Spatial and Temporal Distribution of Singlet Oxygen in Lake Superior

Article pubs.acs.org/est

Spatial and Temporal Distribution of Singlet Oxygen in Lake Superior Britt M. Peterson,†,∥ Ann M. McNally,† Rose M. Cory,†,‡,⊥ John D. Thoemke,§ James B. Cotner,*,‡ and Kristopher McNeill*,†,∥ †

Department of Chemistry, University of Minnesota, Minneapolis, Minnesota, United States Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, Minnesota, United States § Department of Chemistry and Geology, Minnesota State University, Mankato, Minnesota, United States ∥ Institute of Biogeochemistry and Pollutant Dynamics (IBP), ETH Zurich, Zurich, Switzerland ‡

S Supporting Information *

ABSTRACT: A multiyear field study was undertaken on Lake Superior to investigate singlet oxygen (1O2) photoproduction. Specifically, trends within the lake were examined, along with an assessment of whether correlations existed between chromophoric dissolved organic matter (CDOM) characteristics and 1O2 production rates and quantum yields. Quantum yield values were determined and used to estimate noontime surface 1O2 steady-state concentrations ([1O2]ss). Samples were subdivided into three categories based on their absorbance properties (a300): riverine, river-impacted, or open lake sites. Using calculated surface [1O2]ss, photochemical halflives under continuous summer sunlight were calculated for cimetidine, a pharmaceutical whose reaction with 1O2 has been established, to be on the order of hours, days, and a week for the riverine, river-impacted, and open lake waters, respectively. Of the CDOM properties investigated, it was found that dissolved organic carbon (DOC) and a300 were the best parameters for predicting production rates of [1O2]ss. For example, given the correlations found, one could predict [1O2]ss within a factor of 4 using a300 alone. Changes in the quantum efficiency of 1O2 production upon dilution of river water samples with lake water samples demonstrated that the CDOM found in the open lake is not simply diluted riverine organic matter. The open lake pool was characterized by low absorption coefficient, low fluorescence, and low DOC, but more highly efficient 1O2 production and predominates the Lake Superior system spatially. This study establishes that parameters that reflect the quantity of CDOM (e.g., a300 and DOC) correlate with 1O2 production rates, while parameters that characterize the absorbance spectrum (e.g., spectral slope coefficient and E2:E3) correlate with 1O2 production quantum yields.

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INTRODUCTION Due to its potential importance in pollutant fate and biogeochemically relevant transformations,1−6 singlet oxygen (1O2), a reactive oxygen species, has been widely investigated in the environment.7−22 Singlet oxygen reacts with organic contaminants, specifically electron-rich compounds containing phenolic, sulfidic, or olefinic moieties,23 and biological molecules, such as amino acids, DNA, and lipids.5 The incorporation of 1O2 into dissolved organic matter may also play a role in its transformations and biogeochemical cycling.2,3 CDOM, the pool of dissolved organic matter that absorbs ultraviolet and visible light, plays a central role in the photochemistry of natural waters7,24,25 and is the main source of 1O2. Consequently, it is expected that the distribution of 1O2 in a natural water body will map directly onto the distribution of CDOM, which can have high spatial and temporal variability. However, translating CDOM distribution in space and time to the 1O2 spatiotemporal distribution is complicated by the fact that the 1O2 production efficiencies, or quantum yields, have © 2012 American Chemical Society

been found to be dependent on the origin of the CDOM (e.g., terrestrial vs aquatic) and the season.26,27 These quantum yield differences lead to different steady-state concentrations of 1O2 even at the same CDOM concentrations, and therefore different rates of 1O2-mediated transformation processes. The link between CDOM and 1O2 was established in the first report of singlet oxygen in natural waters by Zepp et al.16 The production of 1O2 occurs through an energy transfer reaction between photoexcited CDOM and molecular oxygen (eq 1). +hυ

O2

CDOM ⎯⎯⎯→ CDOM* → 1O2 + CDOM

(1)

The efficiency of singlet oxygen formation, or quantum yield, is defined as the number of moles of singlet oxygen produced per mole of photons absorbed and has been investigated for both Received: Revised: Accepted: Published: 7222

March 21, 2012 May 29, 2012 May 31, 2012 June 15, 2012 dx.doi.org/10.1021/es301105e | Environ. Sci. Technol. 2012, 46, 7222−7229

