The effects of ecosystem characteristics on contaminant distribution in northern freshwater lakes

Ecological factors can modify the effect of airborne contaminants in arctic freshwater lakes. The detention time of contaminants in lake catchments can greatly affect concentrations and time-courses of contaminant distribution. Lake sediments typically contain concentrations of contaminants several orders of magnitude higher than those in overlying waters, and appear to contain good temporal records of contaminant deposition. They also indicate general increases in contaminant inputs from north to south and from west to east in North America. The lower mean temperatures of northern lakes render them more efficient as sinks for volatile contaminants than warmer lakes in the south. Lower temperatures also cause lower growth rates in fish, resulting in higher concentrations of contaminants. Conversely, the ratio of methylation to demethylation declines in colder temperatures, favoring lower concentrations of mercury in fish. Bioaccumulation can increase contaminant concentrations 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 growth rates. Stable isotopes appear to be a useful diagnostic tool for examining biomagnification in arctic food chains. Biological transformation and decomposition affect the quantities and toxicity of some pollutants. The effects of some contaminants can be affected by other human stresses, including acid precipitation, climate change, hydroelectric development, harvesting of fishes and marine mammals, and eutrophication. Management of many contaminants requires tradeoffs, such as the beneficial effects of controlling insect-borne pathogens in the tropics versus the negative effects on northern aboriginal populations relying on wild populations of organisms for food.