Environ. Sci. Technol. 1997, 31, 3712-3718
Influence of ortho-Substitution on Patterns of PCB Accumulation in Sediment, Plankton, and Fish in a Freshwater Estuary ERIC J. WILLMAN, JON B. MANCHESTER-NEESVIG, AND DAVID E. ARMSTRONG* Water Chemistry Program, University of WisconsinsMadison, 660 North Park Street, Madison, Wisconsin 53706
The accumulation of a group of non- and mono-ortho (coplanar) PCB congeners in aquatic food webs is of special interest due to their dioxin-like toxicities. Furthermore, higher octanol-water partition coefficients than homologs with greater ortho-substitution suggest the potential for selective accumulation of the coplanar congeners. We quantified 47 PCB congeners containing 0-3 ortho-chlorines from six homolog groups in sediments, plankton, and fish from Green Bay, WI, using conventional and multidimensional gas chromatography with electron capture detection. Of the congeners exhibiting dioxin-like toxicity, the only nonortho-substituted congener quantifiable in any of the matrices was IUPAC 77, but all of the toxic mono-ortho-substituted congeners were detected and quantified. Penta-, hexa-, and heptachloro congeners were enriched relative to other congeners in sediments and fish located more distant from the main source of the bay, whereas trichloro congeners were depleted. The same homolog groups also became more enriched as PCBs moved to higher levels in the ecosystem from sediment to plankton to fish, while trichloro congeners were depleted. Enrichment patterns of the most toxic (dioxin-like) congeners were also influenced mainly by total chlorine substitution; the degree of orthosubstitution did not systematically affect accumulation in plankton and fish.
Introduction Polychlorobiphenyls (PCBs) continue to be a topic of concern in aquatic ecosystems due to their persistence (1, 2) and toxicity (3-5). Of the 209 possible congeners, only the coplanar congenerssthose with zero or one chlorine in the ortho positionsexhibit dioxin-like toxicity in living organisms (3, 4). The coplanar PCBs are structurally homologous to the planar structure of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) with chlorine substituents at four lateral positions. Structuresactivity studies have shown that PCBs substituted at two para, two or more meta, and no ortho positions exhibit maximum TCDD-like activities. Accordingly, in vivo and in vitro experiments have shown the non-ortho IUPAC congeners 77, 126, and 169 to be the most toxic PCB congeners in mammals and fish (3-6). A group of mono-ortho congeners (IUPACs 105, 114, 118, 123, 156, and 157) also produce toxic responses in mammals (3, 4) but are less active in producing TCDD-like activity in fish (6). Environmental data on many of the toxic coplanar congeners is sparse due to the their * Corresponding author e-mail:
[email protected]; telephone: 608-262-0768; fax: 608-262-0454.
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coelution with other congeners using conventional capillary column gas chromatography (GC). Resolution of the coplanar congeners can be achieved using multidimensional gas chromatography (MDGC), where two GC columns are coupled in series (7). We utilized GC and MDGC to quantify coplanar and nonplanar PCB congener concentrations in sediments, plankton, and fish of Green Bay, WI. The partitioning of PCBs among water, sediments, and aquatic organisms and the bioaccumulation of PCBs during trophic transfer are linked to the physicochemical properties of PCB congeners. Good correlations are found between the octanol-water partition coefficient (Kow) and fish-water (810) and plankton-water (11) partition coefficients or bioconcentration factors (BCF) for congeners with log Kow < 6. Correlations between Kow and BCF reflect the utility of octanol as a surrogate for lipids and other natural organic materials. Correlations between Kow and the partition ratio between trophic levels have also been reported (12-14). Congeners with greater Kow values increase relative to the sum of all congeners (∑PCB) with sequential transfer between trophic levels. Thus, more chlorinated congeners (heavier homolog groups) that have greater Kow values are expected to increase relative to ∑PCB with increasing ecosystem compartment (fish > plankton > sediment). Within homolog groups, congeners with less ortho-substitution have greater Kow values (15) and are predicted to increase relative to their homolog group with increasing compartment. We used the congener concentrations of a wide range of PCB congeners from the Green Bay ecosystem to identify their accumulation trends. Our main purpose was to determine whether selective accumulation of coplanar congeners occurs as PCBs move to higher ecosystem compartments. We compared concentrations in sediment, plankton, and fish. Although transfer between these compartments may involve transfer through intermediate compartments, comparisons of these selected compartments can be used to assess partitioning trends and patterns. The sediment can be viewed as the source to the overlying water and water as the source to the phytoplankton. The contaminant then moves up the food web: phytoplankton and benthic invertebrates, zooplankton, planktivorous fish, and piscivorous or omnivorous fish. In addition, the contaminant is partitioned directly from water to all trophic levels. By analyzing sediment, plankton (a mixture of phytoplankton and zooplankton), and fish (mixture of piscivorous and omnivorous), we examined the local contaminant source, low-order trophic levels, and highorder trophic levels, respectively. Plankton samples were collected about 5 years after fish and sediments were obtained. However, our analysis (see Results) showed that changes in congener patterns in sediments over this time (depth) at a given location were not significant, allowing us to compare congener patterns within these compartments.