Environmental Science & Technology

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DOM isolates13,17,26,27 and filtered water samples.9,16,21 The steady-state concentration is given by the ratio of the production rate and the rate constant for deactivation of 1O2 (eq 2). The formation rate is the product of the rate of photon absorption, Rabs (M s−1), and the quantum yield, ΦΔ, while the deactivation rate constant is the sum of the deactivation by the solvent, ksolv (s−1) and all of the other quenchers, Q, that are present in solution (eq 2). In water, the combined contribution of dissolved quenchers is usually not competitive with deactivation by the solvent and can be ignored. [1O2 ]ss =

ksolv

Rabs·ΦΔ + ∑i kq , i[Q i]

St. Louis River, the major tributary in this part of the lake and one of the main sources of terrestrial CDOM to Lake Superior.31 Station names were chosen to be consistent with prior studies in Lake Superior.32−34 Details of the sampling protocols are given in the Supporting Information. Chemicals. Furfuryl alcohol (FFA) and perinaphthenone were purchased from Aldrich and p-nitroacetophenone was purchased from Acros. FFA was distilled prior to use. Polystyrene sulfonate standards were purchased from Polysciences, Inc. All buffer components were used without further purification, and all solvents were American Chemical Society reagent grade or higher. Optical Measurements and Other Sample Characterization Methods. The optical parameters used in this study, a300 (m−1), spectral slope coefficient (μm−1), E2:E3,35 fluorescence intensity (Raman units, RU), a fluorescence ratio, and fluorescence index,36,37 were each quantified as described in the Supporting Information. Other parameters included were dissolved organic carbon concentration and size exclusion chromatography peak area.38 Details of these characterization methods are also given in the Supporting Information. Singlet Oxygen Measurements. The light source for the 2007−2009 samples was a Rayonet Photoreactor (Southern New England Ultraviolet Company) equipped with 13 bulbs whose maximum intensity was at 365 nm and emitted over a wavelength range of 345−410 nm. Samples spiked with FFA (final concentration 100 μM) were irradiated by the light source and aliquots (100 μL) were taken for analysis by HPLC. Samples (5 mL in 12 × 75 mm borosilicate test tubes) were constantly rotated on a merry-go-round sample holder during the 60−120 min irradiation to ensure uniform light exposure. In addition, a direct photolysis control, Milli-Q water spiked with FFA, was run in parallel. Singlet oxygen steady state concentrations were measured by FFA disappearance where the observed FFA degradation rate constant (kobs,DOM), corrected for direct photochemical loss (kobs,dir), was divided by the bimolecular reaction rate constant between 1O2 and FFA (krxn = 8.3 × 107 M−1 s−1).39

(2)

Steady-state concentrations under sunlight have been estimated for various water bodies.12,14−19,28 While a number of studies have been performed on diverse samples including humic substance isolates,12,26,27 freshwater,12−20 and seawater samples,13,16,21,22 a multiyear field study investigating 1O2 production and CDOM characteristics for one water body has not been published previously. The goal of the present study was to examine the spatial and seasonal variation of singlet oxygen production in Lake Superior, the Earth’s largest freshwater lake with strong spatial and seasonal CDOM dynamics. 29,30 There were two motivations for this study. First, we sought to identify whether readily measured parameters (e.g., spectral slope coefficient, absorption coefficient at 300 nm, absorbance ratios, fluorescence indices, fluorescence intensity, or dissolved organic carbon concentration) could be used to predict singlet oxygen production rates or quantum yields. Second, we sought to delineate the magnitude of the spatial and temporal variability of singlet oxygen production in the Lake Superior system. In this work, we report the first multiyear study of the spatial and temporal distribution of singlet oxygen quantum yields and calculated steady-state concentrations within a large freshwater lake.

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MATERIALS AND METHODS Sample Collection and Study Site. Water samples were collected from Lake Superior (5 m depth) and a selection of its tributaries (surface grab) between June 2, 2005 and May 21, 2009 (Figure 1, Table S1). Most of the samples were collected in the western arm and several samples were collected from the

[1O2 ]ss =

(kobs , DOM − kobs , dir ) krxn , FFA

(3)

Direct photochemical loss was always less than 1% of kobs,DOM. Under the described conditions, where krxn,FFA ≫ ksolv, the solvent-dependent first-order relaxation rate constant (2.5 × 105 M−1 s−1 for H2O),40 the singlet oxygen production rate (Rf) can be simplified to the following equation R f = [1O2 ]ss ksolv

