Lake Rotoiti a Large Volcanic Lake in Central North Island

Lake Rotoiti is a deep (max. depth 124 m) volcanic lake in central North Island which receives most of inflowing water from Lake Rotorua. Despite rapid eutrophication

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Figure 7 A transect taken in summer 2005 from west to east across Lake Rotorua (0-10 km) through the Ohau Channel (10-12 km) and into Lake Rotoiti (12-34 km), with colors representing levels of dissolved oxygen and shaded area representing the lake bed.

of Lake Rotorua in the 1960s, attributed to wastewater discharges from the city of Rotorua, water quality in Lake Rotoiti initially remained relatively unimpacted. However, oxygen levels in the bottom waters of this lake during summer have declined over several decades from an annual minimum around three parts per million in 1956 to being devoid of oxygen (anoxic) for more than four months in 2003. A transect across Lake Rotoiti and Lake Rotorua shows well oxygenated waters of shallow Lake Rotorua and anoxic bottom waters of deep, stratified Lake Rotoiti (Figure 7). Lake Rotoiti, once New Zealand's second-most frequented lake by trout anglers, has in recent years been affected by recurrent blue-green algae (cyano-bacteria) blooms. A diversion wall has recently been

constructed in Lake Rotoiti to direct inflows from Rotorua via the Ohau Channel, directly towards the lake outflow.

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