What effect do calcium ions have on the water in ecosystems? A. They reduce its pH. B. They decrease its salinity. C. They add to its hardness. D. They increase its acidity.

answer: C, They add to its hardness.

Calcium is naturally dissolved in water, it can be found in water as Ca2+ ions and causes water hardness. Comes from rocks such as limestone, marble, calcite, dolomite, gypsum, fluorite and apatite. It can also be a ph stabilizer.