Background
The authors write that "the threats that climate change pose to coral reefs may be magnified by elevated levels of nutrients, sediments and pollutants from terrestrial runoff, or from point sources of pollution such as ship-groundings and mine tailings," citing the findings of the many studies reviewed by Fabricius (2005); but in their study of the subject, they focus their attention solely on trace metal contamination that arises from "agricultural runoff, shipping accidents and operations, and dredging," such as has been described by Howard and Brown (1984), Reichelt and Jones (1994), and Negri and Marshall (2009).

What was done
Negri and Hoogenboom studied larval metamorphosis in two common corals (Acropora millepora and Acropora tenuis) that they collected from depths of 3-5 meters around Magnetic Island -- a near-shore high-turbidity site off the northeast coast of Queensland, Australia -- identifying and evaluating (1) thresholds of water temperature and pollution (specifically, copper contamination) that prevent larval metamorphosis and (2) the synergistic interaction of the two stresses.

What was learned
The two researchers found that "the critical early life stages of coral development, during which corals metamorphose from pelagic larvae into sessile polyps, are more sensitive to high SSTs (sea surface temperatures) in the presence of the common anthropogenic pollutant copper (Cu)," noting that "the combined effects of Cu and increased SST were additive for A. tenuis larvae above 29°C and became synergistic for both species at sea surface temperatures above 31°C, with the combined effect of Cu contamination and excessive SST being stronger than the sum of the independent effects of each stressor."

More specifically, they report that "at a seawater temperature of 28°C, 50% of A. millepora and A. tenuis larvae successfully metamorphosed when Cu concentrations were approximately equal to 25 and 30 µg/L respectively," but they say that "halving Cu concentrations from these values resulted in more than 3.5°C increases in the temperature threshold for both species." In addition, they note that reducing dissolved inorganic nitrogen (a nutrient found in agricultural runoff) by 50-80% "would help to protect inshore corals of the Great Barrier Reef by increasing the bleaching threshold by 2°C," citing Wooldridge (2009).

What it means
In light of these several observations, it would appear that if humanity's many physical and chemical disturbances of the planet's coral reef environments were either eliminated or significantly reduced, there would be much less warming-induced destruction of corals throughout the world; for if a 50% reduction in seawater Cu concentration provides a 3.5°C increase in the degree of heat that can be tolerated by coral larvae, and if a 50-80% reduction in dissolved inorganic nitrogen runoff provides 2°C more relief, imagine what benefits similar reductions in man's many other assaults upon earth's coral reef environments might bring.

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.

Howard, L.S. and Brown, B.E. 1984. Heavy metals and reef corals. Oceanography and Marine Biology: An Annual Review 22: 195-210.

Negri, A. and Marshall, P. 2009. TBT contamination of remote marine environments: Ship groundings and ice-breakers as sources of organotins in the Great Barrier Reef and Antarctica. Journal of Environmental Management 90: S31-S40.

Reichelt, A.J. and Jones, G.B. 1994. Trace metals as tracers of dredging activity in Cleveland Bay -- field and laboratory studies. Australian Journal of Marine & Freshwater Research 45: 1237-1257.

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.