"Ion concentration" refers to the amount of ions (charged particles) present in a solution. It means how many ions are dissolved in a liquid, which can affect the properties and behavior of the solution. Full definition
The uptake of fossil fuel carbon dioxide (CO2) by the ocean increases seawater acidity and causes a decline in carbonate ion concentrations. (sciencedaily.com)
This increase in the ocean's hydrogen ion concentration corresponds to a reduction in pH, or an increase in acidity. (metlink.org)
If carbonate ion concentrations are lower, calcium carbonate minerals are more likely to dissolve. (landscapesandcycles.net)