(4)

which was used to calculate Rf for each sample. 1 O2 Production Quantum Yield Determination. The quantum yield of 1O2 generation at 365 ± 30 nm (ΦΔ, moles of 1 O2 produced per mole of photons absorbed) was calculated for all 2007, 2008, and 2009 samples against perinaphthenone, a singlet oxygen sensitizer with a quantum yield of 0.98.41 Samples and perinaphthenone controls were analyzed simultaneously to ensure the same light intensity. The ΦΔ was calculated according to the following equation ΦΔ =

Figure 1. Map of sampling station locations. See Table S1 in the Supporting Information for latitude, longitude, and sampling dates. 7223

(kobs , DOM − kobs , dir )

·

∑λ Iλaλ , peri

(kobs , peri − kobs , dir ) ∑λ IλSλaλ , DOM

·Φperi

(5)

dx.doi.org/10.1021/es301105e | Environ. Sci. Technol. 2012, 46, 7222−7229


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where kobs is the rate constant for the degradation of FFA in the presence of dissolved organic matter (DOM) or perinaphthenone (peri), Iλ is the intensity of the irradiation source determined with a Jaz radiometer (Ocean Optics) over the wavelength range in 1 nm increments, aλ,peri and aλ,DOM are the Naperian absorption coefficients for the perinaphthenone solution and the Lake Superior water sample respectively, Sλ is the screening factor, and Φperi is the quantum yield for perinaphthenone. The screening factor, a term used to correct for screening of the incident light by any present absorbers, was calculated according to the following equation

Sλ =

1 − e aλ·1.2d aλ ·1.2d

brown water samples. The third group contained samples from the estuaries and transition zones between the rivers and the open lake. These river-impacted samples had intermediate DOC and absorption coefficient values. For the purposes of the following discussion, we defined open lake samples as those with a300 < 2 m−1, riverine samples as those with a300 ≥ 10 m−1, and the river-impacted samples as those with 2 ≤ a300 < 10 m−1. More than ten different descriptors were measured in the present study in an attempt to find a strong correlation between 1 O2 production rates and quantum yields and a chemical or optical parameter. A chemical or optical proxy which would allow the prediction of 1O2 concentrations from a readily measured parameter would greatly facilitate estimation of 1O2mediated reactions with substrates such as organic compounds,4 dissolved organic matter,2 or biological molecules.1 Parameters were chosen that indicated both the CDOM concentration (e.g., DOC, fluorescence intensity, and a300) and CDOM quality (e.g., spectral slope coefficient, E2:E3, and fluorescence index). As detailed below, the parameters that yielded the strongest correlations with singlet oxygen production rate were DOC and a300, which reflected CDOM concentration. The 1O2 formation quantum yield correlated with spectral slope coefficient and E2:E3 values, parameters that are essentially independent of concentration, and are considered to be broad indicators of CDOM chemical composition. Singlet Oxygen Production. The calculated surface noontime steady-state singlet oxygen concentrations for Lake Superior covered 2 orders of magnitude, 5.9 fM (WML, 20 Jun 09) to 710 fM (SLR, 2 Jun 07) (Table S2), and span the range of literature values. Previously determined values, ranked roughly from low to high, include those for Lake Erie (4.6 fM),18 the Gulf of Mexico (10 fM),16 the Rhine (59 fM),12 freshwater peat extract (80 fM),15,17 secondary effluent (140 fM),12 a shallow eutrophic pond (250 fM),14 and the Okefenokee Swamp (710 fM).12,16 The open lake water samples (less than 17 fM) from Lake Superior were from low absorbing, low DOC samples most similar to the Gulf of Mexico and Lake Erie. The river-impacted sites (12−55 fM) were similar to the reported concentrations of the Rhine, while the river concentrations (greater than 120 fM with a maximum of 710 fM) were similar to the secondary effluent and the Okefenokee Swamp. These values were all measured with natural sunlight albeit at different locations. A relationship between the surface steady-state 1 O 2 concentrations and DOC or the absorbance of natural water has been reported previously.12,18 Similar positive correlations were also seen in this study with a300 and DOC (Figure 2). The [1O2]ss correlation with a300 gave a value of 4.7 fM per 1 m−1 absorption coefficient (Figure 2B). The correlation with a300 (92% of the data within a factor of 2 of the best-fit line) was stronger than that of [1O2]ss vs DOC (77% within a factor of 2) (Figure 2A). The a300 correlation would fit all of the steady state concentrations within a factor of 4. However, if a trendline was fit to the open lake, river-impacted, and river samples individually, the slope would vary from 7.1 fM per m−1 absorption coefficient (R2 = 0.64), 5.8 fM per m−1 absorption coefficient (R2 = 0.93), and 2.7 fM per m−1 absorption coefficient (R2 = 0.53), respectively (best-fit lines not shown). At low absorption coefficient values, the greater slope in [1O2]ss vs a300 than at higher absorption coefficient values was likely due to the higher quantum yields in the low absorbing samples

