Over the last couple of decades, a great deal of research has been done on the effect of global warming on coral reefs. The vast majority of that research has focused on the currently observed and potential future effects of climate change on reef-building corals. Coral, however, are not the only organisms that contribute to building a reef. A group of organisms known as the "coralline algae" also secrete calcium carbonate, and contribute to building up reefs. In a paper available online in advance of publication at Nature Geoscience, a group of researchers report on the results of an experiment conducted to observe what will happen to coralline algae by the year 2100 if atmospheric concentrations of carbon dioxide continue to rise at the present rate.
The experiment was carried out at the Hawaii Institute for Marine Biology's Coconut Island facility. The HIMB facility is extremely well suited to this kind of experiment, because it's location and facilities make it relatively easy to set up well-controlled experiments. Coconut Island is located in Kaneohe Bay, and is surrounded by a coral reef. There are both indoor and outdoor lab facilities available there, some of which are located within 10 meters of the reef, and the island has a seawater system that draws water directly from the reef. As a result, it's a lot easier to set up an experiment where you want to look at the effects of a change in one parameter on a reef environment at HIMB than it is in most other locations.
In the experiment that this team of researchers (lead by USGS biologist Ilsa Kuffner) set up a series of outdoor tanks connected to the reef water system. Half of the tanks were maintained under normal (control) conditions, while the other half were kept in a state that simulated what the conditions are likely to be in 2100 under the Intergovernmental Panel on Climate Change's business-as-usual scenario. That model predicts that the concentration of CO2 in the atmosphere will double by then. This higher CO2 concentration, in turn, will cause the pH of the oceans to drop a bit (in other words, the oceans are going to become more acidic). Unfiltered water from the reef was kept constantly flowing through all of the tanks, and surfaces were placed in the tanks to allow algae to naturally settle out from the flowing reef water. The outdoor environment and the use of reef water meant that the control tanks were experiencing the same temperature and light fluctuations as the natural coral reef.
The authors of the study found that, under the experimental conditions, crustose coralline algae were much less likely to settle out and grow than they were under control conditions. They also found that the carbonate-secreting algae were replaced by other species of algae in the more acidic waters of the experimental tanks. This matches the predictions of other studies, they report, which indicate that coral and other carbonate secreting organisms are likely to suffer similarly if the ocean acidifies. These results are not surprising, but the implications are potentially severe.
If this scenario is correct, there is a very real threat of major change to ocean ecosystems. As the study's authors point out, a decrease in coralline algae and an increase in soft algae can - and probably will - have effects of its own on the rest of the reef ecosystem, as the changing algae supply produces changes in the composition of the herbivore community on the reef, which produces changes in the composition of the carnivore community, which, in turn, will produce changes - you get the idea. And remember - all of this is in addition to other effects (such as coral bleaching) that global warming is predicted to have on reefs.
All in all, this is not a good news study.