Here is the abstract:
The Line Islands are calcium carbonate coral reef platforms located in iron-poor regions of the central Pacific. Natural terrestrial run-off of iron is non-existent and aerial deposition is extremely low. However, a number of ship groundings have occurred on these atolls. The reefs surrounding the shipwreck debris are characterized by high benthic cover of turf algae, macroalgae, cyanobacterial mats and corallimorphs, as well as particulate-laden, cloudy water. These sites also have very low coral and crustose coralline algal cover and are call black reefs because of the dark-colored benthic community and reduced clarity of the overlying water column. Here we use a combination of benthic surveys, chemistry, metagenomics and microcosms to investigate if and how shipwrecks initiate and maintain black reefs. Comparative surveys show that the live coral cover was reduced from 40 to 60% to 0.75 km2). The phase shift occurs rapidly; the Kingman black reef formed within 3 years of the ship grounding. Iron concentrations in algae tissue from the Millennium black reef site were six times higher than in algae collected from reference sites. Metagenomic sequencing of the Millennium Atoll black reef-associated microbial community was enriched in iron-associated virulence genes and known pathogens. Microcosm experiments showed that corals were killed by black reef rubble through microbial activity. Together these results demonstrate that shipwrecks and their associated iron pose significant threats to coral reefs in iron-limited regions.
Forest and others have recently been studying the Line Islands because they are relatively undisturbed reefs. Here are a short video about the work there (the work in general, not this specific study per se): http://oceantoday.noaa.gov/swf/flowplayer-latest.swf
Anyway, the new paper does something very different. It focuses on shipwrecks and the impact of these wrecks on reefs. This is of particular interest because as indicated in the abstract, the reefs are very low in iron. And many shipwrecks introduce massive amounts of iron. What they conclude in this new paper is that the iron from the shipwrecks leads to algal blooms, and lead to rapid killing of / damage to the pristine reefs.
For more on the paper there is an article in National Geographic Newswatch by Enric Sala worth checking out.
Forest also wrote me some information by email. He states:
Black reefs are associated with shipwrecks or other debris in this region of the world. These sites are interesting both from a conservation and scientific point of view. As a conservation issue, they are amazingly destructive. Kingman, one of the jewels of the USA coral reefs, has lost >1 km of the lagoon in less than 3 years. An old wreck on Fanning atoll has killed about 10% of their reef.
Visually, the black reefs are some of the eeriest places I’ve ever seen. The bottom is completely covered in different algae (including cyanobacterial mats), the water is filled with marine snow, and dark precipitate on the benthos (probably sulfur). We just published a paper in ISME where we have recreate the precipitate, cloudiness, and
coral death in microcosms by combining rubble from the black reefs, with corals and an iron addition. Addition of antibiotics blocks the coral death, precipitate, and marine snow, suggesting a microbial role.
The black reefs are probably caused by iron-enrichment from the wrecks and debris. We think black reefs are specific to non-emergent coral reefs, where iron is a limiting nutrient. Our current model is that iron stimulation of algae leads to increased microbial activity and coral death. In support of this, metagenomic analysis of the microbial community showed an enrichment of iron-related pathogenicity factors.
Forest also adds a plea to help in conservation of these reefs.
If you are interested in conservation, then please help us petition Congress to support removal of the wrecks and debris. Please contact Emily Douce at the Marine Conservation Biology Institute.
I encourage people to contact her.