Seasonal variation in heavy metal levels in tissues of common guillemots, Uria aalge from northwest Scotland.

Mercury, cadmium, zinc, and copper concentrations were analyzed in three samples of common guillemot (in April, June, and November). Levels measured were uniformly low, and not enough to have any toxic effects. Adult guillemots had significantly more cadmium in their livers and kidneys than juveniles, with juvenile levels ranging from 25% to 89% of adult levels. Mercury concentrations in liver and kidney were also higher in adults. Juvenile levels represented from 80% to 94% of adults, but there were no age differences in feather and muscle mercury. Mercury levels declined throughout the year in internal tissues from April through June to November. There was a strong seasonal fluctuation in cadmium levels in liver and kidney, rising significantly between April and June and declining in both adult and juvenile birds. The influences of seasonal processes (namely breeding and moult) and seasonal dietary differences as causative factors in the changes in metal burdens are discussed. These findings have implications for the use of seabirds as monitors of heavy metals in the marine environment.