Two faces of a lake – Lake Rotoiti

Lake Rotoiti is one of a number of lakes which lie to the east and south of Lake Rotorua, the second largest lake in the North Island. The lakes were created as a result of a volcanic eruption about 240,000 years ago, and were for hundreds of years the ancestral home and source of food and other resources for the Te Arawa people.

Today, the land around the lakes is almost without exception developed – either as urban or rural settlements or farmland – with almost no indigenous forest remaining [click here to view satellite image showing the extent of development around the lake]. With little vegetation buffering it from nutrient run-off  from the surrounding land, and being a relatively shallow lake (about 10 metres in depth), Lake Rotorua is one of New Zealand’s most polluted lakes.

Lake Rotorua links directly to its most immediate neighbour, Lake Rotoiti [click here to view map], and this smaller lake, while quite picturesque, and valued by boaties and others as a recreational destination, it suffers from toxic algal blooms caused by nutrient-laden (mainly nitrogen and phosphorus) water flowing through from its larger neighbour via the Ohau Channel.

So serious had this problem become that a diversion wall was built in 2008 in an attempt to divert Rotorua Lake’s polluted water down the Kaituna River rather than into Rotoiti. It is estimated that 70 percent of the nutrients in the lake come from Lake Rotorua, so it hoped that the wall will see significant improvements over time.

The initiative does not of course tackle the root cause of the pollution, which is a problem that cuts to the heart of historic and ongoing decisions about land use.

A strategy to restore and protect the Rotorua lakes has been developed by the regional council, Environment Bay of Plenty, in partnership with Te Arawa Lakes Trust Board, and the Rotorua District Council. The Rotorua Lakes Protection and Restoration Programme has the vision to restore and protect the water quality of the lakes.

Photo top: the lake seen from the settlement near Okawa Bay. Above left: a sign at Okawa Bay warning lake-users about the toxic algal bloom in the lake. (Photos: C. Knight)

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