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Correspondence on Geisz et al. Melting Glaciers: A Probable Source of DDT to the Antarctic Marine Ecosystem Geisz et al. report an unexpected consequence of climate change, implicating glacier meltwater as a source for DDT in coastal Antarctic seas (1). Hypothetically, glaciers may act as secondary sources for DDT and there may be a link to climate change. However, their data are sparse, variable, and insufficient to either support or reject this hypothesis. Here, we present a different perspective, focusing on dynamics in glacier runoff and concentrations of DDTs in ice, on feeding areas of the affected penguins and on time trends of DDTs in Antarctic seabirds.
Glacier Runoff and Concentrations of DDTs in Glacier Ice and Runoff The estimate of 1-4 kg y-1 of ΣDDT flux by Geisz et al. was calculated by multiplying the net loss of ice by ΣDDT concentrations measured in glacier ice or glacier meltwater from the Antarctic Peninsula. We think that this strongly overestimates the flux. First, the amount of ice loss was based on figures for the entire West Antarctic Ice sheet, and not restricted to the Antarctic Peninsula (see the Supporting Information (SI)). Second, surface melt and runoff are minor components of the total freshwater flux from Antarctica to the ocean and only occur during summer months at low elevation, even on the Antarctic Peninsula. By far the largest loss of ice occurs from a combination of iceberg calving and basal melt of glacier ice (2). Basal melt will contain no DDT because it is the very old and generally pre-DDT ice from the base of the glacier that is lost. The average concentration of ΣDDT in the ice sheet will therefore be much smaller than the concentrations in the precipitation during the time of widespread DDT usage, which were used by Geisz et al. (see SI).
Feeding Areas In order to be affected by the local input of DDTs from glaciers, penguins need to feed in these areas for a long period of time during the year. Penguins from Cape Crozier feed within a few tens of kilometers of the breeding colony only during a relatively short period in the summer (3). Limited published satellite telemetry data suggests that their winter foraging areas are northwest of the Balleny Islands (4). Hence, except for a relatively short period during the breeding period, they feed away from areas thought to be subject to increased melt due to anthropogenic warming. Therefore, the impact of climate change on the DDT concentrations in these birds may be expected to be limited.
Trends of DDTs in Antarctic birds The analysis by Geisz et al. of changes in ΣDDT in Ade´lie penguin fat from 1964 to 2006 includes only four points in time (1). Although data on levels of DDTs in Ade´lie penguins is limited, additional data on other Antarctic bird species are available in the literature. Combining this additional information with new data from our own research shows that levels of DDTs increase from the early 1960s to approximately 1976 (see SI Table S1 and Figure S1). After reaching a peak concentration of approximately 750 ng/g lipids, the concentrations decline. In spite of 3976
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their different conclusion, the data of Geisz et al. (1) are consistent with an overall trend of reducing DDT concentrations in Antarctic seabirds (SI Figure S1). This current overall decline in concentrations is confirmed by our data from Ade´lie penguins and Southern Fulmars from Hop Island collected in 1993/1994 (5) and 2003/2004 (SI Table S2). Concentrations of p,p′-DDE decreased in both species. Perhaps the most persuasive evidence provided by Geisz et al. for exposure of birds from the Antarctic Peninsula to a “fresh” source of DDT from glacier melt is the finding of the parental compound p,p′-DDT in seven out of eight birds from Palmer Station in the part of Antarctica most affected by global warming, but not in birds from Cape Crozier, an area with less glacier influence (6). For this argument to be sustained, p,p′-DDT should be absent or in very low levels in all locations other than those subject to recent anthropogenic warming. This is not the case. DDT:DDE ratios in the range 0.073-0.322 were recently reported in krill from locations covering 50° of longitude in the eastern Antarctic sector (7), indicating that p,p′-DDT is widespread in lower trophic levels in the Southern Ocean and not restricted to the Antarctic Peninsula region.
Conclusions The suggestion by (1) that global warming increases release of DDT from the Antarctic ice sheet to a level affecting concentrations in penguins is not supported by a broader analysis. If Geisz et al. are correct and the DDT is old material that is now released by climate warming, their observations further support the need for action on climate change. However, when accepting this interpretation we risk overlooking global signals of renewed use of DDT. It is therefore important to resolve the source of DDT currently detected in the Antarctic environment. This may have significant implications for both understanding the global life cycle of POPs and for the use of Antarctica as a site for monitoring global background levels of POPs under the Stockholm Convention on Persistent Organic Pollutants (8).
Supporting Information Available Details on references used in the long term trend analysis, and on the calculations of the glacier dynamic. This material is available free of charge via the Internet at http:// pubs.acs.org.
Literature Cited (1) Geisz, H. N.; Dickhut, R.; Cochran, M.; Fraser, W. R.; Ducklow, H. Melting glaciers: A probable source of DDT to the Antarctic marine ecosystem. Environ. Sci. Technol. 2008, 42 (11), 3958– 3962. (2) Rignot, E.; Bamber, J. L.; Van Den Broeke, M. R.; Davis, C.; Li, Y. H.; Van De Berg, W. J.; Van Meijgaard, E. Recent Antarctic ice mass loss from radar interferometry and regional climate modelling. Nat. Geosci. 2008, 1 (2), 106–110. (3) Ainley, D. G.; Ribic, C. A.; Ballard, G.; Heath, S.; Gafney, I.; Karl, B. J.; Barton, K. J.; Wilson, P. R.; Webb, S. Geographic structure of Ade´lie penguin populations: overlap in colony-specific foraging areas. Ecol. Monogr. 2004, 74 (1), 159–178. (4) Davis, L. S.; Harcourt, R. G.; Bradshaw, C. J. A. The winter migration of Adelie penguins breeding in the Ross Sea sector of Antarctica. Polar Biol. 2001, 24, 593–597. (5) Van den Brink, N. W.; Van Franeker, J. A.; De Ruiter-Dijkman, E. M. Fluctuating concentrations of organochlorine pollutants during a breeding season in two Antarctic seabirds: Adelie 10.1021/es8034494 CCC: $40.75
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penguin and southern fulmar. Environ. Toxicol. Chem. 1998, 17 (4), 702–709. (6) Gillet, N. P.; Stone, D. A.; Stott, P. A.; Nozawa, T.; Karpechko, A. Y.; Hegerl, G. C.; Wehner, M. F.; Jones, P. D. Attribution of polar warming to human influence. Nat. Geosci. 2008, 1, 750– 754. (7) Bengtson Nash, S. M.; Poulsen, A. H.; Kawaguchi, S.; Vetter, W.; Schlabach, M. Persistent organohalogen contaminant burdens in Antarctic krill (Euphausia superba) from the eastern Antarctic sector: A baseline study. Sci. Total Environ. 2008, 407, 304-314. (8) UNEP Stockholm Convention on Persistent Organic Pollutants; United Nations Environment Programme: Nairobi, Kenya 2001.
Nico van den Brink,* Martin Riddle, Martine van den Heuvel-Greve, Ian Allison, Ian Snape, and Jan Andries van Franeker Alterra, Wageningen University and Research Centre, P.O. Box 47, NL-6700AA, Wageningen, The Netherlands, Australian Antarctic Division, Channel Highway, Kingston, Tasmania, 7050, Australia, Deltares, P.O. Box 177, NL-2600MH Delft, The Netherlands, and Wageningen IMARES, P.O. Box 167, NL-1790AD Den Burg (Texel), The Netherlands ES8034494
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