THE EFFECTS OF ECOSYSTEM CHARACTERISTICS ON CONTAMINANT DISTRIBUTION IN NORTHERN FRESH-WATER LAKES

Ecological factors can modify the effect of airborne contaminants in a rctic freshwater lakes. The detention time of contaminants in lake cat chments can greatly affect concentrations and time-courses of contamin ant distribution. Lake sediments typically contain concentrations of c ontaminants several orders of magnitude higher than those in overlying waters, and appear to contain good temporal records of contaminant de position. They also indicate general increases in contaminant inputs f rom north to south and from west to east in North America. The lower m ean temperatures of northern lakes render them more efficient as sinks for volatile contaminants than warmer lakes in the south. Lower tempe ratures also cause lower growth rates in fish, resulting in higher con centrations of contaminants. Conversely, the ratio of methylation to d emethylation declines in colder temperatures, favoring lower concentra tions of mercury in fish. Bioaccumulation can increase contaminant con centrations by several orders of magnitude in food chains of 4-5 steps . Contaminant levels increase with trophic position and are positively correlated with age and fat content, but negatively correlated with g rowth rates. Stable isotopes appear to be a useful diagnostic tool for examining biomagnification in arctic food chains. Biological transfor mation and decomposition affect the quantities and toxicity of some po llutants. The effects of some contaminants can be affected by other hu man stresses, including acid precipitation, climate change, hydroelect ric development, harvesting of fishes and marine mammals, and eutrophi cation. Management of many contaminants requires tradeoffs, such as th e beneficial effects of controlling insect-borne pathogens in the trop ics versus the negative effects on northern aboriginal populations rel ying on wild populations of organisms for food.