1975 Stage II commissioned 1978 Stage III commissioned. There has been a steady decline in the water quality since the 1960's. The situation deteriorated from around 1976 due to the increasing loads of nitrogen and phosphorus from the Urban Sewerage System. Phytoplankton growth decreases the clarity of the lake waters. It can also create the conditions where the bottom of the waters of the lake are depleted of oxygen during the calm summer conditions.
Oxygen depletion causes the release of nutrients from the sediment thus compounding the problem.
De-oxygenation of the bottom waters restricts the habitat of trout, driving them to colder waters and river mouths. To restore lake water quality, the nutrient levels in the lake water must be substantially reduced. Reduction in the nutrients entering the lake system via the urban sewerage scheme and land runoff are the only practical ways to address this problem. In 1975 the Upper Kaituna Catchment Control Scheme was released. Arrest the deterioration of and improve the water quality within Lakes Rotorua and Rotoiti.