Feature Story                                                                June 2011 

Hull Fouling and Copper-based Antifouling Paints

Christopher Brown inspects the bottom of a fouled ship in San Francisco.

Are introduced marine species more tolerant of copper than native species?

Most boat owners spend a lot of time and energy cleaning the bottom of their boats and most probably repaint them with antifouling paints every one to five years to reduce the growth of marine invertebrates (barnacles, tunicates, bryozoans) and algae. Maintaining a clean hull is especially important in times of high fuel prices because a clean hull has less drag and is more efficient than a hull that is heavily fouled. But despite our best efforts, some hull fouling still occurs between cleanings, and therefore hull fouling continues to be one of the ways in which marine invertebrates are transported around the world.

Antifouling paints reduce hull fouling because they contain copper, among other metals such as zinc and nickel. Copper is a heavy metal that is toxic to marine invertebrates affecting their reproduction, growth, and abundance. It is also a pollutant in the marine environment, especially close to docks and harbors, because it leaches from the hulls of boats into the surrounding water. Here at the Smithsonian Environmental Research Center (SERC) we designed an experiment to test whether or not introduced species were more tolerant to copper than native species. We suspected that a species that was able to grow on the bottom of a boat painted with copper paint, might produce offspring that were more tolerant to the increased copper concentrations in harbors and marinas where they established after their parent’s overseas voyage.

Dr. Canning-Clode pulling a panel out of the water in Mexico to see what grew on it.
To test this idea Dr. João Canning-Clode, a Post Doctoral Fellow at SERC, painted fiberglass panels with different amounts of copper-based antifouling paint and placed them in marinas in Virginia and Panama for nine weeks to see what species could grow on each type of panel (high, low, and no copper). Dr Canning-Code selected these two locations in order to determine if species in tropical climates responded to copper in the same way as species in more temperate climates.

He found that copper is toxic to all marine species regardless of whether or not they were native or non-native, a discovery which contradicts previous investigations that found that some species were copper tolerant. The diversity of species that grow on the bottom of boats was greatly reduced due to the toxicity of the copper. In the tropical waters of Panama the warm water increased the toxicity of the copper, thus tropical species were more sensitive to the copper than the species in the more temperate waters of Virginia. This means that increased copper in the harbor could reduce the overall diversity of species in the area, not just on the bottom of a boat. This is important because the greater the diversity of species in an area the less likely that an introduced species will be able to establish. However, because Dr. Canning-Clode suspects his finding might be related to site characteristics, he expanded his investigation on a latitude gradient. Last summer, he conducted the same experiment using the same methodology in five regions: Panama, Mexico, Florida, Virginia and Connecticut. This work is still in progress, but the previous study is published in PlosOne.

 Set of fouling plates getting ready to be placed in the water, one replicate of each treatment. 
Fouling plate after nine weeks in the water. The common species are indicated.

Canning-Clode J, P Fofonoff, GF Riedel, M Torchin, and GM Ruiz. 2011. The Effects of Copper Pollution on Fouling Assemblage Diversity: A Tropical-Temperate Comparison. PLoS ONE 6(3): e18026. doi:10.1371/journal.pone.0018026