Shipwrecks can cause long-term biological reef damage

When a ship wrecks on a reef, the impacts go far beyond the mere mechanical damage caused to coral scarred and gouged by the impact and the ship rolling in the surf.

(Image: A shipwreck on the Palmyra reef. USGS photo by Thierry Work.)

New research shows such wrecks can also have long-term effects, including changing the character of life on the reef. And they may require aggressive and somewhat scary responses.

Earlier studies in the Northwestern Hawaiian Islands found that certain seaweeds thrive at wreck sites—outcompeting the corals that would normally be there—perhaps in part because of the nutrient value of iron released from the wreck's presence.

A new study at Palmyra Atoll shows that another form of life also can outcompete corals at wrecks.
Researcher Thierry Work found at a Palmyra shipwreck site an expanding outbreak of a form of life called Rhodactis howesii. This is a form of life related to anemones and to corals, but which is not either one. They are called corallimorpharians.

The report on the find, Phase Shift from a Coral to a Corallimorph-Dominated Reef Associated with a Shipwreck on Palmyra Atoll, was written by Work, of the Honolulu Field Station of the USGS National Wildlife Health Center, along with coral researcher Greta Aeby of the University of Hawai'i's Hawai'i Institute of Marine Biology, and Jim Maragos, of the U. S. Fish & Wildlife Service, Pacific Islands Refuges. The report was published in the online journal PLoS One.

Howesii is rare elsewhere on the Palmyra reef, but common around the 1991 wreck of a longline vessel.

“We documented high densities of R. howesii near the ship that progressively decreased with distance from the ship whereas R. howesii were rare to absent in other parts of the atoll,” the paper says.
Intriguingly, there are also clusters of the corallimorph around buoys at the atoll.

“This is the first time that a phase shift on a coral reef has been unambiguously associated with man-made structures. This association was made, in part, because of the remoteness of Palmyra and its recent history of minimal human habitation or impact,” it says.

Palmyra is an atoll that lies roughly 1,000 miles south of Hawai'i in an island group called the Line Islands. It is jointly managed by the U.S. Fish and Wildlife Service and The Nature Conservancy.

The scientific paper's title uses the term phase shift, which refers to a conversion in this case of the reef from coral being the dominant form of reef life, to the new dominance by the corallimorph.

It is possible, though not proven, that as in the Northwestern Hawaiian Islands, it is the presence of iron or some other component of the metal in a wreck that is feeding the outbreak of howesii. Iron in the hull of the wrecked longliner; iron in the chain and perhaps anchors of the buoys.

One conclusion from the study is that it may make sense when a ship wrecks on the shore to not only stop its physical damage to the reef, but also to quickly remove the ship as a source of nutrients that could change the life of the reef.

“We would hope that the populations of this organism would decrease if the ship were removed, but we are not sure,” Work said in an email.

It might also make sense to chemically kill the corallimorph colony, in hopes the coral reef will re-establish itself at the location. But that's not without consequences.

“Chemical sterilization would kill everything on the benthos. The hope would be that the benthos would then be recolonized by native fauna and flora. Whether this turns out to be the case would have to be seen,” Work said.

While it's not known whether howesii is a natural component of the reef, it seems clear that it is an aggressive species, which can outcompete its neighbors when its populations are dense.

“R. howesii has both anatomic and life history traits that allow it to aggressively compete successfully for the benthos,” the paper said.

Thus, doing nothing may not be a good alternative.
“In the case of Palmyra, the R. howesii infestation is beginning to reach catastrophic proportions,” the paper says.

For managers, the choice is the always difficult one: do nothing and face continued creeping destruction, or do something that could have unanticipated consequences.

“None of the options on the table are really all that palatable, but they may be better than doing nothing,” Work said.

© 2008 Jan TenBruggencate