How Corals Succumb to Sedimentation
Weber, M., de Beer, D., Lott, C., Polerecky, L., Kohls, K. Abed, R.M.M., Ferdelman, T.G. and Fabricius, K.E. 2012. Mechanisms of damage to corals exposed to sedimentation. Proceedings of the National Academy of Sciences USA 109: 10.1073/pnas.1100715109.
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. And what did they learn?
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."
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.
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.