Background
The authors note that the bleaching of coral reefs is often linked to periodic increases in seawater temperature associated with global warming; but they state that local pressures like overfishing and pollution, including eutrophication, are also often involved; and in regard to these latter two phenomena, they say there is "strong evidence" that the input of nutrients and sediments to the lagoons of Australia's Great Barrier Reef "has increased several fold since European farming practices were adopted in adjacent catchments (ca. 150 years ago)," citing Furnas (2003). And, therefore, they felt compelled to study the effects of the most commonly detected herbicides that are present in near-shore areas year round, including diuron and atrazine, which are designed to damage photosystem II (PSII) in terrestrial weeds, but which can affect other photosynthesizing organisms such as the Symbiodinium species found within the corals that host them.

What was done
"In order to examine the potential interactive pressures of local pollution and global climate change," in the words of Negri et al., they exposed Acropora millepora corals to three different agricultural herbicides at four different temperatures ranging from 26 to 32°C.

What was learned
The four Australian researchers found that the combined effects of elevated temperature and reduced water quality (the presence of herbicides) were detrimental to the coral, and that "the reduction in photosynthetic efficiency due to these stressors was additive." In addition, they report that the effects of high temperature in combination with either diuron or atrazine were synergistic (i.e., larger than additive) for the maximum quantum yield of photosynthesis, which is indicative of enhanced damage to PSII under these conditions. And they report that reducing the concentration of diuron from 1 µg per liter to 0 µg per liter at temperatures above 30°C "would protect coral symbionts from an increase in SST [sea surface temperature] of at least 1°C."

What it means
Noting that other field and laboratory studies have shown that "combinations of increased SST and increased pollution: sediments, sewage and nutrients (Nagelkerken, 2006), metals (Nystrom et al., 2001), and nitrate (Nordemar et al., 2003; Wooldridge, 2009) affect corals more severely than thermal stress alone," Negri et al. conclude that their study "adds further evidence to the hypothesis that managing pollution at a local level may help protect corals from the effects of high SST and the projected effects of climate change."

References
Furnas, M.J. 2003. Catchments and Corals: Terrestrial Runoff to the Great Barrier Reef. Australian Institute of Marine Science and CRC Reef Research Centre, Townsville, Australia.

Nagelkerken, I. 2006. Relationship between anthropogenic impacts and bleaching-associated tissue mortality of corals in Curacao (Netherlands, Antilles). International Journal of Tropical Biology 54: 31-44.

Nordemar, I., Nystrom, M. and Dizon, R. 2003. Effects of elevated seawater temperature and nitrate enrichment on the branching coral Porites cylindrica in the absence of particulate food. Marine Biology 142: 669-677.

Nystrom, M., Nordemar, I. and Tedengren, M. 2001. Simultaneous and sequential stress from increased temperature and copper on the metabolism of the hermatypic coral Porites cylindrica. Marine Biology 138: 1225-1231.

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.