Ballast, generally, means anything that adds stability or weight. For example, an air balloon uses sandbags to control altitude and balance the center of gravity, and stone or gravel ballast is used in making concrete. In a ship, ballast is used to add weight for maintaining stability at sea.
The primary function of ballast is to keep the ship balanced when there is insufficient cargo weight. A ship also takes on extra ballast when sailing through rough seas in order to increase its stability, or to make the ship settle lower in the water when it needs to pass under a bridge.
Historically, shipping lines used rocks, sandbags, seashells and lumps of metal in order to weight their vessels. Today, most ships use water, since it is much quicker and easier to load and off-load, making it more economical.
Once cargo has been off-loaded or depleted from a ship, the ship takes on ballast in order to replace the weight. When another cargo is loaded, the water is pumped back into the ocean. Between three and five billion tonnes of water is transported internationally each year, and a similar amount is distributed between domestic ports.
Ballast water is one of the primary causes of importing invasive species into marine ecosystems, which is one of the most severe threats facing the world's oceans. Any form of marine life small enough to be sucked through the pumping system, including microbes, small invertebrates and the eggs and larvae of larger species, can be taken in with the ballast and transferred to another part of the world when the water is discharged.
- Photo Credit ship image by jimcox40 from Fotolia.com bacteria 16 image by chrisharvey from Fotolia.com
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