Global vs. Local Stressors of Calcifying Organisms on Australia's Great Barrier Reef

Uthicke et al: Appendix A
From CO2 Science: (Link)

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 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 and="and" assemblages.="assemblages." different="different" font="font" from="from" inner="inner" intermediate="intermediate" near-shore="near-shore" old="old" older="older" reefs="reefs" significantly="significantly" were="were" years="years">

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.

Uthicke, S., Patel, F. and Ditchburn, R. 2012. Elevated land runoff after European settlement perturbs persistent foraminiferal assemblages on the Great Barrier Reef. Ecology 93: 111-121. 

Read More at CO2 Science: (Link)