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Lake Water Quality in New Zealand

Acknowledgements

Executive summary

1 Introduction

2 Methods

3 Results and discussion

4 Conclusions

References

Appendix 1: Glossary of common limnological terminology used in this report

Appendix 2: Morphological and Catchment Characteristics of the 121 Lakes

Appendix 3: Maps of Water Quality Parameters

Appendix 4: Map of Water Quality Trends (1990-2006) based on Percent Annual Change (PAC) using the Burns et al (2000) Methodology

1 Introduction

New Zealand has a wide variety of lakes, which provide a range of biodiversity, water supply, recreational, aesthetic and food resource values. They are an important part of our clean, green image and economy, contributing to hydropower, tourism and agricultural industries. Although New Zealand lakes vary considerably in their natural water chemistry, colour, clarity and productivity (and there is historical evidence that some were naturally eutrophic¹), land development during the 20th century is known to have significantly altered their character.

Concerns about the condition and water quality of New Zealand lakes are not new. Agriculture, industry point sources, urbanisation and forestry have all been linked to environmental degradation in lakes. Extensive research on lake functioning from the 1960s to 1980s provided a general understanding of the nature of nutrient limitation in different lake types, how productivity changes with change in nutrient input, and how degradation is related to catchment land use. Much of this information was summarised in the 1987 Lake Managers’ Handbook (Vant, 1987) and other contemporary reviews (Viner, 1987).

More recently, tools such as the Trophic Level Index (TLI) and Lake Submerged Plant Indicators (LakeSPI) have been developed to characterise the degree of nutrient enrichment of a water body (eutrophication) more accurately, and to provide statistically valid methods for identifying degradation (Burns et al, 2000; Clayton and Edwards, 2006). Reviews of the status of New Zealand lakes were carried out in 1986 (Livingston et al, 1986a; 1986b) and 1998 (Burns and Rutherford, 1998), and it is now timely to review the available data. Improved mapping, GIS (geographic information system) and environmental classification tools have been developed in the last 10 years that allow a more comprehensive assessment of lakes and can improve the accuracy of interpretations of relationships between land use, lake characteristics and water quality.

The aim of this report is to provide an overview of the current status and recent trends in lake water quality in New Zealand. We have used water quality data from regional council state of the environment monitoring programmes, from recent lake research programmes by NIWA and universities, and submerged aquatic plant data collected by councils and NIWA for LakeSPI analysis. The principal aim of the analysis is to establish the water quality state of lakes in relation to different land-cover classes and human impacts. This has been interpreted in relation to those differences in lake morphology and mixing patterns that control responses and susceptibility to nutrient enrichment. It incorporates a recently developed lake environment classification that categorises lakes based on climatic and physical characteristics that strongly affect their hydrology, chemistry and biological conditions.

This report is one of two linked projects commissioned by the Ministry for the Environment to provide a national snapshot of lake water quality. Ministry for the Environment (2006) has already reported on and analysed the monitoring programmes carried out by councils, and identified patterns in trophic level and lake condition from the council data. In the present report we assume knowledge of the Ministry for the Environment (2006) report and do not repeat its contents. The important conclusions from the previous Ministry for the Environment report and their implications for our analysis are as follows.

• Most of the lake water quality monitoring is occurring in relatively large, deep lakes, due to public interest in these lakes. As a result, lake monitoring is not representative of the national lake resource, in which there are many more small, shallow lakes than large, deep lakes. However, enough small lakes are being monitored to understand their condition, and the lakes monitored span a range of sizes that provides a valid comparison of how size affects condition.

• The lakes monitored span almost the full range of trophic states from microtrophic to hypertrophic (see Table 4 in section 2.2.2 for definitions of trophic level classes). North Island lakes show higher trophic levels than South Island lakes, and a larger deterioration over time (as detected by the TLI) than South Island lakes. Trophic level appears to be higher (ie, water is more nutrient enriched) in developed catchments with human pressures than in native catchments. There are some recent apparent improvements in water quality in some South Island lakes, which appear to be primarily due to short-term climatic fluctuations. In general, trend analysis should be based on a minimum of three years’ data to avoid incorrectly attributing short-term variation to human pressures. Condition recorded by LakeSPI suggests an overall deterioration in the North Island subset for which LakeSPI data are available.

• Shallow lakes (≤ 10 m) have much poorer water quality (higher trophic levels) than deep lakes (> 10 m). Shallow lakes have lower volume than deeper lakes, and will respond more rapidly to catchment land use because there is less dilution, and less contact between water and sediments. Although shallow lakes tend to be naturally more productive than deep lakes, New Zealand has a large number of naturally oligotrophic shallow lakes (especially dune lakes), and not all of the shallow eutrophic, supertrophic and hypertrophic lakes in the data set would have been enriched in their natural condition. In general, the ecological functioning of shallow lakes has received much less research effort than that of deep lakes in New Zealand, and they are not so well understood. Differences in trophic state between different types of lakes (dune lakes, volcanic lakes, glacial lakes, etc.) are likely to be related more directly to their morphometry and catchment land use than their geological origin per se. Dune lakes were strongly represented in the more eutrophic categories because most dune lakes are shallow and have developed catchments, whereas most glacial lakes were low in trophic level because most glacial lakes are deep and have less modified catchments. Otherwise, there was little evidence of systematic differences in trophic levels between different lake types.

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