How does rising atmospheric CO2 affect marine organisms?

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Heat-Induced Stress in Corals is Exacerbated by Eutrophication
Fabricius, K.E., Cseke, S., Humphrey, C. and De'ath, G. 2013. Does trophic status enhance or reduce the thermal tolerance of scleractinian corals? A review, experiment and conceptual framework. PLOS ONE 8: e54399.

The authors report that several lines of study suggest that "corals in waters that are turbid and rich in nutrients and trace metals should be more resistant and resilient to temperature and solar insolation stress," since [1] they "have greater scope to maintain energy reserves through heterotrophy after endosymbiont loss," [2] "their antioxidant enzymes provide protection against oxygen radicals," and [3] they "may be equipped with more thermally tolerant endosymbionts." On the other hand - and it's a very heavy hand - they say there are "at least six other lines of evidence based on physiological or correlative field data, which suggest synergistic or additive effects of temperature stress and nutrients." And "according to these studies," as they continue, "corals in nutrient-enriched or turbid waters are more vulnerable to temperature stress than those in low nutrient environments."

What was done
To help resolve this issue, Fabricius et al. conducted a 90-day-long experiment in which they maintained colonies of two scleractinian corals (Acropora millepora and Montipora tuberculosa) in either (1) normal coastal seawater or (2) normal coastal seawater that was further "organically and nutrient enriched (OE)," both with and without heat stress, where in the former case water temperature was gradually increased from 27 to 31.2°C for another 3-7 weeks.

What was learned
After the 3-7 weeks of heat stress, the four researchers report that colonies of both species had significantly greater reductions in fluorescence yields and lower survival in the OE treatment than in the non-OE-treated coastal water. In addition, they say that photophysiological recovery was incomplete 31-38 days after ending the heat stress only in the OE treatment.

What it means
Fabricius et al. conclude that their study "adds to mounting evidence that eutrophication can worsen thermal stress on inshore reef communities," and that "even without heat stress, the exposure to organical enrichment has strong negative effects on the photophysiology and survival of inshore corals (Fabricius et al., 2003; Weber et al., 2006; Weber et al., 2012)." And they say that "improving water quality, by reducing the loss of fertilizers and soils from farmed and cleared lands, is therefore rightly considered an essential management strategy to enhance the resilience of reefs to warming temperatures and ocean acidification," citing Wooldridge et al. (2006), the State of Queensland and Commonwealth of Australia (2009), and the Secretariat of the Convention on Biological Diversity (2010).

Fabricius, K.E., Wild, C., Wolanski, E. and Abele, D. 2003. Effects of transparent exopolymer particles and muddy terrigenous sediments on the survival of hard coral recruits. Estuarine and Coastal Shelf Science 57: 613-621.

Secretariat of the Convention on Biological Diversity. 2010. COP 10 Decision X/2. Strategic Plan for Biodiversity 2011-2020 and the Aichi Targets. Secretariat of the Convention on Biologicl Diversity, Montreal, Canada.

The State of Queensland and Commonwealth of Australia. 2009. Reef Water Quality Protection Plan for Catchments Adjacent to the Great Barrier Reef World Heritage Area. Queensland Department of premier and Cabinet, Brisbane, Australia.

Weber, M. de Beer, D., Lott, C., Polerecky, I. Kohls, K., Abed, R.M. M, Ferdelman, T.G. and Fabricius, K..E. 2012. Mechanisms of damage to corals exposed to sedimentation. Proceedings of the National Academy of Sciences USA 109: E1558-67.

Weber, M., Lott, C. and Fabricius, K. 2006. Different levels of sedimentation stress in a scleractinian coral exposed to terrestrial and marine sediments with contrasting physical, geochemical and organic properties. Journal of Experimental Marine Biology and Ecology 336: 18-32.

Wooldridge, S., Brodie, J. and Furnas, M. 2006. Exposure of inner-shelf reefs to nutrient enriched runoff entering the Great Barrier Reef Lagoon: Post-European changes and the design of water quality targets. Marine Pollution Bulletin 52: 1467-1479.

Reviewed 10 July 2013