Bioaccumulation patterns and temporal trends of mercury exposure in Wisconsin common loons

A long term field study was initiated by the Wisconsin Department of Natural Resources in 1992 to elucidate patterns of common loon (Gavia immer) mercury (Hg) exposure. Analysis of loon blood and feather samples collected from recaptured adult loons in Wisconsin 1992-2000 found evidence of a decline in overall body burdens of mercury in common loons for this region. The interval between sampling individual loons spanned 2-8 years, a sufficient length of time to observe a change in tissue Hg concentrations. Loon chick blood Hg levels declined by 4.9% annually for chicks sampled on 33 lakes during the period 1992-2000. This is the first evidence we are aware of showing a recent regional annual decrease in common loon Hg exposure. Repeated captures of wild loon chicks in Wisconsin shows that blood Hg concentrations can increase during the period of rapid feather growth (weeks 2-5), although the rate of increase is very slow. Mean egg Hg levels ranged from 0.19 to 0.87 mug Hg/g wet weight (ww) in samples collected 1996-2000. Egg Hg concentration was inversely and significantly related to lake pH (p < 0.0001; r(2) = 0.55). Adults and chicks were often captured simultaneous during the period 1992-2000. Correlations were highest between sibling blood Hg levels (r = 0.88) and chick blood and adult blood (male r = 0.61, female r = 0.52) Hg levels, likely reflecting the influence of the Hg content of prey from the natal lake on loon blood Hg levels. The relationship between feathers and blood of adults and that of chick blood and adult feathers was weaker.