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Lake Quality

Aotearoa’s lakes are steeped in history with significant cultural, recreational, and social values. For Māori, fresh water is a taonga and essential to life and identity. Our economy depends on having plentiful water – agriculture, tourism, and hydroelectricity generation particularly rely on water. New Zealanders and tourists alike enjoy many forms of recreation that use our lakes. These waterbodies also support many indigenous animals, plants, and ecosystems.

Some of our lakes are world class, while others are in a degraded condition.

You can find a summary of the condition of monitored lakes in New Zealand in the National Picture tab, or click on the Regions tab to find out more on the water quality of your favourite monitored lakes. Desktop and tablet users can also view lake information on the map. 

LAWA Lake Water Quality National Picture Summary 2020

26 November 2020

LAWA shows water quality information for lake sites throughout New Zealand. Lakes are monitored for a range of water quality (chemical-physical and bacterial) and ecological indicators by regional and unitary councils.  State and trend results for these indicators have been updated for individual lakes from data up to the end of December 2019 and can be found on the regional lake pages within LAWA.

In this first national picture summary for lakes, LAWA is focusing on the current condition of monitored lakes, and how they have changed over time.  We report on the lake Trophic Level Index (TLI) which combines several indicators to determine overall lake condition.   

While this summary provides some information on how our lakes are tracking, it is important to note that this information only relates to sites where monitoring has been conducted and where there are enough monitoring data to obtain a result for TLI.  Monitored sites with the best data records in New Zealand are often located in areas that are more impacted by human activities, sites that have greatest social and cultural value, or where changes in perceived water quality have resulted in the implementation of monitoring programs.  These sites are not necessarily representative of all New Zealand lakes.  Overall, only a small proportion (124 lakes) of the approximately 3800 lakes (>1 ha) in New Zealand are currently monitored regularly for the indicators required to calculate TLI.

 

Current state

The Trophic Level Index (TLI) combines several measures of lake water quality, to provide an integrated summary of lake condition.  It is calculated using the concentrations of key nutrients (total nitrogen and total phosphorus), an indicator of algae biomass (chlorophyll a - the photosynthetic pigment present in all plants), as well as a measure of water clarity.  These four aspects are combined using an equation that yields a score between 0 and 9 (which is then converted to a grade “very good” to “very poor”), with lower scores indicating better water quality and higher values indicating progressively poorer water quality.  Increasing TLI scores indicate that a lake is becoming nutrient enriched with an increasing likelihood of algae bloom events, which are associated with reduced water clarity.  The TLI is calculated once every year and provides regional councils and lake managers with an integrated measure of water quality that can be tracked over time.

TLI summaries presented for the current state of New Zealand lakes are based on data collected in 2019.  Overall, the condition of less than 20% of monitored lakes can be categorised as either “very good” or “good” (24 lakes), whereas more than 50% of monitored lakes can be categorised as either “poor” or “very poor” (65 lakes).

 

LAWA Lake Trophic Level Index (2019)

Figure 1. TLI (Trophic Level Index) grades for the 124 monitored lakes throughout New Zealand.  TLI grades were calculated from total nitrogen, total phosphorus, chlorophyll a and clarity from data collected in 2019.  The location of these monitored lakes is shown on the map.

 

It is useful to understand the geographical distribution of monitored lakes and how their location in the landscape can affect lake condition.  In this regard, the altitude of lakes in New Zealand provides a useful distinction: lowland lakes are often smaller and shallower than upland lakes.  They are also often located in catchments with higher proportions of agricultural, urban or other development.

Figure 2 shows that the condition of 71% of monitored lowland lakes is either “poor” or “very poor” (55 lakes) compared to 21% of the monitored upland lakes (10 lakes) in those two conditions.  By contrast, the condition of only 6% of monitored lowland lakes (5 lakes) are “very good” or “good”, compared to 40% of the monitored upland lakes (19 lakes).

 

LAWA Lake Trophic Level Index (2019) by Altitude

Figure 2.  Comparison of TLI (Trophic Level Index) grades by altitude.  The number of monitored lakes for each altitude category is shown at the bottom of the bars.  The location of these monitored lakes is shown on the map.

 

How lake condition has changed over the last 10 years

The above plots illustrate current lake condition, but it is also interesting to know how this has changed over time.  To examine this, TLI scores were calculated for each year of the last decade, for lakes where that data was available.  The following figure shows results only for those lakes that have this long and comprehensive sampling history (91 out of 124 monitored lakes).

At the national level, there has not been a great deal of change over the last 10 years but the number of lakes within each TLI grade does vary slightly from year to year as lakes can undergo natural fluctuations in their condition.  There was a slight increase between 2010 and 2019 in the number of lakes in the “very good” grade.  There appears to be no consistent change in the number of lakes that are in the “poor” and “very poor” grades.  For individual lakes you can find information on fluctuations over time on the lake pages on LAWA.

 

LAWA Lake Trophic Level Index Change Over Time (2010-2019)

Figure 3.  Changes in TLI (Trophic Level Index) grades from 2010-2019 at 91 lakes where there were enough data to determine the TLI score each year.  The location of these monitored lakes is shown on the map.   

 

This national summary uses a subset of information on lakes that is freely available on the LAWA website to present a general picture.  The summary focuses on TLI as it is an overall indicator for lake condition. 

You can find more information on the indicators that make up TLI, along with other indicators of lake water quality, for specific lakes on the individual lake pages.  LAWA will be able to show more data for lakes in the future as regional councils and unitary authorities progressively increase lake monitoring efforts.