Experimental Section Sediments. Sediment data and extracts obtained during the Green Bay Mass Balance Study (GBMBS) were utilized in this study. The GBMBS was a multiagency study that applied an ecosystem-scale mass balance to PCBs in Green Bay. Relevant information from a more complete description (16) is summarized briefly. In the GBMBS, sediment core samples were collected between 1987 and 1990. Sampling sites were established by superimposing a grid with 5-km spacing over a map of the bay and assigning station numbers to the intersections. Sediments were analyzed in 1-cm increments for specific PCB congeners. Concentrations were highest near the southeastern section of the bay where sediments from the Fox River are focused (16). In this investigation, sediment
S0013-936X(97)00533-6 CCC: $14.00
1997 American Chemical Society
FIGURE 1. Map of Green Bay, WI. Sediment stations are the numbered squares. Plankton were sampled at stations 6 and 17. Fish were sampled within the zones defined by the indicated transects. Fish collection zones are indicated with an encircled number and letter. extracts from stations within this contaminated zone (see Figure 1) were further analyzed for coplanar congeners. Sediment mixed depth, defined as the layer of sediment mixed by physical processes and/or bioturbation, was also determined by the GBMBS. Green Bay sediments near the stations used in this study have a mixed depth of 2-3 cm (16). If the top three centimeters are well mixed, the PCBs in this layer are most available to partition to the overlying water column and biota. However, mass transfer limits the rate of sediment to water column transfer because the time scales for mixing and desorption may be relatively long. For this analysis, we considered only the top three layers of sediment from five stations (n ) 14, layer 3 of station 9 was not available). Plankton. Vertical tows with plankton nets were taken from the R/V Neeskay on 8/2/1994 and 9/13/1994 near stations 6 and 17 (Figure 1). A 63-µm plankton net was lowered to the bottom (station 6 ) 8 m, station 17 ) 14 m) and slowly raised through the water column. A sample consisted of multiple vertical tow collections (n ) 4) that were filtered through a Gelman Science Type A/E glass fiber filter (142 mm). The plankton sample was stored in pre-ashed and tared sample jars. Replicate samples (n ) 3) were taken on both dates. Further fractionation of the plankton samples into phytoplankton and zooplankton was not attempted. A total of eight composite samples was collected for analysis. To determine water content, a small amount of sample was transferred to an aluminum weighing dish and dried in an oven at 45 °C for 16 h. The water content was used to calculate PCB concentrations on a dry weight basis. For PCB analysis, approximately 30 g of wet sample was mixed with
sodium sulfate that had been ashed at 450 °C for 6.5 h. The mixture was ground with a mortar and pestle to produce a dry homogeneous matrix and Soxhlet extracted for 12 h with dichloromethane. Interferences were removed by silica gel and alumina column chromatography (17). Fish. Fish samples were obtained from the Wisconsin Department of Natural Resources (18). Brown trout (Salmo trutta), walleye (Stizostedion vitreum), and carp (Cyprinus carpio) were collected in the spring, summer, and fall of 1989 from five sections of Green Bay, covering its entirety (Figure 1). Individual samples (n ) 13) consisted of fillet composites of five different fish of the same age and species, collected in the same zone and season. Samples were extracted by the Wisconsin State Laboratory of Hygiene (19). The fillets were ground and further homogenized by freezing with dry ice and blending in a commercial blender. A 10-g subsample was dried with sodium sulfate and extracted in a column with 230 mL of dichloromethane. Lipids were removed using gel permeation chromatography. Further cleanup was performed with florisil and silica gel column chromatography. QA/QC. Surrogate IUPAC congeners 14, 65, and 166 were added to all samples prior to extraction to monitor analytical recoveries (accuracy). Average recoveries for these congeners, respectively, were 84%, 84%, and 90% in sediment; 104%, 101%, and 111% in plankton; and 97%, 101%, and 106% in fish. The minimum recovery for an individual sample was 85%, and the maximum was 138%. Data from two fish samples were discarded due to low recovery. Concentrations were not corrected to recoveries. Precision was ensured on
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TABLE 1. PCB Congeners Selected for Analysis congenera
chlorineb
orthoc
log Kowd
methode
congenera
chlorineb
orthoc
log Kowd
methode
18 22 25 26 33 40 49 52 53 63 74 77 81 82 83 85 87 97 99 105 107 114 118 119
3 3 3 3 3 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5
2 1 1 1 1 2 2 2 3 1 1 0 0 2 2 2 2 2 2 1 1 1 1 2
5.24 5.58 5.67 5.66 5.60 5.66 5.85 5.84 5.62 6.17 6.20 6.36 6.36 6.20 6.26 6.30 6.29 6.29 6.39 6.65 6.71 6.65 6.74 6.58
GC GC GC GC GC GC GC GC GC GC GC MDGC GC GC GC GC GC GC GC MDGC GC MDGC GC GC
123 126 128 130 141 146 149 151 156 157 167 169 174 177 180 183 185 189 191 193 194 198 201
5 5 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 8 8 8
1 0 2 2 2 2 3 3 1 1 1 0 3 3 2 3 3 1 2 2 2 3 3
6.74 6.89 6.74 6.80 6.82 6.89 6.67 6.64 7.18 7.18 7.27 7.42 7.11 7.08 7.36 7.20 7.11 7.71 7.55 7.52 7.80 7.62 7.62
MDGC MDGC GC GC GC GC GC GC MDGC MDGC GC GC GC GC GC GC GC GC GC GC GC GC GC
a IUPAC number. b Degree of chlorination. c Number of ortho-chlorines. chromatography; MDGC is multidimensional GC.
