Climate Change and Ocean Acidification: What Do We Really Know?

Global climate change has the potential to impact the world, especially the oceans, in profound and dramatic ways. Carbon dioxide is a greenhouse gas that acts as insulation for the Earth’s atmosphere. As levels of greenhouse gases in the atmosphere rise, the Earth is more insulated and warms up. If the warming is drastic enough, sea levels can rise as ice melts and drains from the land to the oceans.

Carbon dioxide is also readily dissolved in water, and exchanges freely between the atmosphere and the ocean. The world’s oceans act as a huge sink for carbon dioxide—the oceans hold much more dissolved carbon dioxide than the atmosphere holds. Some of this dissolved carbon dioxide can be fixed into sugars and distributed through marine trophic systems by phytoplankton, photosynthetic bacteria, macroalgae (e.g. kelp) and marine plants.

Dissolved carbon dioxide forms carbonate ions and hydrogen ions. Original image.

Dissolved carbon dioxide also undergoes an equilibrium reaction, where it is converted to carbonic acid, bicarbonate ion and carbonate ion. A large influx of dissolved carbon dioxide shifts the equilibrium towards the production of carbonate and hydrogen ions. An extreme influx of dissolved carbon dioxide in the ocean could effectively lower the pH by increasing the concentration of dissolved hydrogen ions in the water. This poses a threat to organisms with calcium carbonate shells and exoskeletons because calcium carbonate is vulnerable to dissolution at certain pH levels. Shells made of calcium carbonate will not only dissolve in waters with lower pH, they will also be more difficult to construct. Elevated dissolved carbon dioxide concentrations (and the resultant low pH) could render developing invertebrates more vulnerable than they would be under today’s conditions.

Coral reefs are affected by ocean acidification in ways that have been studied extensively. The corals themselves are very sensitive to fluctuations in environmental conditions because they harbor important algal symbionts within their own tissues. The symbiotic algae provide most of the organic material for the corals in exchange for a stable habitat and basic nutrients. While corals can also feed on plankton in the surrounding water, they rely mainly on their symbiotic algae for nutrition. Coral bleaching is a well-known impact of ocean acidification and climate change. When environmental conditions—especially water temperature and local pH—fluctuate, the algae cannot photosynthesize and become stressed, causing the coral host to expel the algae. This causes the coral to lose its source of nutrition and color, and the coral dies soon afterwards.

A coral reef under normal conditions supports a high biodiversity. Photo courtesy of Mikhail Rogov.

Coral reefs that are unaffected by ocean acidification provide protected areas for fish to feed and live, and are often associated with high fish and invertebrate biodiversity. Bleached coral reefs do not display such vibrant diversity, and therefore do not provide the same ecosystem services that unaffected reefs provide. These services are generally important to people and include shoreline protection from wave action and tsunamis, plentiful fish stocks, and high water quality. Coral reefs are ecosystem engineers that provide a substrate for other organisms to occupy. Many invertebrates and fish make their homes within the coral structures, feed directly upon the coral or rely upon organisms associated with reefs to live successfully. When the coral dies, these important interactions may be broken, and species could become locally extinct.

Ocean acidification can affect specific organisms directly (e.g. weakening the shells of invertebrates) and it can impact different interactions between species. The effects of ocean acidification and global climate change on these different interactions have only recently been considered, and therefore the extent of these effects has not been well studied. One recent study by Cripps, Munday and McCormick (2011) explored the potential effect of ocean acidification on the olfactory senses of a coral reef predator.

Olfactory senses in the brown dottyback (Pseudochromis fuscus) are affected by ocean acidification. Original photo.

The brown dottyback (Pseudochromis fucus) is a small mid-level predator that is found in the waters of the Indo-Pacific. It is known to prey upon the lemon damselfish (Pomacentrus moluccensis), which feeds on algae and small crustaceans. In the study by Cripps, Munday and McCormick (2011), adults of P. fucus were exposed to different levels of dissolved carbon dioxide (and therefore different levels of acidified water) and then were exposed to effluent from an injured damselfish. Fish that were exposed to high levels (predicted for both the near and distant future, if carbon dioxide emissions continue at a constant rate) of dissolved carbon dioxide did not respond as well to the prey’s smell. These results suggest that, in this species of fish, olfactory senses may be negatively affected by ocean acidification. The authors hypothesize that the mechanism for this effect may occur on the molecular level, where high levels of dissolved ions interfere with the chemical receptors for smell.

Even alone, the above results would greatly impact the predator-prey relationship between P. fucus and P. moluccensis. However, the effect of ocean acidification on fish predator-prey interaction may have a wider range of impact, thus increasing the effect on such an interspecies relationship. Previous studies on the effect of ocean acidification on larval fish indicate that fish reared in environments with high levels of dissolved carbon dioxide had more difficulty detecting predators and finding suitable habitat for hiding. The combined results could indicate that lower trophic levels may be more affected by ocean acidification than higher trophic levels.

Prey species such as the lemon damselfish (Pomacentrus moluccensis) may be affected by ocean acidification as larvae. Original photo.

In marine habitats especially, an alteration in the environment can impact the entire ecosystem in complex ways. An issue that seems relatively straightforward, such as ocean acidification, can influence the behavior and survival of many different organisms in a variety of ways. Trophic interactions are especially susceptible to impacts related to ocean acidification. A change in one predator-prey relationship can have a cascading effect throughout the entire food web of a habitat, and may potentially alter current dynamics between all the organisms involved in that system.

There has been increasing evidence that anthropogenic carbon dioxide released into the atmosphere is having a profound effect on the world around us. As a sink for carbon dioxide, the ocean is expected to experience huge changes in its chemistry if the current rate of carbon dioxide emissions continues. This could mean catastrophic consequences for the diversity of life in the oceans. These consequences are not limited to coral bleaching, but will likely affect most marine habitats. While the particulars of these results are not fully understood, it is clear that changes in trophic level interactions and habitat formation (especially by ecosystem engineers like the reef-building corals and oysters) can significantly impact the biodiversity of marine life.

Ocean acidification affects marine organisms in ways that are both known and unknown. We know that corals and their symbiotic algae become stressed when environmental conditions are slightly altered. We know that increased levels of dissolved carbon dioxide can prevent invertebrates and plankton from forming calcium-carbonate shells and exoskeletons. We now know that olfactory sensing in fish can be affected by changes in the pH of the ocean. It is unclear how every species will be affected by such changes, and we don’t know what other sensory systems may be affected by altering the ocean’s chemistry. The least understood piece of this puzzle is how greenhouse gas emission and ocean acidification will affect biodiversity on a larger time scale.

Resources:

Cripps IL, Munday PL, McCormick MI (2011) Ocean Acidification Affects Prey Detection by a Predatory Reef Fish. PLoS ONE 6(7): e22736. doi:10.1371/journal.pone.0022736