The chapter "Ocean Acidification: The Other CO₂ Problem" by Richard A. Feely, published in February 2009, discusses the impacts of ocean acidification on marine ecosystems. The text begins with an overview of the chemical reactions involved in ocean acidification, including the saturation state of calcium carbonate and the equilibrium conditions that determine precipitation, equilibrium, and dissolution. Field observations from the WOCE/JGOFS/OACES Global CO₂ Survey in the 1990s provide data on dissolved inorganic carbon (DIC) and total alkalinity (TA). Observations of shoaling saturation horizons, particularly aragonite and calcite, indicate that these horizons are moving closer to the surface due to the penetration of anthropogenic CO₂ into the oceans. The chapter also highlights natural processes that can accelerate ocean acidification in coastal waters, such as coastal upwelling. Experiments on various scales, including aquaria and small mesocosms, have shown that calcification rates in single-celled algae like *Emiliana huxleyi* and *Gephyrocapsa oceanica* decrease significantly at higher CO₂ levels. Shelled planktonic snails, such as *Clio pyramidata*, experience rapid shell dissolution within 48 hours when exposed to elevated CO₂. The response of bivalves like mussels and oysters to elevated CO₂ is also discussed, with both species showing decreases in calcification rates. Juvenile bivalves, such as the hard shell clam *Mercenaria*, are particularly sensitive to undersaturated water, leading to massive dissolution and mortality in estuaries and coastal habitats. potential effects on open ocean food webs and marine fish are also explored, with strong interactive effects on nutrient and trace metal availability, light, and temperature. The chapter concludes by emphasizing the lack of baseline data in coastal regions where CaCO₃ saturation states are expected to decrease dramatically over the next 50-100 years, highlighting the need for more comprehensive research to understand the full extent of ocean acidification's impacts on marine ecosystems.The chapter "Ocean Acidification: The Other CO₂ Problem" by Richard A. Feely, published in February 2009, discusses the impacts of ocean acidification on marine ecosystems. The text begins with an overview of the chemical reactions involved in ocean acidification, including the saturation state of calcium carbonate and the equilibrium conditions that determine precipitation, equilibrium, and dissolution. Field observations from the WOCE/JGOFS/OACES Global CO₂ Survey in the 1990s provide data on dissolved inorganic carbon (DIC) and total alkalinity (TA). Observations of shoaling saturation horizons, particularly aragonite and calcite, indicate that these horizons are moving closer to the surface due to the penetration of anthropogenic CO₂ into the oceans. The chapter also highlights natural processes that can accelerate ocean acidification in coastal waters, such as coastal upwelling. Experiments on various scales, including aquaria and small mesocosms, have shown that calcification rates in single-celled algae like *Emiliana huxleyi* and *Gephyrocapsa oceanica* decrease significantly at higher CO₂ levels. Shelled planktonic snails, such as *Clio pyramidata*, experience rapid shell dissolution within 48 hours when exposed to elevated CO₂. The response of bivalves like mussels and oysters to elevated CO₂ is also discussed, with both species showing decreases in calcification rates. Juvenile bivalves, such as the hard shell clam *Mercenaria*, are particularly sensitive to undersaturated water, leading to massive dissolution and mortality in estuaries and coastal habitats. potential effects on open ocean food webs and marine fish are also explored, with strong interactive effects on nutrient and trace metal availability, light, and temperature. The chapter concludes by emphasizing the lack of baseline data in coastal regions where CaCO₃ saturation states are expected to decrease dramatically over the next 50-100 years, highlighting the need for more comprehensive research to understand the full extent of ocean acidification's impacts on marine ecosystems.