(6)

where 1.2d is the path length of the incident light over depth, d,42,43 and aλ is the Naperian absorption coefficient (cm−1) at each wavelength. For the quantum yield calculations, the depth was 1 cm. The screening factor was included in the quantum yield calculation as the samples’ absorption coefficients vary over 2 orders of magnitude. Noontime 1O2 Steady State Concentration Estimation. Using the quantum yield that was measured, singlet oxygen steady state concentrations, [1O2]ss, were calculated for noon on the date the site was sampled according to eq 7 where the irradiance of the solar spectrum, Iλ (mmol photons cm−2 s−1 nm−1), was calculated for noon using SMARTS44,45 at 47° N, the Naperian absorption coefficient values, aλ (cm−1), and the screening factor, Sλ, were summed over 280−500 nm. The 1O2 quantum yield, ΦΔ, and the screening factor, Sλ, are described above (eqs 5 and 6). 500nm

1

[ O2 ]ss =

ΦΔ ·∑λ = 280nm aλIλSλ ksolv

(7)

Absorbance spectra were adjusted so that the baseline from 600 to 700 nm was zero, and the wavelength range incorporated in this calculation was chosen to minimize the error associated with the low signal-to-noise portion of the absorbance spectrum (λ > 500 nm) and low ΦΔ values.18 Singlet oxygen steady state concentrations were calculated over a depth of 1 cm for surface concentrations and the whole water column for depth-averaged concentrations. The depth-averaged concentrations take into account that all photoactive light was absorbed within the water column and were calculated for river and open lake samples (see below) at 25 m (Duluth-Superior Harbor depth) and 147 m (L. Superior average depth), respectively. Sunlight is not necessarily completely absorbed at the river-impacted sites, so that depth-averaged concentrations were not calculated for these sites. As all incident light was assumed to be absorbed for the depth-averaged concentrations, eq 7 simplifies to eq 8, where z is the water column depth. 500nm

[1O2 ]ss , avg =

■

ΦΔ ·∑λ = 280nm Iλ ksolv·z

(8)

RESULTS AND DISCUSSION General Observations. Over the course of the field studies, it was apparent that the samples could be divided into three groups. The first group comprised the open lake samples, which were blue water samples with low DOC and low absorption coefficient values. The second group contained all of the riverine samples, which were high-DOC, strongly absorbing 7224



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the E2:E3 correlation (Figure 2D, R2 = 0.56) displayed significant scatter, but the overall trend was clear in each case. While both spectral slope coefficient and E2:E3 have been used to characterize the absorbance properties of water samples, they have typically been used to different ends. Spectral slope coefficient has become a common indicator of the absorbance spectra of water samples as it is possible to describe the exponential decrease in absorbance with increase in wavelength with this single value.58 Spectral slope coefficients calculated in a similar manner for samples from the Baltic Sea and Danish fjords covered a similar range of S300−600 values (16−26 μm−1).48 In this study, samples influenced by a river ranged from 16 to 20 μm−1 while open lake samples (e.g., CM, EM, SM, WML, and NM) ranged from 22 to 25 μm−1. The absorbance ratio, E2:E3, has been interpreted in the literature as an indicator of aromaticity and apparent molecular weight, both characteristics of CDOM.35,57 Dalrymple et al. have previously reported a correlation between 1O2 quantum yield and E2:E3 (ΦΔ = 0.871 × E2:E3 −1.53, R2 = 0.65).59 The E2:E3 correlation seen in this study was not as strong as that reported by Dalrymple et al.59 One possible explanation was that the current study includes samples with a wider range of E2:E3 values (5−22) than Dalrymple et al. (2 < E2:E3 < 6).59 Another difference between the studies was the use of isolates by Dalrymple et al. vs whole water in the present study.59 Investigating the singlet oxygen production rates as a function of distance from the St. Louis River, the major source of terrestrial CDOM of Lake Superior, indicates that the production rates were highest for stations located closest to the Duluth-Superior Harbor (Figure 3). This fits the expectation

