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Environmental Toxicology and Chemistry, Vol. 19, No. 4, pp. 920–925, 2000Printed in the USA

0730-7268/00 $9.00 1 .00

QUANTIFYING SOLAR SPECTRAL IRRADIANCE IN AQUATIC HABITATS FOR THEASSESSMENT OF PHOTOENHANCED TOXICITY

MACE G. BARRON,*† EDWARD E. LITTLE,‡ ROBIN CALFEE,‡ and STEVEN DIAMOND§†Stratus Consulting, 1881 Ninth Street, Suite 201, Boulder, Colorado 80302, USA

‡U.S. Geological Survey, Environmental and Contaminants Research Center, 4200 New Haven Road, Columbia, Missouri 65201§U.S. Environmental Protection Agency, 6201 Congdon Boulevard, Duluth, Minnesota 55804

(Received 1 December 1998; Accepted 11 October 1999)

Abstract—The spectra and intensity of solar radiation (solar spectral irradiance [SSI]) was quantified in selected aquatic habitatsin the vicinity of an oil field on the California coast. Solar spectral irradiance measurements consisted of spectral scans (280–700nm) and radiometric measurements of ultraviolet (UV): UVB (280–320 nm) and UVA (320–400 nm). Solar spectral irradiancemeasurements were taken at the surface and at various depths in two marsh ponds, a shallow wetland, an estuary lagoon, and theintertidal area of a high-energy sandy beach. Daily fluctuation in SSI showed a general parabolic relationship with time; maximumstructure–activity relationship (SAR) was observed at approximate solar noon. Solar spectral irradiance measurements taken at 10-cm depth at approximate solar noon in multiple aquatic habitats exhibited only a twofold variation in visible light and UVA anda 4.5-fold variation in UVB. Visible light ranged from 11,000 to 19,000 mW/cm2, UVA ranged from 460 to 1,100 mW/cm2, andUVB ranged from 8.4 to 38 mW/cm2. In each habitat, the attenuation of light intensity with increasing water depth was differentiallyaffected over specific wavelengths of SSI. The study results allowed the development of environmentally realistic light regimesnecessary for photoenhanced toxicity studies.

Keywords—Photoenhanced toxicity Ultraviolet light Photobiology Field assessment

INTRODUCTION

The toxicity of various chemicals and chemical mixtures(e.g., polycyclic aromatic hydrocarbons, heterocycles, and pe-troleum) is photoenhanced; that is, toxicity of a compound ormixture increases two- to 1,000-fold in the presence of ultra-violet light [1–6]. The increase in toxicity can occur as a resultof phototransformations of the chemical caused by UV-inducedstructural changes [4]. Toxicity may also occur through pho-tosensitization caused by the activation of chemicals bioac-cumulated in tissues [3]. Thus, UV can interact directly withthe compound in the water column or can affect organismsthat have accumulated chemical residues. The degree of pho-toenhanced toxicity is dependent on chemical structure (somechemicals are photoactivated, others are not) and the energyand wavelengths of UV radiation (chemical-specific absorptionof specific regions of the light spectra).

The degree of photoenhanced toxicity through a photosen-sitization mechanism and any solar radiation damage to aquaticorganisms are directly related to both the intensity and thespectral distribution of incident light within the water column(solar spectral irradiance [SSI]) rather than at the water surfaceof aquatic habitats [3,7–10]. In general, penetration of thewater column by shorter wavelengths (e.g., UVB, 280–320nm) is much lower than penetration by longer wavelengths(UVA, 320–400 nm; visible, 400–700 nm) [11]. The decreasein light intensity and the alteration of the spectral distributionof SSI with increasing water depth are determined by the at-tenuating characteristics of aquatic habitats [12,13]. For ex-ample, the depth of water required to remove 90% of UVBranges from 20 m in clear and colorous ocean water to a few

* To whom correspondence may be addressed(mbarron@stratusconsulting.com).

