. While bacteria in surface waters can sufferdirect damage from UV, the generation of biolabileDOM by UV photolysis may enhance bacterialproductivity.

UVexposure of a variety of anthropogenic pollutantssuch as polycyclic aromatic hydrocarbons (PAHs) canenhance their toxicity. The primary mechanism of thisphototoxicity is the production of ROS by the photo-sentitizer (PAH or other), with subsequent biologicaldamage such as lipid peroxidation by the ROS. Thephototoxicity of PAHs such as pyrene, fluoranthene,and anthracene is driven much more by UV-A than byUV-B due to a combination of greater absorption byPAHs in the UV-Awavelengths, the greater number ofUV-A versus UV-B photons present in incident solarradiation, and the more rapid attenuation of UV-Bversus UV-A in the water column. Due to the role ofROS, oxygen concentrations may alter phototoxicity.Diel variation in metal speciation and photodegrada-tion of compounds such as methyl mercury suggestthat UV may also play a role in the phototoxicityof heavy metals. Photoxicity may also be ameliorat-ed by complexation of contaminants with DOM, orenhanced byUV photolysis of CDOMand subsequentrelease of toxic compounds.

Climate change can alter the importance of UV ininland waters by modifying cloud cover, aerosols,stratospheric ozone, and CDOM inputs from the sur-rounding watershed. Decreases in precipitation com-bined with increased temperatures reduce thesaturation and inundation of soils, resulting in betteroxygenation of soils andmore complete decompositionof dead organic matter to inorganic C rather thanCDOM. Consequent reductions in CDOM can dra-matically increase the UV transparency of lakes. UVspectral composition can be used as an assay to sort outsome of these responses to climate change because thespectral slope (280–400nm) is generally steeper

Light and Heat in Aquatic Ecosystems _ Ultraviolet Light 713

for CDOM from autochthonous sources than forCDOM from allochthonous sources. Photobleachingof allochthonous CDOM either increases or decreasesthe spectral slope, depending onwhether exposure is tomainly UV-A or the full solar spectrum, respectively.Aquatic organisms ranging from bacteria and phy-

toplankton to zooplankton and fish have UV photo-receptors and can use UV as an environmental cue.Both freshwater and coral reef fish have been shownto use UV in mate selection. Similarly, in the AfricanRift lakes, there is evidence that a narrowing of thespectral transmission of water due to pollution hasinfluenced mate selection and contributed to thedecline of the species richness of African Cichlids.The transparency loss is primarily in the UV rangebut its importance has not been investigated.

Glossary

1%AttenuationDepth (m) –The depth towhich 1%ofsubsurface irradiance of a given wavelength pene-trates in the water column. Using subsurface mea-surements gives a surface reference that excludesalbedo and thusmore accurately reflectswater trans-parency. When estimated from epilimnetic diffuseattenuation coefficients, the attenuation depths maybe somewhat greater than the actual attenuationdepths due to higher absorptivity of the deeper hypo-limnetic waters.

Biolabile – The degree to which a compound is avail-able for biological uptake, usually by microbes.

Chromophore – A molecular structure that absorbsphotons, usually in a wavelength selective fashion.

Diffuse attenuation coefficient (Kd, m�1) – Rate ofdecrease of subsurface irradiance with depth.

Dobson Unit (DU) – A measure of total column con-tent of a gas, frequently used for ozone. The thick-ness of the layer (in 10’s of mms) of the total massof the gas in the column at standard temperatureand pressure (STP, 0 �C, 1 atm). For example, 300DU¼ 3 mm of ozone at STP.

Dose (Jm�3 or J g�1) – Total radiant energy in a givenwavelength range absorbed by a specific chromo-phore (per volume or per mass). Compare expo-sure.

Dose Rate (Wm�3 orW g�1) –Measure of the radiantpower absorbed per unit time by a specific chro-mophore (per volume or per mass). Compare irra-diance.

Erythemal UV index (UVI) – A derived unit thatexpresses the potential for sunburn in Caucasian

humans estimated by multiplying the solar irradi-ance spectrum by the CIE action spectrum forsunburn (McKinlay and Diffey (1987) CIE Jour-nal 6: 17–22) and multiplying by 40 (for irradi-ance in W m�2).

