Lake water quality state and trends in Tāmaki Makaurau / Auckland 2014-2024. State of the environment reporting

Executive summary

There are approximately 108 waterbodies over one hectare in area within the Tāmaki Makaurau / Auckland region. These include natural lakes and lakes formed from human activity, such as dams, constructed or quarry lakes. Several pressures affect the health of lakes across the region including, but not limited to, catchment land cover, coarse fish, presence of invasive species, internal nutrient loading and a changing climate. Auckland Council undertakes long-term monitoring of lake water quality as part of its State of the Environment reporting. This monitoring assesses the current health of lakes in the region, detects trends in water quality and evaluates the effectiveness of council initiatives, policies and lake management strategies. The lake water quality programme started in 1988 with quarterly sampling at seven lakes across the region and expanded in 2020 to include more frequent sampling at thirteen lakes.

This report provides an overview of the current state of lake water quality (e.g. nutrients, algae and clarity) and ecology (e.g. submerged plants) at thirteen lakes and identifies trends in water quality for four lakes over the most recent 10-year period (2014-2024). Analysis includes surface and bottom waters, where available, for each of the lakes. Assessment of lake condition uses water quality parameters and ecological indicators and is graded according to the National Policy Statement for Freshwater Management (NPS-FM 2020).

Most lakes in Tāmaki Makaurau / Auckland were in poor condition (12 out of 13 lakes) according to the lake Trophic Level Index (TLI), with elevated nutrient and algae concentrations, poor water clarity and poor ecological condition of submerged macrophytes. Lake Rototoa had the best water quality in the region and was in a high ecological condition based on assessments of submerged plants (LakeSPI). Lake Keretā had the worst water quality in the region and was in a non-vegetated, algal dominated state.

Lake mixing type influences lake health. Shallow, polymictic (well-mixed) lakes were in worse health than the deeper, seasonally stratified lakes. In seasonally stratified lakes, higher nutrient concentrations in the bottom waters, coupled with persistent anoxia, suggest that internal nutrient loading (nutrient release from lakebed sediments) contributes towards nutrient enrichment in these lakes.

Trend analysis for four lakes that have data available for the 10-year trend period July 2014 – June 2024 (Lake Pupuke, Lake Rototoa, Lake Tomorata and Lake Wainamu) showed mainly degrading trends.

Lake Pupuke had elevated nutrient concentrations in the bottom waters and had a range of algae concentrations, indicating algal blooms occurring at times. Despite this, water clarity was one of the highest in the Auckland region. Degrading trends in bottom water nutrients and turbidity, and temperatures and dissolved oxygen throughout the water column show that the lake is vulnerable to declining water quality. Key drivers of declining water quality in Lake Pupuke include the presence of pest fish, the high biomass of invasive macrophytes, nutrient loading from nutrient-rich sediments within the lake and anoxic conditions that could worsen with a changing climate, fuelling further nutrient loading.

Lake Rototoa had good water quality, with high water clarity. However, degrading trends in nutrient parameters suggest declining water quality may result in adverse changes. There was evidence of internal nutrient loading due to higher nutrient concentrations in the bottom waters of the lake. Auckland Council’s ongoing work on the management of pest fish, invasive macrophyte control, and kākahi conservation are helping to maintain and improve the high ecological values of the lake.

Lake Tomorata was in a poor eutrophic state with nutrient enrichment, algal blooms and poor water clarity. There were mostly degrading trends in nutrient parameters, algae and sediment parameters, suggesting lake health may worsen if there are no interventions in the lake. However, the intensive pest fish removal within the lake is promising. If fish numbers decline, zooplankton populations may increase, which consume phytoplankton, and therefore there may be reductions in algal concentrations within the lake.

Lake Wainamu had elevated nutrient and algae concentrations, high turbidity and poor water clarity. Several pest fish species were present, and no submerged vegetation, resulting in a turbid, algal dominant state. Despite the surrounding native forest catchment, the lake had poor water quality and is vulnerable to further decline. Pressures from pest fish, nutrient enrichment, recreational use of the lake and catchment erosion all contribute towards declining water quality in the lake.

The lake water quality programme expansion in 2020 has provided a more representative assessment of lake health across the Auckland region. Most lakes are in poor health, with several pressures including invasive species and nutrient enrichment. Some lakes are in a good state, but degrading trends in water quality demonstrate their vulnerability to decline in the future. Management efforts are ongoing in some lakes, including pest fish control, invasive plant eradication, biodiversity conservation, and the development of lake-specific management plans. Continued monitoring and management are needed to maintain and enhance the ecological values of Auckland’s lakes.

Auckland Council technical report, TR2025/11

September 2025

See also

Te oranga o te taiao o Tāmaki Makaurau. The health of Tāmaki Makaurau / Auckland’s natural environment in 2025. A synthesis of Auckland Council’s state of the environment reporting