Lake Taupō’s water quality

Methods - how we monitor

• Where and how we collect the data

  Waikato Regional Council contracts the National Institute of Water and Atmospheric Research Ltd (NIWA) to carry out routine monitoring of the lake’s water quality. This monitoring provides long-term information on Lake Taupō’s water so that we can assess its suitability for desired uses (for example, supporting recreation and healthy ecosystems).

  Monitoring sites

  Ecological water quality variables are monitored at a deep-water site (160 m deep) in the open waters of Lake Taupō. This site is regarded as being generally representative of Lake Taupō’s water quality.

  Bacteria levels are monitored at 12 sites commonly used by people for swimming and other contact recreation.

  Monitoring frequency

  Ecological water quality variables including water clarity, nutrients, chlorophyll a, and oxygen depletion are monitored at 2-4 weekly intervals.

  The 12 bathing beach sites were visited at weekly intervals up to 12 times during the summer (December – February).

  Monitoring history

  Ecological water quality variables have been measured for Waikato Regional Council at the deep-water site since 1994.

  Bathing beach surveys were completed annually each summer at the 12 sites, from 1995/96 (summer) to 1998/99 (summer). No horizontal clarity readings were taken for this period. Monitoring began again in 2001/02, and included horizontal clarity readings.

  Since then monitoring has occurred every 2 years.

  Measurement technique

  Water quality is measured by either taking measurements on site (for example water clarity, water temperature, and dissolved oxygen levels) or by taking samples back to the laboratory for analysis.

  More than a dozen water quality variables are measured. They include:

  • from the deep-water site - temperature, dissolved oxygen, pH, clarity, dissolved and total forms of nitrogen and phosphorus, chlorophyll a, algal species (microscopic examination)
  • from the bathing beach sites - Escherichia coli numbers and horizontal clarity readings.

  Samples are analysed by experienced laboratories using standard methods.

  Every 2 – 4 weeks at the deep-water site, temperature and dissolved oxygen levels are measured throughout the water column. The volumetric hypolimnetic oxygen depletion rate (VHOD) is calculated from the temperature and dissolved oxygen profiles (the VHOD is a measure of the rate that oxygen is used up in the bottom waters).

  Secchi depth (water clarity) is also measured, and a sample collected from the 0–10 m layer using a sampling tube to analyse ecological water quality variables such as nitrogen and chlorophyll a. Chlorophyll a provides us with a measure of how many algae are present in the water.

  Sample bottles are used to collect biannual (autumn and spring) water samples at 10 m intervals down to the lake bed. These samples provide ecological health information throughout the entire water column.
• How this indicator is compiled

  Three categories of water quality for ecological health have been developed: ‘excellent’, ‘satisfactory’ and ‘unsatisfactory’. These are based on the ‘critical values’ for four water quality variables identified as relevant to ecological health in this very high quality lake:

  1. water clarity (Secchi disc depth)
  2. algal biomass level (as chlorophyll a) in the 0 -10 m layer
  3. total nitrogen level in the 0 -10 m layer
  4. volumetric hypolimnetic oxygen depletion rate (VHOD).

  The critical values are based on an examination of the long term (1932–99) record of water clarity for the lake. Prior to the 1980s, the average Secchi depth at the deep-water site was 15.2 m (n = 31), while the minimum was 12 m. These values are regarded as being representative of ‘near pristine’ conditions. From an examination of the recent water quality record (1994-99), together with expert judgement, corresponding values for algal biomass and total nitrogen concentration in the upper 10 m of the lake were identified. Critical values for VHOD were identified from an earlier result (1986-87) and recent records.

  For the latest 5-year record (2015–2019), the proportion of samples that met the standard for ‘excellent’, ‘satisfactory’ or ‘unsatisfactory’ water quality for each of the variables is calculated. The average value of the proportions found to be ‘excellent’, ‘satisfactory’ and ‘unsatisfactory’ for each of the four variables is then calculated.

  Three categories of water quality for contact recreation were also developed: ‘excellent’, ‘satisfactory’ and ‘unsatisfactory’. These are based on the critical values for two water quality variables identified as relevant to contact recreation in freshwaters in the region:

  1. water clarity (horizontal range of black disc)
  2. E. coli level - single sample.

  The critical values are derived from national guidelines, and from expert opinion.

  The analysis of the contact recreation variables focuses on the results for the most recent bathing season sampled (2017/18). At each site the average proportions found to be ‘excellent’ for both of the contact recreation variables is calculated, as are the ‘satisfactory’ and ‘unsatisfactory’ proportions. The results for the 12 sites are then aggregated and an average result calculated for all sites.

  Guidelines and standards

  The table below lists the water quality guidelines and standards used.

  Water quality variable (units)	Relevance	Categories
  		Excellent	Satisfactory	Unsatisfactory
  Ecological health
  Secchi disc clarity (m)	Light penetration	>15	12 – 15	<12
  Chlorophyll a (mg/m3)	Algal abundance	<0.7	0.7 – 1.4	>1.4
  Total nitrogen (mg/m3)	Causes nuisance plant growths	<70	70 – 140	>140
  Oxygen depletion (mg/m3/d)	Oxygen for fish to breathe	<5	5 – 15	>15
  Contact recreation
  Clarity, horizontal (m)	Visibility	>4	1.6 – 4	<1.6
  Escherichia coli, single sample (no./100 mL)	Human health	<55	55 – 550	>550

  Notes:

  1. The 'action' level of E. coli in the 2003 national guidelines is 550 units per 100 mL. We use this value to define unsatisfactory conditions.
  2. 'Excellent' conditions have an E. coli level one-tenth of the 'action' value (55 units per 100 mL).
• Limitations

  Lake water quality for ecological health is traditionally assessed using the ‘trophic state’ classification system (the OECD – Organisation for Economic Cooperation and Development - approach), where lakes are classified as:

  • oligotrophic (low nutrient levels)
  • mesotrophic (moderate nutrient levels)
  • eutrophic (high nutrient levels).

  However, this system is too coarse for a high quality lake like Lake Taupō. Reasonably large deteriorations in water quality could occur, but under the OECD system the Lake would still be classed as ‘oligotrophic’.

  To detect reasonably small changes in water quality we have generated our own water quality guidelines for the lake. These guidelines are to some degree arbitrary - ’excellent’ water clarity is that which is greater than the ‘near-pristine’ average value, while ‘unsatisfactory’ is that which is less than the ‘near-pristine’ minimum. Even so, the locally generated guidelines will provide an objective basis for tracking changes over time. For example, if the lake continues to deteriorate, a greater proportion of deep-water samples will fall within the ‘unsatisfactory’ class.

  Quality control procedures

  Samples are taken from the same site on each occasion. Samples are kept chilled and transported to the laboratory as quickly as possible. All bottles are clearly labelled. Samples are transferred to an ISO accredited laboratory for chemical and biological analyses.