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sediment–seagrass–dugong (Dugong dugon) food chain on the Great Barrier Reef (Australia)
Environmental Pollution 113 (2001) 129±134
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PCDDs in the water/sediment±seagrass±dugong (Dugong dugon) food chain on the Great Barrier Reef (Australia) M.S. McLachlan a,*, D. Haynes b, J.F. MuÈller c a
Ecological Chemistry and Geochemistry, University of Bayreuth, D-95440 Bayreuth, Germany b Great Barrier Reef Marine Park Authority, PO Box 1379, Townsville, 4810 Australia c National Research Centre for Environmental Toxicology, University of Queensland, 39 Kessels Road, Coopers Plains, 4108 Australia Received 17 November 1999; accepted 8 July 2000
``Capsule'': Sources of PCDD/Fs to dugongs living o the Great Barrier Reef are considered to explain the unusual array of congeners present. Abstract Polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) concentrations were measured in sediment and seagrass from ®ve locations in or adjacent to the Great Barrier Reef Marine Park. A full spectrum of Cl5 8DDs were present in all samples and, in particular, elevated levels of Cl8DD were found. PCDFs could not be quanti®ed in any samples. The PCDD concentrations ranged over two orders of magnitude between sites, and there was a good correlation between sediment and seagrass levels. There were large quantities of sediment present on the seagrass (20±62% on a dry wt. basis), and it was concluded that this was a primary source of the PCDDs in the seagrass samples. The PCDD levels in the seagrass samples were compared with the levels in the tissue of three dugongs stranded in the same region. The relative accumulation of the 2,3,7,8-substituted PCDD congeners in the dugongs decreased by over two orders of magnitude with increasing degree of chlorination. This was attributed to the reduced absorption of the higher chlorinated congeners in the digestive tract, a behaviour that has been observed in other mammals such as domestic cows. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Great Barrier Reef; Dugong; PCDD/Fs; Bioaccumulation; Food chain; Marine mammals
1. Introduction The dugong (Dugong dugon) is unique among the earth's fauna, in that it is the only completely marine mammal that is herbivorous. It feeds almost exclusively on a number of genera of seagrasses, especially Halodule and Halophila (Marsh et al., 1982; Marsh, 1992). Nearshore seagrass beds along the northeastern coast of Australia provide important habitat and feeding grounds for a signi®cant proportion of existing world stocks of dugong (Marsh). Aerial surveys have indicated that there has been a dramatic decline of at least 50% in dugong numbers in southern Great Barrier Reef waters between 1984 and 1994 (Marsh and Corkeron, 1997). This is of * Corresponding author. Present address: Baltic Sea Research Institute, Postfach 301161, D-18112 Rostock, WarnemuÈnde, Germany. Tel.: +49-381-5197-300; fax: +49-381-5197-302. E-mail address: michael.mclachlan@io- warnemuende.de (M.S. McLachlan).
particular concern as the dugong has been endangered or exterminated over much of its range and is considered to be vulnerable to extinction (IUCN, 1990). The reasons for the decline are unclear, but are likely to include indigenous hunting and accidental capture in ®shing nets, as well as loss of seagrass habitat caused by water quality degradation (Marsh; Preen and Marsh, 1995). A further possible cause of the decline is exposure to persistent organic pollutants. Accumulation of organochlorine pesticides and polychlorinated biphenyls (PCBs) has been implicated in reproductive and immunological abnormalities observed in bird (Kubiak et al., 1989) and marine mammal populations (Ross et al., 1996) in the northern hemisphere. In a study of persistent organic pollutants and heavy metals in dugongs, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) were found at concentrations comparable to those found in carnivorous marine mammals (Haynes et al., 1999). The high levels of octachlorodibenzo-p-dioxin (Cl8DD) present in dugong
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M.S. McLachlan et al. / Environmental Pollution 113 (2001) 129±134
compared to other marine mammals were particularly surprising. Several possible explanations for these high levels were proposed including a high rate of sediment ingestion by the dugongs during feeding, formation of Cl8DD in the digestive tract of the animal, and selective degradation of other PCDD/F congeners (Haynes et al.). In this paper the accumulation of PCDD/Fs in the sediment/water±seagrass±dugong food chain is investigated with the view to resolving this question and, furthermore, gaining information about the accumulation of organic contaminants in marine food chains. 2. Materials and methods Sediment and seagrass samples were collected at ®ve sites in or adjacent to the Great Barrier Reef Marine Park (Fig. 1). Samples were collected between February and May 1997. All sampling locations are in the vicinity of important dugong habitat (Marsh and Corkeron, 1997). At each location three replicate sediment and three replicate seagrass samples were collected into 1-l glass jars. Each sediment sample was a composite of multiple sur®cial sediment samples scooped directly into a solvent-washed glass jar. Sediments were collected randomly over an area of approximately 400 m2. A random sample of the dominant seagrass (Halodule uninervis or Zostera capricorni) was also collected over the same area. Entire plants (leaves, roots, rhizomes) were sampled in accordance with dugong feeding habits. Plants were rinsed in seawater at the time of collection to simulate dugong feeding which appears to eciently remove coarse sediments associated with seagrass leaves and rhizomes prior to plant material ingestion (Heinsohn and Birch, 1972). Aliquots of the sediment and seagrass subsamples were combined in the laboratory to provide one sample per sampling site for dioxin analysis. These samples and a blank consisting of 100 g of anhydrous Na2SO4 were freeze dried at the Queensland Health Services laboratory in Brisbane and then transported in sealed containers by courier to the University of Bayreuth, Germany, for analysis. The quantity of sediment contamination in the seagrass samples was estimated by comparing the per cent dry weight of the composite sample with the per cent dry weight fraction of a representative sub-sample that had been washed to remove the sediment. The sampling jars containing the seagrass were ®lled with water, closed and the contents were shaken for about a minute. The jars were set aside to allow the larger particles to settle out and the seagrass was carefully ®ltered through a sieve. The jar was cleaned of sediments, the seagrass returned to the jar and the washing was repeated three times. A dry weight fraction of 60% was assumed for the sediment.