centimeters in brown humic lakes and rivers [13]. Environ-mental factors affecting light attenuation can vary substantiallybetween habitats and include the concentration and type ofdissolved organic carbon (DOC), humic substances, suspendedparticulates, algae, and total chlorophyll [9,12–17]. For ex-ample, in freshwater the attenuation of UVB in the water col-umn is almost entirely dependent on the concentration of DOC,which can vary by over 10-fold between aquatic habitats [18].Thus, measurement of only surface irradiance is not sufficientto characterize the intensities and spectral qualities of UVtransmitted through the water column. Additionally, light ab-sorption can increase with decreasing wavelength (i.e., UVBattenuated to a greater extent than visible light) [12,13]; thus,the use of constant ratios of visible light, UVA, and UVB inmany photoenhanced toxicity tests may not be ecologicallyrelevant.

To more appropriately assess photoenhanced toxicity, lab-oratory studies and ecological risk assessments will requirequantification of SSI occurring within a variety of aquatichabitats. These data will facilitate both the design of laboratorystudies and the interpretation of photoenhanced toxicity in theenvironment. Additionally, although reduction in SSI can bedramatic in certain habitats, it will also be critical to understandthreshold intensities for the induction of photoenhanced tox-icity because only limited irradiance may be required [19].

Our objective was to measure SSI in several coastal aquatichabitats representing a range of water-quality and physicalconditions. The SSI measurements consisted of spectral scans(280–700 nm) and radiometric measurements of UVA andUVB. Habitats selected for study included two coastal marshponds, a shallow freshwater wetland, an estuary lagoon, andthe intertidal area of a high-energy sandy beach. Measurementlocations were selected because of their importance as habitat

Quantifying solar spectral irradiance in aquatic habitats Environ. Toxicol. Chem. 19, 2000 921

Fig. 1. Solar radiation and water-quality measurement locations.

Table 1. Surface-water-quality measurements in aquatic habitats [20]a

LocationDepth(cm)

Dissolvedoxygen(mg/L)

Temper-ature(8C)

Turbidity(NTU) pH

Conductivi-ty

(mS/cm)Salinity

(‰)

Color(colorunits)

Dis-solvedorganiccarbon(mg/L)

Totalorganiccarbon(mg/L)

Chl-a(mg/L)

Pond CPond AIntertidalEstuaryB/C channel

4290

b

9010

5.37.16.77.06.9

21.420.115.619.621.4

024

024

1

7.58.38.29.08.1

1.63.0

51.42.81.6

0.71.4

33.71.30.7

25110,38335

15.516.2

,5.012.615

25.617

,5.014.416.1

,0.0010.1900.0050.1800.009

a All data reported are means (n 5 2), except for the B/C Channel (n 5 1).b Measurements recorded in a grab sample collected within the swash zone rather than in situ.

for aquatic organisms and exposure to chronic discharges ofdissolved petroleum from an abandoned oil field. We deter-mined the intensity of SSI at ultraviolet and visible wave-lengths, the fluctuation in SSI intensity over the duration of adaily light period, and the wavelength-specific attenuatingcharacteristics of the water of selected aquatic habitats. Ad-ditional field activities included qualitative observations ofenvironmental conditions (e.g., solar disk visibility and watersurface disturbance) and measurement of water quality. Resultsof this study were used in developing environmentally realisticlight regimes in photoenhanced toxicity studies of a spilledoil [19].

MATERIALS AND METHODS

Measurement locations and environmental conditions

Solar spectral irradiance measurements were performed infive coastal aquatic habitats in the vicinity of an abandoned

oil field (Fig. 1). The locations included two marsh ponds(Marsh Pond A and Marsh Pond C), a shallow wetland con-necting two marsh ponds (B/C channel), an estuary lagoon,and the intertidal area of a high-energy sandy beach. Mea-surement locations were selected because of their importanceas habitat for aquatic biota and exposure to chronic dischargesof dissolved petroleum in groundwater from the oil field.Marsh Pond A was an open, mainly freshwater pond connectedto the estuary. Marsh Pond C was a deep, freshwater pondsurrounded by emergent aquatic vegetation (mainly Scirpus)with minimal submerged vegetation. The B/C channel was ashallow, broad wetland of forbs and floating algae. The estuarylagoon was fed by a small river and was intermittently con-nected with the Pacific Ocean because of the formation of asand berm at the mouth. The intertidal area was the swashzone (zone of incoming and outgoing waves) of a high-gradientsandy beach.