Figure 2. Relationships between (A) the steady state singlet oxygen concentrations and dissolved organic carbon concentrations, (B) singlet oxygen steady state concentrations and a300, (C) singlet oxygen quantum yield and E2:E3 ratio, and (D) singlet oxygen quantum yield and spectral slope coefficient (S300−600). The inset in B highlights the higher slope in [1O2]ss vs a300 at lower absorption coefficients. Trendlines are (A) y = −31.9 + 28.4x, R2 = 0.95; (B) y = 12 + 4.7x, R2 = 0.88; (C) y = 0.605 + 0.196x, R2 = 0.592; and (D) y = −3.3 + 0.29x, R2 = 0.66. Black symbols, open lake samples; white symbols, river-impacted samples; dark-gray symbols, river samples.

as discussed below (see inset of Figure 2B). Multiple linear regressions between a300 or DOC and the other measured parameters were also investigated in an attempt to understand the effect of DOM quality on the scatter seen here in 1O2 production. However, this approach did not lead to better fits or greater understanding into the important factors for this data set that was limited by few high DOC/a300 samples. Singlet Oxygen Quantum Yield. Singlet oxygen quantum yields have been measured in previous studies by direct13,26 and indirect9,16,17,21,27 methods for isolates13,17,26,27 and whole water9,16,21 samples using single wavelength,26,27 narrow bandwidth,9,16,17 and broad bandwidth13 irradiation sources. The range of quantum yields measured by various methods reported in the literature range from 0.05%9 to 6.1%.13 Previous research has shown that singlet oxygen quantum yields are wavelength-dependent with decreasing quantum yields with increasing wavelength.9,17 Singlet oxygen quantum yields in this study were determined with an indirect method, furfuryl alcohol disappearance, using a narrow bandwidth (365 ± 30 nm) source. We consider the quantum yields obtained with this irradiation source to represent an approximate averaging over the most relevant part of the action spectrum of the wavelength-dependence that has been found in other studies.9,17 The quantum yields ranged from 0.93% for stations in the St. Louis River and the Duluth-Superior Harbor on 15 May 2008 to 5.5% at Station AB2 on 5 Sep 2007. Singlet oxygen quantum yields were found to correlate with spectral slope coefficient and E2:E3 values, which both characterize the shape of the absorbance spectrum and are thought to be indicators of CDOM structure,35,46 composition,47 origin,48 or history.49−57 Both the spectral slope coefficient, S300−600, correlation (Figure 2C, R2 = 0.66) and

Figure 3. Singlet oxygen production rates for samples collected in 2007−2008 plotted by distance from the Superior entry to the DuluthSuperior Harbor. Stations located within the harbor or the St. Louis River are indicated by negative distances. Trendlines were fit to the data for the river-impacted sites for May−June (y = 2.9 × 10−7 − 2.7 × 10−8x, R2 = 0.81) and Aug−Sept (y = 9.6 × 10−8 − 1.7 × 10−8x, R2 = 0.91) samples while a third trendline was fit to all the data with a300 < 2 m−1. ● May−June, ◊ Aug−Sept.

that higher absorbing waters give higher production rates.16 We observed an approximately linear gradient of 1O2 production rates along the St. Louis River estuary (Figure 3). A linear gradient in production rates was also seen in laboratory mixtures of river and lake water (see Supporting Information for results and characterization of the laboratory mixtures). This linear decrease was found in spite of the increase in the quantum efficiency of the samples, reflecting the fact that the 7225



