Calcium is a necessary component for shell secretion and formation. Zebra mussels do not survive when there is an insufficient calcium level. If the level of calcium in the water column falls below 5 mg/l, there will be no survival. Sprung (1987) reported that 40-55 mg of calcium per liter of water was sufficient for larval development and that a minimum of 24 mg of calcium per liter of water was necessary for 10-percent larval survival. Stanczykowska (1977) found that a calcium level of 28-109 mg/l in lakes was sufficient for successful zebra mussel populations. The following table taken from Claudi and Mackie (1994) illustrates the growth and survival of zebra mussels from waters with varying calcium levels and is the basis for the Risk Assessment Calculator.