Water-quality measurements included both in situ mea-surements (dissolved oxygen [DO], temperature, turbidity, pH,conductivity, and salinity) and laboratory analytical chemistry(color, total organic carbon [TOC], DOC, and chl a) (methodsare provided in [20]; see Table 1). A qualitative assessmentof environmental conditions was performed for each SSI mea-surement, including water surface conditions (e.g., surface rip-ple), atmospheric haze and cloud type, visibility of the solardisk, wind direction and speed, cover (e.g., vegetation andfoam), and wave height.

Radiometric instrumentation

Solar spectral irradiance measurements were performedwith either a scanning spectroradiometer (SSR) or a broad-band radiometer (BBR). The SSR was an Optronics 754 Spec-troradiometer (Optronic Laboratories, Orlando, FL, USA) thatmeasured light intensity from 280 to 700 nm wavelengths in1-nm increments at each measurement depth. The SSR mea-sure light intensity and spectral with a downwelling integratingsphere. Before conducting light measurements, a wavelengthcheck at 296.7 nm was performed to ensure wavelength ac-curacy of the instrument, and a response check was conductedto ensure conformance to a calibrated intensity of 550 nm usinga National Institute of Standards and Technology traceablestandard lamp. Measurements were conducted using the meth-ods of Little and Fabacher [21], and irradiance values wererecorded as W/cm2. Total light intensity as UVB, UVA, andvisible was quantified by summing the individual energy read-ings of each wavelength for 280 to 320 nm, 320 to 400 nm,and 400 to 700 nm, respectively. Spectral data and integrationvalues were collected, stored, and calculated using the SSR’s

922 Environ. Toxicol. Chem. 19, 2000 M.G. Barron et al.

internal software. Subsurface measurements were performedwith the SSR to depths of 1 m with a minimum water depthof about 0.15 m because of the size of the instrument’s opticalintegrating sphere.

The BBR was a Macam Photometrics Model UV-103 ra-diometer (Macam Photometrics Ltd., Livingston, Scotland)that measured radiation intensity at two bandwidths of ultra-violet light (UVA, 328–402 nm; UVB, 276–344 nm) usingseparate UVA and UVB sensors connected to the radiometerby 10 m of coaxial cable. Data were recorded through a datalogger. The BBR was used to quantify SSI in habitats that wereeither too shallow (B/C channel) or too energetic (intertidal)for the SSR. Broad-band radiometer measurements of UVAand UVB were calibrated against simultaneously collectedSSR measurements (UVB, 280–320 nm; UVA, 320–400 nm)in the estuary, and BBR values were corrected using calibrationcoefficients developed for UVA and UVB bandwidths.

SSI measurements

Solar spectral irradiance measurements were taken on Au-gust 5, 6, and 7, 1996; all measurements within an aquatichabitat were taken on a single day. The SAR measurementswere taken at the water surface (0 cm; sensor or integratingsphere never submerged) and at various water column depthsat each location. During SSI measurements, the optical inte-grating sphere (SSR) and the sensors (BBR) were orientedperpendicular to the water surface and held stationary (attachedto a metal bar or rod or placed on the substrate surface). Tominimize substrate disturbance, measurement locations weremarked more than 24 h before field measurements with anangle iron pounded securely into the substrate. The marshponds and estuary were entered using a small boat and paddlesat a point greater than 10 m from the measurement location.The B/C channel and the intertidal area were entered on footand then monitored remotely.

