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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

3 Lake Monitoring in New Zealand

3.1 General

Monitoring of lake water quality is an important aspect of lake management; it alerts us to potential problems and helps us measure the success of any management efforts. Two tools have recently been developed for monitoring New Zealand lakes. These are the Protocol For Monitoring Lake Trophic Levels And Assessing Trends In Trophic State (Burns et al 2000), and development of a method for using submerged aquatic macrophytes to assess lake condition, called LakeSPI (Clayton et al 2002). This snapshot survey identifies what lakes are currently being monitored by regional councils around New Zealand, the type of monitoring and how it is being undertaken. We found that the recently developed protocols for assessing trophic state (TLI) and lake condition (LakeSPI) are being widely adopted around the country.

3.2 Protocols for assessing lake trophic state and ecological condition

3.2.1 Protocols for monitoring trophic levels of New Zealand lakes and reservoirs

The protocol for monitoring trophic levels of New Zealand lakes and reservoirs (Burns et al 2000) provides procedures for monitoring lake water quality, assessing trophic state and determining trends. The protocol summarises water quality information into a Trophic Level Index (TLI). The TLI is an indicator of lake water quality compiled from annual values of total nitrogen, total phosphorus, algal biomass (as measured by concentrations of chlorophyll a) and water clarity (measured as secchi disc depth). A TLI score is calculated for each of these parameters and summarised into a single overall TLI score for the lake. The overall score is categorised into seven trophic states indicating progressively more nutrient enrichment, more algal productivity and reduced water clarity. These are:

• ultra-microtrophic TLI score <1 (pristine)
• microtrophic TLI score = 1-2
• oligotrophic TLI score = 2-3
• mesotrophic TLI score = 3-4 (moderately productive)
• eutrophic TLI score = 4-5
• supertrophic TLI score = 5-6
• hypertrophic TLI score >6 (extremely degraded, algae blooms common).

The monitoring protocols promote reasonably intensive monitoring, initially in order to obtain a baseline of lake water quality and to understand seasonal changes and lake dynamics. After several years, the monitoring intensity is sometimes reduced to focus on tracking water quality changes over the long term.

3.2.2 LakeSPI - a method for monitoring ecological condition in New Zealand lakes

LakeSPI is a management tool that uses Submerged Plant Indicators (SPI) to assess the ecological condition of New Zealand lakes and monitor trends in lake ecological condition (Clayton et al 2002). Key features of aquatic plant structure and composition are used to generate three LakeSPI indices. These are:

1. Native Condition Index - this captures the native character of vegetation in a lake. A higher score means a healthier, more diverse community of native plants growing to greater depths.
2. Invasive Condition Index - this captures the invasive character of vegetation in a lake. A higher score means more impact from exotic plants, which is often undesirable.
3. LakeSPI Index - this is a synthesis of the Native Condition Index and the Invasive Condition Index and provides an overall indication of lake condition. The higher the score, the better the condition of the lake.

Lakes have differing physical characteristics that influence the extent and type of vegetation that can potentially be present. In this report, the LakeSPI indices are expressed as a percentage of a lake's maximum scoring potential and correspond to the depth of each lake. The highest possible LakeSPI Index is 100 percent.

3.3 Types of monitoring

There are generally two objectives for lake water quality monitoring programmes undertaken by regional councils. The first is state of the environment (SoE) monitoring, and the second is bathing beach monitoring to assess the suitability for contact recreation such as swimming.

SoE monitoring aims to assess state and trends, ie, how healthy are the lakes and are they getting better or worse? Assessing water quality (ie, trophic state) requires a different approach to the assessment of ecological conditions which focuses on submerged aquatic macrophytes, fish or koura. To assess water quality, sampling is usually done throughout the year and at locations either representative of the whole lake or an area of the lake most likely to respond to pressures, ie, shore or near inlet. The monitoring programmes are often designed to also increase our knowledge of the functioning and dynamics of a particular lake, hence a wide range of parameters are often sampled. LakeSPI is the most widely-used indicator of biological condition and it is likely to be more widely adopted. Some councils also collect information on the populations of koura, freshwater mussels and fish, but there is currently no commonly reported indicator for these organisms.

Bathing beach monitoring is designed to assess the suitability of a lake for contact recreation such as swimming, boating or fishing. Generally the sites monitored are at popular beaches with samples collected approximately weekly during the summer. Samples are analysed for parameters indicating the suitability for human health, ie, E. coli bacteria and in some cases cyanobacteria and cyanotoxins. This monitoring and the response to an exceedance of guideline values are based on the New Zealand Microbiological Water Quality Guidelines for Marine and Freshwater Recreational Areas (MfE 2003). More information on bathing beach monitoring can be found on the MfE website (www.mfe.govt.nz) and on council websites.

