LAWA Lake Health - National Picture 2022

 

Published: 25 September 2022

LAWA shows lake health information for nearly 200 lakes throughout New Zealand. Lakes are monitored for a range of water quality (chemical-physical and bacterial) and ecological indicators by regional and unitary councils.  State and trend results for these indicators have been updated for individual lakes using data up to the end of June 2021.  This year, data has been analysed by hydrological year (1 July - 30 June), which provides an opportunity to switch to a more meaningful time period for understanding lake dynamics, as summer is the main growth period for algae and as many lakes stratify over the summer months. 

This national picture summary focuses on the current and developing state of monitored lakes.  Here we report on five indicators: 

An indicator of overall lake condition: 

• Trophic Level Index (TLI) which combines several indicators to determine overall lake condition. We present how TLI has changed over time and compare upland with lowland lakes 

Indicators of water quality contributing to lake health: 

• Ammonia toxicity – A nutrient that can be toxic to in-stream life 

• Total phosphorus and total nitrogen – Nutrients that can lead to elevated plant and algae growth 

An indicator of aquatic life in lakes: 

• Chlorophyll a – A measure of phytoplankton (algae) growth that impacts ecological communities and water clarity 

While this summary provides some information on how our lakes are tracking, it is important to note it only relates to sites where monitoring has been conducted and where there are enough data to determine state. Monitored sites with the best data records in New Zealand are often located in areas that are more impacted by human activities, have significant social and cultural value, and/or where changes in perceived water quality have resulted in the implementation of monitoring programs. These sites are not necessarily representative of all New Zealand lakes. 

 

Lake condition current state

The Trophic Level Index (TLI) combines several measures of lake water quality, to provide an integrated summary of lake trophic condition. It is calculated using the concentrations of key nutrients (total nitrogen and total phosphorus), an indicator of phytoplankton / algae biomass (chlorophyll a - the photosynthetic pigment present in plants and algae), as well as a measure of water clarity. These four measures are combined, using an equation that yields a score between 0 and 9 (which is then converted to a grade ranging from “very good” to “very poor”), with lower scores indicating better water quality and higher values indicating progressively poorer water quality. Increasing TLI scores indicate that a lake is becoming nutrient enriched with an increasing likelihood of algae bloom events, which are associated with reduced water clarity. The TLI is calculated once every year and provides regional councils, unitary authorities and lake managers with an integrated measure of water quality that can be tracked over time.

TLI summaries presented for the current state of New Zealand lakes are based on data collected between 1 July 2020 and 30 June 2021.  Overall, the condition of 14% of monitored lakes can be categorised as either “very good” or “good” (22 lakes), whereas 64% of monitored lakes can be categorised as either “poor” or “very poor” (98 lakes).

 

LAWA Lake Trophic Level Index (2021)

Figure 1. TLI (Trophic Level Index) grades for the 152 monitored lakes throughout New Zealand. TLI grades were calculated from data collected in the latest available hydrological year (1 July 2020 – 30 June 2021). The location of these monitored lakes is shown on the map.

 

A lake’s condition can be affected by its location in the landscape. In New Zealand, lowland lakes are often shallow (< 10 m deep) and located in catchments with higher proportions of agricultural, urban or other development. Upland lakes are often deeper and are more likely to have fewer pressures on their water quality.

Figure 2 shows that 82% of monitored lowland lakes are in either “poor” or “very poor” condition (81 lakes), compared to 32% of the monitored upland lakes (17 lakes). By contrast, only 5% of monitored lowland lakes (5 lakes) are in “very good” or “good” condition, compared to 32% of the monitored upland lakes (17 lakes).

 

LAWA Lake Trophic Level Index (2021) by Altitude

Figure 2.  Comparison of TLI (Trophic Level Index) grades by altitude. TLI grades were calculated from data collected in the latest available hydrological year (1 July 2020 – 30 June 2021). The number of monitored lakes for each altitude category is shown at the bottom of the bars. The location of these monitored lakes is shown on the map.

 

Lake condition over time

The above plots illustrate current lake condition, but it is also interesting to know how this has changed over time. To examine this, TLI scores were calculated for each year of the last decade. The following figure shows the results for those lakes that have this long and comprehensive sampling history (92 monitored lakes).