The SSI measurements in the intertidal area were performedby attaching the UVA and UVB sensors of the BBR to anangle iron crossbar connected to a metal post pounded intothe sand. The rod was positioned in the approximate middleof the swash zone such that the crossbar contacted the sand.The water depth ranged from about 0 to 100 cm, dependingon wave movement. The BBR sensors were periodically re-positioned in response to the incoming or outgoing tide tomaintain a position in the approximate middle of the swashzone. The sensors were positioned to achieve a minimal depthwith only limited exposure above the water surface (e.g., onceper 30 waves). The UVA and UVB intensities were quantifiedby averaging measurements recorded continuously at sequen-tial 1-min intervals during a 30-min period.

Daily fluctuation in light intensity and spectra was antici-pated to be dependent on the solar angle of incident light. Toquantify this fluctuation, a time series of SSI measurementswere taken with the SSR integrating sphere and BBR sensorsat the water surface of the estuary (0930 and 1845 PacificDaylight Time [PDT]; simultaneous SSR and BBR measure-ments) and within the intertidal area (0915 and 1800 PDT;BBR only).

Solar spectral irradiance measurements were performed atapproximately solar noon (1300 PDT 6 1.5 h). The SSI mea-surements in Marsh Pond A were performed with the SSRintegrating sphere in open water at 0-, 10-, 45-, and 90-cmdepths. The SSI measurements in the B/C channel were per-formed with the BBR sensors in an open-water location at 0-

and 10-cm depths. The SSI measurements in Marsh Pond Cwere performed with the SSR integrating sphere at two lo-cations: (1) open water about 0.5 m perpendicular to emergentvegetation (Scirpus) encircling the pond edge at 0-, 10-, and50-cm depths and (2) 0.5 m within the Scirpus at a 10-cmdepth. The SSI measurements in the estuary were performedin open water with the SSR integrating sphere at 0, 10, 50,and 90 cm. The SSI measurements in the intertidal area wereperformed with the BBR sensors in the approximate middleof the swash zone (water depth ranged from about 0 to 100cm, depending on wave movement).

Attenuation of UVA, UVB, and visible light was deter-mined using the SSR in situ in Pond C, Pond A, and the estuarylagoon. Attenuation of UVA and UVB was also determinedin an open-water location in Pond C with the BBR (measure-ments at 3-, 34-, and 76-cm depths at 1500 PDT). Because ofthe varying depth of the intertidal area, light attenuation wasestimated using surrogate measurements performed adjacentto the swash zone. Surrogate measurements were taken in aplastic basin (2.4 m diameter 3 45.7 cm deep) filled with beachsand and water collected from the swash zone (0-, 2-, 16-cmdepths; simultaneous BBR and SSR measurements). Diffuseattenuation coefficients (K values) for UVB in each habitatwere computed from the slope of the log-linear regression oflight intensity (mW/cm2) and measurement depth (0–50 cm)[22].

RESULTS

Daily fluctuations in SSI

Measurements of the daily fluctuations in SSI at the surfacewater of the estuary and within the intertidal zone showed ageneral parabolic relationship over time (Fig. 2). Linear re-gression analysis of simultaneous measurements of SSI withthe BBR and SSR at the estuary surface showed that uncali-brated BBR measurements underestimated UVA by about 50%and overestimated UVB by about 65% (Fig. 3). MinimumUVA and UVB levels were observed in the morning and even-ing, and maximum values were observed at approximate solarnoon (Fig. 2). Maximum average SSI values in the intertidalarea (UVA, 670 mW/cm2; UVB, 26 mW/cm2) were 3.8 and 3.1times greater than minimum values measured for UVA andUVB, respectively. Maximum SSI values at the surface of theestuary lagoon (UVA, 3,000 mW/cm2; UVB, 160 mW/cm2)were 4.7 and 12 times greater than minimum values for UVAand UVB, respectively, observed in the morning and evening.

