Ballast water
Ballast water is water carried in ships’ tanks to control stability, trim, and structural loads as cargo, fuel, or water supplies change.
Ship stability: A vessel’s ability to resist capsizing and return toward an upright position after disturbance. Ballast water changes a vessel’s weight distribution and therefore its resistance to rolling or capsizing.
Invasive species: Organisms introduced outside their native range that establish populations and cause ecological, economic, or other harm. Ballast discharge is a major pathway by which aquatic species reach new ecosystems.
Ballast water treatment: Physical, chemical, or combined processes used to reduce viable organisms in ballast water. Treatment is the main onboard approach for limiting organisms released during discharge.
Ballast: Material carried or placed in a vessel to improve stability, trim, or balance. Water is one form of ballast, alongside solid materials such as iron or concrete.
Metacentric height: The distance between a vessel’s center of gravity and its metacenter, used to assess initial stability. Adding or moving ballast alters the center of gravity and can change this measure of stability.
Zebra mussel: A freshwater mussel native to Eurasia that has become invasive in parts of North America and Europe. Its spread in ship ballast helped demonstrate the ecological consequences of organism transport.
International Convention for the Control and Management of Ships' Ballast Water and Sediments: A 2004 International Maritime Organization treaty that sets requirements for managing ships’ ballast water and sediments. The convention establishes global standards and plans for ballast-water management.
Trim (ship): A ship’s orientation in the water measured by the difference between its forward and aft drafts. Ballast distribution can raise or lower the bow or stern to achieve a desired trim.
Ballast tank: A tank built into a vessel to hold ballast water or other ballast material. These tanks store and distribute the water used to adjust draft, trim, and stability.
Comb jelly: A gelatinous marine animal; the North American species Mnemiopsis leidyi became invasive in the Black Sea. Its introduction through shipping, including ballast-water transport, contributed to major food-web changes.