Background
The authors write that "tropical coral reefs are currently under threat by a variety of regional and global stressors," with examples of the former being "land runoff and overfishing (e.g., Pandolfi et al., 2003; Fabricius, 2005)," while examples of the latter are "sea temperature increase and ocean acidification (Hoegh-Guldberg et al., 2007; Fabricius et al., 2011)." So which set of stressors is the most significant or bigger threat?

What was done
In an effort designed to answer this most important question, Uthicke et al. analyzed sediment cores collected from inshore fringing coral reefs in the Whitsunday area of the Great Barrier Reef (GBR) of Australia. These cores were collected from three different locations: (1) inner near-shore reefs with low coral cover and high macroalgal abundance, (2) intermediate reefs, and (3) reefs at outer islands with low algal and high coral cover. These three locations were chosen based on the facts that (a) inner near-shore reefs are typically the first to produce evidence of regional human impacts, that (b) reefs at outer islands are the last to experience the negative effects of human influence, as well as the most likely to exhibit evidence of global stressors, while (c) intermediate reefs often show evidence of both.

What was learned
The three researchers report that benthic foraminiferal assemblages found in the cores of outer-island reefs that are unaffected by increased land runoff have been "naturally highly persistent over long (>2000 years) timescales." In both of the other zones, assemblages were also rather persistent, but only until 150 years ago; and they say that assemblages <55 years old from inner near-shore and intermediate reefs were significantly different from older assemblages.

What it means
In concluding their report, Uthicke et al. write that they found support for the likelihood that "increased land runoff since the start of land clearing and agriculture in the catchment of the Whitsunday Region of the GBR has left a signature in the foraminiferal assemblages of inner and intermediate areas of the study area," when previously the assemblages of these areas had been "persistent for at least several thousand years." In addition, and based on the fact that "no changes were observed on outer reefs located away from land runoff," they propose that "changes observed on inner and intermediate reefs were mainly driven by enhanced agricultural runoff after European settlement." And topping off everything else, they affirm that "the hypothesis that global forcing, such as sea temperature increase or ocean acidification, altered the foraminiferal community found little support." In fact, it found none.

References
Fabricius, K.E. 2005. Effects of terrestrial runoff on the ecology of corals and coral reefs: review and synthesis. Marine Pollution Bulletin 50: 125-146.

Fabricius, K.E., Langdon, C., Uthicke, S., Humphrey, C., Noonan, S., De'ath, G., Okazaki, R., Muehllehner, N., Glas, M.S. and Lough, J.M. 2011. Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations. Nature Climate Change 1: 165-169.

Hoegh-Guldberg, O., Mumby, P.J., Hooten, A.J., Steneck, R.S., Greenfield, P., Gomez, E., Harvell, C.D., Sale, P.F., Edwards, A.J., Caldeira, K., Knowlton, N, Eakin, C.M., Iglesias-Prieto, R., Muthiga, N., Bradbury, R.H., Dubi, A. and Hatziolos, M.E. 2007. Coral reefs under rapid climate change and ocean acidification. Science 318: 1737-1742.

Pandolfi, J.M., Bradbury, R.H., Sala, E., Hughes, T.P., Bjorndal, K.A., Cooke, R.G., McArdle, D., McClenachan, L., Newman, M.J.H., Paredes, G., Warner, R.R. and Jackson, J.B.C. 2003. Global trajectories of the long-term decline of coral reef ecosystems. Science 301: 955-958.