Nutrient enrichment of shallow lakes

Why we monitor nutrient enrichment of shallow lakes

There are over 100 lakes in the Waikato region, which vary greatly in their physical, chemical and biological characteristics. The majority of the region’s lakes are shallow and less than 10 hectares in size.

Many of these shallow lakes are valuable conservation refuges for unique plant and animal species. They are valued for their unique genetic diversity, cultural and spiritual importance, scientific interest, recreational opportunities and intrinsic value.

This indicator focuses on nutrient enrichment (which influences trophic state) as a measure of water quality in shallow lakes. Monitoring nutrient enrichment in shallow lakes tells us about the ability of a lake to support native plant and animal life and identifies long-term trends in water quality. This information helps us develop management responses to avoid or reverse adverse affects on shallow lake water quality.

The indicator also tells us about the presence of potentially harmful blue-green algae in certain lakes.

What's happening?

Shallow lakes are defined as generally:

having an average depth of less than three metres
being able to support submerged aquatic plants over large areas of the lake bed
not being stratified - their shallow depth means the lake’s water is stirred up regularly due to wind and wave action.

The Waikato region’s shallow lakes are generally nutrient enriched. They have high levels of nutrients such as phosphorus and nitrogen. The amount of nutrients entering a lake from its catchment mainly determines its trophic state. Nutrient enrichment results in poor water quality and a high trophic state.

Most of the 12 shallow lakes monitored are highly to extremely nutrient enriched (a trophic status of eutrophic, supertrophic or hypertrophic). They have high nutrient levels and poor water clarity. In contrast lakes with low-to-moderate nutrient levels and clear water are classed as oligotrophic or mesotrophic.

Increasing nutrient enrichment or ‘eutrophication’, results from run off and leaching of contaminants such as effluent, fertiliser and sediment from land use in a lake's catchment. Nutrients can also be recycled from the bottom sediments of shallow lakes, adding to the levels found in the overlying water. Farmland now surrounds most shallow lakes in the region.

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 2014–20. Trends in nutrient enrichment between 1995 and 2020 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. Since 2019, monitoring has been carried out monthly (see table 1).

For blue-green algae, the five lakes were generally sampled 10 times per year (monthly except for July and September) until 2019, when monitoring changed to monthly throughout the entire year.

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 2014-2020. The trends in nutrient enrichment indicator uses data collected between 1995 and 2020 (noting that the records in several lakes did not begin until 2002: see Table 1). The blue-green algae indicator uses data collected during 2010-2020.

Table 1: Records used in this indicator

Lake name	Frequency	Period
Horseshoe Lake	Bi-monthly	2014-2018
Lake Areare	Bi-monthly	2014-2020
Lake Hakanoa	Bi-monthly	2002-2020
Lake Harihari	Bi-monthly	2010-2020
Lake Mangakaware	Bi-monthly	2012-2020
Lake Maratoto	Bi-monthly	2002-2020
Lake Rotomanuka	Bi-monthly	1995–2020
Lake Serpentine North	Bi-monthly	2002–2020
Lake Serpentine South	Bi-monthly	2010–2020
Lake Waahi	Bi-monthly	1995–2020
Lake Waikare	Bi-monthly	1998–2020
Lake Whangape	Bi-monthly	2002–2020

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 (below) 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 2020 (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 the software ‘R‘ (www.r-project.org) using the ‘EnvStats’ package for Environmental Statistics including US EPA Guidance. 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 of the trophic indicators. PAC values >1% were deemed definite changes, whereas PAC values <1% were deemed undetermined (i.e. no changes).

Frequency of cyanobacteria blooms

The frequency of cyanobacteria blooms indicator uses data collected between 2010 and 2020. 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 (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. 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 category	Trophic state	Trophic level	Chla (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

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 18 years; the other three lakes had 22-25 year records (number of samples varied 81 and 169 for these lakes). There were 106-111 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.

Data and trends

The upper graph shows how many of a group of 12 shallow lakes are currently found in each nutrient enrichment category. For seven of these lakes there is enough information to measure any changes in water quality over time. The middle graph shows the proportion of the lakes that have improved (nutrient levels have decreased), have not changed or have deteriorated (nutrient levels have increased) in their level of nutrient enrichment. The bottom graph shows the proportion of samples collected from five lakes during 2010-2020 contained levels of potentially toxic cyanobacteria that exceed guideline values. Note that these five lakes are sampled because cyanobacteria blooms are known to have occurred in them in the past.

The data were collected between 1995 and 2020.
Find out more about our shallow lakes and where they are.

No changes in water quality were found in Lake Hakanoa and Lake Waahi between 2002 and 2020. Water quality in Lake Rotomanuka showed deterioration for Secchi depth and total nitrogen concentration. While total phosphorus concentrations in Lake Waikare improved between 1998 and 2020, total nitrogen and chlorophyll a concentrations showed deterioration. Lake Serpentine North showed deterioration for Secchi depth and total nitrogen concentrations but improvements in chlorophyll a concentrations.