Dependence of SSI on measurement depth and habitat

Visible light, UVA, and UVB were greatest at surface leveland declined with water depth and varied considerably betweenhabitats (Fig. 4). The attenuation of UVA and UVB exhibiteda negative log-linear relationship with measurement depth, andestimated 1% attenuation depths (depth at 99% attenuation)ranged from 55 to 25 cm for UVA and 45 to 20 cm for UVB(Fig. 5). Pond A and the B/C channel exhibited the greatestattenuation of UVA, and Marsh Pond C exhibited the greatestattenuation of UVB. Diffuse attenuation coefficients (K; mW/cm2/cm; optical depths between 0 and 50 cm) were 0.02 forthe intertidal area, 0.07 for the B/C channel, 0.09 for Pond Cand the estuary, and 0.11 for Pond A.

At a depth of 10 cm, SSI exhibited only limited variabilitybetween habitats despite differences in atmospheric conditionsduring measurement (e.g., solar disk visibility, clouds, and

Quantifying solar spectral irradiance in aquatic habitats Environ. Toxicol. Chem. 19, 2000 923

Fig. 2. Daily fluctuations in ultraviolet intensity at the estuary andintertidal (surf) areas. Each panel shows intensity of UVA or UVBover time; solar noon is 1300 (PDT). Shaded areas are quadratic fitsof the measured values.

Fig. 3. Linear regression models of the relationship between scanningspectroradiometer (SSR) and broad-band radiometer (BBR) measure-ments of UVA (top) and UVB (bottom) intensity. Simultaneous mea-surements were taken at the surface of the estuary lagoon and usedin interinstrument calibration of the BBR measurements. The dashedline shows the expected relationship for equivalent BBR and SSRvalues.

haze) (Fig. 6). In open water at a 10-cm depth, visible lightranged from 11,247 (Marsh Pond A) to 19,281 mW/cm2 (MarshPond C), UVA levels ranged from 463 (Pond A) to 998 mW/cm2 (Pond C), and UVB ranged from 8.4 (Pond A) to 38 mW/cm2 (B/C channel) (Fig. 6). The SSI measured within the emer-gent vegetation of Marsh Pond C (10-cm depth) was reducedabout 90% for both UVA (138 mW/cm2) and UVB (1.3 mW/cm2) relative to unshaded surface levels. The swash zone ofthe intertidal area had relatively high levels of UVA (670 mW/cm2) and UVB (26 mW/cm2) despite continual resuspensionof sand caused by wave action. At depths greater than 10 cm,SSI intensities were dependent on habitat and wavelength-specific attenuation (Fig. 6).

DISCUSSION

The SSI varied with the specific habitat, time of day, andmeasurement depth. Daily fluctuation in SSI exhibited a par-abolic form typical of these data; maximum SSI was observedat approximate solar noon. Daily fluctuations in SSI monitoredfrom 0900 to 1800 PDT showed that maximum UVA and UVBvalues were 3 to 12 times greater than minimum values. De-spite changing environmental conditions (e.g., solar disk vis-ibility) and habit-specific water quality, SSI measurements tak-en at a 10-cm depth at approximate solar noon exhibited onlya twofold variation in visible and UVA light and a 4.5-foldvariation in UVB. Despite continuous resuspension of sandand varying water depth, intertidal levels of UVA and UVB

were within the ranges of values observed in other habitats ata 10-cm depth. Solar spectral irradiance measured within theemergent vegetation (10-cm depth in dense 1-m-high Scirpus)of Marsh Pond C was reduced by about 90% of the UVA andUVB levels measured in the open water of this marsh pond.These data suggest the potential for substantial UVA and UVBexposures to aquatic organisms near the water surface, suchas basking frogs (e.g., Rana tadpoles), zooplankton, larval fish,and intertidal suspension feeders, such as sand crabs (e.g.,Emerita).

