Climate change, coral bleaching and ocean acidification: will the Great Barrier Reef remain great?

The Great Barrier Reef is the world’s largest coral reef, supporting an enormously biodiverse community of organisms. It provides a multitude of ecosystem functions and services, both biotic and abiotic, benefitting the species of the marine ecosystem and the human population surrounding it. The effect of climate change on coral reefs has been widely studied, and multiple detrimental stressors identified. The extreme extent of recent coral bleaching events, additional stress posed by ocean acidification and other anthropogenic drivers make it hard to see a bright future for the GBR. Nevertheless, the discovery of restoration techniques such as microfragmentation and 3D reef printing give some hope to the conservation of the GBR and the ecosystem services it provides.

Article Highlights

· New modelling technology demonstrates that the level of atmospheric CO2 concentrations are nearing a critical point for the Great Barrier Reefs survival

· The extent of recent coral bleaching on the Great Barrier Reef is considerably more severe than bleaching events of the past

· The synergistic effect of warming oceans and increasing ocean acidity on coral are compounding at an alarmingly high rate

· The recent discovery of microfragmentation gives some hope for restoring thermally tolerant reefs

· 3D printing technology opens new doors to creating structurally complex artificial reefs and maintaining the ecosystem services they provide

(…)

2) Ocean acidification (OA)

Atmospheric carbon dioxide (CO2) concentrations have been continuously increasing since they were first recorded in 1958. Recordings (as of 5th December 2019) measured 410.27 ppm [54], pre-industrial levels are estimated to have been 280 ppm [33]. All emissions scenario projections by the IPCC suggest atmospheric CO2 levels will continue to rise in the near future [33]. If there is not drastic global change, they have the potential to reach 936 ppm by the year 2100 [33]. The ocean is an enormous atmospheric CO2 sink, it has absorbed approximately 30-40% of all anthropogenic CO2 emissions since the industrial revolution [33, 55-57].

Hydration and hydrolysis of CO2 that is absorbed into the ocean forms carbonic acid [H2CO3] [58]. This results in a higher concentration of oceanic hydrogen ions [H+], thus reducing ocean pH levels [7, 58-60]. Bicarbonate ions [HCO3-] and protons are formed as carbonic acid dissociates [7], the protons then react with carbonate ions [CO32-] to form more bicarbonate ions [7, 58-60]. This reaction decreases the concentration of oceanic carbonate ions which are essential for coral growth [7, 58-60]. Carbonate ions have not dropped below 240 mmol kg-1 in oceanic regions housing coral reefs in the past 420,000 years. That is until today, where concentrations are already less than 210 mmol kg-1 in key coral reef habitats [7]. Projections indicate the concentration will continue to decline as atmospheric CO2 increases [7].

Coral utilise carbonate ions in the ocean to form and deposit the framework of aragonite [CaCO3] crystals that later become their skeleton [7, 61]. Shallow tropical seas are highly saturated with carbonate ions and aragonite, this high saturation level is essential for coral growth [58, 60, 61]. Due to OA, the GBR has already started to display evidence of a decreased rate of calcification and growth [63, 64]. Albright et al. [64] manipulated the ocean chemistry in two isolated coralline lagoons on the GBR. To represent more alkaline pre-industrial conditions, the carbonate ion and aragonite saturation levels in the lagoons were artificially increased. Following manipulation, calcification rates increased by approximately 6% [64]. This indicates that OA is already negatively impacting GBR corals. At the time of this research’s publishing (March 2016), CO2 concentrations were at 404.87 ppm [54].

The oceanic temperature and acidification response to increasing atmospheric CO2 concentrations is subject to a lag-time before the full effect is evident [20]. The lag-time is estimated to be in the range of several decades, the lowest estimations suggest a single decade [20]. Given this lag time, studies regarding the effects of OA on coral growth may in actuality represent the effects of atmospheric CO2 concentration at least 10 years prior. It has previously been reported by Veron et al. [20] that atmospheric CO2 of 350 ppm is the maximum concentration at which coral reefs can exist in a healthy state. Considering the minimum ten-year lag time, therefore we may be beyond the point of no return concerning ensuring the survival of a healthy GBR.

It has been suggested that the lethal dissolution effects of OA on coral reefs will not be witnessed under natural conditions [27]. In some species, extreme OA has been shown to cause bleaching and subsequent mortality before disrupting calcification enough to trigger the reef to enter dissolution [67]. Unfortunately, this is in part because the atmospheric CO2 concentrations required to generate a state of dissolution may have already driven global temperatures beyond the level coral can survive [27]. Atmospheric CO2 concentrations of 560 ppm have been proposed as the level at which the coral reefs will enter a state of dissolution [65]. Fig. 5 demonstrates that an atmospheric CO2 concentration of 450 ppm has a 34% chance of driving a global temperature increase of ~1.5°C [27]. Warming surpassing 1.5°C could result in the death of 90% or more of the worlds coral reefs irrespective of dissolution [53].

In terms of potential adaptive responses of coral to OA, the topic is largely unknown [27]. This is in part due to experimental difficulties in isolating the effects of OA from temperature stress [27]. OA is a significant stressor on coral reef ecosystems [7] and given the GBR’s southerly range, is potentially an even more prominent stressor than on other coral reef ecosystems [7, 58, 64, 66].

(…)

Fraser Woodburn, Environy, 15 October 2020. Full article.

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