NOAA's Reporter's Coral Reef Tip Sheet
July 28, 1997 - Week 30

High concentrations of metals such as lead, mercury, tin, zinc, cadmium, copper, cobalt, iron, manganese, nickel, aluminum, vanadium, and silver can kill corals and other coral reef organisms. Lower concentrations can inhibit coral growth and reproduction, and may even contribute to coral bleaching. Corals absorb chemicals across their outer tissue layer, so whatever contaminants are suspended (floating) in the surrounding water will soon be absorbed into the coral itself.

Although there are some natural sources of these toxic metals, the majority of the heavy metal found in nearshore waters is anthropogenic (created by humans). Some of these anthropogenic heavy metals come from sources at sea such as the drilling fluid used in offshore oil production, however, the majority of the sources are land-based. Heavy metals come from mining and smelting operations, oil refineries, power plants, shipyards, domestic and industrial sewage, and agricultural activities. They are found in the antifouling and anticorrosive paints used to protect ships and coastal structures, and are a normal component of fertilizers, lime, and pesticides.

If most of these activities occur on land, some of them many miles inland, how do they threaten coral reefs? The main mechanism for the transportation of metals from inland regions to the nearshore environment is rivers. Runoff from agricultural lands, urban areas, storm drains, and sewage pipes all flow into rivers, and eventually into the sea. Along the way, many of the metals settle into the sediment on river and estuary bottoms, or the sediment trapped in mangrove forests. However, as soon as a channel is dredged for larger ships, or a mangrove forest is cut down to make way for a shrimp farm, those metals are resuspended (stirred up) and once again become a threat to the health of coral reefs.

The biggest problems with heavy metals are that they are persistent (remain in the environment unchanged for years), and they bioaccumulate (increase in concentration as they go up the food chain). So, in addition to posing a threat to the health of coral reefs around the world, toxic metals also pose a long-term public health risk, especially for those human populations that rely on fish for protein.

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