Blubber samples were collected from three mature dugongs (two males and one female) that were stranded in the Great Barrier Reef Marine Park (Fig. 1). All three animals were suspected or con®rmed net drownings and the carcasses were in good condition at the time of sampling. The analytical methodology employed and the results for these samples have been presented elsewhere (Haynes et al., 1999). In Bayreuth, Germany, the seagrass samples were milled in a blender. The sediment and seagrass samples were homogenised and aliquots of approximately 40 g (seagrass) or 10 g (sediment) were soxhlet-extracted in toluene for 16 h following addition of a mixture of 12 13 C12-labelled 2,3,7,8-substituted PCDD/F congeners to the extraction solvent. The extracts were cleaned up using a H2SO4/silica gel+NaOH/silica gel mixed column and an alox column as described in Horstmann et al. (1992). The puri®ed extracts were concentrated to 10 ml, a recovery standard was added, and the samples were analysed for the 2,3,7,8-substituted PCDD/F congeners using HRGC/HRMS on a VG Autospec Ultima at a resolution of 10,000. The analytical parameters employed are described in Horstmann and McLachlan (1995). The quality assurance programme includes the determination of internal standard recoveries, the veri®cation of relative retention times and proper isotope ratios, and laboratory blanks. The recovery of the labelled internal standards was >80%, and no evidence of interferences was found. The limiting factor for the quanti®cation of the analytes were the laboratory blanks. An analyte was only quanti®ed if the amount in the sample was at least a factor of 5 above the laboratory blank. 3. Results and discussion 3.1. Uptake in seagrass The PCDD/F concentrations in both sediment and seagrass were dominated by Cl8DD, which with concentrations ranging from 14 to 2900 pg/g dry weight accounted for over 90% of the 2,3,7,8-substituted congeners in all samples (Table 1). The levels of 1,2,3,4,6, 7,8-Cl7DD were 11±35 times lower, and the concentrations of the Cl6DD congeners were at least another order of magnitude less. The PCDFs and 2,3,7,8-Cl4DD were below the limit of quanti®cation in most samples. The PCDD concentrations in the sediments ranged over more than two orders of magnitude between the sampling sites. A good correlation was observed between the concentrations in sediment and in seagrass from the same sampling point, as shown for Cl7DD and Cl8DD in Fig. 2. One possible explanation for this observation could be that the concentrations in the seagrass samples were
M.S. McLachlan et al. / Environmental Pollution 113 (2001) 129±134
131
Fig. 1. Map showing the sediment, seagrass and dugong sampling sites (F, Flinders Island; CW, Cardwell; P, Pallarenda; U, Upstart Bay; N, Newry Bay).
primarily determined by the sediment particles that adhered to the seagrass. Although visual examination of the seagrass samples did not indicate that it was heavily contaminated with sediment, the laboratory determination of sediment content indicated that sediment made up between 20 and 62% of the dry weight of the samples as analysed. Furthermore, the dry weight-based concentrations in the two matrices were similar. In Fig. 3,
the quotient of the seagrass and sediment concentrations is plotted for each of the PCDD congeners that were quanti®ed in both matrices. The quotients were close to 1 with the exception of the Upstart site. Here the levels in seagrass were close to the limit of quanti®cation and consequently were likely overestimated, which may explain the higher quotients. Also, at a given site the quotients were similar for the dierent
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Table 1 PCDD concentrations in the sediment and seagrass samplesa Sampling station
2,3,7,8-Cl4DD 1,2,3,7,8-Cl5DD 1,2,3,4,7,8-Cl6DD 1,2,3,6,7,8-Cl6DD 1,2,3,7,8,9-Cl6DD 1,2,3,4,6,7,8-Cl7DD Cl8DD a
Sediment (pg/g dry wt.)
