Metals such as mercury, lead, copper, cadmium, zinc, selenium, and arsenic can be introduced into streams through industrial wastewater discharges, runoff from urban and industrial areas, mining wastes, and landfills. In addition, metals can be transported long distances into remote sections of streams by atmospheric deposition in rain, snow, or dust. Metals can undergo chemical alterations to form more harmful substances once they enter aquatic systems. For example, inorganic mercury, which has a strong affinity for sediments, can be changed to organic methyl mercury (CH3Hg) by sulfate reducing bacteria in anoxic sediments. This toxic form of mercury is lipophilic and is the major form of mercury that bioaccumulates in the tissues of aquatic organisms. Biomagnification, the increase in con centration with increasing trophic status, results in top predators becoming highly enriched in mercury.

Metals on aquatic systems tend to accumulate on benthic organic sediments, where they can persist for long periods of time even though water column concentrations are rela tively low due to continuous flushing. Fluctuations in the release of metals from the sediment are quite variable and depend on the physical characteristics of sediments (e.g., texture and composition), environmental conditions (e.g., redox state and microbial composition), and individual metal properties.

Metal accumulation in aquatic organisms can have both acute and chronic effects, and negatively affect all components to the ecosystem. For example, copper levels near 2 mg l 1 can greatly reduce algal productivity, and the bioaccumulation of mercury, cadmium, and zinc causes reproductive and juvenile developmental problems in macroinvertebrates, mussels, and fish. Metal toxicity can also change with different environmental conditions such as temperature and pH. Much is still unknown about the effect of metals on aquatic systems, including the effect of chronic low doses and the inter active effects of multiple metals.

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