Background
The authors write that "increasing nutrient availability can negatively impact coral reproduction, growth and mortality (Koop et al., 2001) and hasten the progression of coral disease (Bruno et al., 2003; Voss and Richardson, 2006)." And they say that evidence also suggests that "nutrients may increase corals' susceptibility to bleaching," citing the work of Marubini and Davies (1996), especially "when sea surface temperatures rise," as reveled by the studies of Wooldridge (2009), Cunning and Baker (2012) and Wiedenmann et al. (2012). Yet they indicate that "despite these proposed links between nutrient availability and coral disease and bleaching, there has been no empirical evidence to date that nutrients can cause an increase in the prevalence of either disease or bleaching in the field." And, therefore, they set about to see if they could fill this data void.

What was done
Vega Thurber et al. conducted "a long-term nutrient enrichment experiment on a coral reef in the Florida Keys, USA, designed to examine the impacts of anthropogenic nutrient loading on benthic community structure." There, over the course of three years, they continually added nitrogen and phosphorus to four nine-m² enrichment plots that were paired with control plots subject to ambient levels of nutrients, after which - at the end of the experiment - they surveyed the coral community for signs of coral disease and bleaching.

What was learned
The six scientists determined that "Siderastrea siderea corals within enrichment plots had a two-fold increase in both the prevalence and severity of disease compared with corals in unenriched control plots," and that "Agaricia spp. of corals exposed to nutrients suffered a 3.5-fold increase in bleaching frequency relative to control corals." But one year later, after nutrient enrichment had been terminated for ten months, they say "there were no differences in coral disease or coral bleaching prevalence between the previously enriched and control treatments."

What it means
In addition to demonstrating the debilitating effects of nutrient pollution of coastal waters on indigenous corals, and "given that coral disease and bleaching are some of the primary killers of corals worldwide (Bruno et al., 2007; Harvell et al., 2007)," according to Vega Thurber et al., their study's results reveal that "conservation efforts that reduce nutrient loading and lower the prevalence and severity of disease and bleaching may be effective strategies for helping preserve reef ecosystems."

References
Bruno, J.F., Petes, L.E., Harvell, C.D. and Hettinger, A. 2003. Nutrient enrichment can increase the severity of coral diseases. Ecology Letters 6: 1056-1061.

Bruno, J.F., Selig, E.R., Casey, K.S., Page, C.A., Willis, B.L., Harvell, C.D., Sweatman, H. and Melendy, A.M. 2007. Thermal stress and coral cover as drivers of coral disease outbreaks. PLOS Biology 5: 1220-1227.

Cunning, R. and Baker, A.C. 2012. Excess algal symbionts increase the susceptibility of reef corals to bleaching. Nature Climate Change 3: 259-262.

Harvell, C.D., Jordan-Dahlgren, E., Merkel, S., Rosenberg, E., Raymundo, L., Smith, G., Well, E. and Willis, B. 2007. Coral disease, environmental drivers, and the balance between coral and microbial associates. Oceanography 20: 172-195.

Koop, K., Booth, D., Broadbent, A.D., Brodie, J., Bucher, D.J., Capone, D., Coll, J., Dennison, W., Erdmann, M., Harrison, P.L., Hoegh-Guldberg, O., Hutchings, P., Jones, G.B., Larkum, A.W.D., O'Neil, J., Steven, A., Tentori, E., Ward, S., Yellowlees, D. and Williamson, J. 2001. ENCORE: the effect of nutrient enrichment on coral reefs. Synthesis of results and conclusions. Marine Pollution Bulletin 42: 91-120.

Marubini, F. and Davies, P.S. 1996. Nitrate increases zooxanthellae population density and reduces skeletogenesis in corals. Marine Biology 127: 319-328.

Voss, J.D. and Richardson, L.L. 2006. Nutrient enrichment enhances black band disease progression in corals. Coral Reefs 25: 569-576.

Wiedenmann, J., D'Angelo, C., Smith, E.G., Hunt, A.N., Legiret, F.-E., Postle, A.D. and Achterberg, E.P. 2012. Nutrient enrichment can increase the susceptibility of reef corals to bleaching. Nature Climate Change 3: 160-164.

Wooldridge, S.A. 2009. Water quality and coral bleaching thresholds: formalizing the linkage for the inshore reefs of the Great Barrier Reef, Australia. Marine Pollution Bulletin 58: 745-751.