Seagrass (pg/g dry wt.)
U
P
F
N
CW
U
P
F
N
CW
0.01 0.01 0.02 0.02 0.04 0.67 14
0.02 0.07 0.09 0.21 0.29 6.2 130
0.03 0.15 0.32 0.48 1.2 12 190
0.09 0.40 0.95 1.5 2.7 57 1200
0.06 0.39 1.2 1.8 3.7 77 2900
0.01 0.03 0.08 0.11 0.18 2.5 49
0.02 0.14 0.29 0.40 0.52 8.2 90
0.03 0.26 0.43 0.66 1.5 15 200
0.07 0.44 1.1 1.5 2.5 47 890
0.04 0.38 1.1 1.5 2.4 44 1300
U, Upstart Bay; P, Pallarenda; F, Flinders Island; N, Newrny Bay; CW, Cardwell.
fractions typically have a higher organic carbon content and higher dry weight-based concentrations. It is considered unlikely that the congener pattern in the relatively short-lived seagrass resulting from uptake of dissolved chemical from the water column would be identical to the congener pattern in sediment. On the balance of the evidence it is concluded that ingestion of sediment associated with seagrass is a major vector of PCDDs to dugongs. Fig. 2. Plot of the sediment concentration versus the seagrass concentration of 1,2,3,4,6,7,8-Cl7DD and Cl8DD at the ®ve sampling sites.
Fig. 3. Histogram of the quotient of the dry weight-based seagrass and sediment concentrations for the 2,3,7,8-substituted polychlorinated dibenzo-p-dioxins at each of the sampling sites. 2,3,7,8Cl4DD is not included on the diagram because it was below the limit of quanti®cation in the sediment samples.
congeners, demonstrating that the congener patterns in the seagrass and the sediment were similar. All of these observations are consistent with the hypothesis that the sediment was a major source of the PCDDs in the samples. The contribution of the sediment to the PCDD concentrations in seagrass was estimated by multiplying the PCDD concentration in sediment by the dry weight fraction of sediment in the seagrass. The contribution of the sediment exceeded 15% at all sites and 50% at two of the sites. This calculation may yield an underestimation, since the sediment adhering to the seagrass would be expected to have a smaller average particle size than the bulk sediment, and the ®ner sediment
3.2. Uptake in dugongs The PCDD/F levels found in the three stranded dugongs have been presented elsewhere (Haynes et al., 1999). Brie¯y, only two of the PCDF congeners were detected in the dugong blubber, whereas all 2,3,7,8substituted PCDDs were quanti®ed. Among the PCDDs, the levels of C18DD were unusually high compared with other marine mammals. A quantitative estimate of the uptake of PCDDs by the dugongs was not possible due to the uncertainties in the animals age and exposure, particularly in view of the wide range of seagrass concentrations measured. However, a comparison of the relative accumulation of the dierent PCDD congeners gives important insights into the accumulation behaviour. Such a comparison is possible because the PCDD pattern (the relative concentrations of the dierent congeners) was similar in each of the seagrass samples as well as in each of the dugong samples (although the patterns in dugong tissue and seagrass were dierent). In Fig. 4, the quotient of the average concentration in the dugongs samples and the average concentration in the seagrass samples is plotted for each of the 2,3,7,8-substituted PCDD congeners. This quotient decreases by more than two orders of magnitude with increasing degree of chlorination, indicating that a much larger fraction of the lower chlorinated congeners in the food supply had accumulated in the dugongs' tissue. There are a number of dierent explanations for this observation. One is that the PCDD pattern measured in the food supply is not representative of the pattern in the
M.S. McLachlan et al. / Environmental Pollution 113 (2001) 129±134
Fig. 4. Histogram of the quotient of the average concentrations measured in dugongs (three animals, pg/g lipid) and the average concentration measured in seagrass (®ve samples, pg/g dry wt.).
