Although, as we will see, ocean acidification is only just beginning to affect reef growth, there is already another very serious affliction in many reefs around the world---coral bleaching. One of the key changes in corals over the last few decades has been an increase in the frequency and severity of bleaching events. Bleaching occurs when the dinoflagellate temporarily or permanently leaves the polyp, resulting in a loss of color.
Bleaching today is common when temperatures increase very slightly (by about 1oC) in summer months and thus is often an annual summer event. Bleaching is accompanied by slower growth and increased coral mortality. The response to bleaching differs considerably between different species. Some species are able to recover normal growth rates quite rapidly, whereas others are more vulnerable and unable to recover. Bleaching also changes the ecology of a reef, promoting the growth of algae that blanket corals, making the recovery of corals more difficult.
One of the most significant bleaching events took place in 2016 and 2017 in the Great Barrier Reef. This catastrophic event caused by warmer than average water caused up to 50% coral mortality in some areas. The extent of bleaching shocked ecologists and is clearly a window into the future.
Deep Water Corals
Not all corals live in shallow water. A very diverse group of corals are able to thrive in the deep realms of the oceans. These corals lack Zooxanthellae and are able to calcify at cooler temperatures and lower levels of saturation than exist in the shallower waters. The number of known deep-water species has increased dramatically with new methods of exploring the deep oceans, including ROVs (remotely operated vehicles) and AUVs (autonomous underwater vehicles).
Examples of Deep Water Corals
These organisms exist in isolated patches at depths down to 2000 meters. In these conditions, the corals grow considerably slower than shallow water reefs. They feed on zooplankton and in some cases use chemicals coming out of the sea floor for a source of nutrition. Like their shallow-water counterparts, deep coral reefs provide a habitat for a diverse array of creatures. However, at the same time, it is clear that deep corals are in a particularly precarious situation, as we will see later on.
Corals and Calcification
The growth rate of corals, technically known as the rate of calcification, corresponds more closely to aragonite Ω rather than any other variable. Cores of corals from the Great Barrier Reef indicate a 14% decrease in calcification rates from 1990 to 2005. Corals generally maintain a high Ω at the site of calcification, but with decreasing CO32-, precipitation of aragonite requires more energy, hence the decrease in calcification rate. Moreover, lab experiments show that the larval stages of corals grow more slowly under elevated CO2. Overall, the net calcification rates of reefs decrease to zero when the Ω of aragonite is <1, and thus we can predict that worldwide reefs will switch from actively growing via calcification to shrinking via dissolution when CO2 doubles to 560 ppm (the year 2050 in emission scenario A1B). The same logic leads to predictions of decreases in calcification rates of 10-50% by 2050. Experiments show that modern coral species have very different abilities to grow in water with lower saturation, with some species able to continue growing at lower pH and CO32- levels.
Many of the predictions about calcification and saturation are based on experiments in the laboratory. Experiments are one of the best ways to forecast into the future, but they have significant limitations. In particular, it is impossible to replicate natural growing conditions in the lab, and further, experiments are conducted at intervals that are significantly shorter than the changes that are occurring in nature. In fact, saturation experiments conducted over longer time periods demonstrate less significant decreases in calcification than those that are conducted at short intervals. Moreover, experiments show that certain species of coral and coralline algae are less sensitive to decreasing levels of saturation and thus may be more likely to survive in the future.
Field studies and experiments often show very different results. Strangely, field results indicate much higher susceptibility of growth rates to decreasing levels of saturation than do laboratory experiments. These studies suggest that there is much left to learn about calcification and how it will change in the future. At the same time, it is clear that there is significant variability between species; some species having a much greater likelihood of survival in the future. Moreover, even within individual species, it is apparent that some reefs likely have the ability to survive better than others. Factors such as community composition, growth rate, nutrient levels and local variations in seawater chemistry and sediment composition will also play a vital role in determining which reefs will survive.
Since the coralline algae are more tolerant of colder waters, they have a very widespread distribution in the oceans, extending from the tropics to polar regions. The algae are also able to use low light levels for photosynthesis and therefore live considerably deeper than most corals. Their distribution suggests that they have great potential to adapt to variable environments. As it turns out, however, they may be in more precarious position than the corals.
The recent decrease in CO32- has also begun to lower calcification rates of the coralline algae. These species are composed of high Mg calcite, which is the most soluble form of CaCO3 (more so than low Mg calcite and aragonite), so they are particularly prone to ocean acidification. Experimental work confirms that calcification of the coralline algae is particularly sensitive to CO2 levels with growth rates slowing significantly, and actually, dissolution beginning at moderately high CO2 contents.
Perhaps the coralline algae will be the “canary in the coal mine” for dissolution of all framework structures under rising CO2. For the corals, it is very apparent that the threat varies considerably from reef to reef. Deep-water corals are in particular jeopardy because calcification rates are lower in colder waters, because saturation levels at these depths are lower, and because, in the future, saturation stands to decrease more rapidly in deep than in shallow waters. However, the fate of reefs in the tropics is also likely to vary significantly.
Making predictions about the exact impact of ocean acidification in the future of coral reefs of all types is inherently difficult. Acidification will occur in parallel with other deleterious effects such as bleaching and sea level rise. Species that are most susceptible to the effects of bleaching may turn out to also be more susceptible to extinction. For example, by impacting zooxanthellae, bleaching impacts calcification and thus may exacerbate the impact of acidification. However, these same corals also have the ability to recover from bleaching events more quickly, possess shorter generation times, and thus may have the ability to evolve more rapidly to tolerate bleaching in the future. At the same time, nutrient levels in reef environments have and will continue to decrease as stratification of the upper ocean increases, and this will put considerable strain on future reefs. Temperatures, nutrient levels, local rates of sea level rise, and human activities are factors that will likely decide which reefs are the most vulnerable, possibly with acidification pushing the most threatened systems over the edge into extinction.
Probably the largest uncertainty with respect to the future of reefs is whether corals will be able to adapt to lower saturation levels, increased temperatures, and lower nutrient levels, and if so, how rapidly. As we have seen earlier, corals have shown great resilience in surviving numerous environmental threats in the past. So, why do reefs recovered after times of environmental stress in the past, at times when temperatures were even warmer than they are today, and CO2 levels higher, appear to be in such dangerous territory today? The answer to this question is that in the past, temperature and CO2 perturbations occurred slowly enough that ultimate increases in weathering (remember feedbacks we discussed in Module 3) essentially decreased the rate of CO2 addition and buffered the ocean with CO32- before extinction occurred. What concerns scientists is that warming today and CO2 addition are rapid enough that the weathering feedback will lag the decrease in saturation by several thousand years.
So, we are left with a lot of questions: will the rates of saturation decrease and temperature rise be too rapid for modern species to adapt? Will algae take over the niche of shallow water corals and dominate the low pH oceans of the future? Or will a few species of coral and possibly coralline algae develop the ability to calcify rapidly enough to survive the current threats and take over the niche of species that do not? Will Zooxanthellae themselves be able to adapt and assist corals in calcifying? Finally, when will feedbacks, largely through weathering, come into play and make conditions more favorable for calcifying organisms? For some of these questions, it’s a matter of wait and see. However, ongoing research should shed light on others. For example, the genetics of coral populations are currently being explored to understand the ability of corals to adapt to environmental change.
At this stage, however, any outcome is possible for corals, ranging from complete extinction by late in the 21st century to adaptation to a new set of environmental parameters. Ultimately, like many elements of the ecosystem, the fate of reefs may rest on how well we manage CO2 emissions in the future.