The purpose of this report is to provide a synthesis of the detailed technical reports produced for the Lake Rotorua Science Review. Short summaries are presented for each technical report while the synthesis of these, including discussion of key findings and recommendations, is provided in this Executive Summary.
This science review is required under Method LR M2 in ‘Plan Change 10 Lake Rotorua Nutrient Management’ (PC10), and under an agreement with key stakeholders. The review followed a set methodology leading to 12 reports which are summarised here. Each of the technical reports and this summary document were independently peer reviewed by Professor Warwick Vincent from Laval University, Canada (see Appendix 1).
The water quality in Lake Rotorua deteriorated from the 1960s onwards, with algal blooms driven by land use change, farm intensification and city sewage discharges. This decline was due to excess nutrient inputs and caused widespread public concern and support for restoring water quality to levels prevailing in the 1960s, prior to the onset of major decline. The policy response to these issues is documented by Lamb (2018) and is based on a TLI (Trophic Level Index) target of 4.2 reflecting water quality conditions in the 1960s, with sustainable lake loads for nitrogen and phosphorus of 435 t per year and 37 t per year respectively. These estimates of sustainable lake loads were based on a key paper by Rutherford et al. (1989) and have been supported by subsequent studies, including those presented here.