both GC systems by running replicate samples and confirming small relative standard deviations (typically sediment) and multiplying by 100. In Figures 3 and 5 the percent changes for each congener were normalized by dividing by the average of the two compartment means. (1) Congener Concentrations Relative to ∑PCB. For the within compartment comparisons, few or no significant differences in congener relative concentrations were found for sediment collection location, sediment depth, plankton collection date, or fish species. However, differences between collection zones in fish concentrations were significant. The light congeners made up a larger fraction of ∑PCB in zones 1 and 2, and the heavier congeners were more abundant in the fish collected in the northern bay in zones 3 and 4. Between compartments, many significant differences in relative concentrations were found (Figure 3). Thus, changes in relative distributions of congeners are caused mainly by transfer between compartments rather than by withincompartment variability. The plankton and fish are enriched with the high IUPAC (high Kow) congeners and depleted in the low IUPAC (low Kow) congeners when compared to the
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FIGURE 3. Enrichment or depletion (%) of congener concentration between compartments. % change ) {(UCmean - LCmean)/[(UCmean + LCmean)/2]} × 100 where mean ) ∑(congeneri/∑PCB in the sample)/n, UC ) upper compartment, LC ) lower compartment, and n ) number of samples of the compartment (sediment, plankton, or fish). An asterisk indicates statistical significance.
FIGURE 4. Enrichment or depletion (%) of homolog group concentration between compartments. % change ) (UCmean - LCmean) × 100, where mean ) ∑(homolog groupi/∑PCB in the sample)/n. An asterisk indicates statistical significance. sediment. Relative to sediment, the non- and mono-ortho congeners greater than IUPAC 81 generally increase in abundance in plankton and fish along with the multipleortho congeners. Heavier congeners were also enriched in fish relative to plankton, but the differences were less pronounced. (2) Homolog Concentrations Relative to ∑PCB. Within compartments, no significant differences in relative concentrations were found for plankton collection dates, fish species, or sediment collection depth. However, significant differences in relative concentrations were found between
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fish collection zones with tri- and tetrachloro congeners being depleted in zones 3 and 4 while hexa- and heptachloro congeners were enriched in these zones. Significant differences in sediment collection location were also observed. Tri- and tetrachloro congeners were depleted at station 17, and hexa- and heptachloro congeners were enriched at this station. Comparing between compartments, we observed a systematic enrichment of penta-, hexa-, and heptachloro congeners and a systematic depletion of trichloro congeners with increasing compartment (Figure 4). The homolog composition of Aroclor 1242 is also shown for comparison but was not included in the statistical analysis. If Aroclor 1242 is representative of the composition of the PCB source to Green Bay, a further depletion of lighter trichloro congeners is apparent in all compartments. (3) Congener Concentrations Relative to Homolog Groups. Within compartments, few significant differences were observed. Between compartments many significant differences were found (Figure 5). However, patterns related to ortho-substitution within a homolog group were not evident. If the mono-ortho and non-ortho congeners were systematically enriched with increasing compartment, the shaded bars would be positive and significant while the unshaded multiple-ortho congeners would be negative and significant. Thus, we did not find a systematic enrichment in the mono- and non-ortho-substituted congeners with increasing compartment. In fact, many coplanar congeners, notably IUPAC 77, are depleted with increasing compartment. As stated earlier, congeners with less ortho-substitution generally have greater Kow values (15) within homolog groups. Based on this, we expected that concentrations of these congeners might increase relative to their homolog group with increasing trophic level. The contrary results may be due to uncertainty in the Kow values, uncertainty in our data, or other factors such as steric effects on adsorption (discussed below). The similarity in accumulation patterns in plankton and fish relative to sediment and high analytical reproduc-
FIGURE 5. Enrichment or depletion (%) between compartments of congener concentration within a homolog group. % change ) {(UCmean - LCmean)/[(UCmean + LCmean)/2]} × 100, where mean ) ∑(congeneri/∑PCB in its homolog group)/n. An asterisk indicates statistical significance.
