The Ratio of DDE to PCB Concentrations in Great Lakes Herring Gull Eggs and its Use in Interpreting Contaminants Data

Abstract

The ratio of DDE to PCB (DDE.PCB) concentrations was examined in herring gull ( Larus argentatus) eggs collected from thirteen sites on the Great Lakes from 1979 to 1996. This ratio has been shown to have had a number of biological interpretations in the past and the data in this study have been used to test the validity of these interpretations. The findings suggest that the consistency of DDE:PCB over many years reflects the relative availability of DDT and PCB in different geographical areas and provides an indication of a bird's general foraging ecology. The ratio can also be used as a reflection of the relative rates of increase or decrease of DDE and PCBs in food over time. A significant increase was found in the ratio in eggs sampled from sites on the upper Great Lakes and Lake Ontario during the study period. This is attributed to PCB levels decreasing faster than DDE levels at these sites. At Lake Erie and its two connecting channels, a significant increase was detected in the ratio but at a rate 56% or less than that found at other Great Lakes sites. This lower rate is attributed to DDE and PCB levels decreasing at high and equal rates. Similarities/differences in the fates of these contaminants among the Great Lakes would never have been realized upon an examination of individual contaminant levels alone. The ratio is valuable as a measure of the relative exposure of the two contaminants in non-migratory birds such as herring gulls. The use of the ratio as an indication of contamination movement through the food chain could not be assessed; the ratio cannot be used as a reflection of the interspecific differences in the accumulation and metabolism of the two contaminants. Similarities in the patterns of these ratios shown by eggs collected from sites in close proximity or within the same lake reinforce the fact that herring gulls are important as monitors of regional contaminant conditions.