Zooplankton are small animals that feed on algae and bacteria in lakes, and are in turn food for small fish. Three major zooplankton groups exist in lakes; the cladocerans and copepods, which are both small crustaceans, and the rotifers (Figure 1). Like other animals, many zooplankton species have naturally distinct geographies, meaning New Zealand has species that are not known from other parts of the world. Many zooplankton species are sensitive to changes in water quality and fish introductions.