food that the dugongs consumed over their lifetime. It could be hypothesised that the contamination in the sediments and seagrass is recent and that the dugongs, which were at least 10 years old, had previously been exposed to a dierent PCDD pattern in food. However, given the huge spatial extent of the contamination (the sampling sites extended over 1000 km of coastline), the limited and heterogeneous anthropogenic activity in this region, and the homogeneity of the PCDD pattern in the seagrass/sediment, we consider it unlikely that the contamination was very recent. Another possible explanation for the dierent degree of accumulation in dugong tissue is metabolism. More rapidly metabolised compounds would accumulate less than more persistent compounds. This would imply that the metabolism of Cl8DD is more than two orders of magnitude faster than metabolism of 2,3,7,8-Cl4DD. However, this is contrary to the published literature on PCDD metabolism which shows that the metabolism of the higher chlorinated 2,3,7,8-substituted congeners is slower than for the lower chlorinated (Van den Berg et al., 1994). While large-scale metabolism of the 2,3,7,8-substituted PCDDs is considered unlikely, there was evidence of metabolism of some of the non-2,3,7,8-substituted congeners. The chromatograms of the homologues in the seagrass and sediment samples contained more isomers and dierent isomer patterns than the chromatograms of the dugong tissue samples. The absorption and tissue distribution of the 2,3,7,8-substituted and non2,3,7,8-substituted congeners in a given PCDD homologue are expected to have been similar since their physical±chemical properties are similar. Therefore, the changes in the isomer patterns in the chromatograms were likely due to metabolism. There were, however, some non-2,3,7,8-substituted congeners which were as persistent in the dugong as the 2,3,7,8-substituted congeners. A more likely explanation for the dierent accumulation behaviour of the PCDD congeners is dierences in absorption in the digestive tract. Decreasing net
133
absorption of PCDDs with increasing degree of chlorination has been observed in a number of mammals including another herbivore, the domestic cow (McLachlan et al., 1990; McLachlan and Richter, 1998). Net absorption is the result of two competing processes: advection through the digestive tract, and transport from the lumen across the intestinal wall to the lymph and blood. It is believed that the transport of very hydrophobic compounds like PCDDs across the intestinal wall is limited by an aqueous diusion resistance (Gobas et al., 1988). The higher the lipid/water partition coecient of the compound, the smaller the quantity that partitions out of the lipophilic phases in the lumen into these aqueous phases and the lower the rate of transport across the aqueous barrier. The result is a decrease in net absorption with increasing lipid/water partition coecient. 4. Concluding discussion Three hypotheses to explain the high levels of Cl8DD in dugongs were presented in the introduction: ingestion of contaminated sediment, formation in the digestive tract, and selective degradation of other PCDD/F congeners. The evidence gathered in this study supports the ®rst hypothesis. The fact that the dry weight-based concentrations of the PCDDs in seagrass correlated with those in the sediment, that the PCDD concentrations and the PCDD patterns in seagrass were similar to those in the sediment at a given site, and that a large fraction of the seagrass sample dry weight was contributed by sediment all support the conclusion that a large fraction of the PCDDs ingested by the dugongs originates from sediment. Selective formation of Cl8DD in the digestive tract or selective degradation of other congeners are unlikely explanations since the transfer of Cl8DD from the diet to dugong tissue is actually much less than for the other 2,3,7,8-substituted congeners. The high levels of Cl8DD in dugongs are due to unusually high levels in the sediments which are ingested with the seagrass and transferred to the dugongs in accordance with established environmental chemistry principles for persistent hydrophobic pollutants. Acknowledgements We thank Frieder BoÈhme for cleaning up the samples, Stefan Will for his assistance with the analysis, and Leon Jackson (GBRMPA) for assistance with Fig. 1. References Gobas, F.A.P.C., Muir, D.C.G., Mackay, D., 1988. Dynamics of dietary bioaccumulation and faecal elimination of hydrophobic chemicals in ®sh. Chemosphere 17, 943±962.
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Haynes, D., MuÈller, J.F., McLachlan, M.S., 1999. Polychlorinated dibenzo-p-dioxins and dibenzofurans in Great Barrier Reef (Australia) dugongs (Dugong dugon). Chemosphere 38, 255±262. Heinsohn, G.E., Birch, W.R., 1972. Food and feeding habits of the dugong, Dugong dugong (Erxleben), in northern Queensland, Australia. Mammalia 36, 414±422. Horstmann, M., McLachlan, M.S., 1995. Results of an initial survey of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) in textiles. Chemosphere 31, 2579±2589. Horstmann, M., Kaune, A., McLachlan, M.S., Ressinger, M., Hutzinger, O., 1992. Temporal variability of PCDD/F concentrations in sewage sludge. Chemosphere 25, 1463±1468. IUCN (International Union for the Conservation of Nature Natural Resources), 1990. 1990 IUCN Red List of Threatened Animals. World Conservation Union, Gland, Switzerland. Kubiak, T.G., Harris, H.J., Smith, L.M., Schartz, T.R., Stalling, D.L., Trick, J.A., Sileo, L., Docherty, D.E., Erdman, T.C., 1989. Microcontaminants and reproductive impairment of the Foster's tern on Green Bay, Lake Michigan Ð 1983. Archives of Environmental Contamination and Toxicology 18, 706±727. Marsh, H., 1992. Biological Basis for Managing Dugongs and Other Large Vertebrates in the Great Barrier Reef Marine Park (Research Publication No. 21). Great Barrier Reef Marine Park Authority, Townsville, Queensland, Australia.