924 Environ. Toxicol. Chem. 19, 2000 M.G. Barron et al.

Fig. 4. Intensity and spectral distribution of light in the estuary (top)and Marsh Pond C (bottom). Measurements were performed in openwater at approximately solar noon (1300 PDT 6 1.5 h).

Fig. 6. Summary of SSI measurements in aquatic habitats performedin open water at approximately solar noon (1300 PDT 6 1.5 h).Intensity was measured with the BBR in the B/C channel and intertidalhabitats (no measurement of visible light); measurements at all otherlocations were performed with the SSR. Additional measurements arenoted by an asterisk (* 5 duplicate measurement; ** 5 measurementin vegetation).

Fig. 5. Depth (cm) at 99% attenuation of UVA and UVB in aquatichabitats. Data for the intertidal area were surrogate measurementsadjacent to the intertidal area (see text).

Solar spectral irradiance at 50 and 90 cm exhibited sub-stantially greater variation between habitats because of habitat-specific attenuation. In each habitat, the attenuation of lightintensity with increasing water depth was differentially af-fected over specific wavelengths of SSI, and the attenuationof UVA and UVB exhibited a negative log-linear relationshipwith measurement depth. Estimated 99% attenuation depths

ranged from 55 to 25 cm for UVA and 45 to 20 cm for UVB.Consistent with these data, Williamson et al. [9] reported that75% of the lakes in some regions of North America have 99%attenuation of UVB at depths of less than 50 cm. In contrast,25% of the lakes in some regions of North America have 99%attenuation at depths greater than 4,000 cm [9].

Water-quality and physical conditions can reduce light pen-etrance, thus reducing photoenhanced toxicity in aquatic hab-itats [23]. Water-quality parameters measured in our study pro-vided a general characterization of conditions but could onlybe qualitatively associated with light attenuation in aquatichabitats. For example, Marsh Pond A exhibited relatively highattenuation of UVA and UVB and had comparatively highDOC, color, turbidity, and chl a levels. Conversely, the inter-tidal area exhibited the lowest attenuation (surrogate deter-mination) of UVA and UVB and had relatively low DOC,color, turbidity, and chl a levels. Williamson et al. [9] reportedan exponential increase in UVB attenuation with increasingDOC. Because of wavelength-specific attenuation of SSI, ra-tios of visible light, UVA, and UVB intensities will not beconstant with changing water depth [9].

The UVA and UVB levels applied in prior laboratory pho-toenhanced toxicity studies are within the range of SSI mea-sured in our study at depths of 10 to 50 cm. For example, Orisand Giesy [1], Ankley et al. [8], Boese et al. [24], and Pelletieret al. [6] tested applied UVA levels of 17 to 397 mW/cm2 andUVB ranges of 20 to 58 mW/cm2. However, previous labo-ratory studies determining photoenhanced toxicity have typi-cally applied lower visible light levels (e.g., ,1,500 mW/cm2)and higher UVB levels relative to UVA than we measured in

Quantifying solar spectral irradiance in aquatic habitats Environ. Toxicol. Chem. 19, 2000 925

five aquatic habitats. The intensity of visible light and ratiosof visible to UVB light intensity are of concern because UVBis damaging to aquatic organisms and the test organisms maybe deprived of sufficient visible radiation for photorepair [21].

The BBR used in this study (Macam photometrics) has beenfrequently used in quantifying incident light and light dosesin previous photoenhanced toxicity studies. We determinedthat the BBR underestimated UVA by 50% and overestimatedUVB by 65%. Systematic bias in the BBR measurements wasanticipated because the ultraviolet bandwidths of this radi-ometer do not correspond to true UVA and UVB [25]. Forexample, the UVB bandwidth extends into the UVA range,resulting in overestimation of UVB [25]. All BBR measure-ments reported in our study were corrected for measurementbias using calibration factors developed with simultaneousmeasurements with the SSR.

