19 August 2008

Ship-wrecked reefs

ResearchBlogging.orgLast week in a brilliant, ranting post on the fiasco that is EU fishing policy, lunartalks concluded, "Extensive marine reserves should be declared policed and anyone fishing in them punished severely, their boat confiscated, cleaned and sunk as an artificial reef. We don’t need fish on our plates. We do need a functioning marine ecosystem."

Most of the comments on this post circled around whether such a dramatic response as stopping eating fish was actually part of the solution, but I couldn't help notice another point to pick on--the one about sinking ships as artificial reefs. See, as luck would have it, there's a paper out today in PLoS ONE that addresses the impact of shipwrecks on reef ecosystems. And fittingly, the particular shipwreck examined in the paper was even a fishing vessel.

In "Phase Shift from a Coral to a Corallimorph-Dominated Reef Associated with a Shipwreck on Palmyra Atoll" authors from the U.S. Geological Survey provide evidence to support the claim that shipwrecks may seed or facilitate the spread of "unwanted species" on coral reefs in a phenomenon known as a "phase shift".

In a phase shift, the dominant biota of a coral reef is swapped for something else. More often than not, this something else is a bad something else, and long term negative consequences ensue. Potential causes of such shifts include human disturbance, pollution, or changes in particular groups of ecologically important subsets of the coral biota that trigger some kind of imbalance that nucleates the phase shift.

An example of such a phase shift can be found at the site of a long line fishing vessel that wrecked in 1991 on the remote Palmyra Atoll National Wildlife Refuge in the central Pacific:

1991 wreck of long line fishing vessel off Palmyra atoll.
Credit: Thierry M. Work (USGS)

In their PLoS ONE paper, the authors document a transition at the site of the shipwreck from coral to the invasive corallimorpharian Rhodactis howesii (normally rare or absent in Palmyra). Their evidence spans both space and time: this unwelcome invader is not only more abundant near the ship than farther away from it, but its abundance also increased exponentially, as documented by subsequent surveying. R. howesii was also found around buoys installed on the atoll as recently as 2001.

Reef covered by non-native Rhodactis howesii. Native corals are completely absent.
Credit: Thierry M. Work (USGS)

The authors claim that "this is the first time that a phase shift on a coral reef has been unambiguously associated with man-made structures." Their claim is based not only on their observations as outlined above, but also on the fact that Palmyra is so remote and has suffered minimal human impact in recent times (except, apparently, for the shipwreck and the buoys).

It will be interesting (and rather depressing, I suspect) to see and how quickly R. howesii spreads in the future, and I'm guessing the authors have every intention of tracking this. Also interesting is the question of why this phenomenon occurs. The PLoS ONE press replease quotes the lead author as saying that one possibility is that iron leaching from the ship and mooring buoy chains, accompanied with other environmental factors particular to Palmyra atoll, are somehow promoting the growth of Rhodactis.

So let's see, iron might be bad for reefs then? Don't tell anyone at Planktos. Actually, do.

In addition to fortuitously highlighting the huge black box that is the potential risk of feritilising the ocean with iron in a hubris-clad bid to solve the climate change crisis, the authors' intended message is (lunartalks take note) that "the extensive R. howesii invasion and subsequent loss of coral reef habitat at Palmyra also highlights the importance of rapid removal of shipwrecks on corals reefs to mitigate the potential of reef overgrowth by invasives."

Work, T.M., Aeby, G.S., Maragos, J.E., McClain, C.R. (2008). Phase Shift from a Coral to a Corallimorph-Dominated Reef Associated with a Shipwreck on Palmyra Atoll. PLoS ONE, 3(8), e2989. DOI: 10.1371/journal.pone.0002989

1 comment:

lunartalks said...

The first and second world wars, along with the general iron-bound nature of this coast (rough, rocky shoreline, few ports of refuge) have put far more iron shipwrecks on the seabed here than my seeing off any number of trawlers could.

Coral? What's that? All the charts say that the seabed hereabout is 'black shelly ooze'. Which will inherit the earth, when we have pizened all them poncy corals.