Magnesium is the third most abundant ion in seawater, surpassed only by sodium and chloride. Its abundance is critical in the formation of weak ion pairs with carbonate ions. This allows calcium and carbonate ions to be available for uptake by corals and invertebrates. Low levels of magnesium lead to low or unstable levels of calcium and alkalinity. Signs of low magnesium levels include precipitation of calcium as insoluble calcium carbonate and impaired inhabitant growth and health.
