Lake water quality – published April 2019
What is measured
We report on six measures of lake water quality based on measurements made at monitored lake sites: chlorophyll-a, total nitrogen, ammoniacal nitrogen, total phosphorus, water clarity, and lake trophic level index (TLI3). For these measures we report on:
- median values for the period 2013 to 2017
- for selected measures, how these values compare to the National Objectives Framework (NOF) (MfE, 2017) bands related to ecosystem health
- trends during the 10-year period from 2008 to 2017.
Why it is important
When nitrogen and phosphorus accumulate above certain concentrations in lakes (referred to as ‘nutrient enrichment’), they can stimulate excessive growth of algae and cyanobacteria. Chlorophyll-a is a measure of the phytoplankton (algae) biomass. The lake trophic level index (TLI) indicates the health of a lake based on concentrations of three variables:
- total nitrogen
- total phosphorus
- chlorophyll-a.
Water clarity is a measure of underwater visibility. Lakes with poor clarity and TLI are poor habitats for some species of animals and plants, and they may not be suitable for recreation. Ammoniacal nitrogen can be toxic to aquatic life if concentrations are high enough.
Key findings
The median trophic level index (TLI3) rating was very good or good at 16 percent of lake sites, average at 28 percent, and poor or very poor at 57 percent of 58 monitored lake sites from 2013 to 2017.
The NOF ‘national bottom line’ (NOF band D) for total phosphorus, total nitrogen, and chlorophyll-a was exceeded at 17 percent, 30 percent, and 35 percent, respectively, of the 63 monitored lake sites, indicating high risk of degradation in lake ecological communities.
- The NOF national bottom line for toxicity to sensitive aquatic species was not exceeded at any monitoring sites measured for ammoniacal nitrogen.
Ten-year trends (2008–2017) showed that more sites had improving rather than worsening trends for TLI3, chlorophyll-a, ammoniacal nitrogen, and total phosphorus.
- Roughly the same proportion of sites had improving or worsening trends for total nitrogen and clarity.
Where this data comes from
Regional councils, NIWA
View data tables
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
Technical report
About the data
We report on six measures of lake water quality:
- chlorophyll-a – measure of phytoplankton (algae) biomass in a lake
- total nitrogen – sum of all nitrogen forms found, including organic nitrogen from plant tissue
- ammoniacal nitrogen (ammonia in water) – a form of nitrogen that is immediately available to support algae and plant growth, and at high concentrations can be toxic to aquatic life
- total phosphorus – sum of all phosphorus forms, including phosphorus bound to sediment
- water clarity – measured by Secchi depth, which is the maximum depth at which a black and white Secchi disk is visible to an observer at the lake surface
- trophic level index 3 rating (TLI3) – composite index that describes the state of nutrient enrichment of lakes.
Two versions of TLI are used in New Zealand, one with three variables (TLI3) and one with four (TLI4) (Burns et al, 2000). TLI3 comprises total nitrogen, total phosphorus, and chlorophyll-a, whereas TLI4 includes water clarity as a fourth variable.
The TLI rating is used to place lakes into nutrient-enrichment categories known as trophic states (Burns et al, 2000):
- microtrophic (TLI rating <2; very good) lakes are very clean and often have snow or glacial sources (eg Lake Pukaki in Canterbury)
- oligotrophic (TLI rating 2–3; good) lakes are clear and blue, with low concentrations of nutrients and algae (eg Lake Rotoma in Bay of Plenty)
- mesotrophic (TLI rating 3–4; average) lakes have moderate concentrations of nutrients and algae (eg Lake Rerewhakaaitu in Bay of Plenty)
- eutrophic (TLI rating 4–5; poor) lakes are murky, with high concentrations of nutrients and algae (eg Lake Rotoroa in Northland)
- supertrophic or hypertrophic (TLI rating >5; very poor) lakes have extremely high concentrations of phosphorus and nitrogen, and are overly fertile; they are rarely suitable for recreation and lack habitats for desirable aquatic species (eg Lake Forsyth in Canterbury).
