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Snapshot of Lake Water Quality in New Zealand

Acknowledgements

Executive Summary

1 Introduction

2 Lakes in New Zealand

3 Lake Monitoring in New Zealand

4 State and Trends of New Zealand Lakes

5 Discussion

6 Recommended Directions of Further Work

Appendix 1: Summary of Council Monitoring Programmes

Appendix 2: Summary of Monitoring for Individual Lakes

References

4 State and Trends of New Zealand Lakes

4.1 Water quality (TLI)

Trophic state was assessed using the Trophic Level Index (TLI) or a modified version of the TLI for 134 lakes. Thirteen of these lakes are not currently monitored and the trophic state is based on either the results of a survey (the case for nine lakes in Northland) or based on the results of the New Zealand Lake Water Quality Monitoring Programme (NZLWQMP) conducted between 1992 and 1996 (Burns and Rutherford 1998). [This is the case for five lakes: Lake Taharoa (Waikato), Lake Dudding (Manawatu/Wanganui), Lake Tutira (Hawke's Bay), Lake Rotoiti (Tasman District) and Lake and Lady Lake (West Coast).]

A summary of trophic state for New Zealand lakes is shown in Tables 3 and 4; lake specific information is in Appendix 1. The trophic level of New Zealand lakes is mapped in Figures 6 and 7. For display purposes, a buffer has been placed around each lake so that they appear larger then they are in reality.

There is a relatively even spread of lakes in each trophic type with about half of the 134 lakes monitored being eutrophic or worse. The monitored lakes with the best water quality (ie, microtrophic) are all in the South Island, reflecting their depth and fewer land-use pressures. In the South Island, the two most pristine lakes are Lake Tekapo and Lake Coleridge in the Canterbury high country. In the North Island, the lakes with the best water quality (ie, oligotrophic or better) are Lake Taupo, four Rotorua lakes (Lake Okatina, Lake Rotoma, Lake Tarawera and Lake Tikitapu) and two dune lakes in Northland (Lake Taharoa and Lake Waikere).

The most eutrophic lakes (ie, hypertrophic) are eight shallow lakes in the Waikato region (eg, Lake Hakanoa and Mangakawhere), several dune lakes in Northland (Lake Omapere, Kapoai, Rotokawau and Waiporohita) and Auckland (Lake Specticle), and two coastal lagoons in Canterbury (Lake Ellesmere/Te Waihora and Forsyth/Te Wairewa).

The trophic status of a lake is strongly related to depth. This is illustrated in Figure 5 which graphs the percentage of lakes in each trophic category that are either deep (>10 metres deep) or shallow (<10 metres deep). This shows that the more eutrophic lakes (ie, with a higher TLI) are more commonly shallow lakes and that the more pristine lakes are more commonly deep lakes. The association with depth corresponds with shallow lakes being both naturally more eutrophic and having less capacity to absorb incoming nutrient loads due to their smaller volume.

Table 3: Trophic state assessed using the TLI for each region

View trophic state assessed using the TLI for each region (large table)

Table 4: Trophic state assessed using the TLI for each lake type

Note:

'*' indicates that a modified version of the TLI or a limited data set was used for one or more of the lakes.

nd = lake type is not determined.

4.2 Ecological condition (LakeSPI)

Ecological condition was assessed using LakeSPI for 76 lakes. These lakes are all within the Northland, Auckland, Waikato and the Bay of Plenty regions. There are 14 lakes in the Northland region where LakeSPI is assessed but was not available for this survey. For the purpose of this report, ecological condition is categorised into four levels corresponding to the following LakeSPI scores. The four levels are:

• 'Excellent' >85%
• 'High' 50-85%
• 'Moderate' 20-50%
• 'Poor' <20%.

A summary of ecological condition based on LakeSPI is shown in Tables 5 and 6 and mapped in Figure 8. For display purposes, a buffer has been placed around the lakes so they appear larger than in reality. Few lakes were in the excellent category, but lakes were spread evenly over the other categories. The six lakes with excellent ecological condition are the Northland dune lakes - Kuhuparere, Pretty, Waiporohita, and Te Paki - and the Waikato peat lakes - Rotopiko east and Rotopiko north. The trophic state of these lakes ranges from mesotrophic (ie, Lake Pretty) to hypertrophic (ie, Lake Waipophita).

