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  Environment » Environmental Information » Environmental indicators » Lakes and wetlands: monitoring and reporting » Nutrient enrichment of shallow lakes » How we monitor

How we monitor

Where and how we collect the data

Monitoring sites

This indicator reports on the monitoring of 13 shallow lakes:

  • Horseshoe Lake
  • Lake Areare
  • Lake Hakanoa
  • Lake Harihari
  • Lake Mangakaware
  • Lake Maratoto
  • Lake Ngaroto*
  • Lake Rotomanuka
  • Lake Serpentine North
  • Lake Serpentine South
  • Lake Waahi
  • Lake Waikare
  • Lake Whangape

*Lake Ngaroto is only sampled for cyanobacteria

The state of nutrient enrichment is described for 12 lakes for the period 2014–18. Trends in nutrient enrichment between 1995 and 2014 are described for the seven lakes shown in bold. Note that for four of these latter lakes the water quality records began in 1995–96, while for the other five lakes the records began in 2002. Monitoring of blue-green algae in five lakes began in 2003, but it wasn't until 2010 that measurements were made of algal biovolume (as well as algal numbers).

 

Monitoring frequency

For nutrients, the lakes were mostly sampled bi-monthly. The other 11 lakes were sampled up to three times per year (see table 1).

For blue-green algae, the five lakes were generally sampled 10 times per year (monthly except for July and September).

Monitoring history

Waikato Regional Council began monitoring shallow lakes under the current water quality monitoring programme in 1995. The nutrient enrichment indicator uses data collected during 2014-2018. The trends in nutrient enrichment indicator uses data collected between 1995 and 2018 (noting that the records in several lakes did not begin until 2002:  see Table 1).  The blue-green algae indicator uses data collected during 2010-2018.

Table 1: Records used in this indicator

Lake name Frequency Period
Horseshoe Lake Bi-monthly 2014-2018
Lake Areare Bi-monthly 2014-2018
Lake Hakanoa Bi-monthly 2002-2018
Lake Harihari Bi-monthly 2010-2018
Lake Mangakaware Bi-monthly 2012-2018
Lake Maratoto Bi-monthly 2002-2018
Lake Rotomanuka Bi-monthly 1995–2018
Lake Serpentine North Bi-monthly 2002–2018
Lake Serpentine South Bi-monthly 2010–2018
Lake Waahi Bi-monthly 1995–2018
Lake Waikare Bi-monthly 1998–2018
Lake Whangape Bi-monthly 2002–2018

Measurement technique

Monitoring of water quality follows the method established by the New Zealand Lakes Water Quality Monitoring Programme, which was subsequently adopted as a Ministry for the Environment protocol (Burns et al., 2000).

Water quality is measured by taking some measurements on site (for example, water temperature and water clarity) and also taking water samples back to the laboratory for analysis. In total 14 water quality variables (including those necessary to determine trophic state) were measured. The trophic state of a shallow lake is calculated for each of the four trophic indicators:

  • chlorophyll a (Chla)
  • secchi depth (SD)
  • total nitrogen (TN)
  • total phosphorus (TP).

How this indicator is compiled

The following equations were used to determine each individual trophic value for Chlorophyll a (TLc), Secchi depth (TLs), total nitrogen (TLn) and total phosphorus (TLp):

  • TLc = 2.22 + 2.54log(Chla)
  • TLs = 5.56 + 2.60log(1/SD 1/40)
  • TLp = 0.218 + 2.92log(TP)
  • TLn = -3.61 + 3.01log (TN)

The average trophic level index (TLI) for each lake was calculated by:

  • TLI = Σ (TLc + TLs + TLp + TLn)/4

Check out the thresholds for determining a lake’s trophic state in table 2 that were derived from standards, guidelines, and expert opinion.

Trends in nutrient enrichment

The trends in nutrient enrichment indicator uses data collected between 1995 and 2014 (noting that the records in several lakes did not begin until 2002:  see Table 1). Trends in water quality were examined using the seasonal Kendall trend test in ‘Time Trends(external link)’ (NIWA 2014). Percent annual change (PAC) was calculated by dividing the median annual Sen slope by the overall median value. This was done for each of the trophic indicators (Chla, SD, TN, TP).  

Only PAC values calculated from statistically-significant trends (p< 0.05) were considered indicative of a trend. For each lake the PAC values determined for each of the trophic indicators were averaged (non-significant PAC values being replaced with zero). PAC values >1% were deemed definite changes, whereas PAC values >1% were deemed probable changes.

Frequency of cyanobacteria blooms

The frequency of cyanobacteria blooms indicator uses data collected between 2010 and 2018. Samples were examined using a microscope, and the number and biovolume of the main species of cyanobacteria were determined.  The combined biovolume of cyanobacteria was then compared with the Ministry for the Environment’s interim guideline for blue-green algae for recreational uses of lakes(external link) (issued in 2010). 

Guidelines and standards

Table 2 lists the thresholds used to determine the trophic status of a shallow lake. For this indicator a trophic level less than 5 is regarded as a “moderate-to-high” level of nutrient enrichment, a trophic level of 5-to-6 is regarded as a “very high” level of enrichment, while a trophic level more than 6 is regarded as an “extremely high” level of enrichment.  Table 3 provides the scale of probabilities developed for determining whether a shallow lake has undergone a significant change in water quality over time. The Ministry for the Environment's interim (2010) guideline for the combined biovolume of cyanobacteria species in a water sample is 1.8mm3/L.

Table 2: Values of variables defining the boundaries of different trophic levels (Burns et al., 2000)

Nutrient enrichment categoryTrophic stateTrophic levelChla
(mg/m3)
Secchi depth
(m)
TP
(mg/m3)
TN
(mg/m3)
Low Oligotrophic 2-3 < 2 > 7 < 10 < 200
Medium Mesotrophic 3-4 2–5 3-7 10–20 200–300
High Eutrophic 4-5 5–15 1–3 20–50 300–500
Very high Supertrophic 5-6 15-30 0.5–1 50–100 500–1500
Extremely high Hypertrophic 6-7 > 30 < 0.5 > 100 > 1500

 

Limitations

The datasets for several of the 12 lakes used to calculate the nutrient enrichment indicator were reasonably small based on the bi-monthly sampling regime. Although the datasets for the seven lakes that were examined for trends were substantially larger than this, in four cases these records only covered 14–15 years; the other three lakes had 19-22 year records. There were 79-81 samples available for each of the five lakes considered for the blue-green algal indicator.

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.

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