FIGURE 6. Observed changes between fish and sediment in relative congener concentrations (relative to ∑PCB) and the predicted changes from the least-squares regression: change ) 2929 - 414 log Kow - 3700 SEC + 537KowSEC (p ) 0.000, R 2 ) 54.5%). (b) Fish-sediment; (O) regression fits. ibility suggest that the observed patterns are not due to experimental error. Larger uncertainties may exist for the congener Kow values, as many were obtained by regression rather than experimental measurement. General Trends. As Fox River water and sediments contaminated with Aroclor 1242 are washed into the bay, the lighter congeners are depleted, and a mixture enriched with the heavier congeners is deposited into sediments in an area near Station 17. A systematic loss of lighter congeners with increasing distance from the source (Fox River) is apparent in fish as well as in sediments. Further enrichment of the
heavier congeners occurs through transfer up the food chain. However, less chlorine substitution at ortho positions does not result in increased accumulation. For those mono- and non-ortho congeners exhibiting enrichment, their increased total chlorine substitution rather than coplanar structure appears to be the main factor controlling accumulation. The non-ortho, tetrachloro congener 77 decreased in relative abundance both within ∑PCB and within its homolog group with increasing compartment. Similarly, Bright et al. (26) observed a decrease in congener 77 relative to ∑PCB in coastal arctic sediment, sea urchins, and four-horn sculpins
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from the Northwest Territories, Canada. Furthermore, Falandysz et al. (27) observed selective enrichment of the pentachlorobiphenyl 126 and of the hexachlorobiphenyl 169 but depletion of congener 77 in cod liver oil from the Baltic Sea relative to Kanechlor technical mixtures. Although we did not detect congeners 126 and 169 in Green Bay, the results are consistent with our observations of enrichment of pentaand hexachloro congeners. In the yellow eel from The Netherlands, de Boer et al. (24) suggested metabolic degradation of 77. The depletion of congener 77 in fish suggests that metabolic degradation may also be occurring in Green Bay. Accumulation of PCB congeners may be influenced by their ability to adsorb onto a surface in addition to their tendency to partition into an organic-rich matrix. Shaw et al. (28) found that the former effect was dependent on stereochemistry and developed empirical coefficients for chlorines in different positions to quantitatively estimate the effect of PCB structure on adsorption to the surfaces of solids. The coefficients for each chlorine position are multiplied to give a steric effect coefficient (SEC) for the congener. We calculated SECs for the 47 congeners that were analyzed in our study and regressed the SEC along with log Kow and an interaction term against the changes plotted in Figures 3 and 5. The regressions were significant (p ) 0.000) for fishsediment changes relative to ∑PCB (Figure 6) and for plankton-sediment changes relative to ∑PCB (p ) 0.000). The data demonstrate that steric effects become significant for the highly chlorinated congeners and decrease the accumulation in plankton and fish that would be predicted from octanol-water partitioning alone. Regressions were not significant for the changes within homolog groups, implying that steric effect coefficients as estimated by Shaw et al. are not good predictors of the more subtle trends within homolog groups for our data set. The low R2 values for plankton-sediment and fish-sediment concentration changes relative to ∑PCB (45.0% and 54.5%, respectively) also reflect the more subtle lack of fit within homolog groups (Figure 6). We conclude that accumulation patterns of PCB in plankton and fish from their sediment source are largely determined by chlorine content, which affects partitioning to organic phases and surface adsorption. Maximum preferential accumulation occurs for the penta-, hexa-, and heptachloro congeners, respectively. The tri-, tetra-, and octachloro congeners are depleted due to low lipophilicity and high steric effects, respectively. The accumulation of toxic non- and mono-ortho-substituted congeners is also largely predicted by degree of chlorination rather than orthosubstitution.
Acknowledgments This work was funded by the University of Wisconsin Sea Grant Institute under grants from the National Sea Grant College Program, the National Oceanic and Atmospheric Administration, the U.S. Department of Commerce, and the State of Wisconsin. Federal Grant NA90AA-D-SG469, Project Number R/GB-38. The authors would like to thank William Sonzogni, David Degenhart, Carol Buelow, and David Rogers
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of the Wisconsin State Laboratory of Hygiene for their assistance and for the use of the Siemens multidimensional gas chromatograph.
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Received for review June 16, 1997. Revised manuscript received September 8, 1997. Accepted September 11, 1997.X ES970533G X
Abstract published in Advance ACS Abstracts, October 15, 1997.
