Lake water quality
What is measured
We report on eight biological, chemical, and physical lake water quality attributes based on measurements made at monitored lake sites and spatially modelled data: trophic level index (TLI), chlorophyll-a, E. coli, total phosphorus, total nitrogen, ammoniacal nitrogen, nitrate-nitrogen, and clarity.
For all attributes we report on the median values for monitored sites during the period 2016–20. For selected attributes we also report:
- how these values compare to the National Objectives Framework (NOF) bands related to ecosystem health (trophic level), toxicity, and E. coli (MfE, 2020)
- a comparison of NOF and trophic level index (TLI) bands at monitored sites with the proportion of human modified landcover in the upstream catchment area
- trends based on measurements made at monitoring sites over a 10-year period (2011–2020).
We also report median values and TLI and NOF bands for selected attributes using spatially modelled data with predictions for 3,813 lakes in New Zealand that are larger than 1 hectare.
Why it is important
High concentrations of the measured biological and chemical attributes or low clarity indicates poor habitats for some species of animals and plants, risks for human recreation, potential toxic effects on aquatic life, and poor overall lake health.
Water quality attributes – DataInfo+ provides further information.
Key findings
Trophic level
For the period 2016–20, the median trophic level index (TLI) rating was good or very good for:
- 11.9 percent of 101 monitored lake sites
- 2.4 percent of 3,813 lakes based on national modelled data.
The TLI rating was poor or very poor at:
- 62.4 percent of 101 monitored lake sites
- 45.8 percent of 3,813 lakes based on national modelled data.
Monitored sites with lower trophic levels (linked to better ecosystem health) had significantly lower proportions of human modified landcover in the upstream catchment area compared to sites with higher trophic levels. The average proportion of human modified landcover in the upstream catchment area was:
- 34.9 percent for sites with a TLI rating of good or very good (12 sites)
- 78.4 percent for sites with a TLI rating of poor or very poor (63 sites).
Attributes related to trophic level that were in NOF band A (indicating healthy and resilient ecological communities) were:
- total phosphorus: 20.8 percent of 101 monitored sites, 10.2 percent of 3,813 modelled lakes
- total nitrogen: 6.9 percent of 101 monitored sites, 6.3 percent of 3,813 modelled lakes
- chlorophyll-a: 12.6 percent of 101 monitored sites, 1.7 percent of 3,813 modelled lakes.
The proportion that exceeded the NOF ‘national bottom line’ (and therefore require action to mitigate ecosystem degradation) was:
- total phosphorus: 24.8 percent of 101 monitored sites, 13.6 percent of 3,813 modelled lakes
- total nitrogen: 35.6 percent of 101 monitored sites, 28.2 percent of 3,813 modelled lakes
- chlorophyll-a: 42.7 percent of 103 monitored sites, 33.9 percent of 3,813 modelled lakes.
Trends are classified as improving or worsening where the trend is ‘likely’ or ‘very likely’. We use the term ‘indeterminate’ when there is either no trend direction or not enough statistical certainty to determine trend direction. Ten-year trends (2011–2020) at monitored sites showed that for TLI:
- 36 percent were improving
- 45.3 percent were worsening
- 18.7 percent were indeterminate.
Toxicity
For ammoniacal nitrogen, 29.3 percent of 82 monitored sites were in NOF band A (indicating a high level of species protection), and 22.0 percent of 82 monitored sites exceeded the ‘national bottom line’ (and therefore require action to improve water quality).
Bacteria
For E. coli, 67.5 percent of 40 monitored sites were in NOF band A (indicating low infection risk).
Where this data comes from
Regional councils, NIWA, Land Water People
View data tables
Lake water quality – monitored state (added 7 September 2022)
Lake water quality – modelled state (added 7 September 2022)
Lake water quality – trends (added 7 September 2022)
Related indicators
Nitrate leaching from livestock
River water quality: nitrogen
River water quality: phosphorus
River water quality: clarity and turbidity
River water quality: macroinvertebrate community index
River water quality: Escherichia coli
Related content
Environment Aotearoa 2022
Our freshwater 2020
Environment Aotearoa 2019
Our fresh water 2017
Environment Aotearoa 2015
Technical reports
Water quality state and trends in New Zealand lakes: Analyses of national data ending in 2020
Spatial modelling of lake water quality state: Incorporating monitoring data for the period 2016 to 2020
Update to REC Land Cover categories and review of category membership rules
About the data
We report on the following lake water quality attributes:
- trophic level index (TLI)
- chlorophyll-a
- E. coli
- total phosphorus
- total nitrogen
- ammoniacal nitrogen
- nitrate-nitrogen
- clarity.