At the national level, TLI shows fairly constant grades over the last 10 years, suggesting no overall improvement or degradation of lake condition. The number of lakes within each TLI grade does vary from year to year as lakes can undergo natural fluctuations in their condition. For each lake you can find information on fluctuations over the whole sampling record on the individual lake pages on LAWA.

 

LAWA Lake Trophic Level Index Change Over Time (2012 - 2021)

Figure 3.  Changes in TLI (Trophic Level Index) grades from 2012 - 2021 at 92 lakes where there were enough data to determine the TLI score each year. The location of these monitored lakes is shown on the map.  

 

Current state of lake health indicators

As lake monitoring efforts across New Zealand increase, there are more sites nationwide with enough data to evaluate state against attribute bands described in the National Policy Statement for Freshwater Management 2020 (NPS-FM 2020). Attribute bands range from A (good) to D (poor), and the 'current state' for 2021 at each site is based on monitoring data over the previous five hydrological years (from 1 July 2016 to 30 June 2021).

This is the first year LAWA has presented a lake national summary for the following indicators: phytoplankton (chlorophyll a), total phosphorus, total nitrogen and ammonia (toxicity). Over time as lake monitoring programmes continue to grow, we will extend our reporting (more indicators from more sites) in this national picture summary.

Those lakes with state indicators in band A are expected to have healthy and resilient ecological communities, while those in band B are slightly impacted by algal and plant growth arising from elevated nutrient concentrations. Band C suggests lake ecological communities are moderately impacted by algal and plant growth arising from elevated nutrient concentrations, with subsequent reduced water clarity, and band D indicates lakes in a degraded state due to impacts of elevated nutrients, excessive algal growth and loss of oxygen from water at the bottom of lakes.

The current state of our lakes varies by indicators for water quality and aquatic life (Figure 4).

Chlorophyll a is the green pigment in plants and algae that is used for photosynthesis. Measuring how much of this pigment is in the water provides a good indicator of the total amount (or biomass) of algae (phytoplankton) in a lake. Over 60% of the monitored lakes assessed (59 lakes) were classified in the C and D bands, indicating that their ecological communities are impacted by algal and plant growth arising from elevated nutrient concentrations, with subsequent reduced water clarity. Lakes in the D band are likely to have undergone or are at high risk of a regime shift to a persistent, degraded state.

Both nitrogen and phosphorus are important nutrients contributing to the growth of algae and aquatic plants in freshwater systems. When nitrogen and phosphorus concentrations are too high (through a process called eutrophication) there is an increased risk of nuisance algal blooms occurring and lakes shifting to a permanently degraded, turbid state without native aquatic plant cover. Of monitored lakes whose state could be assessed for total phosphorus and total nitrogen, around 60% (55 and 59 lakes respectively) were classified in the C and D bands, suggesting these lakes are impacted by nutrient enrichment, with lakes in the D band at risk, or already in a degraded state.

Ammonia is one of several forms of nitrogen that exist in aquatic environments. Ammonia is also a plant nutrient and can have enriching effects in the ecosystem, but at high concentrations ammonia is toxic to aquatic fauna. The NPS-FM 2020 ammonia bands only assess direct toxicity effects. More than 80% of monitored lakes (76 lakes) had levels of ammonia where toxicity effects are low (A and B bands). Toxic effects of ammonia on aquatic ecosystems can be an issue for sensitive species at 18% of the monitored lakes in C band (17 lakes).

LAWA National Lake Health Summary of State (2021)

Figure 4.  Percentages of monitored sites by attribute bands for the four indicators discussed in this national picture summary. Bands were calculated over the five-year hydrological period (1 July 2016 – 30 June 2021). The number of sites with suitable data to determine an attribute band for each indicator is shown below each bar.

 

Lake health monitoring and reporting

This national summary uses information on lakes that is freely available on the LAWA website to present a general picture.  Looking at individual site data provides further context alongside information on the lake type, size, depth, mixing pattern, and ecological condition. For the first time this year, ecological condition on individual lake pages have been evaluated by attribute bands for two submerged plant indicators (natives and invasive species). As lakes are surveyed for submerged plants more frequently, a national picture of lake ecological condition may be feasible in the future.

Information specific to monitored sites can be accessed either by clicking on a site dot on the map to the left of the LAWA main screen (desktop and tablet users), or by navigating menus to the region, then lake, and then site of interest.  The organisation responsible for monitoring an individual site can provide further contextual information on the results shown here.