Background
The authors write that "worldwide, coastal coral reefs are threatened by eutrophication and sedimentation from terrestrial runoff," citing Burke et al. (2011) and Howarth et al. (2011), while noting that "the extent of photo-physiological stress in corals correlates with the amount of sedimentation multiplied by exposure time," citing Philipp and Fabricius (2003). However, they indicate that "earlier studies either did not characterize the sediments or regarded them mainly as mineral particles, and the microbial and chemical components have not been analyzed," which suggests, as they note, that "the mechanisms leading to coral mortality after sediment exposure still are poorly understood."

What was done
In an effort to find the true mechanisms of death by sedimentation, Weber et al. made numerous microsensor measurements in mesocosm experiments and in naturally accumulated sediment on corals.

What was learned
The eight researchers - hailing from Australia, Germany, Italy and Oman - found that (1) in organic-rich sediments, pH and oxygen started to decrease as soon as the sediment accumulated on the coral, that (2) "the combination of anoxia and low pH led to colony death within 24 hours," and that (3) "when hydrogen sulfide was added after 12 hours of anoxia and low pH, colonies died after an additional three hours." And based on these observations, they suggest that (4) "sedimentation kills corals through microbial processes triggered by the organic matter in the sediments, namely respiration and presumably fermentation and desulfurylation of products from tissue degradation." Put another way, they say that "first, increased microbial respiration results in reduced O2 and pH, initiating tissue degradation," and that "subsequently, the hydrogen sulfide formed by bacterial decomposition of coral tissue and mucus diffuses to the neighboring tissues, accelerating the spread of colony mortality."

What it means
Weber et al. conclude that "the organic enrichment of coastal sediments is a key process in the degradation of coral reefs exposed to terrestrial runoff." And we suggest that striving to mitigate this problem, as well as a number of other localized assaults on reef environments will prove far more effective than focusing on the more nebulous and tenuous global concern of anthropogenic CO2 emissions. Therefore, we say: think locally and act locally, for we all are stewards of our own backyards.

References
Burke, L., Reytar, K., Spalding, M. and Perry, A. 2011. Reefs at Risk Revisited. World Resources Institute, Washington, D.C.

Howarth, R., Chan, F., Conley, D., Garnier, J., Doney ,S.C., Marino, R. and Billen, G. 2011. Coupled biogeochemical cycles: Eutrophication and hypoxia in temperate estuaries and coastal marine ecosystems. Frontiers in Ecology and the Environment 9: 18-26.

Philipp, E. and Fabricius, K. 2003. Photophysiological stress in scleractinian corals in response to short-term sedimentation. Journal of Experimental Marine Biology and Ecology 287: 57-78.