[Coral-List] concurring with Steve and Alina

[Charles Birkeland]

As with Steve and Alina, my experience with corals on massive metal shipwrecks has been quite different from the experiences being reported by others on this website. In Chuuk (Truk) Lagoon (Caroline Islands), 41 ships (a total of about 216,515 tons of metal) were sunk on 17-18 February 1944.  The rich coral communities that developed on these ships by the 1980s were a diverse and spectacular variety of populations. These rusted ships served as a beautiful array of gigantic settlement panels at various angles and depths. Although many surfaces were rich with scleractinians and octocorals, their compositions varied abruptly with angle of the substratum. The change in generic composition seemed to vary more with depth and angle of substratum than with nature of the metal. I remember a ship near the entrance of Apra Harbor in Guam which had a nice population of Plerogyra sinuosa on the outer vertical rusted surfaces, but with Tubastraea aurea inside the pilot house. It seemed that shade and angle of the substrata, rather than rust, was the major factor determining distribution.
Nine large (> 30 m) ships were grounded on the coral reef in Pago Pago Harbor during Hurricane Val in 1991.  Their rusting hulls remained for nine years on both sides of a permanent coral transect that had been monitored for decades. The algae, corals and other invertebrates were examined between the nearest shipwreck and the transect, but no signs of any effects could be discerned other than the physical abrasion where the rusty hull dragged over the reef. The 9 ships were finally removed in 2000, thanks to the U.S. Coral Reef Task Force. 
Chang-Feng Dai and I made a dive in Beqa Lagoon, Fiji, on a ship that was recently sunk to be a diving tourist attraction.  I remember we were impressed with the size of some of the scleractinian and octocoral colonies given the relatively short time they had to grow. Perhaps positioning in the current high above the sediment outweighed the unpleasantness of metal.
Although I have not seen them firsthand, I am aware of reports of long-term effects of rusty shipwrecks on coral reefs, especially favoring cyanobacteria. In 1984, Bruce Hatcher reported evidence that a maritime accident produced an alternative stable state (phase shift) in a benthic community on a coral reef. Likewise, a shipwreck on Rose Atoll released fuel oil and dropped metal debris and fishing lines onto the reef. The effects on algal community structure were visible for years. I agree with Alina that cyanobacteria can occasionally be favored by rusty iron, but it is not always the case. 
In contrast to the diverse coral communities on 216,515 tons of rusted metal in Chuuk Lagoon, it was noted in the  September 3 Coral-List that rebar in an aquarium will have a substantial negative effect. When extrapolating from the laboratory to the field, matters of scale (absolute volume of water) and context (whether or not the system is iron-limited, and the nature of the water motion) should be considered.
There are many local products of human activities such as shipwrecks that might have negative effects on corals in some contexts, and coral reefs might be dying from a thousand small cuts. But what is depressing is that these are serving as a diversion from the more fundamental problem of CO2 deposition into the biosphere. Like the old cliché - if we cannot keep the Titanic from sinking, we can at least tidy things up as she goes down. 
Chuck