For this analysis, NIWA used lake water quality data acquired from regional councils and Land Air Water Aotearoa (LAWA) (Larned et al, 2018). Most regional councils monitor lake water quality to manage environmental impacts. We adjust some datasets to ensure our reports are nationally consistent. As a result, our evaluations may differ from those produced by regional councils and 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 2013 to 2017. The number of lake monitoring sites that met filtering rules for inclusion in the analysis using five-year medians was: 63 (chlorophyll-a), 63 (total nitrogen), 62 (ammoniacal nitrogen), 63 (total phosphorus), and 58 (TLI3). The small number of sites, the restriction to only a few regions, and the bias towards monitoring lakes with known water quality issues mean that the results don’t represent New Zealand lakes in general (there are 3,821 lakes in New Zealand that are larger than 1 hectare).
We report on the state of our lakes by comparing median values of selected indicators to thresholds as outlined in the National policy statement for freshwater management 2014 (amended in 2017) (MfE, 2017). The National Objectives Framework (NOF) in the National policy statement groups sites by their levels of nutrient enrichment and toxicity to aquatic species. Band A is the best state and band D the worst. The national bottom line is the boundary between bands C and D. All sites in band D must have actions in place to improve lake water quality.
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. NOF includes five compulsory attributes for lake ecosystem health: total nitrogen, total phosphorus, chlorophyll-a, ammoniacal nitrogen, and cyanobacteria. We do not report on cyanobacteria due to insufficient data for analysis. We also don’t report on the predicted risk of Campylobacter infection (based on Escherichia coli as an indicator).
Lake water quality trends were assessed for 2008 to 2017 (10-year period). Sites with sufficient data to compute state analysis and 10-year trends were almost entirely restricted to the Northland, Auckland, Waikato, and Bay of Plenty regions. The method we used for trend analysis is different from previous reports. A trend is classified as ‘very likely’ when there is 90–100 percent certainty of an improving or worsening trend. Where the trend is assessed as ‘likely’, the certainty of the trend is 67–89 percent. ‘Indeterminate’ means there is not enough statistical certainty to determine trend direction (less than 67 percent certainty). The categories used to describe the certainty of trends are adapted from the IPCC (2013). For technical details, see McBride (2018) and Snelder & Fraser (2018). This method is consistent with the LAWA approach used for river water quality. For reporting on trend likelihood, we included all sites where the trend was assessed as ‘likely’ or ‘very likely’. Sites with insufficient data were excluded from the trend analysis.
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
Burns, N, Bryers, G, & Bowman, E (2000). Protocol for monitoring trophic levels of New Zealand lakes and reservoirs. Retrieved from http://www.mfe.govt.nz/.
Intergovernmental Panel on Climate Change (IPCC) (2013). Climate change 2013: The physical science basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [TF Stocker, D Qin, G-K Plattner, MMB Tignor, SK Allen, J Boschung, … PM Midgley, (Eds)]. Cambridge and New York: Cambridge University Press. Retrieved from www.cambridge.org
Larned, S, Snelder, T, Whitehead, A, & Fraser, C (2018). Water quality state and trends in New Zealand lakes: Analyses of national data ending in 2017. Prepared for the Ministry for the Environment. NIWA Client Report No: 2018359CH.
McBride, GB (2018). Has water quality improved or been maintained? A quantitative assessment procedure. Journal of Environmental Quality, 1–36. https://doi.org/10.2134/jeq2018.03.0101
Ministry for the Environment (MfE) (2017). National policy statement for freshwater management 2014 (amended 2017). Retrieved from http://www.mfe.govt.nz.
Snelder, T, & Fraser, C (2018). Aggregating trend data for environmental reporting. LWP Client Report 2018-01. Retrieved from http://www.mfe.govt.nz.
Archived page
Archived April 2019:
