Environment Counts | Ocean Acidification

Ocean acidification—the changes in carbonate chemistry and acidity (pH) of seawater resulting from entry of atmospheric CO2 into the ocean—is an inevitable consequence of the rapid rate of CO2 release into the atmosphere through anthropogenic activities like fossil fuel combustion. Atmospheric CO2 concentrations are higher than they have been for at least 800,000 years, and the rate of release of CO2 is the greatest for at least the past 55 million years. Since the start of the Industrial Revolution in the middle of the 18th century, atmospheric CO2 levels have risen by ~40% and the pH of seawater has decreased by ~0.12 pH units, which corresponds to an approximately 30% rise in acidity. By the end of this century, models based on “business as usual” scenarios for CO2 release predict a further decrease in pH that would lead to an approximately 100-150%rise in ocean acidity relative to the mid-18th century. Models show a continuing fall in seawater pH over the coming few centuries (if not longer), even though with rising CO2 levels in seawater the capacity of the ocean to absorb additional CO2 is diminished. The consequences of ocean acidification—which is sometimes referred to as “the other CO2 problem”—have received much less attention than CO2’s effects as a greenhouse gas. Whereas public acceptance of climate change is increasing rapidly, at the time of this writing polls indicate that less than ten percent of the US public is even aware of the process of ocean acidification, much less concerned about its known or potential impacts. However, the effects of ocean acidification have become of increasing concern to a wide range of scientists over the past two decades. Review of the Federal Ocean Acidification Research and Monitoring Plan

Atmospheric carbon dioxide (CO2) levels are currently approaching 395 ppm, a value that is 40% higher than those of the preindustrial period and exceeds CO2 levels of at least the past 800,000 years. Perhaps more significant is the rapid rate of increase in atmospheric CO2 concentration, a rate that is unprecedented over the last 55 million years of the Earth’s history. The ocean plays a critical role in governing atmospheric CO2 levels. By absorbing a substantial share of the CO2 released through varied human activities, the ocean reduces atmospheric levels of this greenhouse gas and thus moderates human-induced climate change. However, this beneficial effect of CO2 uptake by the ocean has resulted in potentially damaging consequences due to a lowering of ocean pH and related changes in ocean carbonate chemistry, collectively known as “ocean acidification.”
Since the start of the Industrial Revolution in the mid-18 century, the average pH of the upper ocean has decreased by about 0.1 pH unit, corresponding to an approximately 30% rise in acidity, and is projected to decrease by an additional 0.3 to 0.4 units by the end of this century, corresponding to a 100 to150% rise in acidity since preindustrial times. The current and expected magnitude and rate of ocean acidification argue for an expeditious and detailed investigation of ocean acidification and its associated impacts on ecosystems and natural resources. Additional environmental stressors – such as rising temperatures and decreases in dissolved oxygen – that may exacerbate the effects of acidification on marine organisms further highlight the urgency of this challenge.
Decreasing seawater pH has already been shown in laboratory experiments to have widespread influences on biological processes and numerous types of additional effects likely remain to be discovered. Many—and perhaps most—of these effects of acidification will have negative impacts on individual species and the ecosystems in which they are enmeshed. For example, ocean acidification decreases the availability of carbonate ions at calcification sites, making it increasingly difficult for many calcifying organisms such as corals, oysters and calcifying phytoplankton to build their calcium carbonate skeletons and shells. However, calcifiers differ in their responses to ocean acidification, notably in the case of different species of reef-building corals. In contrast, some processes or certain species, e.g., photosynthetic carbon fixation in some plants, may benefit from rising CO2 and bicarbonate levels. Disparity among species in responses to ocean acidification remains a critical unknown for efforts to predict what ocean acidification portends for marine life. In addition, many metabolic and cellular processes besides calcification and photosynthesis are affected by ocean acidification due to decreases in the pH of blood and cellular fluids. Falling pH can impede oxygen uptake by certain marine animals and directly or indirectly reduce metabolic rates.
The full suite of biological processes and structures perturbed by ocean acidification is difficult to predict. Some recently discovered effects of ocean acidification were wholly unanticipated. For example, laboratory studies of coral reef fish have revealed that neurological and behavioral processes can be affected by a decrease of seawater pH. These behavioral abnormalities may translate into changes in predator-prey interactions and capacities for locating suitable sites for settlement and recruitment.
Most of our knowledge of the effects of decreasing pH on marine organisms is from controlled laboratory and field mesocosm studies; we know much less about ocean acidification’s effects on natural (“wild”) communities and ecosystems. Thus, efforts are underway to extrapolate from controlled laboratory experiments and limited in situ observations to impacts at the community and ecosystem level. However, predicting the future consequences of ocean acidification for the marine environment and society is a challenging endeavor due to the complexity and dynamic nature of marine ecosystems and the likelihood that the effects of acidification will differ among species and ecosystems. Furthermore, interaction of stresses from acidification with other simultaneous stressors such as warming, eutrophication, and deoxygenation remains poorly understood.
Ocean Acidification and Changes in the CO2 and Carbonate System: Atmospheric carbon dioxide is absorbed by the ocean, where it reacts with seawater to form carbonic acid, which then dissociates to form bicarbonate ions (HCO3-) and hydrogen ions (H+). The increase in hydrogen ion activity (decrease in pH) is buffered by the carbonate system : some of the added hydrogen ions react with carbonate ions (CO32-) to form more bicarbonate, which makes CO32- less abundant. If atmospheric carbon dioxide rises slowly, ocean pH and carbonate ion levels will remain relatively stable due to dissolution of existing calcium carbonate deposits in the ocean (1,000s+ of years), weathering of terrestrial rock (100,000s+ years), and tectonic processes (millions of years). However, the current rapid rise in atmospheric CO2 is faster than the time required for natural processes to buffer changes in the ocean carbonate system and avoid large changes in pH or ocean carbonate levels. Increased nutrient input from runoff can result in larger than usual algal blooms (i.e., eutrophication) that produce organic matter, which contributes to increases in CO2 when respired.
Effects on Biological Processes: The increase in CO2 and HCO3- availability has the potential to increase photosynthesis by some but not all photosynthesizers in the ocean. The decreased availability of CO32- at calcification sites makes it more difficult for many types of calcifying organisms, including some phytoplankton, corals and bivalves (clams and mussels) to build their calcareous shells or skeletons. Lastly, a decrease in pH may cause important physiological changes, many of which are associated with negative impacts such as increased energetic costs for regulating internal H+ concentrations.
Simultaneous Changes Impacting Biological Processes: Global increase in ocean temperature and decrease in dissolved oxygen are stressors for many marine organisms that will likely add to or amplify the impacts of ocean acidification, resulting in changes in the composition, abundance, and production of
biological communities. In addition, regional human impacts—such as overfishing, eutrophication, pollution, or oil spills to name a few—also affect biological processes.

Review of the Federal Ocean Acidification Research and Monitoring Plan