Changes in food web structure affect rate of PCB decline in herring gull (Larus argentatus) eggs

Biological monitors provide important information regarding temporal trends in levels of persistent organic pollutants. Correct interpretation of thes e trends is critical if we are to accurately assess our progress in elimina ting these contaminants from the environment. In the Laurentian Great Lakes , polychlorinated biphenyl concentrations in herring gull eggs declined dur ing the 1970s and early 1980s. By the mid-1980s, further declines were not as obvious. An exception to this trend was observed in eggs from Lake Erie. On that lake, egg PCB concentrations continued to decline rapidly during t he 1980s/1990s. Evidence from stable isotope analysis indicated that tempor al changes in the composition of the herring gull diet occurred on Lake Eri e. In the eastern basin, declines in fish availability may have forced the gulls to incorporate a greater proportion of terrestrial food into their di ets. Decreases in the proportion of fish in the gull diet would have result ed in reduced PCB exposure. This may be partially responsible for the conti nuing rapid rate of decline in egg PCB concentrations. This decline should be interpreted with caution. These trends may not he indicative of lake-wid e declines in PCB bioavailability hut only reflect changes in dietary expos ure brought about by alterations in food web structure.