Measurement of SSI in diverse coastal aquatic habitats inthis study allowed the development of environmentally real-istic light regimes necessary for photoenhanced toxicity stud-ies. Habitat-specific measurements or estimates of SSI (e.g.,UVB intensity estimated from DOC concentrations) are nec-essary because of the extreme variation in light attenuation indifferent aquatic habitats [9,11]. Surface-level measurementssubstantially overestimate water column irradiance. For ex-ample, UV intensity at the surface can be reduced 50 to 90%even at shallow depths (i.e., 10-cm depth). At a depth of 1 m,SSI at the water surface can be reduced greater than 99%.Even at these levels of attenuation, we found that UV wassufficient to induce photoenhanced toxicity [19]. The use ofconstant ratios of UVA to UVB in photoenhanced toxicitystudies does not appear to be environmentally realistic. Lightattenuation is more pronounced at short wavelengths than atlonger wavelengths [12,13]; thus, ratios of UVB to UVA ap-plied in laboratory tests should reflect observed environmentallevels. In addition, irradiance intensity for UVB, UVA, andvisible radiation varies with the solar angle (angle of sun rel-ative to the water surface). We recommend that separate pho-toperiods be utilized for UVB (# 5 h) and UVA (# 16 h) tomore appropriately simulate natural sunlight and to limit solarradiation damage caused by UVB [7]. Because adequate visiblelight is necessary for biological repair [26], we recommend avisible light photoperiod of at least 8 h and a minimum visiblelight intensity of 500 mW/cm2. We also recommend that mul-tiple light treatments be used to account for varying SSI inaquatic habitats and to demonstrate a causal linkage betweenlight dose and photoenhanced toxicity.

Acknowledgement—We thank T. Podrabsky, D. Cacela, B. Sanders,A. Jahn, W. Kicklighter, N. Scott, and C. Swift for advice and assis-tance. This study was undertaken at the direction of the CaliforniaDepartment of Fish and Game Office of Oil Spill Prevention andResponse in cooperation with Unocal Corporation; the field work wasfunded directly by Unocal.

REFERENCES

1. Oris JT, Giesy JP. 1987. The photo-induced toxicity of polycyclicaromatic hydrocarbons to larvae of the fathead minnow (Pime-phales promelas). Chemosphere 16:1395–1404.

2. Larson RA, Berenbaum MR. 1988. Environmental phototoxicity:Solar ultraviolet radiation affects the toxicity of natural and man-made chemicals. Environ Sci Technol 22:354–360.

3. Landrum PF, Giesy JP, Oris, JT, Allred PM. 1987. Photoinducedtoxicity of polycyclic aromatic hydrocarbons to aquatic organ-isms. In Vandermeulen JH, Hrudey SE, eds, Oil in Freshwater.Pergamon, New York, NY, USA, pp 304–318.

4. Huang XD, Dixon DG, Greenberg BM. 1993. Impacts of UV

radiation and photomodification on the toxicity of PAHs to thehigher plant. Lemna gibba (duckweed). Environ Toxicol Chem12:1067–1077.

5. Arfsten DP, Schaeffer DJ, Mulveny DC. 1996. The effects of nearultraviolet radiation on the toxic effects of poly-cyclic aromatichydrocarbons in animals and plants: A review. Ecotoxicol En-viron Saf 33:1–24.

6. Pelletier MC, Burgess RM, Ho KT, Kuhn A, McKinney RA, RybaSA. 1997. Phototoxicity of individual polycyclic aromatic hy-drocarbons and petroleum to marine invertebrate larvae and ju-veniles. Environ Toxicol Chem 16:2190–2199.

7. Tevini M. 1993. Molecular biological effects of ultraviolet ra-diation. In Tevini M, ed, UV-B Radiation and Ozone Depletion:Effects on Humans, Animals, Plants, Microorganisms, and Ma-terials. Lewis, Boca Raton, FL, USA, pp 1–15.

8. Ankley GT, Erickson RJ, Phipps GL, Mattson VR, Kosian PA,Sheedy BR, Cox JS. 1995. Effects of light intensity on the pho-totoxicity of fluoranthene to a benthic invertebrate. Environ SciTechnol 29:2828–2833.

