The biological implications of ocean acidification

A couple of posts ago, I explored the chemistry behind ocean acidification but did not touch on any of the implications on marine life. Hence, I will be covering this today, focusing mainly on calcification rates and acidosis. If you missed my explanation of ocean acidification, you can find it here.

Firstly, marine calcifiers such as reef-forming corals, protozoans, molluscs, crustaceans, echinoderms and some algae are greatly affected by the decrease in carbonate ions (CO_­3^2-) associated with CO₂ dissolution (Bijma et al., 2013). They use CO₃^2- to build and maintain their calcium carbonate shells and skeletons, but this becomes increasingly difficult with falling CO₃^2- concentrations. Consequently, many calcifiers have shown reduced rates of calcification in response to the increasing partial pressure of CO₂ (Fabry et al., 2008).

For tropical reef-building corals, it is predicted that calcification rates will have reduced by 20-60% once CO₂ concentrations reach double that of preindustrial levels. A change of this magnitude could slow growth rates, making reefs much more vulnerable to erosion. Furthermore, with the associated reductions in CO₃^2- ions, it is likely that much weaker skeletons will form and thus, allow erosional processes to occur at quicker rates than previously (Guinotte and Fabry, 2008). A report issued on Friday, following last year’s Third Symposium on the Ocean in a High-CO₂ World, delivers some extremely worrying facts. They state with high confidence that ‘If CO₂ emissions continue on the current trajectory, coral reef erosion is likely to outpace reef building sometime this century’. Take ocean warming into account and the prospect is even more dire – erosion could potentially outpace growth by mid-century, once CO₂ levels reach 560 ppm. If this happens, there would be consequences for coral biodiversity, with many species losing their habitat (IGBP et al., 2013).

Coral reef near Fiji

However, it is not all doom and gloom for tropical corals. Fine and Tchernov found that two species of scleractinian corals were able to survive under acidic conditions (pH values of 7.3-7.6) (2007). Initially, their skeletons dissolved leaving the coral polyps exposed, but when pH returned back to normal, the polyps were able to recalcify with no lasting effects. This offers a glimmer of hope for corals’ future in a high CO₂ world. Still, we have to remain cautious as the study did not consider the effects of predation on the naked polyps and there has been discrepancy about its representativeness (Guinotte and Fabry, 2008).

As for cold-water corals, 70% will experience unsaturated waters by 2100 (high confidence), with this beginning as early as 2020 for some (IGBP et al., 2013). As highly productive ecosystems with a rich biodiversity, this would impact the many deep-water organisms that rely on them as feeding grounds, habitat and nursery areas (Guinotte and Fabry, 2008).

Many plankton and zooplankton species are also affected by changes to the carbonate chemistry of the ocean (see Figure 1). This is potentially detrimental considering they form integral components of the marine food chain. Amongst those most sensitive to ocean acidification are pteropods – the major planktonic producers of aragonite. In waters that are unsaturated with respect to aragonite, it has been observed that pteropods are unable to maintain their shells and consequently, have begun dissolving (Orr et al., 2005). This is occurring in the Southern Ocean today, impacting predators such as pink salmon (IGBP et al., 2013). The future for pteropods that are endemic to polar regions depends on their ability to transition to lower latitudes, where warmer, carbonate-rich waters await (Orr et al., 2005).

Figure 1. Photos showing scanning electron microscopy photographs of the coccolithophorid Gephyrocapsa oceanica under different carbon dioxide concentrations; normal in the top image (300 ppm) and elevated in the bottom (780-850 ppm). The distinct structural malformations can be clearly seen under higher carbon dioxide concentrations (Riebesell et al., 2000). 

However, once again, it is not all bad. Whilst the majority of marine calcifiers do see decreasing calcification rates under acidic waters, some organisms actually benefit; these include some fleshy algae, seagrasses and phytoplankton groups (although further research is still required to fully understand the underlying mechanisms) (IGBP et al., 2013). It appears that under higher CO₂ concentrations, these experience increased photosynthesis and, therefore, growth (Gattuso and Hansson, 2011).

Lastly, for many marine animals, including invertebrates and some fish, the acidification of ocean waters can result in acidosis. This is where high environmental CO₂ levels cause an increase in carbonic acid in the bloodstream. Consequently, the blood's pH is lowered, affecting many cellular processes. Acidosis can lead to metabolic and behavioural depression, lowered resistance and asphyxiation (Howard et al., 2012). Compared to less mobile organisms, fish appear to be more resistant to ocean acidification as they do not have extensive calcium carbonate shells (IGBP et al., 2013). However, particularly in larval fish, acidic seawater has led to impaired sensory performance, altered behaviour and decreased growth rates, all of which impact predator-prey relationships (Howard et al., 2012). Overall though, this area remains insufficiently explored and further research is required to fully understand the physiological mechanisms behind individual animal responses to elevated CO₂ levels.

To round things up, whilst a few organisms can tolerate or actually benefit from acidic waters, many respond negatively to ocean acidification. Survival, growth, abundance and larval development for many organisms is reduced, alongside their ability to form and maintain shells and skeletons. These impacts have repercussions on marine food webs and biodiversity, which, in turn, ultimately affects society. It has been suggested that over generations, acclimatization and evolutionary adaption may occur that mitigates or even compensates for ocean acidification. To support this, there are a few examples of limited adaption from the paleo-record, but knowledge in this area remains patchy, not helped by the fact that every species differs in its potential to adapt (Wittmann and Pörtner, 2013). What is most problematic, however, is that the rate of ocean acidification today is unprecedented; 30-100 times faster than at any point in the recent geological past (Bijma et al., 2013). It is, therefore, hard to imagine how marine life could possibly ever adapt quick enough to these rapid changes in ocean chemistry.