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Such high quantum yield samples were not measured in the St. Louis River, Harbor, or any of the stations outside of the western arm suggesting a different chemical composition of these lake samples collected in Aug−Sept compared to other samples. Possible interpretations of such high quantum yields include in situ-produced CDOM61,62 that is highly efficient in singlet oxygen production, photochemical (abiotic) transformation63 of the CDOM that leads to production of organic matter with a high quantum efficiency, or a combination of these processes. Evidence for in situ-produced organic matter could potentially come from the fluorescence index of the samples, which has been used by others with some success in characterizing DOM of different origins.36,37 The FI values did not reveal any patterns in this study, as there was very little change in FI values across all samples, including the high quantum yield samples. Another possibility for the high quantum yields is an anthropogenic source, such as an oil spill or sewage treatment outfall (We thank an anonymous reviewer for pointing out this possibility.). However, the samples’ fluorescence and absorbance spectra did not indicate anything unusual. Several trends between quantum yield and the source of an isolate have been noted in previous quantum yield studies.13,27 In contrast to the present work, in which we observed higher quantum yield values for waters with a lower terrestrial contribution to the CDOM, others measured higher quantum yields (albeit at 480 nm) for terrestrial than aquatic isolates.27 In another study, quantum yields did not vary along the salinity gradient in the Mississippi River estuary.13 These differing trends suggest there is no general rule as of yet to predict in which direction quantum yields will vary as one moves from a river into the receiving water. It is noted that these prior studies examined organic matter isolates and it is possible that isolates may not fully represent the natural sample.64 Indeed, an effect of the isolation method has been seen in the subsequent measurements of the photoproduction of singlet oxygen13,26 and hydroxyl radical.65 Further work will be needed to assess the effect of organic matter isolation on the production rate and quantum yield of 1 O2. Sensitized Production of Singlet Oxygen. The relationship between [1O2]ss and DOC or a300 seen here matches the theoretical prediction, in which the production rate is first order with the concentration of the sensitizer (e.g., CDOM) or indicators of the sensitizer concentration (e.g., DOC or a300). The rate of singlet oxygen production is determined by ΦΔ and rate of light absorption (Rabs).66 The rate of light absorption is given by

relative changes in the CDOM concentration along this transect were much larger than the changes in quantum yield. Magnitude of Spatial and Temporal Variability of Singlet Oxygen Production. Because terrestrial CDOM is generally thought to mix conservatively, water properties were expected to vary from the riverine and open lake endmembers according to the mixing gradient.60 The 1O2 production rates appeared to meet this expectation near the outlet of the St. Louis River. In the western arm of Lake Superior, the influence of the riverine water on the production rates extended several kilometers into the lake, which is a short distance given the lake’s overall scale (Figure 3). Judging from the data in Figure 3, the riverine water appeared to have an impact approximately 11 km into the western arm during the May−June time frame. During the Aug−Sept time frame, the extent of impact was approximately 6 km from the Superior Harbor, presumably due to lower river flow as well as lower CDOM concentrations. As expected the production rates did indeed scale with the CDOM as indicated by absorbance (data not shown). Looking at the 1O2 quantum yields, a slightly more complicated picture was uncovered. Instead of monotonically varying quantum yield values spanning the river and open lake endmember values, late season (Aug−Sep) quantum yields along a transect from the Duluth-Superior Harbor to the open lake showed a maximum in the middle of the transect at station AB2 (5.6 km from the Duluth-Superior Harbor) (Figure 4).

Figure 4. Singlet oxygen quantum yields (ΦΔ) plotted as a function of distance from the Superior harbor entrance. Negative distances indicate stations in the harbor or the St. Louis River. ● May−June (2007, 2008, 2009), ◊ Aug−Sept (2007, 2008).

Table 1. Seasonal Singlet Oxygen Distribution and Calculated Degradation Half-Lives of Cimetidine 1 cm

a

water column

water

[1O2]ss (fM)

k′ (s−1)

t1/2 (hr)

riverine river-impacted open lake

580 55 15

5.3 × 10−5 5.0 × 10−6 1.4 × 10−7

3.6 38 140

riverine river-impacted open lake

310 27 18

2.9 × 10−5 2.5 × 10−6 1.7 × 10−6

6.8 78 120

[1O2]ss (fM) May−June 1.6 -a 0.49 Aug−Sept 2 -a 0.70

k′ (s−1)

t1/2 (hr)

O2 flux (mol photons m−2 s−1)

1.5 × 10−7 -a 4.5 × 10−8

1300 -a 4300

1.5 × 10−6 -a 2.2 × 10−6

1.8 × 10−7 -a 6.4 × 10−8

1000 -a 3000

7.7 × 10−7 -a 2.2 × 10−6

1

These values were not calculated, because sunlight is not completely absorbed within the water column. 7226


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the rivers. In the bulk of Lake Superior, CDOM-initiated indirect photochemistry transformations will be relatively slow and other transformation processes (e.g., biodegradation or direct photoprocesses) will therefore be of relatively increased importance.