There is no clear relationship between ecological condition and lake type. Neither does there appear to be any clear relationship between ecological condition and lake depth. [The number of deep and shallow lakes in each category are respectively: excellent (0 and 6), high (12 and 14), moderate (12 and 11), poor (2 and 19).] This probably reflects the fact that many of the factors affecting ecological condition, such as exotic (invasive) plants and exotic fish, can impact on any lake.

From the information available, no strong relationship can be observed between ecological condition and trophic state. Lakes with 'poor' ecological condition correspond to trophic states of eutrophic or worse, however lakes with 'excellent' and 'high' ecological condition have a wide range of trophic states. This is not surprising as the two indices (LakeSPI and the TLI) measure different aspects of lake health. A correlation might be expected between water clarity (secchi depth) and maximum depth of plants, but past studies have found no obvious link between eutrophication and the potential for invasion by introduced plants (Hughes 1976) - which forms a major component of the overall LakeSPI score.

Table 5: Ecological condition assessed using LakeSPI for each region

Table 6: Ecological condition assessed using LakeSPI for each lake type

A bloom of cyanobacteria ( Microcyctis spp.) in Lake Horowhenua, Manawatu-Wanganui

Source: Horizons MW

4.3 Trends

4.3.1 Changes in trophic state of New Zealand lakes

Information on trends in trophic state is available for 70 lakes. This report has not analysed data but has instead relied on results from published reports. Where water quality trends have not been assessed by councils, it was usually because the length of record was too short.

Changes in trophic state are identified in this report based on the results of trend analysis from published reports (primarily using methods in Burns et al (2000)) or, where these were not available, from changes in the reported trophic state since the New Zealand Lake Monitoring Programme (NZLMP) (Burns and Rutherford 1998). The trends identified by this report are for the most recent period of reporting, typically 1995-2002, but the precise period was different for each lake and region. Consequently, caution is needed when comparing the results across different lakes.

Furthermore, these trends only show past changes and can not automatically be used to extrapolate future changes. It is worth noting that the water quality of some lakes can fluctuate over time, for example, in many shallow lakes water quality changes in response to changes in the macrophytes community.

A summary of trends in trophic state for New Zealand lakes is shown in Tables 7 and 8 and in Figures 9 and 10. For display purposes, the lakes on the map have been buffered so they appear larger than in reality. On a national scale, there are more lakes with improving quality than declining quality. Most of the lakes with improving water quality are in the Canterbury high country [Note that information on water quality trends for Canterbury lakes is based on changes in reported trophic state over the lake over 10 years rather than on a comprehensive trend analysis of the data.] which, as previously reported, already has near pristine water quality.

The lakes with declining water quality are: Lake Ngatu, Lake Omapere and Lake Rotokawau West in Northland; Lake Spectacle and Lake Ototoa in Auckland; Lake Rotomanuka South, Lake Waikare and Lake Whangape in the Waikato; Lake Okataina, Lake Rotoiti and Lake Tikitapu (in the Bay of Plenty); and Lake Brunner on the West Coast. Recent monitoring data suggest that water quality is also deteriorating in Lake Taupo (in the central North Island).

Table 7: Changes in trophic state for each region

View changes in trophic state for each region (large table)

Table 8: Changes in trophic state for each lake type

4.3.2 Changes in lake ecological condition

Information on changes in ecological condition (LakeSPI) is available for 44 lakes. A summary of these changes is shown in Table 9 and in Figure 11. The figures are based on LakeSPI surveys done by NIWA for Environment Waikato and for Environment Bay of Plenty (Edwards et al 2005; Scholes and Bloxham 2005). We did not interpret changes from macrophyte surveys in Auckland lakes [Note that Auckland Regional Council has LakeSPI scores for Lake Wainamu (calculated on four occasions), however, this data has not been published.] (Gibbs et al 1999) and changes in ecological condition were not determined with LakeSPI assessments for Northland lakes. For this report, a trend was reported if there was a change of more than five percent in the LakeSPI score compared to previous surveys. The time period between surveys differs for each lake and typically ranges from five to 20 years.

