Lake water quality

  • Image, Lake water quality: trophic level index.

    The lake trophic level index (TLI) indicates the health of a lake based on its degree of nutrient enrichment. When nitrogen and phosphorus accumulate in lakes (referred to as ‘nutrient enrichment’) above certain concentrations, they can stimulate the growth of algae and cyanobacteria. Chlorophyll-a is a measure of the phytoplankton (algae) biomass. Lakes with very high concentrations of nutrients and algae are rarely suitable for recreation and provide poor habitats for aquatic species, particularly through reduction in dissolved oxygen concentrations. Ammoniacal nitrogen and nitrate-nitrogen can be toxic to aquatic life if concentrations are high enough. Water clarity is a measure of underwater visibility in lakes.

    We report on the states and trends for nine indicators of lake water quality: TLI, total nitrogen, total phosphorus, chlorophyll-a, nitrate-nitrogen, ammoniacal nitrogen, dissolved reactive phosphorus, water clarity, and bottom water dissolved oxygen.

    We classified Lake water quality as a case study.

    Key findings

    Median TLI was very good or good at 36.9 percent of lake sites, moderate at 26.2 percent, and poor or very poor at 36.9 percent of 65 monitored lake sites from 2009–13.

    Over the five-year period 2009–13:

    • 15.8 percent of sites measured for total phosphorus, 15.5 percent of sites measured for total nitrogen, and 15.3 percent of sites measured for chlorophyll-a exceeded the national bottom line for ecosystem health.
    • 2.1 percent of sites measured for ammoniacal nitrogen exceeded the national bottom line for toxicity to aquatic species in the same period.

    Trends, over the 10-year period 2004–13:

    • For total nitrogen, dissolved reactive phosphorus, total phosphorus, ammmonical nitrogen, chlorophyll-a, trophic level index, and visual clarity, more trends at monitored sites were improving than worsening over the period.
    • For bottom-water dissolved oxygen and nitrate-nitrogen, more trends at monitored sites were worsening than improving over the period.

    Figure 1

    Note: The trophic states shown from left to right represent increasing nutrient enrichment, from a state of microtrophic (very good) to supertrophic (very poor).


    Figure 2

    Note: The National Objectives Framework (NOF) in the National Policy Statement for Freshwater Management 2014  groups sites by their levels of nutrient enrichment and toxicity to aquatic species. Bands A through D represent different states, with A being the best state and D the worst. The national bottom line is the boundary between bands C and D, meaning all sites in band D must have actions put in place to improve lake water quality. The states of monitored lake sites are assessed for chlorophyll-a (72 sites), ammoniacal nitrogen (48 sites), total nitrogen (71 sites), and total phosphorus (76 sites).


    Figure 3

    Figure 4

    Note: Over the 10-year period 2004–13, monitored lake sites were assessed for trends in chlorophyll-a (39 sites), TLI (30), ammoniacal nitrogen (31), total nitrogen (31), total phosphorus (44), Secchi depth (visual clarity) (35), nitrate-nitrogen (13), dissolved reactive phosphorus (19), and bottom water dissolved oxygen (24). 

    Definition and methodology

    We report on nine indicators of lake water quality: 

    • total nitrogen – sum of all nitrogen forms found, including organic nitrogen from plant tissue
    • ammoniacal nitrogen (ammonia in water) – can be toxic to aquatic life
    • nitrate-nitrogen – can be toxic to aquatic life 
    • total phosphorus – sum of all phosphorus forms, including phosphorus bound to sediment 
    • dissolved reactive phosphorus – indicates how much phosphorus is immediately available to support algae and plant growth
    • chlorophyll-a – measure of phytoplankton (algae) biomass in a lake
    • bottom water dissolved oxygen – the oxygen concentration at the deepest point in lakes where oxygen-depth profiles were measured
    • water clarity – measured by Secchi depth, which is the maximum depth at which a black and white Secchi disk is visible to an observer at the lake surface
    • trophic level index (TLI) – composite index that describes the state of nutrient enrichment of lakes.

