Coral Reefs, Temperature and Ocean pH

By Andy May

Georgiou, et al. 2015 have reported that coral reefs in the Australian Great Barrier Reef, near Heron Island, are insensitive to ocean pH changes. The location of Heron Island, about 257 miles (414 km) north of Brisbane, Queensland, Australia, is shown in figure 1 using Google maps.

Figure 1

Figure 2 shows the island and a portion of the reef using Google Earth.

Figure 2

Georgiou and colleagues observed that while the pH of the ocean around the reef may vary dramatically, the pH at the site of calcification remains in a narrow range. This suggests that the animals (coral polyps) can actively modify the pH of the water at the calcification site to maximize their growth rate. To quote the paper:

“This result reflects the capacity of these coral to homeostatically maintain a pH_sw [pH of seawater] of ~8.4–8.6 at the site of calcification … and thus near constant up-regulation of pH_cf [pH at cite of calcification] during the calcification process. As such, these findings are in marked contrast to earlier laboratory studies in which corals grown under stable and constant pH conditions exhibited a stronger sensitivity to ambient pH, whereby pH_cf decreased by up to 0.5 units for each unit decrease in ambient seawater pH.”

Georgiou, et al. did their experiments in situ the open ocean by building enclosures that are open on two sides and on the bottom and raising the level of CO₂ in the “treated” enclosures. They compared the results to nearby corals that were in enclosures with no added CO₂ (the “controls”). They found no reef growth differences between the two environments. This implies a high degree of tolerance to ocean acidification. The enclosures for injecting CO₂ and the control enclosures are called FOCE (Free Ocean Carbon Enrichment) and are described in Kline, et al., 2012. They describe a very ingenious way to measure the effects of sea-water carbon dioxide concentration differences on corals in-situ.

They constructed four FOCE systems near the island. There were two controls and two treatment systems. They were all oriented parallel to the shore with each flume open at both ends and on the bottom. Figure 3, which is a portion of figure 5 in Kline, et al., 2012, shows a photograph of the flumes and a schematic of the system.

Figure 3, source Kline, et al., 2012

The area chosen for the experiment has a lot of diel (daily) variation and a lot of seasonal variation in both pH and temperature. This can be seen in figure 4A and figure 4C below. Figure 4 is a portion of figure 1 in Georgiou, et al., 2015.

It is a little difficult to see the main point of the paper from figure 4, so I downloaded the supplementary data and made figure 5. In figure 5, the orange and yellow curves are the average monthly calcification site pH for the control and treated (added CO₂) areas. The blue and gray curves are the average monthly pH for the ambient ocean around the experiment. There are two things to notice about figure 5. First, the calcification site on the corals has a controlled pH of about 8.5, regardless of the ambient ocean pH. Second, the ambient ocean pH, over the reef, varies 0.2 units, without CO₂ treatment, over the six months of the experiment.

Figure 5: Monthly average pH for the controls and the CO₂ treated areas.

There are two differences between this study and earlier laboratory studies that are significant. First this study was done on an actual dynamic, growing reef. Second and just as important, this study was done in an area where there are natural diel and seasonal changes. Thus, the corals in this environment are used to changes in pH and deal with it routinely and the additional changes due to CO₂ injection do not affect them. In the laboratory, the animals adapted to a constant pH, had no access to alternative symbiotic algae and when it is radically changed they are affected. As discussed by Jim Steele here, corals live with a variety of symbiotic algae species, each uniquely adapted to a particular environment and the corals receive up to 90% of their food from their algae. Thus, as their natural environment changes the corals will expel their current symbiotic algae and repopulate with a more suitable species. This is an action they can do in their native environment, but not in the laboratory. The ability to maintain a relatively constant pH at the calcification site varies from species to species, the Porites coral species seems to be particularly resilient to ocean acidification.

It has been reported by Hofmann, et al., 2011, that the natural daily and seasonal range of pH in the oceans is much larger than model predictions of future changes in pH. They illustrate the daily range in their figure 2, shown here as figure 6. Currently, the open ocean pH is between 8.01 and 8.08, as seen in figure 6 (top left). Climate model projections predict that if we continue emitting CO₂ on the current trend, ocean pH will drop to 7.8 by the end of this century according to the European Project on Ocean Acidification (EPOCA).

Figure 6, source: Hofmann, et al., 2011

These graphs are for 30-day periods, so they do not reflect seasonal variability which can be significant. Compare these to the blue and gray curves in figure 5, which cover six months on the Great Barrier Reef.

