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Monitored lake water quality

New Zealand has more than 50,000 lakes, but only 3,821 of them are larger than one hectare (10,000 m2). The different lake types, geology, topography, and human activities are factors that contribute to lake water quality (see Land-based activities are putting pressure on water quality).

Monitored lakes make up a small proportion (less than 5 percent) of all lakes in New Zealand larger than one hectare. Due to the processing and filtering rules for creating a national dataset, the data we used for this section were almost entirely restricted to the Northland, Auckland, Waikato, Bay of Plenty, and Canterbury regions (Larned et al, 2015).

We compare results for lake water quality to attribute states for compulsory values in the National Objectives Framework (New Zealand Government, 2014). These are described in table 3.

Table 3: Lake water quality attribute states

Lake water quality compulsory value

Status

Water quality attribute

What it means

Ecosystem health (trophic state) – National Objectives Framework national bottom lines in the National Policy Statement for Freshwater Management (New Zealand Government, 2014)

Regulatory

Algae (phytoplankton, measured as chlorophyll-a), total nitrogen, total phosphorus

The attribute states consist of four bands, A–D, with A being the best state and D the worst. The national bottom line for phytoplankton is the level at which lake ecological communities have undergone or are at high risk of a regime shift to a persistent, degraded state.

Ecosystem health (toxicity) – National Objectives Framework national bottom lines in the National Policy Statement for Freshwater Management (New Zealand Government, 2014)

Regulatory

Ammoniacal nitrogen

The attribute states consist of four bands, A–D, with A being the best state and D the worst. The bottom line for median nitrate toxicity and pH adjusted ammonia toxicity is the level that would have some growth effects on up to 20 percent of sensitive fish species.

Trophic level index at lake sites varies

When nitrogen and phosphorus enter lakes (referred to as ‘nutrient enrichment’), they stimulate algal growth, which can make lakes turbid and green. In New Zealand, the trophic level index (TLI) is widely used to measure the nutrient status of lakes, and provides an indication of the health of a lake.

Of 65 lake monitoring sites between 2009 and 2013, 24 sites had median TLI scores of very good or good, 17 monitored sites had moderate scores, and 24 monitored sites had poor or very poor scores (Larned et al, 2015). Lakes rated good or very good are clear (unless they have natural turbidity, eg from glacial silt) and have low concentrations of nutrients and algae (eg Lake Pukaki in Canterbury). Those rated poor or very poor tend to be turbid, with high concentrations of nutrients that promote frequent algal blooms. These lakes are rarely suitable for recreation and have habitats unsuitable for some native freshwater species (eg Lake Horowhenua in Manawatu-Wanganui).

For more detail see Environmental indicators Te taiao AotearoaLake water quality [Stats NZ].

Most lake sites monitored for water quality meet national bottom lines

We assessed the variables for lake water quality against the attribute states in the National Objectives Framework. Total phosphorus, total nitrogen, and chlorophyll-a concentrations are National Objectives Framework attributes commonly used to assess nutrient enrichment in lakes. We also measured ammoniacal nitrogen in lakes, as it can be toxic to sensitive freshwater species.

For monitored lake sites assessed for water quality from 2009 to 2013, the following did not meet the national bottom lines for ecosystem health: 12 of 76 sites for total phosphorus; 11 of 71 sites for total nitrogen, and 11 of 72 sites for chlorophyll-a (a measure of phytoplankton (algae) biomass in a lake) (see table 3 and figure 18). One of 48 monitored sites did not meet the national bottom line for ammonia toxicity to sensitive freshwater species.

Figure 18

monitored lake sites grouped by National Objectives Framework bands for total phosphorus, total nitrogen, chlorophyll-a, and ammoniacal nitrogen for the period 2009–13
Click to enlarge view
 

This graph shows monitored lake sites grouped by National Objectives Framework bands for total phosphorus, total nitrogen, chlorophyll-a, and ammoniacal nitrogen for the period 2009–13. Visit the MfE data service for the full breakdown of the data.

Note: 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. At this level, negative impacts on growth and mortality of multiple sensitive species are expected.

For more detail see Environmental indicators Te taiao AotearoaLake water quality [Stats NZ].

Trends in water quality at monitored lake sites show mixed results

Along with the water quality variables and trophic level index discussed in the previous two sections, we also report on the trends in lake visual clarity, nitrate-nitrogen, dissolved reactive phosphorus, and bottom-water dissolved oxygen for monitored lake sites.

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 10 years from 2004 to 2013 (see figure 19). For bottom-water dissolved oxygen and nitrate-nitrogen, more trends at monitored sites were worsening than improving over the same 10-year period.

For more detail see Environmental indicators Te taiao AotearoaLake water quality [Stats NZ].

Figure 19

shows trends in concentrations of nine water quality variables at monitored lake sites between 2004 and 2013
Click to enlarge view

This graph shows trends in concentrations of nine water quality variables at monitored lake sites between 2004 and 2013. Visit the MfE data service for the full breakdown of the data.