Zebra Mussel
An exotic species originally introduced into the Great Lakes via the ballast water of transoceanic ships. This small bivalve mussel poses a multibillion dollar threat to industrial, agricultural, and municipal water supplies across North America by clogging water intake pipes. It can also have impacts on fisheries, native freshwater mussels, and natural ecosystems. By moving along contiguous waters of the Great Lakes, attached to ships, barges and recreational boats, this Eurasian native has rapidly spread throughout the Mississippi River Basin and many of its major tributaries such as the Ohio River. Free-swimming larvae are also spread by river currents. Boater education campaigns focus on preventing further spread of this species.

Zero Discharge
Zero discharge refers to halting all inputs from all human sources and pathways to prevent any opportunity for persistent toxic substances to enter the environment from human activity. To completely prevent such releases, the manufacture, use, transport, and disposal of these substances would have to stop. The Binational Program has designated nine toxic substances (critical pollutants) to be part of the Zero Discharge Demonstration Program for the Lake Superior Basin. These substances are chlordane, dieldrin, dichloro-diphenyl-trichloro-ethane (DDT and its metabolites such as DDE), hexachlorobenzene (HCB), mercury, octachlorostryrene (OCS), polychlorinated biphenyls (PCBs), 2, 3, 7, 8 tetrachlorodibenzo-p-dioxin (TCDD), and toxaphene.

Zero Discharge Demonstration Program
This international program is in response to the recommendation by the International Joint Commission that Lake Superior be designated a zero discharge demonstration zone where no point source discharge of any persistent bioaccumulative toxic substance be permitted. Nine persistent toxic substances (critical pollutants) have been designated as critical for the program. The first priority of the program is the goal of achieving zero discharge of the nine substances from point sources. To completely prevent such releases, the manufacture, use, transport, and disposal of these substances must stop. This objective is to be met by:

1. pollution prevention;
2. enhanced controls and regulations, and;
3. protection through special designations of all or part of the basin (See also Outstanding International Resource Waters and Outstanding National Resource Waters).

Zone of Initial Dilution ZID
The region of initial mixing surrounding or adjacent to the end of an outfall pipe or diffuser. The ZID may not be larger than allowed by mixing zone restrictions in applicable water quality standards.

Zooplankton
Small, mostly microscopic animals that swim or float freely in open water. Zooplankton eat algae, detritus, and other zooplankton and in turn are eaten by fish.