A number of organisations undertake research, investigations and surveys on lake water quality, hydrology and biology which we have not identified in this snapshot survey but the information is extremely valuable. Capturing this information was beyond the scope of this report but could be addressed with an update of the inventory of New Zealand lakes (Livingston et al 1986).

While this snapshot survey has focused on SoE monitoring, it also reports information on bathing beach monitoring in order to assess the extent to which cyanobacteria are being monitored. Cyanobacteria are also monitored in some water supplies by city and district councils to ensure the suitability of the drinking water. In order to gain a more complete picture of cyanobacteria monitoring, a search was made of the Water Information New Zealand (WINZ) database to identify where cyanobacteria are being monitored in natural lakes.

3.4 State of environment (SoE) monitoring

3.4.1 General

New Zealand has 17 regional councils/unitary authorities (including the Chatham Islands Council) with 13 of these councils monitoring lake water quality. Those council which don't monitor lakes either have no natural lakes (ie, Nelson City), have lakes with very few pressures due to their location in the conservation estate (ie, Tasman District and Marlborough District), or they have plans to initiate lake monitoring in the near future (ie, Hawke's Bay Regional Council). [See Appendix 1 for the list of lakes planned to be monitored.]

This survey found a total of 153 lakes were being monitored by 13 councils for trophic status, ecological condition or cyanobacteria through bathing beach monitoring. This number is expanding; prior to 2000 about 87 lakes were being monitored (68 for trophic status). Since then, lake monitoring has been initiated in Northland (30 lakes), Southland (three lakes) and the Chatham Islands (10 lakes) and expanded in other regions. Furthermore, there are firm intentions by Hawke's Bay Regional Council to monitor a further eight lakes starting in 2006 and Otago Regional Council is planning to monitor a further two lakes. [Otago Regional Council is also changing the method of lake monitoring to be consistent with the protocol (Burns et al 2000).] The extension of monitoring to additional lakes is also being considered by Greater Wellington Regional Council and Tasman District Council - contingent on the outcome of monitoring reviews.

A detailed summary of lake monitoring being undertaken by regional councils is provided in Tables 10 and 11 in Appendix 1 along with lake specific information. These tables also identify the key publications used in this survey for each council.

Lakes with trophic level monitoring are mapped in Figures 1 and 2 (in red) along with all lakes greater than one hectare (in blue). Figures 3 and 4 map the same information but only show lakes greater than 50 hectares in size. For display purposes, a buffer has been placed around each lake so they appear larger than they actually are.

3.4.2 Distribution of monitored lakes

The largest lake monitored is Lake Taupo (62,000 hectares) and the smallest, Lake Rotopiko South (1.4 hectares). Although small, this lake is 3.5 metres deep and has a high ecological condition. About half (55 percent) of all the lakes monitored are shallow (less than 10 metres deep) and about half (49 percent) are less than 50 hectares in size.

Out of all lakes over 50 hectares in size, about one-third are being monitored (75 out of 229 lakes). Many of the large lakes not being monitored are in national parks (particularly in Fiordland) or are reservoirs for hydroelectric power generation. The Waikato River is monitored for water quality but the hydro lakes on the Waikato are not specifically monitored for water quality.

3.4.3 Monitoring of different lake types

Lake types differ across the country and this is reflected in the monitoring programmes. Table 1 shows the number of lakes and different types of lakes monitored in each region. A breakdown of monitoring according to lake type is given in Table 2. Council priorities for deciding which lakes to sample are based on the pressures on lakes, their values and uses rather than morphologically defined lake type. However, analysing the types of lakes monitored provides an indication of how representative a monitoring programme is being. For example, in the Waikato region there are no west coast dune lakes and few volcanic lakes monitored for water quality. Similarly, the numbers in Table 2 suggest there is value in extending the use of LakeSPI ecological assessments to glacial lakes, landslide lakes and lagoons.

Table 1: Types of lakes monitored by each regional council

Notes:

* In the Waikato region, no dune lakes were monitored for water quality and Lake Taupo was the only volcanic lake monitored for water quality.

** These figures do not include sampling from the Waikato River downstream of the tailrace of two hydroelectric lakes (part of Environment Waikato's river water quality monitoring network).

