Ocean Acidification: Chemistry Impacts on Marine Life and Ecosystem Consequences
Ocean Acidification: Chemistry Impacts on Marine Life and Ecosystem Consequences
Ocean acidification is the ongoing decrease in the pH of Earth’s oceans caused by the uptake of carbon dioxide from the atmosphere. Since the Industrial Revolution, the ocean has absorbed about thirty percent of the carbon dioxide released by human activities, fundamentally altering seawater chemistry. This chemical change, often called climate change’s evil twin, has profound implications for marine organisms, particularly those that build shells and skeletons from calcium carbonate. The rate of ocean acidification is unprecedented in the geological record, and its effects are already being observed in marine ecosystems worldwide. This guide explores the chemistry of ocean acidification, its effects on marine life, the consequences for marine ecosystems and human societies, and the actions needed to address this growing threat.
The Chemistry of Ocean Acidification
When carbon dioxide dissolves in seawater, it forms carbonic acid, which dissociates into hydrogen ions and bicarbonate ions. The increased concentration of hydrogen ions reduces the pH of seawater, making it more acidic. Since the Industrial Revolution, the average pH of ocean surface waters has decreased by about zero point one units, representing a thirty percent increase in hydrogen ion concentration. Projections suggest that by the end of the century, ocean pH could decrease by an additional zero point three to zero point four units if emissions continue unabated.
The process of ocean acidification also reduces the availability of carbonate ions, which are essential for marine organisms to build calcium carbonate shells and skeletons. As pH decreases, carbonate ions combine with hydrogen ions to form bicarbonate, reducing the saturation state of calcium carbonate minerals. This makes it more difficult for organisms to precipitate calcium carbonate and can cause existing shells and skeletons to dissolve.
Calcification and Shell Formation
Many marine organisms rely on calcium carbonate to build their shells, skeletons, and other structures. Corals, mollusks, echinoderms, and some planktonic organisms including foraminifera and coccolithophores are calcifiers that are vulnerable to ocean acidification. The saturation state of seawater with respect to calcium carbonate determines whether these organisms can easily build and maintain their structures.
Laboratory studies have shown that reduced pH impairs calcification in many species. Coral growth rates decline as saturation state decreases. Oyster larvae have difficulty forming their initial shells in acidified conditions. Pteropods, small swimming snails that are important components of polar food webs, show shell dissolution when exposed to acidified waters. The effects are species-specific, with some species showing greater tolerance than others, raising questions about which organisms will be able to adapt.
Effects on Coral Reefs
Coral reefs are particularly vulnerable to ocean acidification because the calcium carbonate skeletons that build the reef structure become more difficult to form and maintain. Reduced calcification rates slow reef growth, making it harder for reefs to keep pace with sea level rise. Weakened skeletons are more susceptible to erosion from storms and bioerosion. The combination of ocean acidification with warming, which causes coral bleaching, creates a double threat to reef ecosystems.
The geological record provides evidence of the devastating effects of ocean acidification on reefs. During the Paleocene-Eocene Thermal Maximum about fifty-five million years ago, a massive release of carbon caused ocean acidification and widespread extinction of deep-sea calcifying organisms. The recovery took hundreds of thousands of years. This historical precedent underscores the severity of the threat posed by current acidification.
Impacts on Marine Food Webs
Ocean acidification can affect marine food webs through multiple pathways. Direct effects on calcifying organisms at the base of food webs, including pteropods and foraminifera, can have cascading effects throughout the ecosystem. These organisms are important food sources for fish, birds, and marine mammals in many regions. Reductions in their abundance or nutritional quality could affect higher trophic levels.
The effects of acidification are not limited to calcifiers. Studies have shown that acidification can affect fish behavior, including the ability to detect predators, navigate, and make decisions. These behavioral effects appear to result from interference with the GABA-A receptor in fish brains, which affects sensory processing. The ecological significance of these behavioral changes in wild populations is an active area of research.
Regional Variations and Hotspots
Ocean acidification is not uniform across the globe. Polar regions are experiencing acidification faster than other areas because cold water absorbs more carbon dioxide. The Arctic Ocean is particularly vulnerable, with some areas already experiencing undersaturation with respect to aragonite, a form of calcium carbonate used by many polar organisms. The Southern Ocean is also acidifying rapidly.
Upwelling regions, where deep waters rich in carbon dioxide rise to the surface, experience naturally lower pH that is being exacerbated by anthropogenic acidification. The California Current and the Humboldt Current are among the upwelling regions where acidification is already affecting marine life. Coastal waters can experience additional acidification from nutrient pollution, which stimulates respiration and produces carbon dioxide, creating local hot spots.
Human Dimensions
Ocean acidification has direct implications for human societies, particularly through its effects on fisheries and aquaculture. Shellfish fisheries, including oysters, clams, and mussels, are directly threatened by reduced calcification and increased larval mortality. The Pacific Northwest oyster industry experienced significant losses in the mid-2000s due to acidification, providing an early warning of the economic impacts.
Coral reef fisheries, which support the livelihoods and food security of hundreds of millions of people, are threatened by the combined effects of acidification and warming. The loss of reef habitat reduces fish abundance and diversity. The economic costs of ocean acidification, including losses in fisheries, aquaculture, tourism, and coastal protection, are projected to be substantial.
Frequently Asked Questions
How does ocean acidification differ from climate change? Ocean acidification and climate change are both caused by increased atmospheric carbon dioxide, but they are different phenomena. Climate change results from the greenhouse effect of carbon dioxide in the atmosphere. Ocean acidification results from the chemical reaction of carbon dioxide with seawater.
Will the ocean become acidic? No. The ocean is currently slightly alkaline, with a pH of about eight point one. Ocean acidification refers to the decrease in pH toward neutral, not to the ocean becoming acidic. Even under worst-case projections, the ocean will remain alkaline.
Can ocean acidification be reversed? The only way to reverse ocean acidification is to reduce atmospheric carbon dioxide levels. Natural processes that neutralize acidity operate on timescales of thousands to hundreds of thousands of years.
What can individuals do about ocean acidification? Reducing individual carbon footprint by using less energy, choosing renewable energy, reducing consumption, and supporting climate-friendly policies helps address the root cause of ocean acidification.
Conclusion
Ocean acidification is a fundamental change in seawater chemistry driven by carbon dioxide emissions, with far-reaching consequences for marine life and human societies. The rapid rate of change, unprecedented in geological history, challenges the capacity of marine organisms to adapt. Addressing ocean acidification requires reducing carbon dioxide emissions, the same actions needed to address climate change. The future of marine ecosystems, from coral reefs to polar food webs, depends on the actions taken today to reduce emissions and protect the ocean from this pervasive threat.