    Two versions of TLI are used in New Zealand, one with three variables (TLI3) and one with four (TLI4) (Burns et al, 2000; Verburg et al, 2010). We report using TLI3 (comprising total nitrogen, total phosphorus, and chlorophyll-a). Verburg et al (2010) found that TLI3 did not differ significantly from TLI4, which includes water clarity as a fourth parameter.

    The TLI is used to place lakes into nutrient-enrichment categories known as trophic states (Burns et al, 2000):

    • microtrophic (TLI < 2; very good) lakes are very clean and often have snow or glacial sources (eg Lake Pukaki in Canterbury) 
    • oligotrophic (TLI 2–3; good) lakes are clear and blue, with low concentrations of nutrients and algae (eg Lake Rotoma in Bay of Plenty) 
    • mesotrophic (TLI 3–4; average) lakes have moderate concentrations of nutrients and algae (eg Lake Rerewhakaaitu in Bay of Plenty) 
    • eutrophic (TLI 4–5; poor) lakes are murky, with high concentrations of nutrients and algae (eg Lake Rotoroa in Northland) 
    • supertrophic or hypertrophic (TLI > 5; very poor) lakes have extremely high concentrations of phosphorus and nitrogen, and are overly fertile; they are rarely suitable for recreation and lack habitats for desirable aquatic species (eg Lake Forsyth in Canterbury).

    Four lake water quality indicators are compulsory attributes in the National Objectives Framework in the National Policy Statement for Freshwater Management 2014: total nitrogen, total phosphorus, chlorophyll-a, and ammoniacal nitrogen.

    For the analysis presented here, NIWA used lake water quality data monitored by regional councils in the state and trend analysis with consistent time periods and comparable methods (Larned et al, 2015). Most regional councils monitor lake water quality to manage environmental impacts.

    The number of lake monitoring sites that met filtering rules for inclusion in the analysis for states were 72 (chlorophyll-a), 65 (TLI), 48 (ammoniacal nitrogen), 71 (total nitrogen), 76 (total phosphorus), and 61 (Secchi depth). The number of those that met filtering rules for inclusion in the analysis for trends for the 10-year period 2004-13 were 39 (chlorophyll-a), 30 (TLI), 31 (ammoniacal nitrogen), 31 (total nitrogen), 44 (total phosphorus), and 35 (Secchi depth). Increasing and decreasing trends at sites are inferred with 95 percent confidence using the Relative Seasonal Sen Slope Estimator. Indeterminate trends mean we have insufficient data to determine trend direction.

    Sites with sufficient data to compute 10-year trends were almost entirely restricted to the Northland, Auckland, Waikato, and Bay of Plenty regions. Sites with sufficient data to analyse state were similarly biased, with additional sites in the Canterbury region. The small number of sites and restriction to only a few regions mean we cannot extrapolate results to infer the state of New Zealand lakes in general.

    If you want detailed regional-level information, we recommend you review the relevant regional council’s environmental reports.

    This is because although our data are sourced from regional councils, we adjust some datasets to ensure our reports are nationally consistent. The adjustments may include omitting information produced by non-comparable methods. As a result, our evaluations may differ from those produced by regional councils.

    Data quality 

    Topic Classification   Relevance Accuracy 
     Freshwater quality, quantity, and flows  Case study



     Image, High accuracy.



    See Data quality information for more detail.


    Burns, N, Bryers, G, & Bowman, E (2000). Protocols for monitoring trophic levels of New Zealand lakes and reservoirs. Retrieved from

    Larned, S, Snelder, T, Unwin, M, McBride, G, Verburg, P, & McMillan, H (2015). Analysis of water quality in New Zealand lakes and rivers: Data sources, data sets, assumptions, limitations, methods and results. Prepared for the Ministry for the Environment. NIWA Client Report no. CHC2015-033. Wellington: NIWA. Retrieved from

    Verburg, P, Hamil, K, Unwin, M, & Abell, J (2010). Lake water quality in New Zealand 2010: Status and trends. NIWA Client Report HAM2010-107. Retrieved from

    Archived pages

    See Lake water quality (archived April 2017).

    Updated 27 April 2017

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