CURRENT DAY THREATS TO CORAL REEFS: NATURE OR NURTURE? (PART 2)

A Review of Hoegh-Guldberg et al. (2007) CORAL REEFS UNDER RAPID CLIMATE CHANGE AND OCEAN ACIDIFICATION.

Introduction: Atmospheric carbon dioxide concentration is expected to exceed 500 parts per million and global temperatures to rise by 2 degrees between 2050 and 2100. This significantly exceed conditions of the last 420,000 years during which most extant marine organisms evolved. Under expected conditions for the 21^st century, global warming and ocean acidification will compromise carbonate accretion, with corals becoming increasingly rare on reef systems.

What is ocean acidification? During the 20th century, increasing atmospheric CO₂ concentrations has driven an increase in the global oceans’ average temperature by 0.74°C, sea level by 17 cm, and has depleted seawater carbonate concentrations and increased acidity by 0.1 pH units. Approximately 25% of the CO₂ emitted from all anthropogenic sources currently enters the ocean where it reacts with water to produce carbonic acid. Carbonic acid dissociates to form bicarbonate ions and protons, which in turn react with carbonate ions to produce more bicarbonate ions, reducing the availability of carbonate to biological systems (Fig. 1A). Decreasing carbonate ion concentrations reduce the rate of calcification of marine organisms such as reef-building corals, ultimately favouring erosion.

Figure 1: Links between the buildup of atmospheric CO2 and the slowing of coral calcification due to ocean acidification. 

Modeled scenarios: Hoegh-Guldberg et al. (2007) projected three scenarios for coral reefs over the coming decays and centuries.

1)      375ppc and 1˚C increase (stabilising at the present atmospheric CO₂ concentration) coral reefs will continue to change but will remain coral dominated and carbonate accreting in most areas of their current distribution (Fig 2A).

2)      However if we move towards higher CO₂ concentrations (450-500ppm and 2˚ increase) coral community compositions will change with some areas becoming dominated by more thermal tolerant corals and others potentially dominated by thermally sensitive but rapidly colonising genera. The density and diversity of corals on reefs are likely to decline leading to vastly reduced complexity and loss of biodiversity including losses of coral associated fish and invertebrates (Fig 2B).

3)      As atmospheric carbon dioxide levels of more than 500ppm are approached levels of carbonate ion concentrations fall well below today’s value and we exhibit an increase of more than 3˚ in sea surface temperature. These changes will reduce coral reef ecosystems to a crumbling frame work with few calcareous corals. Continuously changing climate (which might not stabilise for hundreds of years) is likely to impede migration of corals. Reefs will be rapidly eroded and ultimately become drowned reefs (corals and reef growth fail to keep up with rising sea levels). Under this climate scenario coral reefs become non-functioning (Fig 2C).

Figure 2: Extant reefs from the Great Barrier Reef that are used as analogs for the ecological structures under scenarious 1-3. A). reef slope communities at Heron island. B). mixed algal and coral community  & C). Inshore reef slope.

Impacts: These three scenarios are likely to have serious consequences not only on the corals reefs diversity and density but through wider regional economies e.g. coastal protection, fisheries, and tourism. These consequences become progressively worse as we move through the three potential scenarios.  

 

Conclusion: It is worrying to think that when using the lowest rage of IPCC scenarios there is still devastating effects for coral reefs. At carbon dioxide atmospheric concentration of more than 500 ppm the consequences for coral reefs are extremely risky for both corals and those that depend on them. Climate change also exacerbates local stresses from declining water quality and overexploitation of key species, driving reef increasingly toward the tipping point for functional collapse.

*ppm – parts per million.