Lake water quality in New Zealand 2010: Status and trends

This 2010 Lake water quality in New Zealand: Status and trends report updates the 2006 report ‘A snapshot of lake water quality in New Zealand’.

The report provides a national picture of current status (2005–2009) and trends in water quality of New Zealand lakes.  It uses Trophic Level Index (TLI) and LakeSPI (submerged plant indicators) to provide a picture of nutrient levels and overall ecological condition, and discusses the results in the context of different lake types and land uses.  In addition, a comparison of New Zealand lake water quality with a subset of lakes in Europe, USA and Canada is made.

Key findings

  • Of the 4000 lakes in New Zealand over 1 hectare in size, 43 per cent are likely to have very good or excellent water quality (very low levels of nutrients), and 32 per cent are likely to have poor or very poor water quality (are nutrient enriched).
  • The lakes with the poorest water quality and ecological condition tend to be surrounded by pastoral land cover.
  • Between 2005 and 2009, 12 per cent of the 68 lakes assessed showed an improvement in water quality and 28 per cent deteriorated.
  • The decline was most marked in lakes with predominantly native catchment cover (40 percent of lakes with native cover declined, compared with 25 per cent of lakes with pastoral cover).

Executive summary

This report provides a national picture of the current status (2005-2009) and long term trends of water quality in New Zealand lakes. The status is reported mainly in terms of recognised national indicators – the Trophic Level Index (TLI) and LakeSPI (Lake Submerged Plant Indicator). The results are discussed in the context of differences in lake types and land use and are extrapolated to estimate water quality for all lakes in New Zealand.

Lake water quality was assessed for 112 lakes for the period 2005-2009. There were a significant number of lakes for which Secchi depth data was not available. Excluding Secchi depth from the calculation of the TLI made little difference to overall scores; consequently trophic state was assessed using a modified version of the TLI calculated from total nitrogen (TN), total phosphorus (TP) and chlorophyll a.

Forty four percent (49/112) of currently monitored lakes were found to be eutrophic or worse (i.e., TLI >4), and 33% oligotrophic or better (i.e., TLI <3). The TLI score increased with increasing percentage pastoral land cover and decreased with increasing percentage native or alpine land cover.

The water quality results were extrapolated to all New Zealand lakes (>1 ha) using a regression tree analysis based on climate, lake morphology and land cover. This provided statistics that out of 3820 NZ lakes 32% would be eutrophic or worse (TLI >4), while 43% would be oligotrophic or better (TLI <3).

Ecological condition was assessed using LakeSPI for 155 lakes since 2005. Thirty-three percent of lakes had ‘high’ or ‘excellent’ ecological condition (i.e., LakeSPI score >50%); while 37% of lakes had poor ecological condition or were not vegetated with submerged plants (i.e., LakeSPI score <20%). In contrast half the lakes with pastoral land cover had poor ecological condition or were not vegetated.

Dissolved oxygen-temperature profiles were examined for 63 lakes. Eleven percent of lakes had mean bottom dissolved oxygen (DO) concentrations <3 mg/L. However, under stratified conditions (recorded in 46 lakes) 63% of the lakes had mean bottom water dissolved oxygen concentrations <3 mg/L. Hypoxic bottom water in these lakes likely enhances the release of phosphorus from lake sediments and may accelerate eutrophication.

Trends in TLI were analysed for 18 lakes over ten years since 2000. Three of these lakes (17%) had deteriorated significantly and one lake (6%) had improved significantly (Lake Omapere). Trends were analysed for 68 lakes for five years since 2005. The trend analysis found 19 lakes (28%) had deteriorated and eight lakes (12%) had improved. While there were more lakes that declined than lakes that improved, there was a slight but significant decrease in mean TLI (indicating improved condition) because the lakes that declined were on average oligotrophic while the improving lakes were on average eutrophic.

The status of NZ lake water quality was compared with a subset of lakes from Europe, USA and Canada. The comparison showed that median TN is lower in NZ lakes compared to lakes in Europe and USA and median TP concentration is similar in NZ lakes and European lakes. The median chlorophyll a concentration was slightly higher in European lakes (5.4 mg m-3) than in NZ lakes (4.2 mg m-3).