These attributes provide information on the trophic level of the lake (TLI, chlorophyll-a, total phosphorus, total nitrogen), potential toxic effects on aquatic species (ammoniacal nitrogen, nitrate-nitrogen), bacteria (E. coli), and the physical state of the lake (clarity).
Water quality attributes – DataInfo+ provides further information.
We report on lake water quality monitoring sites operated by regional councils in New Zealand. To ensure our reports are nationally consistent, we only report on data collected over consistent time periods and with comparable methods. As a result, our evaluations may differ from those published separately by regional councils and Land Air Water Aotearoa (LAWA). If detailed regional-level information is required, we recommend consulting the relevant regional council’s environmental reports, or the LAWA website.
Lake water quality was assessed by determining the median values of the measures from 2016 to 2020 (Whitehead et al., 2021). The number of lake sites that met filtering rules for inclusion in the analysis using five-year medians was: 101 (TLI), 103 (chlorophyll-a), 40 (E. coli), 101 (total phosphorus), 101 (total nitrogen), 96 (ammoniacal nitrogen), 82 (ammoniacal nitrogen - pH adjusted), 20 (nitrate-nitrogen), and 85 (clarity).
To explore the relationship between landcover and lake water quality, we looked at the extent of human modification in the upstream catchment for each lake. We used the Land Cover Database version 5.0 classes to assess extent of human modification. Human modification includes the following land cover types:
- urban
- pastoral
- exotic forest.
Landcover – DataInfo+ provides further information.
In addition to reporting on monitored sites, NIWA modelled attribute concentrations for 3,813 lakes in New Zealand that are larger than 1 hectare for the 2016–20 period (Snelder et al., 2022). This allowed us to estimate water quality state at lakes that do not have monitoring sites. Attribute values were predicted for 3,813 lakes using Random Forest models and predictors (explanatory variables) such as climate, geology, topography, hydrology, stock intensity, and landcover. In this indicator page we only report on modelled attributes that had sufficiently good model performance.
Modelled lake water quality – DataInfo+ provides further information.
Lake water quality trends were assessed for 2011 to 2020 (10-year period). The number of lake sites with sufficient data to estimate trends varied by attribute: 75 (TLI), 77 (chlorophyll-a), 15 (E. coli), 76 (total phosphorus), 76 (total nitrogen), 65 (ammoniacal nitrogen), 5 (nitrate-nitrogen), and 66 (clarity). Trends are classified as improving or worsening if the trend certainty is above 66 percent (‘likely’) or above 90 percent (‘very likely’). A worsening trend means concentrations of the attribute are increasing over time (decreasing in the case of clarity). For this indicator we use the term ‘indeterminate’ when there is either no trend direction or not enough statistical certainty to determine trend direction (less than 66 percent certainty).
Trends – DataInfo+ provides further information.
Our lake water quality pages report concentrations of attributes against TLI categories and bands of the National Objectives Framework (NOF) in the National policy statement for freshwater management 2020 (MfE, 2020). The TLI rating is used to place lakes into nutrient-enrichment categories known as trophic states – higher numbers indicate more nutrient enrichment and poorer water quality. The NOF groups sites by their levels of nutrient enrichment and toxicity to aquatic species. Band A is the best state and bands D and E the worst. The NOF bands are designed to help communities make decisions on how to manage water quality, including setting minimum acceptable states called ‘national bottom lines’ which councils must meet, or work towards meeting over time. The national bottom line for attributes associated with trophic state (total nitrogen, total phosphorus, and chlorophyll-a) is between NOF bands C and D. The bottom line for toxicity (ammoniacal nitrogen) is between NOF bands B and C. All sites below the bottom line must have actions in place to improve lake water quality. The National policy statement for freshwater management does not specify a national bottom line for E. coli.
Thresholds – DataInfo+ provides further information.
Data quality
The accuracy of the data source is of medium quality.
Lake water quality is a partial measure of the ‘Freshwater quality, quantity and flows’ topic.
Stats NZ and the Ministry for the Environment must report on topics related to the five environmental domains: air, atmosphere and climate, fresh water, land, and marine. These topics identify key issues within each domain.
Topics for environmental reporting describes the topics for each domain.
Data quality information has more information about the criteria we use to assess data quality.
References
Ministry for the Environment (MfE). (2020). National policy statement for freshwater management 2020. http://www.mfe.govt.nz
Snelder, T., Fraser, C., & Whitehead, A. (2022). Spatial modelling of lake water quality state: Incorporating monitoring data for the period 2016 to 2020 (No. 2021-15). LWP client report prepared for Ministry for the Environment. LWP, Christchurch.
Whitehead, A., Fraser, C., Snelder, T., & White, R. (2021). Water quality state and trends in New Zealand lakes: Analyses of national lakes data ending in 2020. NIWA client report prepared for Ministry for the Environment. NIWA, Christchurch.
Archived pages
Archived April 2022:
