5 Discussion
5.1 Lake monitoring
This survey identified that 13 of the 17 regional authorities in New Zealand currently monitor 153 lakes for various indices and analyses including trophic status, ecological condition and/or cyanobacteria through bathing beach monitoring programmes. State of Environment monitoring programmes assessed trophic status in 119 lakes and ecological condition in 86 lakes. The number of lakes monitored by regional councils is increasing. Prior to 2000, about 87 lakes were being monitored (68 for trophic status). Since then, lake water quality monitoring has been initiated in Northland, Southland and the Chatham Islands and extended in other regions.
The New Zealand protocols for monitoring lakes and reservoirs (Burns et al 2000) is widely used throughout New Zealand as a tool for monitoring and reporting lake water quality. However, resourcing constrains the way lakes are monitored and which lakes are monitored by councils. For example, Canterbury Regional Council has chosen to monitor water quality at a large number of high-country lakes by taking surface grab samples rather than applying the more intensive method in the protocol to fewer lakes. Similarly, up until recently many of the lakes monitored by Otago Regional Council involved samples from the lake outlet as part of the river monitoring programme. While sampling methods differ, in many cases a modified version of the TLI can still be calculated for the sake of national reporting, although some caution is needed in interpreting the results.
Most of New Zealand's large lakes are monitored but it is clear that a considerable number of smaller lakes are not being monitored. While it is not feasible to monitor all these small lakes, it is important that a representative number are monitored on an ongoing basis. There are a range of reasons why some lakes may not be monitored; these include:
- a lack of land-use/pollution pressures not warranting an investment in monitoring programmes
- the fact that some lakes are already in poor condition and no further monitoring investment is necessary to ascertain their condition
- some lakes are considered too small or to have too few values or uses to be monitored
- regional councils prioritising their resources for monitoring.
Some further work would be required to address key issues identified by this report, in particular how do lake monitoring networks compare with the values and management objectives set for lakes in the planning process? If lakes are not monitored is there monitoring of the river entering or leaving them? Does the national monitoring network cover a full range of lake types and pressures? This last question may be best addressed using the multivariate classification system being developed by NIWA and the Department of Conservation.
LakeSPI (Clayton et al 2002) is a recent tool offering considerable value to monitor and report on ecological condition; it is also gaining wide acceptance in New Zealand. Its use by regional councils is currently limited to Northland, Waikato and the Bay of Plenty However, some councils have plans to initiate LakeSPI monitoring in the future. As a result, there is likely to be an increase in the types of lakes being monitored for ecological condition - perhaps extending to glacial lakes or lagoons.
5.2 Reporting of results
There are few targets or reference conditions for reporting lake water quality on a national scale. This is more of an issue for reporting water quality than ecological condition. The LakeSPI scores can be adjusted for lake depth which overcomes a major source of variability between different lakes and allows more accurate comparison of lakes. The TLI, on the other hand, provides a good measure of trophic state but does not provide management targets or adjustments for naturally eutrophic lakes. It is an excellent tool for managing lakes and reporting on individual lakes but there are difficulties with national reporting because some lakes are naturally eutrophic even without human pressures. This was partially illustrated earlier in the report by the correlation between trophic status and lake depth. Similarly, trophic state and ecological condition do not necessarily correlate; Lake Rotopiko North is eutrophic but has one of the best LakeSPI scores in the Waikato. Eutrophication will be a key issue to address for a number of important lakes and it would be helpful to have realistic targets to help water managers measure progress.
This issue of reporting against appropriate reference conditions may be partially addressed by current and future work on lake classification currently being undertaken by NIWA for the Department of Conservation. It potentially allows the classification of lakes according to physical attributes (eg, area, depth) and catchment attributes (eg, proportion of catchment in beach forest, glaciers, peat, pasture, geology with high phosphorus, etc). These, and other attributes, affect the clarity and nutrient levels that a lake can naturally and realistically attain.
An alternative approach to reporting on trophic state would be to report on the suitability of lake water quality for different uses. This is done when reporting the results of bathing beach monitoring and there are clear guidelines for acceptable levels of faecal indicator bacteria and cyanobacteria for contact recreation. Using this approach, the targets would relate to achievement of guidelines for intended uses of the lake and may be identified through the planning process.
Regional authorities have a fundamental role in identifying values and setting goals for water management. Targets for lake management are generally set on a regional or catchment scale through regional plans or strategies for specific lakes (eg, Rotorua lakes protection and restoration programme, Protecting Lake Taupo Strategy), and it may be appropriate to use these targets for reporting on a national scale. This would require reviewing the approaches to managing lake water quality and condition, collating the regional, catchment and lake-specific targets set for lakes and, if possible, using these targets to report current condition.
When reporting on the State of the Environment (SoE) it would also be valuable to gain a better understanding of the response to monitoring results. To what extent are the results of SoE monitoring feeding back into the planning process (particularly regarding management of lakes with declining quality)? Is SoE monitoring providing sufficient information for the planning process, and are planning mechanisms responding to the results of SoE monitoring? These questions would need to be addressed in a separate review.
Finally, it became clear while undertaking this work that there is a considerable amount of new information available on New Zealand lakes, but it is distributed amongst many organisations around the country and not all of it is published. This information would be much more accessible if summarised in a single location such as an updated version of the Inventory of New Zealand Lakes produced in 1986 by Livingston et al (1986).
