Bill Bement

At the same time, the government was forced to admit that some of the pollution targets set for this year will not be met. Specifically, Australia wanted to reduce the fertiliser runoff by 50% by this year under the socalled Reef Plan, but this target has now been postponed to 2018. Only small reductions in nitrogen and sediment runoff have been achieved so far. These problems can be clearly linked to agricultural practices introduced relatively recently. “Historical records from the skeletons of corals show a 5to 10-fold increase in the delivery of sediments after the arrival of Europeans and their livestock in the last 1800s,” explains Hughes. “Many coastal reefs and seagrass beds have been destroyed, often long before modern monitoring or management began. Sediment can smother juvenile corals, hindering the capacity of reefs to recover from natural and man-made disasters like cyclones or bleaching events.” In a recent study of several reef systems in the Caribbean, Chris Perry from the University of Exeter, UK, and colleagues from several other institutions estimated the long-term speed of accretion of carbonate to the reefs and compared it to the rate observed on geological timescales throughout the Holocene (Nat. Commun. (2013) doi 10.1038/ ncomms/24090). The researchers found that the net production rate (as measured in kilogram carbonate added per year) was around half the historic rate, while the accretion rate (millimetres growth per year) is even an entire order of magnitude smaller than the historic rate. The authors attribute this weak growth to the changes in reef ecosystems, which today are generally impoverished. Even the Caribbean reefs with a healthy amount of live coral cover, the authors report, rely now on just one species (Monastraea) for the bulk of their carbonate production, while the higher production rates in historic times were due to corals from two different genera, Monastraea and Acropora. Combined with natural erosion rates, the weak growth of the reefs may actually mean that some of them are eroding faster than they can grow and are ultimately disappearing. Furthermore, slow growth can become a problem if it is outpaced by sea level rise. Apart from bleaching, other diseases of corals also seem to be spreading, as Caroline Rogers and Jeff Miller have pointed out in a recent letter to Science (Science (2013) 340, 1522). Some of these seem to be encouraged by pollution, or possibly overfishing in ways that remain to be explored. For instance, research published in 2008 suggested that active ingredients from sunscreen, which washes off swimmers on the scale of thousands of tonnes per year, can activate dormant viruses in the corals’ zooxanthellae symbionts and thus trigger a deadly disease. At the Great Barrier Reef, fertiliser pollution has encouraged the spread of a native starfish called crown of thorns (Acanthaster planci), which preys on coral polyps. Further unexpected ways in which humans harm corals may yet surface. Given the important ecosystem services that coral reefs provide to industries, including fisheries and tourism — not to mention the fact that some island nations are literally built on them — the continuing danger to their health remains a pressing concern, even if some reefs can sometimes recover from disturbance or adapt to mitigate the impact of climate change. One ray of hope is offered by the as yet little explored deeper reefs, found at depths between 30 and 150 metres. These socalled mesophotic reefs are better shielded from human impact than the shallower ones, both by their depth and by greater distance from the shore. However, they are also less well understood scientifically, and often not even mapped, let alone granted protection. Researchers hope that these little known reefs may act as a species reserve to re-seed shallow reefs. In a recent commentary (Nature Climate Change (2013) 3, 528–530), Hughes and colleagues call to extend protection status to all coral reefs, at all depths.