How did zebra mussels reach the Great Lakes from Europe?

Zebra mussels (Dreissena) were introduced to the Great Lakes from Europe around 1986. They were most likely transported to North America in the ballast water of ships.12

Ballast water is used to stabilize vessels after cargo has been off-loaded. The indiscriminate release of this water into new environments is a common practice that has frequently led to the introduction of invasive species into novel ecosystems. Once established in the Great Lakes, these mussels proliferated rapidly, causing significant ecological and commercial damage.312