Coral reefs are one of the most diverse marine ecosystems of the world. The coral reef ecosystem is formed by the calcifying organisms inhabiting it, such as corals and algae. Even though, coral reefs occupy a small percentage (0.1%) of the ocean, they provide habitat for a wide variety of fish and invertebrate species. Recent studies have identified a closed link between ocean acidification and the degradation of coral reefs. During the past three decades, the increase of CO2 emissions has increased and almost one third of these emissions have been absorbed by the ocean. Ocean acidification affects the coral reef structure, especially corals and coralline algae, which are important calcifying organisms in the coral reef ecosystem.
Effects of ocean acidification on biodiversity
The principal contributing organisms in the formation and cementation of the calcium carbonate (CaCO3) in coral reefs include corals, macroalgae, benthic foraminifera, mollusks and echinoderms. The calcium carbonate formations in coral reefs provide the habitat for a wide variety of fish and invertebrates. Ocean acidification, temperature, light and nutrients are factors affecting the calcification rates of organisms inhabiting coral reefs. Changes in the chemistry of sea water stemming from increased CO2 uptake prevent the production of CaCO3 by many reef organisms, affecting the structure and biodiversity of coral reefs.
Studies have shown that doubling the amount of pre-industrial atmospheric CO2 results in a 10-50% decrease in the calcification rate of corals and coralline algae, two of the most important contributors to the structure of the coral reef ecosystem. Scientists think that the effect is reversible if acidification is reversed, that is to say, if the amounts of atmospheric CO2 are reduced; however, scientists predict that the reversibility of the acidification of the ocean is unlikely to occur. Scientists think that a record of the different coral reef ecosystems around the world is needed to improve our understanding of the effects of ocean acidification in the coral reef ecosystem.
Other calcifying organisms
Calcareous benthic foraminifera contribute to reef sediments by producing carbonate sands. Studies performed on calcareous foraminifera proved their sensitivity to high CO2 concentrations. Other studies performed on mollusks suggest that many will produce thinner shells under high concentrations of CO2. Echinoderms, such as starfish, sea urchins, brittle stars, crinoids, holothurians and sand dollars are responsible for the distribution of calcium carbonate on coral reefs; however, their high-Mg calcite structures show great vulnerability under ocean acidification.
Effects on biodiversity
The effects of ocean acidification seem to affect primarily corals and coralline algae, which are the main building blocks in the coral reef ecosystem. The effects of ocean acidification on these two main biological calcifying producers may affect other marine species related to them. When corals die, other species are affected and the effects extend throughout the whole coral reef ecosystem, which in turn may leave the ecosystem vulnerable to other minor disturbances, such as ocean currents and erosion. Reef structure depletion may decrease the habitat of marine species, decreasing biodiversity.
Scientists cannot foresee the real consequences of ocean acidification on coral reefs, especially because the change in ocean water chemistry is occurring along other factors, such as higher temperatures, overfishing, tourist developments along coasts and mining. The structure of reef ecosystems is likely to change in the future, mostly due to a decrease in carbonate production and an increase in chemical dissolution. According to the U. S. Geological Survey, calcification rates are expected to decrease for a variety of marine organisms in response to a lower pH in sea water. Experiments and computer/deled simulations indicate that low calcification rates a loss in the coral reef ecosystem could occur in the next decades. Moreover, scientists estimate that coral reefs will be in danger once atmospheric concentration of CO2 reaches 560 ppm, which is predicted to occur in the year 2050.