∑ Iλaλ λ

(9)

where the Naperian absorptivity, aλ, in a system with a welldefined sensitizer would be substituted by the product of the concentration of the sensitizer ([Sens]), the molar absorptivity coefficient (ε) of the sensitizer, and ln (10) to convert the decadic absorptivity coefficient to a Naperian one. The same theory also applies to mixtures of sensitizers or the CDOM in this system. Changes in the mixture would be expected to result in changes of Rf as in Figure 3 where the temporal difference in slopes indicates that the CDOM pool is not constant. In addition to the changes in Rf, the variation seen in the “quality” for CDOM (Figure 2), indicates that the CDOM pool in Lake Superior varies with time and space. Mixing of two endmember Lake Superior samples provides additional experimental validation of this photochemical theory (see the Supporting Information). Implications for the Variability of Singlet Oxygen in Lake Superior. To illustrate the importance of the spatial and temporal variability of 1O2 concentrations in Lake Superior, we used steady state concentrations to predict the half-life of cimetidine in the riverine, river-impacted, and open lake areas at the surface and in the water column (Table 1). Cimetidine, a pharmaceutical sold under the trade name Tagamet, is a compound that is known to degrade primarily by a 1O2mediated mechanism (eq 10).39

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ASSOCIATED CONTENT

S Supporting Information *

Study site and sample collection details, additional methods, water characterization and singlet oxygen production trends and data tables, data from the laboratory mixtures of lake water, plot of singlet oxygen formation rates vs SEC peak area, and plot of singlet oxygen formation rates from laboratory mixtures of lake water. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.B.C.) or [email protected]. ch (K.M.). Present Address ⊥

Environmental Sciences & Engineering, University of North Carolina, Chapel Hill, Chapel Hill, North Carolina, USA.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Meghan Jacobson, Rachel Lundeen, Andrea Little, Kristen Thoreson, Sarah Kliegman, Elodie Marlier, Angela Follett, Betsy Edhlund, and Kari Ann Torma who participated in the fieldwork, as well as the crews of the CCGS Limnos and R/V Blue Heron. We are grateful to Profs. R. W. Sterner and E. Minor for ship time aboard the R/V Blue Heron and for the ship time aboard the CCGS Limnos. We thank Prof. Charles Sharpless (University of Mary Washington) for helpful comments that improved the manuscript. This work was supported by the National Science Foundation under Grant OCE-0527196 and Minnesota Sea Grant under GrantR/EH04-05. We also gratefully acknowledge the University of Minnesota and the Large Lakes Observatory for support of this work.

The reaction rate constant that was used in the calculation was 9.2 × 107 M−1 s−1.39 The boundary conditions were calculated for the surface (1 cm) and for complete sunlight absorption in the water column to indicate the range of potential degradation times. Cimetidine half-lives in the top 1 cm of the water column were calculated to be 4 h, 38 h, and 140 h for riverine, river-impacted, and open lake sites in May, respectively. In August, the half-lives of cimetidine were calculated to be 7 h for the riverine sites, 78 h for the riverimpacted sites, and 120 h for the open lake sites. Based on this calculation for Lake Superior, the 1O2-based surface transformation of cimetidine, and other similarly reactive compounds, is expected to occur relatively rapidly in rivers, an order of magnitude slower at river-impacted lake sites, and another 1.5−4 times slower in the open lake. If the whole water column is taken into account, the half-lives increase to 55 days in May and 44 days in August for river sites and 178 days in May and 125 days in August for open lake sites. In the western arm of Lake Superior, singlet oxygen dynamics appeared to be controlled by an input of terrestrial CDOM. Based on the correlations seen with DOC and a300, the singlet oxygen steady state concentrations will be slightly higher where the CDOM concentration is higher, namely in riverine and river-impacted areas, and not necessarily where the quantum yield and flux is higher (Table 1). This is potentially significant, given that some pollutants, such as those associated with wastewater, are typically introduced to lakes, rivers, and estuaries in these “high” 1O2 areas. While the degradation of 1 O2-reactive compounds is higher in the river and river-impacted sites, it is important to keep in mind that the open lake sites represent 1000 times the area of the river-impacted areas where open lake singlet oxygen fluxes are slightly higher than those in

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REFERENCES

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Spatial and Temporal Distribution of Singlet Oxygen in Lake Superior

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