Is there a future for coral reefs in acid oceans?
Research into the multiple environmental factors affecting coral reefs and how these factors interact is bringing a greater understanding as to how these fragile ecosystems are likely to fare under increasingly acid ocean conditions.
Until recently, global warming was seen as the greatest threat from climate change to coral reefs; however, another related phenomenon termed ‘ocean acidification’ (OA) has since stolen the headlines.
OA is derived from excess atmospheric CO2 dissolving into the oceans to form a very weak acid. Ocean pH has already decreased from ~8.2 at the start of the Industrial Revolution to a present day value of ~8.1; however, models predict this will further fall to ~7.6 by the year 2100.
OA will substantially limit the ability of fish to use their sense of smell to detect predators and locate the best sites for larval development
This potentially presents a big problem to coral reefs, which are effectively built from chalk (which is highly soluble in acid). In fact, the first signs of OA are now beginning to be seen in long term ‘climate records’ hidden within the skeletons of some of the largest coral colonies on earth, which have been growing since before the Industrial Revolution.
In order to predict how OA will impact coral reefs, researchers have performed experiments in which key organisms are incubated under conditions that replicate elevated CO2 (reduced pH) conditions expected for 50-100 years’ time. This effort has proved extremely fruitful in terms of identifying responses with significant ecological or geochemical consequences. For example, coral calcification consistently declines under OA, while, simultaneously, productivity and growth of macroalgae and seagrasses is enhanced.
Together these analyses predict a future shift in how reef habitats are structured, and in turn in the ‘ecosystem services’ that are provided. Not only corals and plants will be affected; OA will substantially limit the ability of fish to use their sense of smell to detect predators and locate the best sites for larval development. Therefore, OA will likely affect how reef habitats appear as well as their inhabitants.
Whilst experiments have hugely improved our understanding of how OA will affect reefs, they share several common weaknesses. Most OA experiments do not ‘replicate’ climate change well; they only operate over relatively short timescales (weeks), and they typically only examine changes of CO2/pH in isolation.
We know, however, that climate change affects multiple environmental factors alongside pH. Many coastal reefs will experience warmer waters, more intense El Niño events, and changing precipitation patterns, which affect river run off and therefore light availability and nutrient loading.
Understanding how all these changes act together to govern the OA response is now a priority. Experiments led by the University of Essex are now demonstrating that interactions of light, temperature and choice of coral species affect the rate at which coral calcification declines with OA.
Fortunately, nature has also provided researchers with an array of ‘natural experiments’ which provide glimpses of how OA could affect reefs. Volcanic activity produces natural CO2 seeps creating reef sites with naturally elevated CO2/lower pH – for example the cool water CO2 seeps that fringe the D’Entrecastraux Islands, Papua New Guinea. Observations here have shown that hard coral cover is the same as for neighbouring sites at ambient CO2, but diversity is lower at the high CO2 sites. Thus, some coral species can still successfully compete under OA conditions.
It is not just volcanic activity that naturally produces extreme pH conditions; even intertidal reef flats can exhibit large daily/weekly changes in pH as a result of tidal and biological activity. Coral reef diversity/abundance is very different in intertidal reef flats compared to subtidal reef complexes but again it is clear that some coral species are already adapted to environments with some of the characteristics of future environments.
Research in these more ‘extreme’ reef environments based at Operation Wallacea’s Hoga facility has demonstrated the tolerance of corals to many stressors (although the additional role of pH has yet to be examined).
Management of such reefs already adapted to extreme environmental conditions against more immediate stressors, eg pollution and over-harvesting, is thus an obvious priority to give reefs their best chance against our rapidly changing climate.