Global warming is one of the most commonly and extensively discussed environmental issue of the 21st century. Episodes of mass coral bleaching since the 1980’s have led to extensive mortality and apprehension about the capability of coral reef ecosystems during a period of rapid climate change (Donner et al. 2018). A number of studies have been presented and analysed in both Pandolfi et al. (2011) and Hoegh-Guldberg et al. (2007) where both papers aim to predict the response of coral reefs to ocean acidification (OA). However, Pandolfi et al. (2011) presents a more robust prediction of the changes to coral reefs and how they vary through space and time, by providing a broader examination of the geological record, making more comparable inferences between species and geographic locations in terms of OA and bleaching effects, whilst presenting a more optimistic perspective and expansive suggestions for adaptable management strategies within coral reef ecosystems.
Geological historical information is crucial and a necessity when predicting responses to climate change and how regions are sensitive to global climate change over glacial- interglacial and millennial timescales (Placzek et al. 2009). Hoegh-Guldberg et al. (2007) use “data from the Vostok Ice Core study to explore the ocean temperature and carbonate-ion concentration”, as their main historical data set. In comparison Pandolfi et al. (2011) looks in detail at the past episodes of increased OA and then further utilise both fossil and proxy records along with the analysis of the physiology of past victims and survivors to present a cohesive examination. Lima et al.(2017) state that using fossil records will help improve biodiversity risk assessment in response to future climate change scenarios, and Knoll et al. (2007) state that hypercapnia best accounts for the selective survival of marine invertebrates in past extinctions. It is from these geological records that this can present an insight into the physiology of past ancient organisms and their responses to hypercapnic stress, particularly where the rates of CO2 were much higher than todays (Pandolfi et al. 2011).
Pandolfi et al. (2011) goes on to describe that fossil records provide evidence that rates of change are crucial for ecological outcomes, and highlights that previous cases show that some reefs were more resilient to past rapid warming and acidification than previously thought. Pandolfi analyses multiple historical episodes and provides profound detail for these events in comparison to Hoegh-Guldberg et al. (2007), where only the “Permian-Triassic extinction” is the only geologic event to be mentioned. A study done by Abellan, Arribas & Svenning (2012) who use geological data in relation to climate change, state that “geological long term history is an important constraint on the distribution and range and dynamics of species over long term climate change”. However both papers by Pandolfi et al. (2011) and Hoegh-Gulburg et al. (2007) do highlight that rate of change is critical, yet Pandolfi presents a broader and more comprehensive range of studies with evidence extending over millennial time scales, arguably presenting a more robust foundation for predicting future responses to climate change.
Furthermore both papers concur that OA is considered a crucial threat to coral reef ecosystems, as it reduces the availability of carbonate ions for reef building corals to use in order to produce their skeletons. Additionally Mollica et al. (2018) argue that OA threatens the future of coral reefs, however there have been mixed responses to corals by OA in experiments and field observations. For example, species responses can differ between sites such as where the rates of Porites at Papua New Guinea, differed to Palau, where the range of the calcification rate was higher at Palau (Mollica et al. 2018). Furthermore, they used Porities and Favia, two different species in their research and two different geographical locations (Mollica et al. 2018). Pandolfi et al. (2011) uses a compilation of data from several sources, from Florida Keys, Dry Tortugas, Kenya, Australia, Bahamas and the Red sea, highlighting effects of bleaching across multiple species and coral reef calcification, as well as emphasising the importance of the scale of time that studies are conducted over, overall resulting a thorough in depth and well researched projection. Hoegh-Gulburg et al. (2007) uses multiple sources as well, however they only use one set of data from a paper which only looked at coral calcification in one coral species (massive Porites) in two regions of the great Barrier Reef to form their conclusions on decreased linear extension rate and skeletal density. A more robust argument would probably present a comparison of multiple species as every species is potentially different in its responses, and across a wider geographical range.
Mizerek et al. (2018) state that in “efforts to protect corals, focus needs to be on understanding variability in bleaching within coral assemblages and taxa”, and that even though bleaching and responses to OA may be different it is the intraspecific differences in coral colony susceptibilities that suggest sensitivity is solely species specific. Furthermore Hoegh-Gulburg et al. (2007) uses a paper published in 1990 by Szmant & Gassman to support their statement that “corals may maintain their physical extension or growth rate by reducing skeletal density”. Not only is this paper potentially dated, Szmant & Gassman only looked at one species (Montastrea annularis) in one location (Carysfort Reef, Florida), where data was collected in 1987, and their sample size was reported as ‘limited’ (Szmant & Gassman 1990). Mcneish & Stapleton (2016) looked at the effect of small sample size and stated that to estimate without bias, adequate sample sizes must be obtained which could have the potential to affect inferences in applied research. However with both papers it is supported through out literature that aragonite saturation, pH and pCO2 all have an effect on coral growth. For example, Leclercq et al. (2000) also reported linear declines, resulting in around a 30% decrease in calcification. This is supported with a study done by Schneider and Erez (2006) who found a 35% decrease in calcification in Acropora eurystoma. These and other studies show that lowering aragonite saturation, as a function of increasing CO2 in our atmosphere can lead to declines in coral growth.
The approach to management is extremely vital in coral reef conservation on a local, regional and global scale, especially managing the impacts of climate change on coral reefs as they will require integration with our understanding of other anthropogenic stresses (Pandolfi et al. 2011). Hoegh-Gulburg et al. (2007) suggest that management approaches need to be focused on grazing herbivores which would result in an improved ability of coral reefs to bounce back from disturbances. Further analysis of Hoegh-Gulburg et al. (2007) paper (figure 3), shows that there is a two way interaction between grazing fish and macro algae in response to climate change and OA. Perhaps this is a limitation of the model by not accounting for external factors or influences that might occur in reality. This notion is supported by Araujo & Luoto (2007) who state that important biotic interactions and interspecific competition need to be accounted for when modelling. Pandolfi et al. (2011) goes even further to suggest that “the most achievable thing we can do is manage anthropogenic impacts” looking at implementing a wider scope of factors such as more Marine Protected Areas, further fisheries management and marine spatial planning.
In conclusion it be ultimately agreed upon that both papers highlight that climate change and ocean acidification bring considerable threat to coral reef ecosystems, and that the rate of change will ultimately be a significant and determining factor in the projections of coral reefs responses coinciding to such change. However both papers state that there is a lack of information in predicting the rate of environmental change and models that incorporate such shifts. Whilst Hoegh-Gulburg et al. (2007) puts forward a strong argument, it is Pandolfi et al. (2011) that provides a detailed a range of information and a variety of comparisons from geological historical data and across geographic locations as well as providing a wider approach to management, to present a more robust and vigorous projection on the future of our coral reefs.