THE CALCIUM QUESTION©

By Adolf F. Klostermann

(as appeared in FAMA April 91)

Calcification; the ability of certain organisms and algae to extract calcium and carbonate ions from the surrounding seawater and deposit these ions as solid crystalline structures of calcite or aragonite depends on a multitude of factors. The most obvious is the availability of the base raw materials, calcium, and carbonate ions. When a solid such as calcite or aragonite is in contact with solution (seawater), the solid dissolves, remains unchanged or more solid is deposited. The process that is favored depends on the saturation condition of the ions with respect to either solid. The saturation condition or saturation index can be determined by use of the specific solids' solubility product constant, and the mole concentration of the ions in question:

The concentration of calcium can quite easily be measured and is generally expressed in milligrams (mg) per liter of water. Seawater at a salinity of 35 parts per thousand (0/00) and temperature of 25ºC has a specific gravity of 1.023. Therefore, the calcium concentration of 412 mg per Kg of seawater becomes 420 mg per liter of seawater. The mole concentration is the weight per liter divided by the formula weight of the ion and becomes 420 mg divided by 40, or 10.5 millimoles (mM) per liter. The carbonate ion can not be measured directly but may be calculated with reasonable accuracy from measurements of total alkalinity and pH by the use of the acidity constants of carbonic acid (aqueous carbon dioxide)².

The carbonate concentration at a total alkalinity of 2.8 milliequivalents (mequ) and pH of 8.3 is 19 mg/l. In mole concentration, 19 divided by 60 results in 0.32 mM/l. The solubility product constants for calcite and aragonite at a salinity of 35 0/00 and temperature of 25ºC are 5.77 x 10^-7 and 9.27 x 10^-7 respectively.

Applying the measured and calculated mole concentrations and the solubility product constant for calcite, the calcite saturation index (CSI) is found to be:

And by using the solubility product constant for aragonite, the aragonite saturation index becomes:

In either ease, since the saturation index is greater than one, the ions are oversaturated with respect to the solid phase. It can be noted however, that the degree of oversaturation of calcite is much greater than that for aragonite. Therefore, calcite extraction thermodynamically speaking, takes place much more readily than aragonite extraction. Corals and other aragonite depositing organisms must expend more energy than for example mollusks. The above indices apply to "standard" seawater with salinity alkalinity, pH, calcium concentration, and temperature of 35 0/00 2.8 mequ, 8.3, 420 mg/l, and 25ºC, respectively. If any of these parameters change, new indices must be calculated. To examine the effect of any one parameter we can hold the others constant and evaluate a new index.

A drop in pH from 8.3 to 8.0 would leave the calcium concentration unchanged, but the carbonate concentration drops to 0.19 mM/I. With this new data the saturation index for calcite becomes 3.5 and for aragonite 2.2. This is a significant reduction in the degree of saturation and is easily brought about by the sudden influx of carbon dioxide.

A drop in total alkalinity from 2.8 to 2.0 mMequ/l would again leave the calcium concentration unchanged but the carbonates would drop to 0.23 mM/l. The resultant indices for calcite and aragonite are 4.2 and 2.5, respectively. We again observe a significant drop. However, the affect is less than that produced by the change in pH.

A drop in calcium concentration produces an equivalent percentage drop in both indices. A reduction of 10% in calcium would drop the indices to 5.2 and 3.2. A change in salinity or temperature affects calcium and carbonates, as well as the solubility product constants. A decrease in salinity to 33 0/00 equates to a solids contents change of 5.7% per Kg of seawater and results in a calcium level of 9.9 mM/l. The carbonates are reduced to 0.31 mM/I and the solubility product constants become 5.43 x 10^-7 and 8.93 x 10^-7 for calcite and aragonite, respectively. Calculations of the indices reveal only minor changes in saturation levels. CSI is 5.7 and ASI is 3.4. The drop in calcium due to the decrease in salinity is mostly offset by the change in the solubility product constants. Whereas the acidity constants are but slightly affected by salinity changes or this magnitude.

A drop in temperature from 25ºC to 20ºC results in calcium and carbonate levels of 10.6 mM and 0.30 mM/l, respectively. The solubility product constants increase to 6.09 x 10^-7 and 9 .59 x 10^-7 with resultant indices of 5.2 and 3.3.

In all cases considered, the indices remained above one, indicating that calcium deposition is still possible. However, downward changes in pH and alkalinity produce a much greater effect than changes in salinity and temperature that may be normally encountered. If the changes that have been considered singly occurred in aggregate, the following conditions would prevail: pH 8.0, alkalinity 2 mequ/l, salinity 33 0/00, temperature 20ºC, and calcium 10% low from nominal. The calculated CSI is 1.9 and the ASI is 1.2. Neither index indicates the possibility of active calcification. As a matter of fact, the index for aragonite at 1.2, is so near unity that dissolution may be favored.

The factors discussed are the most obvious indicators we can use to predict if calcification can take place. There are known inorganic inhibitors and most likely a multitude of organics that even at low concentrations prevent calcification. Nevertheless, if the basic building blocks of calcium and carbonate ions are in short supply we don't have a chance at all.

ABOUT THE AUTHOR…

The author was born in Germany in 1942 and immigrated to America in 1955. After finishing his education he joined the United States Air Force and, during his travels had the opportunity to see and experience his first coral reef off the island of Okinawa in the Pacific Ocean. The spark of interest in the mysterious creatures of the sea, that had always been there, was finally ignited. A hobbyist was born in the early sixties. After leaving the Air Force the author pursued a career in electronics. He picked a company in Ft. Lauderdale to be near the only reefs in the United States. In 1986, and just prior to his retirement as Vice President of Sunair Electronics, he founded Coral Reef Research. The company is dedicated, through continuing and from prior years of research, to the development of a saltwater processing system that maintains the original, essential, water chemistry of closed systems indefinitely.

. . . At this date, indefinitely is 14 years.

References:

1) Calcite Solubility Product Constant* (0.1614 + 0.02892Cl -0.0063t) x l0^-6

Aragonite Solubility Product Constant* (0.5115 + 0.02892Cl -0.0063t) x 10^-6

2) First Acidity Constant* ( H⁺ )(HCO₃^-) / H₂CO₃ *= 6.00 and 6.02 at 25 and 20ºC.

Second Acidity Constant* (H⁺)(C0₃^-2) / (HCO₃^- = 9.10 and 9.17 at 25 and 20ºC.

Acidity Constants are expressed as -logK

a) J.M. Gieskes, in The Sea, Vol. 5, E.D. Goldberg, Ed., Wiley Interscience, 1974.