We measured the toxic metal cadmium (Cd) over a 13-year interval in lakes located near two metal smelters to determine whether reductions in atmospheric Cd emissions have led to reductions in the Cd present in aquatic food webs. Although Cd in the lake water consistently declined over time, Cd in animals increased in some lakes. This apparent contradiction was explained when we considered the simultaneous reductions that have occurred in lake water acidity; under these conditions, animal Cd can increase if there are insufficient hydrogen ions to out-compete Cd ions at biological uptake sites. We conclude that the risk to organisms from metals has increased in some lakes recovering from smelter emissions.
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