9. Williamson CE, Stemberger RS, Morris DP, Frost TM, PaulsenSG. 1996. Ultraviolet radiation in North American lakes: Atten-uation estimates from DOC measurements and implications forplankton communities. Limnol Oceanogr 41:1024–1034.

10. American Society for Testing and Materials. 1996. Standard guidefor use of lighting in laboratory testing. E 1733-95. In AnnualBook of ASTM Standards, Vol 14.02. Philadelphia, PA, pp 1279–1289.

11. Lean DRS. 1998. Influence of ultraviolet radiation on aquaticecosystems. In Little EE, Greenburg BM, De Lonay AJ, eds,Environmental Toxicology and Risk Assessment, Vol 7. STP1333. American Society for Testing and Materials, Philadelphia,PA, pp 1–20.

12. Kirk JTO. 1994. Light and Photosynthesis in Aquatic Ecosys-tems, 2nd ed. Cambridge University Press, Cambridge, UK.

13. Kirk JTO. 1994. Optics of UV-B radiation in natural waters. ArchHydrobiol Ergeb Limnol 43:1–16.

14. Davies-Colley RJ, Vant WN. 1987. Absorption of light by yellowsubstance in freshwater lakes. Limnol Oceanogr 32:416–425.

15. Vertucci FA, Likens GE. 1989. Spectral reflectance and waterquality of Adirondack mountain region lakes. Limnol Oceonogr34:1656–1672.

16. Morris DP, Zagarese H, Williamson CE, Balseiro EG, HargreavesBR, Modenutti B, Moeller R, Queimalinos C. 1995. The atten-uation of solar UV radiation in lakes and the role of dissolvedorganic carbon. Limnol Oceanogr 40:1381–1391.

17. Morris DP, Hargreaves BR. 1997. The role of photochemical deg-radation of dissolved organic carbon in regulating the UV trans-parency of three lakes in the Pocono Plateau. Limnol Oceanogr42:239–249.

18. Scully NH, Lean DRS. 1994. The attenuation of ultraviolet ra-diation in temperate lakes. Arch Hydrobiol Ergeb Limnol 43:135–144.

19. Little EE, Cleveland L, Calfee R, Barron MG. 2000. Assessmentof the photoenhanced toxicity of a weathered oil to the tidewatersilverside. Environ Toxicol Chem 19:926–932.

20. ENTRIX. 1996. UV field study water quality measurements andGPS Locations. Report 307411. Walnut Creek, CA, USA.

21. Little EE, Fabacher DE. 1996. Exposure of freshwater fish tosimulated solar UVB radiation. In Ostrander GK, ed, Techniquesin Aquatic Toxicology. Lewis, Boca Raton, FL, USA, pp 141–158.

22. Smith RC, Calkins J. 1976. The use of the Roberson meter tomeasure the penetration of solar middle-ultraviolet radiation (UV-B) into natural waters. Limnol Oceanogr 21:746–749.

23. Ireland DS, Burton GA Jr, Hess GG. 1996. In situ toxicity eval-uation of turbidity and photoinduction of polycyclic aromatichydrocarbons. Environ Toxicol Chem 15:574–581.

24. Boese BL, Lamberson JO, Swartz RC, Ozretich RJ. 1997. Pho-toinduced toxicity of fluoranthene to seven marine benthic crus-taceans. Arch Environ Contam Toxicol 32:389–393.

25. Bullock AM. 1988. Solar ultraviolet radiation: A potential en-vironmental hazard in the cultivation of farmed finfish. In MuirJF, Roberts RJ, eds, Recent Advances in Aquaculture. CroomHelm, Beckenham, Kent, UK, pp 139–224.

26. Applegate LA, Ley RD. 1988. Ultraviolet radiation-induced le-thality and repair of pyrimidine dimers in fish embryos. MutatRes 198:85–92.