Radiant Exposure (H or Hl, J m�2 or J m�2 nm�1) –

The total amount of radiant energy within a wave-length range or at a given wavelength, l, incidenton a surface. Compare dose. Fluence is the analo-gous term for radiant energy received from alldirections to a point.

Irradiance (E or El, W m�2 or W m�2 nm�1) – Mea-sure of the radiant powerwithin awavelength rangeor at a given wavelength, l, incident per unit areaon a surface; exposure rate. Compare dose rate.Fluence rate is the analogous term for radiantpower received from all directions to a point.

Ozone mass deficit (megatons) – The mass of ozonethat would have to be added to an area of ozonedepletion such as the Antarctic Ozone Hole tobring the total column ozone up to 220 DU.

Photobleaching – The removal of a chromophoricmoiety in a chemical compound induced by theabsorption of photons.

Photodegradation – Photochemical transformation ofa molecule into lower molecular weight fragments,usually by an oxidation process.

Photolabile – The degree to which a chemical com-pound is susceptible to being broken down by pho-tolysis.

Photolysis – The cleavage of chemical bonds inducedby the absorption of photons.

Photooxidation – Oxidation reactions induced by ab-sorption of photons.

Reactive Oxygen Species (ROS) – Potentially damag-ing highly reactive forms of oxygen such as hydro-gen peroxide, superoxide or hydroxyl radicals, orsinglet oxygen.

Spectral Slope (S, nm�1) – The slope of the plot of thenatural log of the absorbance versus wavelengthfor a specified wavelength range such as all UV(280–400 nm), UV-A (320–400 nm), or UV-B(280–320 nm).

Total Column Ozone – The amount of ozone in theentire atmospheric column over a given area,expressed in Dobson Units.

Zenith – The point in the sky directly overhead fromthe location of interest.

714 Light and Heat in Aquatic Ecosystems _ Ultraviolet Light

See also: Color of Aquatic Ecosystems; Light, BiologicalReceptors.

Further Reading

De Mora S, Demers S, and Vernet M (eds.) (2000) The Effects ofUV Radiation in the Marine Environment. Cambridge, UK:Cambridge University Press.

Hader D-P (ed.) (1997) The Effects of Ozone Depletion on AquaticEcosystems. Austin TX: R.G. Landes Company.

Helbling EWand Zagarese HE (eds.) (2003) UV Effects in AquaticOrganisms and Ecosystems. Cambridge, UK: Royal Society of

Chemistry.

Kirk JTO (1994) Light and Photosynthesis in Aquatic Ecosystems.Cambridge, UK: Cambridge University Press.

Shick JM and Dunlap WC (2002) Mycosporine-like amino acids

and related gadusols: Biosynthesis, accumulation, and UV-

protective functions in aquatic organisms. Annual Review ofPhysiology 64: 223–262.

Smith RC, Tyler JE, and Goldman CR (1973) Optical properties

and color of Lake Tahoe and Crater Lake. Limnology andOceanography 18: 189–199.

United Nations Environment Programme (2007) Environmental

effects of ozone depletion and its interactions with climate

change: 2006 assessment. Photochemical and PhotobiologicalSciences 6: 201–332.

Williamson CE, Neale PJ, Grad G, DeLange HJ, and Hargreaves

BR (2001) Beneficial and detrimental effects of UV radiation onaquatic organisms: Implications of variation in spectral compo-

sition. Ecological Applications 11: 1843–1857.Williamson CE and Zagarese HE (eds.) (1994) Impact of UV-B

radiation on pelagic freshwater ecosystems. Archiv fur Hydro-biologie Ergebnisse der Limnologie 43: 1–226.

Relevant Websites

http://www.nasa.gov/vision/earth/environment/ozone_resource_

page.html – United States National Aeronautics and Space

Administration (NASA) Ozone Resources.http://jwocky.gsfc.nasa.gov/ – Total Ozone Mapping Spectrometer

(TOMS): data on ozone at any given time and location.

http://www.epa.gov/sunwise/uvindex.html – United States Envi-

ronmental Protection Agency (US EPA) UV Index Page.http://www.cpc.ncep.noaa.gov/products/stratosphere/uv_index/uv_

current_map.shtml – United States National Oceanic and

Atmospheric Administration UV Index Page.