Marsh, H., Corkeron, P., 1997. The status of the dugong in the Great Barrier Reef Marine Park. In: Wachenfeld, D., Oliver, J., Davis, K. (Eds.), Proceedings of the State of the Great Barrier Reef World Heritage Area Workshop, November 1995. Great Barrier Reef Marine Park Authority, Townsville, Queensland, Australia, pp. 231±247. Marsh, H., Channels, P., Heinsohn, G., Morrissey, J., 1982. Analysis of stomach contents of dugongs from Queensland. Australian Wildlife Research 9, 55±67. McLachlan, M.S., Richter, W., 1998. Uptake and transfer of PCDD/ Fs by cattle fed naturally contaminated feedstus and feed contaminated as a result of sewage sludge application. 1. Lactating cows. Journal of Agriculture and Food Chemistry 46, 1166±1172. McLachlan, M.S., Thoma, H., Reissinger, M., Hutzinger, O., 1990. PCDD/F in an agricultural food chain. Part 1: PCDD/F mass balance of a lactating cow. Chemosphere 20, 1013±1020. Preen, A., Marsh, H., 1995. Response of dugongs to large-scale loss of seagrass from Hervey Bay, Queensland, Australia. Australian Wildlife Research 22, 507±519. Ross, P., De Swart, R., Addison, R., Van Loveren, H., Vos, J.G., Osterhaus, A., 1996. Contaminant-induced immunotoxicity in harbour seals: wildlife at risk? Toxicology 112, 157±169. Van den Berg, M., De Jongh, J., Poiger, H., Olson, J.R., 1994. The toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and their relevance for toxicity. Critical Reviews in Toxicology 24, 1±74.
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PCDDs in the water/sediment±seagrass±dugong (Dugong dugon) food chain on the Great Barrier Reef (Australia) M.S. McLachlan a,*, D. Haynes b, J.F. MuÈller c a
Ecological Chemistry and Geochemistry, University of Bayreuth, D-95440 Bayreuth, Germany b Great Barrier Reef Marine Park Authority, PO Box 1379, Townsville, 4810 Australia c National Research Centre for Environmental Toxicology, University of Queensland, 39 Kessels Road, Coopers Plains, 4108 Australia Received 17 November 1999; accepted 8 July 2000
``Capsule'': Sources of PCDD/Fs to dugongs living o the Great Barrier Reef are considered to explain the unusual array of congeners present. Abstract Polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) concentrations were measured in sediment and seagrass from ®ve locations in or adjacent to the Great Barrier Reef Marine Park. A full spectrum of Cl5 8DDs were present in all samples and, in particular, elevated levels of Cl8DD were found. PCDFs could not be quanti®ed in any samples. The PCDD concentrations ranged over two orders of magnitude between sites, and there was a good correlation between sediment and seagrass levels. There were large quantities of sediment present on the seagrass (20±62% on a dry wt. basis), and it was concluded that this was a primary source of the PCDDs in the seagrass samples. The PCDD levels in the seagrass samples were compared with the levels in the tissue of three dugongs stranded in the same region. The relative accumulation of the 2,3,7,8-substituted PCDD congeners in the dugongs decreased by over two orders of magnitude with increasing degree of chlorination. This was attributed to the reduced absorption of the higher chlorinated congeners in the digestive tract, a behaviour that has been observed in other mammals such as domestic cows. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Great Barrier Reef; Dugong; PCDD/Fs; Bioaccumulation; Food chain; Marine mammals
1. Introduction The dugong (Dugong dugon) is unique among the earth's fauna, in that it is the only completely marine mammal that is herbivorous. It feeds almost exclusively on a number of genera of seagrasses, especially Halodule and Halophila (Marsh et al., 1982; Marsh, 1992). Nearshore seagrass beds along the northeastern coast of Australia provide important habitat and feeding grounds for a signi®cant proportion of existing world stocks of dugong (Marsh). Aerial surveys have indicated that there has been a dramatic decline of at least 50% in dugong numbers in southern Great Barrier Reef waters between 1984 and 1994 (Marsh and Corkeron, 1997). This is of * Corresponding author. Present address: Baltic Sea Research Institute, Postfach 301161, D-18112 Rostock, WarnemuÈnde, Germany. Tel.: +49-381-5197-300; fax: +49-381-5197-302. E-mail address: michael.mclachlan@io- warnemuende.de (M.S. McLachlan).
