Ocean acidification, caused by rising atmospheric CO2, is making our seas more acidic. This process threatens marine life, especially organisms that build shells or skeletons from calcium carbonate. The effects ripple through ecosystems, impacting biodiversity and food webs.

The consequences of ocean acidification extend beyond ecology. It poses risks to fisheries, coastal tourism, and infrastructure. Understanding these impacts is crucial for developing strategies to protect our oceans and the communities that depend on them.

Ocean Acidification and Its Effects

Ocean acidification and atmospheric CO2

• Ocean acidification is the ongoing decrease in the pH levels of the Earth's oceans primarily caused by the absorption of carbon dioxide (CO2) from the atmosphere
• Atmospheric CO2 levels have been increasing due to human activities such as burning of fossil fuels (coal, oil, natural gas), deforestation, and cement production
• When atmospheric CO2 dissolves in seawater, it forms carbonic acid (H2CO3) which dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-)
• Increased H+ concentration lowers the pH of the seawater, making it more acidic (lemon juice, vinegar)
• Since the industrial revolution, the average pH of the ocean surface has decreased by approximately 0.1 units corresponding to a 30% increase in acidity

Calcification in marine organisms

• Calcification is the process by which marine organisms build calcium carbonate (CaCO3) structures such as shells (oysters, mussels), exoskeletons (crabs, lobsters), and coral reefs
• Calcifying organisms use dissolved carbonate ions (CO32-) and calcium ions (Ca2+) from seawater to form CaCO3 through the reaction: $Ca^{2+} + CO_3^{2-} \rightarrow CaCO_3$
• Ocean acidification reduces the availability of carbonate ions in seawater as increased H+ concentration favors the formation of bicarbonate ions (HCO3-) over carbonate ions (CO32-)
• Lower CO32- concentration makes it more difficult for calcifying organisms to build and maintain their CaCO3 structures leading to thinner, weaker, or deformed shells and skeletons
• Reduced calcification rates increase vulnerability to predation (crabs, fish), physical damage (waves, storms), and other stressors (temperature, pollution)

Vulnerable species to acidification

• Corals secrete CaCO3 exoskeletons to form the foundation of coral reefs and reduced calcification rates can slow coral growth and weaken reef structures (Great Barrier Reef, Caribbean reefs)
• Mollusks such as oysters, mussels, and pteropods (sea butterflies) rely on CaCO3 shells for protection and support with thinning or deformed shells increasing vulnerability
• Calcifying plankton including coccolithophores and foraminifera form CaCO3 plates or tests that contribute to marine sediments and carbon cycling with decreased calcification affecting their survival and role in marine food webs (base of the food chain)

Consequences of ocean acidification

• Ecological consequences include:

1. Reduced biodiversity in marine ecosystems from loss of sensitive calcifying species and alteration of community structure and food web dynamics
2. Degradation of coral reefs leading to loss of habitat for numerous marine species (fish, invertebrates) and decreased coastal protection from storms and erosion
3. Changes in biogeochemical cycles with altered carbon and nutrient cycling due to shifts in plankton communities

• Economic consequences encompass:

1. Impacts on fisheries and aquaculture from declines in commercially valuable shellfish populations (oysters, mussels, clams) and reduced yields and increased production costs
2. Tourism and recreation losses due to degradation of coral reefs and other marine attractions resulting in decreased revenue for coastal communities and businesses
3. Coastal infrastructure damage with increased vulnerability to storms and sea-level rise due to weakened coral reefs and higher costs for coastal protection and restoration projects