What do we mean by 'state'?
On LAWA, we use the term 'state' to describe the current condition of a lake for a given water quality indicator.
Why do we assess the state of lake health?
Understanding the current state of our lakes helps to:
- describe how healthy our monitored lakes are across Aotearoa
- identify pressures on lake ecosystems and risks to human use
- track progress towards national and regional goals for freshwater
- support decisions about how we use our natural resources
Human activities will always have some impact on waterways because of the way we live and use land. It is unlikely that many water bodies in New Zealand can be restored to their pre-human state. Even in places where human pressures could almost be completely removed, such as within national parks, introduced aquatic species, along with native and exotic birds, insects, pest mammals, and natural processes such as erosion and atmospheric pollution, continue to affect the health of our waterways to some extent.
How is state calculated for our lakes?
National framework and thresholds
LAWA evaluates state (current conditions) at sites nationwide against attribute bands described in the National Policy Statement for Freshwater Management 2020 (NPS-FM 2020), and other water quality standards where no NPS-FM 2020 attribute bands exist for an indicator (e.g., clarity).
The NPS-FM 2020 requires the management of freshwater in a way that ‘gives effect’ to Te Mana o te Wai, and sets up requirements for regional councils and unitary authorities in setting objectives, policies and rules to manage freshwater in their regions. The characteristics of the water that need to be managed are referred to as ‘attributes’ in the NPS-FM 2020, and 'indicators' on LAWA.
Each indicator's state is defined by attribute bands, from A (good) to D or E (poor).
Councils, along with their communities, need to set ‘target attribute states’ (attribute bands) for sites in their region. For many indicators there is a national bottom line value and councils and their communities need to aim above that bottom line, unless there is a specific justification why that would not be achievable (e.g., the existing freshwater quality is caused by naturally occurring processes, or a specific time-limited exemption applies). As part of setting target attribute states, councils will also set timeframes as to when these will be achieved.
Data preparation and quality control
Data is collated from monitoring data supplied by councils. Before state is calculated, all results are screened to remove data of a lesser quality based on the Quality Code (QC) information provided by councils:
- All measurements with standard NEMS* QC codes: 100 (missing data) and 400 (measured value may have been compromised and/or undergone significant modification and therefore may not be representative of the intended measurement) are removed.
- Related internal council QC 400 child codes (e.g. 403, 404, 450) are also removed.
- Equivalent 'poor quality' codes from council-specific QC systems (e.g. Auckland codes 41, 61, 100, 42 and 151) are excluded as well.
*NEMS are National Environmental Monitoring Standards which are part of ongoing data improvement processes that councils are actively working on to implement.
Minimum data requirements
The 'current state' for 2024 at each site is based on data from the past three to five hydrological years (July-June).
Minimum data abundance requirements must be met before a grade can be assigned, so not all sites featured on LAWA can be assigned a state result.
LAWA calculates the state of lake water quality sites using monthly data, or quarterly data if monthly data are not available. When multiple measurements are collected on the same day, a daily average is calculated before assessing whether the data meet the specified data abundance criteria.
For ammoniacal nitrogen (as ammonia toxicity*), total nitrogen, total phosphorus, chlorophyll a, and clarity, LAWA requires:
- at least 80% of monthly data available (48 of 60 monthly measurements), or
- if monthly data requirements are not met, then at least 80% of quarterly data available (16 of 20 quarterly measurements) from the five hydrological year period (July 2019-June 2024).
*To determine ammonia toxicity attribute bands, LAWA uses pH adjusted ammoniacal nitrogen data, meaning that pH measures must also be available.
For E. coli, LAWA requires:
- at least 90% of monthly samples (i.e., at least 54 of 60 measurements) from the previous five hydrological years (July 2019-June 2024).
For cyanobacteria, LAWA requires:
- at least 30% of monthly samples (i.e., at least 12 of 36 monthly measurements) from the previous three hydrological years (July 2021-June 2024).
Assigning attribute bands
When attributes (indicators) in the NPS-FM 2020 include multiple statistics calculated from monthly data or quarterly data, the overall attribute band for the indicator is determined by the worst of those statistics. For example, if the ammonia toxicity median result qualifies as an A band and the 95th percentile result qualifies as a B band, the site is assigned a B attribute band.
For chlorophyll a, LAWA uses the median and the 95th percentile, rather than the maximum value prescribed in the NPS-FM 2020. This approach is intended to reduce the influence of a single unusually high value in the dataset across the five hydrological years used for band calculation.
What do the state bands tell us?
- Band A represents the best conditions (high ecological health or low risk to human use).
- Bands B - D (or E) represent progressively worse conditions and higher risk.
- State provides a snapshot, not a long-term trend. For changes over time, see factsheet on calculating water quality trends in lakes.
On LAWA, attribute bands are colour-coded to make interpretation easier. Here is an example of the coloured bands, alongside descriptions of state attribute bands (for ammonia toxicity):