particular concern as the dugong has been endangered or exterminated over much of its range and is considered to be vulnerable to extinction (IUCN, 1990). The reasons for the decline are unclear, but are likely to include indigenous hunting and accidental capture in ®shing nets, as well as loss of seagrass habitat caused by water quality degradation (Marsh; Preen and Marsh, 1995). A further possible cause of the decline is exposure to persistent organic pollutants. Accumulation of organochlorine pesticides and polychlorinated biphenyls (PCBs) has been implicated in reproductive and immunological abnormalities observed in bird (Kubiak et al., 1989) and marine mammal populations (Ross et al., 1996) in the northern hemisphere. In a study of persistent organic pollutants and heavy metals in dugongs, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) were found at concentrations comparable to those found in carnivorous marine mammals (Haynes et al., 1999). The high levels of octachlorodibenzo-p-dioxin (Cl8DD) present in dugong
0269-7491/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0269-7491(00)00172-X
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M.S. McLachlan et al. / Environmental Pollution 113 (2001) 129±134
compared to other marine mammals were particularly surprising. Several possible explanations for these high levels were proposed including a high rate of sediment ingestion by the dugongs during feeding, formation of Cl8DD in the digestive tract of the animal, and selective degradation of other PCDD/F congeners (Haynes et al.). In this paper the accumulation of PCDD/Fs in the sediment/water±seagrass±dugong food chain is investigated with the view to resolving this question and, furthermore, gaining information about the accumulation of organic contaminants in marine food chains. 2. Materials and methods Sediment and seagrass samples were collected at ®ve sites in or adjacent to the Great Barrier Reef Marine Park (Fig. 1). Samples were collected between February and May 1997. All sampling locations are in the vicinity of important dugong habitat (Marsh and Corkeron, 1997). At each location three replicate sediment and three replicate seagrass samples were collected into 1-l glass jars. Each sediment sample was a composite of multiple sur®cial sediment samples scooped directly into a solvent-washed glass jar. Sediments were collected randomly over an area of approximately 400 m2. A random sample of the dominant seagrass (Halodule uninervis or Zostera capricorni) was also collected over the same area. Entire plants (leaves, roots, rhizomes) were sampled in accordance with dugong feeding habits. Plants were rinsed in seawater at the time of collection to simulate dugong feeding which appears to eciently remove coarse sediments associated with seagrass leaves and rhizomes prior to plant material ingestion (Heinsohn and Birch, 1972). Aliquots of the sediment and seagrass subsamples were combined in the laboratory to provide one sample per sampling site for dioxin analysis. These samples and a blank consisting of 100 g of anhydrous Na2SO4 were freeze dried at the Queensland Health Services laboratory in Brisbane and then transported in sealed containers by courier to the University of Bayreuth, Germany, for analysis. The quantity of sediment contamination in the seagrass samples was estimated by comparing the per cent dry weight of the composite sample with the per cent dry weight fraction of a representative sub-sample that had been washed to remove the sediment. The sampling jars containing the seagrass were ®lled with water, closed and the contents were shaken for about a minute. The jars were set aside to allow the larger particles to settle out and the seagrass was carefully ®ltered through a sieve. The jar was cleaned of sediments, the seagrass returned to the jar and the washing was repeated three times. A dry weight fraction of 60% was assumed for the sediment.
Blubber samples were collected from three mature dugongs (two males and one female) that were stranded in the Great Barrier Reef Marine Park (Fig. 1). All three animals were suspected or con®rmed net drownings and the carcasses were in good condition at the time of sampling. The analytical methodology employed and the results for these samples have been presented elsewhere (Haynes et al., 1999). In Bayreuth, Germany, the seagrass samples were milled in a blender. The sediment and seagrass samples were homogenised and aliquots of approximately 40 g (seagrass) or 10 g (sediment) were soxhlet-extracted in toluene for 16 h following addition of a mixture of 12 13 C12-labelled 2,3,7,8-substituted PCDD/F congeners to the extraction solvent. The extracts were cleaned up using a H2SO4/silica gel+NaOH/silica gel mixed column and an alox column as described in Horstmann et al. (1992). The puri®ed extracts were concentrated to 10 ml, a recovery standard was added, and the samples were analysed for the 2,3,7,8-substituted PCDD/F congeners using HRGC/HRMS on a VG Autospec Ultima at a resolution of 10,000. The analytical parameters employed are described in Horstmann and McLachlan (1995). The quality assurance programme includes the determination of internal standard recoveries, the veri®cation of relative retention times and proper isotope ratios, and laboratory blanks. The recovery of the labelled internal standards was >80%, and no evidence of interferences was found. The limiting factor for the quanti®cation of the analytes were the laboratory blanks. An analyte was only quanti®ed if the amount in the sample was at least a factor of 5 above the laboratory blank. 3. Results and discussion 3.1. Uptake in seagrass The PCDD/F concentrations in both sediment and seagrass were dominated by Cl8DD, which with concentrations ranging from 14 to 2900 pg/g dry weight accounted for over 90% of the 2,3,7,8-substituted congeners in all samples (Table 1). The levels of 1,2,3,4,6, 7,8-Cl7DD were 11±35 times lower, and the concentrations of the Cl6DD congeners were at least another order of magnitude less. The PCDFs and 2,3,7,8-Cl4DD were below the limit of quanti®cation in most samples. The PCDD concentrations in the sediments ranged over more than two orders of magnitude between the sampling sites. A good correlation was observed between the concentrations in sediment and in seagrass from the same sampling point, as shown for Cl7DD and Cl8DD in Fig. 2. One possible explanation for this observation could be that the concentrations in the seagrass samples were
M.S. McLachlan et al. / Environmental Pollution 113 (2001) 129±134
131
Fig. 1. Map showing the sediment, seagrass and dugong sampling sites (F, Flinders Island; CW, Cardwell; P, Pallarenda; U, Upstart Bay; N, Newry Bay).
