Ballast Water and Alien Species

Ballast water is pumped into and out of ships, depending upon whether the ship is  loaded with cargo or empty, and used to weight and balance the ship to ensure its stability.  A less direct but perhaps even more insidious export of waste occurs when ships pump their ballast water out in the ports where they take on cargo.
Ballast water is largely harmless - besides rust residue or a small amount of fuel, the water itself is not the problem. But when ballast water is pumped into a vessel, many eggs, larvae, and adult plants and animals go with it.  When these organisms are pumped out in ports far away, they can out-compete local flora and fauna and wreak havoc.  For example the zebra mussel, which invaded the U.S. Great Lakes and major rivers, was carried in ballast water from the Black Sea region.

In return for that favor, subsequent ballast water originating in the western Atlantic brought a dreaded jelly-bodied creature into the Black Sea, where it devoured the young of all the local commercially important fish species and caused the ecosystem to collapse.  Ballast-borne alien species invasions have reshuffled the world's marine ecosystems, and caused amazing ecological and economic damage in some places.