# = Macrophyte surveys have been done prior to the development of LakeSPI in four additional lakes.

Table 2: Lake monitoring according to lake type

3.4.4 Water quality (trophic level index)

This survey found that trophic status is being assessed in 119 New Zealand lakes. In most cases, (ie, for 78 lakes and 10 councils) this was assessed using the Trophic Level Index. In some cases, a modified version of the Trophic Level Index is used. This is the case for 36 lakes in the Canterbury high country, the Chatham Islands and Southland's Waituna Lagoon. The reduced number of parameters was primarily because the system which collects the water quality samples makes measurements of secchi depth impractical - in the case of the Chatham Islands samples were collected from the lake edge, and lakes in the Canterbury high country were sampled by helicopter. [Alternative measurements are made such as black disc distance or turbidity.] A further four lakes are monitored for water quality but do not include two key TLI parameters of secchi depth and chlorophyll a. These lakes are in Southland (outlet to Mavora Lake South), Otago (Lake Dunstan and Lake Hawea,) and Gisborne (Ratahi Lagoon).

The last comprehensive national monitoring programme of New Zealand lakes monitored 27 lakes between 1992 and 1996, providing a good baseline of lake water quality information (Burns and Rutherford 1998). Water quality monitoring has continued on all but five of these lakes: Lake Tahroa (Waikato), Lake Tutira (Hawke's Bay), Lake Dudding (Manawatu-Wanganui), Lake Rotoiti (Tasman district) and Lady Lake (West Coast).

Most monitoring programmes were designed to characterise a lake's temporal and spatial variability, establish baseline water quality information, and assess state and trends. However, there were a variety of sample frequencies, parameters and a choice of sample sites. The lakes chosen for monitoring were generally chosen because they were one or more of the following:

• the lakes of highest value or quality (eg, Northland)
• the largest lakes in the region (eg, Auckland, Greater Wellington)
• lakes at high risk of eutrophication (eg, shallow lakes in the Waikato) or
• lakes of high use and public interest (eg, Canterbury high-country lakes).

In many cases, lakes were monitored on a roving rather than continuous basis, with several years of monitoring to collect baseline data followed by a period of no monitoring. Since 2000, new lake monitoring programmes have been initiated by Southland Regional Council, Northland Regional Council, Otago Regional Council, Canterbury Regional Council, Chatham Islands Council [Canterbury Regional Council has recently been contracted to provide regional council services to Chatham Island Council.] and (in 2006) Hawke's Bay Regional Council. In most cases, the new programmes are consistent with the lake monitoring protocols (Burns et al 2000). However, in some cases (eg, Canterbury high country and Chatham Islands) cost and practicalities demand a different approach such as sampling from the lake edge rather than from a central lake location or not doing profiles of dissolved oxygen-temperature.

Several regional councils have extended the parameters monitored beyond the traditional physical and chemical variables. Phytoplankton analysis is being done for lakes in Northland, Auckland, Waikato, Bay of Plenty, Taranaki and Canterbury. Bay of Plenty has started monitoring koura (freshwater crayfish) and kakai (freshwater mussels) to help assess ecological condition in some Rotorua lakes.

3.4.5 Ecological condition (LakeSPI)

This survey found that ecological condition was being assessed in 86 New Zealand lakes using LakeSPI with macrophyte monitoring in a further eight lakes prior to development of LakeSPI. Hawke's Bay is considering using LakeSPI in the future. Four regions (Northland, Auckland, Waikato and Bay of Plenty) are currently using both LakeSPI and the TLI to monitor a total of 57 lakes.

LakeSPI is a relatively new method and there is considerable scope to extend its use. Where it has been used, it has generally been done as a one-off survey. Northland Regional Council intends to repeat LakeSPI surveys approximately every five years.

3.5 Cyanobacteria monitoring for public health

Cyanobacteria can potentially produce toxins. When blooms occur and large numbers of cyanobacteria are present, this can be an issue for human and animal health as well as affecting the wider ecosystem (eg, by inhibiting the feeding of zooplankton). Five councils are monitoring phytoplankton and cyanobacteria as part of their state of environment monitoring programmes but most cyanobacteria monitoring is undertaken as part of bathing beach monitoring.

This survey identifies 41 lakes where cyanobacteria cell counts (or scan) and identification is being undertaken. For 23 of these lakes, this monitoring is part of a bathing beach monitoring programme. Cyanobacteria monitoring from bathing beach monitoring programmes provides water quality information for an additional four lakes where water quality monitoring does not otherwise occur. Scanning for cyanobacteria has not detected any issues in Lake Rotomanu in Taranaki; however, high cyanobacteria cell counts have occurred in Lake Dudding, Lake Pauri and Lake Wiritoa in the Manawatu-Wanganui region.

Where lakes or reservoirs are used to supply drinking water the suppliers often monitor for cyanobacteria to ensure the suitability of the supply. A review of the WINZ database and a survey of district health boards identified four natural lakes where cyanobacteria are monitored as part of drinking-water supplies. These are: Lake Taupo, Lake Karapiro (Waikato), Lake Te Anau (Southland), and Lake Mangamahoe (Taranaki).