primarily determined by the sediment particles that adhered to the seagrass. Although visual examination of the seagrass samples did not indicate that it was heavily contaminated with sediment, the laboratory determination of sediment content indicated that sediment made up between 20 and 62% of the dry weight of the samples as analysed. Furthermore, the dry weight-based concentrations in the two matrices were similar. In Fig. 3,
the quotient of the seagrass and sediment concentrations is plotted for each of the PCDD congeners that were quanti®ed in both matrices. The quotients were close to 1 with the exception of the Upstart site. Here the levels in seagrass were close to the limit of quanti®cation and consequently were likely overestimated, which may explain the higher quotients. Also, at a given site the quotients were similar for the dierent
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Table 1 PCDD concentrations in the sediment and seagrass samplesa Sampling station
2,3,7,8-Cl4DD 1,2,3,7,8-Cl5DD 1,2,3,4,7,8-Cl6DD 1,2,3,6,7,8-Cl6DD 1,2,3,7,8,9-Cl6DD 1,2,3,4,6,7,8-Cl7DD Cl8DD a
Sediment (pg/g dry wt.)
Seagrass (pg/g dry wt.)
U
P
F
N
CW
U
P
F
N
CW
0.01 0.01 0.02 0.02 0.04 0.67 14
0.02 0.07 0.09 0.21 0.29 6.2 130
0.03 0.15 0.32 0.48 1.2 12 190
0.09 0.40 0.95 1.5 2.7 57 1200
0.06 0.39 1.2 1.8 3.7 77 2900
0.01 0.03 0.08 0.11 0.18 2.5 49
0.02 0.14 0.29 0.40 0.52 8.2 90
0.03 0.26 0.43 0.66 1.5 15 200
0.07 0.44 1.1 1.5 2.5 47 890
0.04 0.38 1.1 1.5 2.4 44 1300
U, Upstart Bay; P, Pallarenda; F, Flinders Island; N, Newrny Bay; CW, Cardwell.
fractions typically have a higher organic carbon content and higher dry weight-based concentrations. It is considered unlikely that the congener pattern in the relatively short-lived seagrass resulting from uptake of dissolved chemical from the water column would be identical to the congener pattern in sediment. On the balance of the evidence it is concluded that ingestion of sediment associated with seagrass is a major vector of PCDDs to dugongs. Fig. 2. Plot of the sediment concentration versus the seagrass concentration of 1,2,3,4,6,7,8-Cl7DD and Cl8DD at the ®ve sampling sites.
Fig. 3. Histogram of the quotient of the dry weight-based seagrass and sediment concentrations for the 2,3,7,8-substituted polychlorinated dibenzo-p-dioxins at each of the sampling sites. 2,3,7,8Cl4DD is not included on the diagram because it was below the limit of quanti®cation in the sediment samples.
congeners, demonstrating that the congener patterns in the seagrass and the sediment were similar. All of these observations are consistent with the hypothesis that the sediment was a major source of the PCDDs in the samples. The contribution of the sediment to the PCDD concentrations in seagrass was estimated by multiplying the PCDD concentration in sediment by the dry weight fraction of sediment in the seagrass. The contribution of the sediment exceeded 15% at all sites and 50% at two of the sites. This calculation may yield an underestimation, since the sediment adhering to the seagrass would be expected to have a smaller average particle size than the bulk sediment, and the ®ner sediment
3.2. Uptake in dugongs The PCDD/F levels found in the three stranded dugongs have been presented elsewhere (Haynes et al., 1999). Brie¯y, only two of the PCDF congeners were detected in the dugong blubber, whereas all 2,3,7,8substituted PCDDs were quanti®ed. Among the PCDDs, the levels of C18DD were unusually high compared with other marine mammals. A quantitative estimate of the uptake of PCDDs by the dugongs was not possible due to the uncertainties in the animals age and exposure, particularly in view of the wide range of seagrass concentrations measured. However, a comparison of the relative accumulation of the dierent PCDD congeners gives important insights into the accumulation behaviour. Such a comparison is possible because the PCDD pattern (the relative concentrations of the dierent congeners) was similar in each of the seagrass samples as well as in each of the dugong samples (although the patterns in dugong tissue and seagrass were dierent). In Fig. 4, the quotient of the average concentration in the dugongs samples and the average concentration in the seagrass samples is plotted for each of the 2,3,7,8-substituted PCDD congeners. This quotient decreases by more than two orders of magnitude with increasing degree of chlorination, indicating that a much larger fraction of the lower chlorinated congeners in the food supply had accumulated in the dugongs' tissue. There are a number of dierent explanations for this observation. One is that the PCDD pattern measured in the food supply is not representative of the pattern in the
M.S. McLachlan et al. / Environmental Pollution 113 (2001) 129±134
Fig. 4. Histogram of the quotient of the average concentrations measured in dugongs (three animals, pg/g lipid) and the average concentration measured in seagrass (®ve samples, pg/g dry wt.).