EPOCA claims in an alarming tone:

“Modeling demonstrates that if CO2 continues to be released on current trends, ocean average pH will reach 7.8 by the end of this century, corresponding to 0.5 units below the pre-industrial level, a pH level that has not been experienced for several millions of years. A change of 0.5 units might not sound as a very big change, but the pH scale is logarithmic meaning that such a change is equivalent to a three-fold increase in H+ concentration. All this is happening at a speed 100 times greater than has ever been observed during the geological past. Several marine species, communities and ecosystems might not have the time to acclimate or adapt to these fast changes in ocean chemistry.”

Figures 5 and 6 show changes much larger than EPOCA have forecast occur on both a monthly basis and on a seasonal basis. These changes occur near existing reefs as well as in other parts of the ocean. Joint, et al., 2011, have concluded:

“However, it is important to place these changes within the context of pH in the present-day ocean, which is not constant; it varies systematically with season, depth and along productivity gradients. Yet this natural variability in pH has rarely been considered in assessments of the effect of ocean acidification on marine microbes. Surface pH can change as a consequence of microbial utilization and production of carbon dioxide, and to a lesser extent other microbially mediated processes such as nitrification. Useful comparisons can be made with microbes in other aquatic environments that readily accommodate very large and rapid pH change. For example, in many freshwater lakes, pH changes that are orders of magnitude greater than those projected for the twenty-second century oceans can occur over periods of hours. Marine and freshwater assemblages have always experienced variable pH conditions.”

Georgiou, et al. found that the growth rates of the coral nubbins (coral seedlings) in the CO₂ flumes, the control flumes, and in the surrounding area were very similar. More importantly, the density of the low pH FOCE corals was within 3% of the control corals. Thus, the corals were able to grow at normal rates, even when the ambient pH was low (ΔpH = -0.25).

There are many unknowns about how reefs will react to higher CO₂ concentrations in sea water and to higher temperatures. It is also apparent from the study that different species of corals will react differently. However, nearly all of the reef-building Scleractinia coral species alive today survived the “PETM” or the Paleocene-Eocene Thermal Maximum, 50 million years ago, when global temperatures were 9°C warmer than today (Paleomap Project) and the atmospheric CO₂ was twice what it is today (Black Hills Institute). A temperature history of the Phanerozoic can be seen in figure 7 here. The Scleractinia Coral family tree is shown in figure 7.

Figure 7: The dominant reef building animal today is the Scleractinian coral. The various types of Scleractinian corals are shown in the family tree. Many went extinct at the beginning of the Paleocene or Eocene, but with a few exceptions, if they survived the extinction event at the end of the Cretaceous and the cooling at the end of the PETM (Paleocene-Eocene Thermal Maximum) they still exist today. Source: AIMS.

Discussion and Conclusions

While the study performed by Georgiou, et al. only involves a small area near Heron Island on the Australian Great Barrier Reef and only a few species of corals, it does demonstrate that some corals actively modify their environment and grow in a lower pH environment. Further, the study highlights the diel and seasonal variability in pH in our oceans and shows it is greater than the modeled changes in pH ascribed to increasing atmospheric CO₂ concentrations.

We do not contest the idea that additional CO₂ in the atmosphere will lower ocean pH, this is easily demonstrated. Although, the amount of change in pH is in question, the ocean is buffered, so the ultimate change in pH may be very small.  We see no evidence that this will affect marine life, as marine organisms have demonstrated an ability to tolerate rapid changes in ocean pH. They demonstrate this on a daily and seasonal basis. Further, existing species of reef-building Scleractinian corals have thrived in much higher temperatures and much higher carbon dioxide concentrations as (geologically) recently as 50 million years ago.

Like other aspects of the climate change scare, or really any environmental scare, someone finds something (anything) that changes due to man’s influence and assumes it changes for the worse. This assumption that change is bad, is proclaimed without checking the geological record to see if it has happened before. Or, in this case, without even checking to see if it happens today on a daily or seasonal basis.

We do not contest the idea that rapidly rising temperatures cause corals to undergo thermal stress, but this is temporary and corals do adapt with time, as discussed by Jim Steele here. The symbiotic algae that corals depend on for most of their food vary with depth, temperature, pH, and other factors. Laboratory experiments conducted in tanks remove corals from a source of alternative symbionts (algae) that will allow them to adapt, essentially freezing their environment. So, removed from their natural state, and their main adaptation mechanism, they are harmed by rapidly rising temperatures and changes in pH. Corals, in the ocean, have no problem with the same changes.

Further, reefs are composed of many different species and they each adapt to changes in pH and temperature (and other environmental factors) in different ways, but they do adapt. A very good discussion of the adaptability of corals, also by Jim Steele, can be found here. As Steele notes, most reef deaths are due to cold temperatures, not warm temperatures, a fact often ignored by NOAA and others with an agenda.