In the Waikato and Bay of Plenty, there are greater declines in ecological condition than in water quality. Almost half the lakes (45%) show a decline in their ecological condition compared to previous surveys. In contrast, about a third of the lakes in the Waikato and Bay of Plenty show a decline in water quality. This worsening of ecological condition may reflect the expansion of exotic (invasive) plants and exotic fish into many lakes. For example, an invasion of hornwort was responsible for the decline in LakeSPI in Lake Tarawera.

No significant correlation was found between trends in TLI and trends in the LakeSPI. [The cumulative binomial distribution probability of 0.9 was calculated for trends from TLI and from LakeSPI. For the 21 lakes with trend information for both TLI and LakeSPI, 13 lakes showed the same trend (improving, declining or no change) for both index.] This is not surprising because the trends for each index are based on different time periods (LakeSPI generally having a longer period), the sample size was small and past studies have found no obvious link between eutrophication and the potential for invasion by introduced plants (Hughes 1976). Nevertheless, a collapse of macrophyte populations in shallow lakes is typically followed by deterioration in water quality which improves again after macrophytes re-establish. In Lake Whangape, macrophytes collapsed in 1987 and there was a corresponding decline in water quality. Water quality improved as macrophytes re-established again in the mid-1990s. The pattern has been repeated more recently with the collapse of macrophytes in Lake Rotokauri and Lake Rotomanuka (Waikato) in 1996/97 and Lake Omapere (Northland) in 2000. Some lakes are also returning to more stable macrophyte dominated state, for example, native macrophytes are re-establishing in Lake Rotorua (Hamilton) and the water quality is improving.

Table 9: Changes in ecological condition as assessed by LakeSPI by lake types

4.4 Pressures on lake water quality

The monitoring programmes and reports from regional councils have identified a number of key pressures on lake water quality. These are:

• invasive plants
• exotic fish
• increased sediment and nutrient loadings
• drainage and the lowering of water levels.

Controlling the loads of sediment and nutrients entering lakes is a universal concern for lake managers in New Zealand and a key to preventing and reducing algae blooms. External nutrient loads of both nitrogen and phosphorus are a concern for eutrophication of lakes, however, lakes differ as to which particular nutrient limits phytoplankton growth. Nitrogen is typically limiting in lakes on the central volcanic plateau and Rotorua lakes because the pumice soils are naturally high in phosphorus. Phosphorus, or both phosphorus and nitrogen, tend to be the limiting nutrient in many other parts of the country (eg, Canterbury high-country lakes).

This survey found only two lakes that receive a direct effluent discharge, Ratahi Lagoon on the East Coast and Lake Waikare in the Waikato. Effluent discharges to Lake Horowhenua and Lake Rotorua stopped many years ago but their impacts are still being felt as nutrients are internally recycled within the lakes.

A more extensive problem being tackled by councils is the control of contaminants from diffuse pollution entering lakes directly or via rivers. Diffuse pollution can be difficult to define but for the purposes of this report it refers to pollution arising from land-use activities which are dispersed across a catchment and do not arise from municipal sewage or process effluents. Some examples of diffuse pollution sources include application of fertiliser and pesticides to farmland and forestry, road surface runoff, soil erosion, septic tanks, organic waste applied to farmland, effluent from farm stock, and surface water outfalls from field drains.

Controlling diffuse pollution is a major focus of the Rotorua Lakes Protection and Restoration Programme. There is some concern about the possible impact of on-site wastewater treatment systems from communities around some Canterbury high-country lakes (Lake Alexandrina and Lake Clearwater) and around some Central Otago lakes (eg, Lake Hayes). Protecting the lake margins from direct run-off and from stock access is a key first step in controlling diffuse pollution. These measures have shown positive results in Lake Emma (Canterbury).

Lowering of water levels and drainage of surrounding land is a major issue for many lakes. Lowered water levels due to drainage are a particular concern for peat lakes and in many lagoons water levels are managed by opening to the sea. This year, there was also concern about the dry summer causing many lakes in the Canterbury high country to be about half their normal water levels with potential impacts on lake ecology.