A THREE-YEAR study conducted by theWorld Wide Fund for Nature-International for the EnvironmentProtection Agency (EPA) reveals that seabirds inhabiting the Midway Islands inthe,north Pacific have concentrations ofpersistant organic pollutants ajhigh asthose in the birds found around theshores of the Great Lakes, NorthAmerica .(Environmental Science &Technology, Vol 30, No 1).
The chemicals had earlier beendetected in high concentrations amongbirds of the northern hemisphere, butthis report is the first to indicate thatglobal atmospheric and cycling processes (largescale wind circulation overmost of the earth, responsible formaintaining the different climatic zonesof the world) can disperse high levels ofcontaminants even to remote tropicalregions far from the area of origin. Itwas found that concentrations of polychlorinated biphenyls (PCBS), dioxins,furans, and other dioxin-like chemicalswere as high as 124 picograms (onepicogram, = I X 10-1.2 gm) which makesthese toxic equivalents. According toJohn Geisy, a toxicologist at theMichigan State University,'us, "Ourresearch indicates that the environmentis at a threshold. There is no moreassimilative capacity. Any TCDD (tetraPolychlorinated dibenzo para-dioxin) -like pollutant added now, will push itabove the limit and produce adverseeffects."
The research team studied twomarine bird species - the black-footedalbatross and Laysan albatross.Concentrations of the above mentionedchemicals were measured in the eggs,tissue and @lood samples of the birds.Present and historical population dataon the birds was also studied. In theGreat Lakes and other areas, high levelsof expos6re were found to have seriously affected the reproductive potential ofseveral species of birds, and this hasconsequently led to a decrease in theirpopulations.
The reproductive progress of thebirds was monitored by examining arather large sample of 500- 1,000 eggs ofeach species. It was found that theblack-footed albatross hatched eggs2.5 per cent.lesser than the Laysan albatross. The albatrosses were chosen for the study as they matefor life, return to the same nesteach year and do not fearhumans.
Concentrations of the contaminants were found in theirblood and the resulti obtainedwere compared with'the doseresponse relationships (thecalulation of a measure calledthe lethal dose (LD) 50, wherein@O per cent of the animalsadministered a certain dosageof the test compound die withina predetermined length of time)in other birds. It was found thatthe total toxicity burden (theamount of toxin borne by ananimal) on the black-footedalbatross was twice that ofthe Laysan albatross. The lowertoxicity burden found in the case ofthe iAysan albatross saved its reproductive capability from being adverselyaffected.
This difference in the levels of exposure to pollutants, according to theresearchers, is due to the dietary habitsof the two species. Though similar innature, the Laysan albatross eats squidwhile the black-footed albatross eats -besides squid - flying fish and its eggs,which, are more than half a trophic level(in the food chain) higher than thesquid. Thus, accumulation of contaminants in the food chain would result intheir increased concentrations in black-footed albatrosses.
The results obtained from the studycame as a surprise because the birds ofthis region were specially chosen forinhabiting the relatively pristine environment. This just confirms the theorythat pollutants cannot be confined toone region, and that their effects can befelt on a global scale.