Is Copper to Blame for Fish Declines in San Francisco Bay?

Residents of the San Francisco Bay region have long suspected that its waters might be contaminated with copper, particularly when the native fish populations in the north bay began declining two decades ago. Such declines can typically be attributed to changes in water temperature; however – in this case – such changes weren’t observed, leading some to believe that copper toxicity might be to blame. Adding to their suspicions is the fact that copper levels in the waters periodically exceed the U.S. EPA water quality guidelines.

Dr. Kristen Buck, Assistant Scientist at BIOS in the Trace Metal Biogeochemistry Lab, received a fellowship from the CALFED Bay-Delta Science Program to study the water in the San Francisco Bay delta and provide insight into whether copper contamination is causing these problems. Her results, although preliminary, just might surprise you.

Copper – a heavy metal – can become bound in terrestrial and marine sediments where it can lay dormant for years until disturbed by human or environmental forces. San Francisco Bay is a relatively shallow body of water, measuring only a few meters deep on average. As a result, wind-driven mixing of the water often stirs up the aquatic sediments that lay not far below the surface, bringing copper and other materials into the water column.

In order to find out whether copper toxicity was an issue, Dr. Buck first had to determine how much of the dissolved copper was present in its “bioavailable form” – that is, what percentage of copper in the water is available for uptake and use by aquatic organisms. She discovered that, in San Francisco Bay, 99% of dissolved copper in the water ends up bound to other organic molecules – called ligands – in a process called “organic complexation.”

Once bound to ligands, the bioavailability and corresponding toxicity of copper is reduced to a level that is not considered toxic to aquatic organisms. So although the concentration of dissolved copper was quite high, it was not reaching levels that would be toxic to either the fish or the plankton they feed on.