Lake Rotorua – Wai Ora Mo A Maatau Mokopuna
Stormwater Conference
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Lake Rotorua is the largest lake in the Rotorua district at 8,085 hectares. It was returned to Te Arawa through the 2004 Deed of Settlement, and the lakebed is vested in Te Arawa. The management of the Lakes is shared between “the Partners” to the Rotorua Lakes Strategy Group (Te Arawa Lakes Trust, Bay of Plenty Regional Council, Rotorua Lakes Council). The Statutory Acknowledgement acknowledges Te Arawa’s cultural, spiritual, historical and traditional association with the Lakes.
The city of Rotorua is on the south-western shore of the lake and covers about eight percent of the 53,789 hectare lake groundwater catchment. The Lake Rotorua catchment is dominated by pastoral farming and forestry but includes around 9,000 hectares of indigenous vegetation. Forestry occupies around 23,129 ha (43%) within the catchment. Dairy farming occurs on 6,590 ha (12%) and dry stock land-use occupies 19,770 ha (37%).
The water quality for Lake Rotorua has been the subject of concern for over 35 years due to weed growth and algal blooms.
Through research and community engagement the water quality of the 1960s was identified as a suitable and achievable target for the lake. This was established as a target Trophic Level Index1 (TLI) of 4.2.
To achieve the TLI target the concentration of two key nutrients feeding algal populations in the lake need to be reduced. Science research has shown that the sustainable annual nutrient load required to achieve the target is 435tN (nitrogen) and a range of 33.7 to 38.7tP (phosphorus). The peak nitrogen load flowing to Lake Rotorua is predicted by NIWA’s catchment nitrogen model (ROTAN) is 755tN/year. A high percentage of phosphorus in the catchment comes from natural sources and the capacity to achieve any required reduction from the catchment’s land-uses is likely to be extremely difficult. Phosphorus needs to be reduced by 47% to 64% of the anthropogenic load.
The challenge for “the Partners” and the community is firstly, how to achieve the target and then how to maintain this level for future generations.