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How Best to "Weatherproof" Earth's Corals Against Warming-Induced Bleaching
Reference
Wooldridge, S.A. and Done, T.J. 2009. Improved water quality can ameliorate effects of climate change on corals. Ecological Applications 19: 1492-1499.

Background
In our Editorial of 26 Mar 2003, we wrote "we have long contended that any increase in coral bleaching that may have occurred in response to periods of elevated water temperature over the past two decades have only occurred because of a long-term weakening of coral resistance to thermal stress caused by the gradual intensification of a multitude of local anthropogenic assaults upon the watery environments in which corals live (Idso et al., 2000)," listing, among other things, "chemical insults to once-pristine reef environments ... such as the rising levels of nutrients and toxins in coastal waters caused by runoff from agricultural activities on land and associated increases in sediment delivery."

What was done
In a study that addresses the concerns we have been expressing about this matter over the past decade, Wooldridge (2009a) developed an hypothesis that suggests that reduced dissolved inorganic nitrogen (DIN) content in seawater surrounding reefs could "directly benefit corals by enhancing their resistance to heat stress, i.e., raising the temperature thresholds that trigger bleaching," while Wooldridge and Done (2009) investigated the implications of this suggestion "at the scale of sea-scapes and regions on [Australia's] Great Barrier Reef [GBR]," where they say "the coastal waters are highly 'DIN enriched' as a consequence of a century and a half of European settlement of north Queensland." More specifically, they say they "used a spatially explicit Bayesian belief network (BBN) model (Pearl 1988; Woldridge and Done, 2004) to investigate the benefits of inclusion of DIN in explaining and predicting variability in complex patterns of coral bleaching documented on the GBR in 1998 and 2002."

What was learned
The two researchers from the Australian Institute of Marine Science found that "corals bathed in nutrient-rich coastal waters had a decreased bleaching resistance (per degree of heating) during the 1998 and 2002 bleaching events compared to reefs in oligotrophic oceanic waters, effectively lowering the upper thermal bleaching threshold by ~1.0-1.5C," while they report that one of them (Wooldridge, 2009b) further found that "a complementary investigation suggests these figures could be as much as 2.0-2.5C in the most DIN-enriched locations."

What it means
Woldridge and Done state that "the new conceptual picture that emerges from this paper is of the fundamental importance of nutrient loading, in particular DIN, in defining the bleaching resistance of corals to heat stress." We agree, although we would suggest that the broad outline of the concept has been around for about a decade, which is -- as they aptly put it -- that "coral reef resilience to climate change may be improved by good local management of coral reefs, including management of water quality."

References
Idso, S.B., Idso, C.D. and Idso, K.E. 2000. CO2, global warming and coral reefs: Prospects for the future. Technology 75S: 71-93.

Pearl, J. 1988. Probabilistic Reasoning in Intelligent Systems: Networks of Plausible Inference. Morgan Kaufmann Publishers, San Francisco, California, USA.

Wooldridge, S.A. 2009a. A new conceptual model for the warm-water breakdown of the coral-algal endosymbiosis. Marine and Freshwater Research 60: 483-496.

Wooldridge, S.A. 2009b. 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.

Wooldridge, S.A. and Done, T.J. 2004. Learning to predict large-scale coral bleaching from past events: A Bayesian approach using remotely sensed data, in-situ data, and environmental proxies. Coral Reefs 23: 96-108.

Reviewed 16 December 2009