food that the dugongs consumed over their lifetime. It could be hypothesised that the contamination in the sediments and seagrass is recent and that the dugongs, which were at least 10 years old, had previously been exposed to a dierent PCDD pattern in food. However, given the huge spatial extent of the contamination (the sampling sites extended over 1000 km of coastline), the limited and heterogeneous anthropogenic activity in this region, and the homogeneity of the PCDD pattern in the seagrass/sediment, we consider it unlikely that the contamination was very recent. Another possible explanation for the dierent degree of accumulation in dugong tissue is metabolism. More rapidly metabolised compounds would accumulate less than more persistent compounds. This would imply that the metabolism of Cl8DD is more than two orders of magnitude faster than metabolism of 2,3,7,8-Cl4DD. However, this is contrary to the published literature on PCDD metabolism which shows that the metabolism of the higher chlorinated 2,3,7,8-substituted congeners is slower than for the lower chlorinated (Van den Berg et al., 1994). While large-scale metabolism of the 2,3,7,8-substituted PCDDs is considered unlikely, there was evidence of metabolism of some of the non-2,3,7,8-substituted congeners. The chromatograms of the homologues in the seagrass and sediment samples contained more isomers and dierent isomer patterns than the chromatograms of the dugong tissue samples. The absorption and tissue distribution of the 2,3,7,8-substituted and non2,3,7,8-substituted congeners in a given PCDD homologue are expected to have been similar since their physical±chemical properties are similar. Therefore, the changes in the isomer patterns in the chromatograms were likely due to metabolism. There were, however, some non-2,3,7,8-substituted congeners which were as persistent in the dugong as the 2,3,7,8-substituted congeners. A more likely explanation for the dierent accumulation behaviour of the PCDD congeners is dierences in absorption in the digestive tract. Decreasing net
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absorption of PCDDs with increasing degree of chlorination has been observed in a number of mammals including another herbivore, the domestic cow (McLachlan et al., 1990; McLachlan and Richter, 1998). Net absorption is the result of two competing processes: advection through the digestive tract, and transport from the lumen across the intestinal wall to the lymph and blood. It is believed that the transport of very hydrophobic compounds like PCDDs across the intestinal wall is limited by an aqueous diusion resistance (Gobas et al., 1988). The higher the lipid/water partition coecient of the compound, the smaller the quantity that partitions out of the lipophilic phases in the lumen into these aqueous phases and the lower the rate of transport across the aqueous barrier. The result is a decrease in net absorption with increasing lipid/water partition coecient. 4. Concluding discussion Three hypotheses to explain the high levels of Cl8DD in dugongs were presented in the introduction: ingestion of contaminated sediment, formation in the digestive tract, and selective degradation of other PCDD/F congeners. The evidence gathered in this study supports the ®rst hypothesis. The fact that the dry weight-based concentrations of the PCDDs in seagrass correlated with those in the sediment, that the PCDD concentrations and the PCDD patterns in seagrass were similar to those in the sediment at a given site, and that a large fraction of the seagrass sample dry weight was contributed by sediment all support the conclusion that a large fraction of the PCDDs ingested by the dugongs originates from sediment. Selective formation of Cl8DD in the digestive tract or selective degradation of other congeners are unlikely explanations since the transfer of Cl8DD from the diet to dugong tissue is actually much less than for the other 2,3,7,8-substituted congeners. The high levels of Cl8DD in dugongs are due to unusually high levels in the sediments which are ingested with the seagrass and transferred to the dugongs in accordance with established environmental chemistry principles for persistent hydrophobic pollutants. Acknowledgements We thank Frieder BoÈhme for cleaning up the samples, Stefan Will for his assistance with the analysis, and Leon Jackson (GBRMPA) for assistance with Fig. 1. References Gobas, F.A.P.C., Muir, D.C.G., Mackay, D., 1988. Dynamics of dietary bioaccumulation and faecal elimination of hydrophobic chemicals in ®sh. Chemosphere 17, 943±962.
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