LAWA Lake Health - National Picture 2024

 

Published: 22 September 2024

LAWA presents lake health information for nearly 160 lakes throughout New Zealand.  Some lakes are monitored at more than one site, and data presented comes from around 200 sites.  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 2023.  Data are analysed by hydrological year (1 July - 30 June) rather than calendar year to show 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 state of monitored lakes, and how they have changed over time.  Here we report on five indicators: 

An indicator of overall lake condition: 

• Trophic Level Index (TLI) - a measure which combines chlorophyll a, total phosphorus, total nitrogen and water clarity (Secchi disc depth) to determine overall lake condition 

An indicator of aquatic life in lakes: 

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

Indicators of water quality contributing to lake health: 

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

• Ammonia toxicity – a nutrient that can be toxic to aquatic life  

 

LAWA reports on overall lake condition by calculating the Trophic Level Index (TLI) at lake sites.  TLI results for each year are determined from annual data over hydrological years (e.g. 1 July 2022 to 30 June 2023).  The measures are combined and then converted to a score, with lower scores indicating better water quality and higher scores indicating progressively poorer water quality.  This translates into a lake’s condition assessment ranging from 'very good' to 'very poor'. 

LAWA also evaluates conditions (state) at representative lake sites against attribute bands described in the National Policy Statement for Freshwater Management 2020 (NPS-FM 2020), from A (good) to D (poor).  The 'current state' for 2023 at each site is based on data over the previous five hydrological years (from 1 July 2018 to 30 June 2023).  This evaluation requires a five-year history of monthly or quarterly measurements, before a state can be assigned.  See factsheets on TLI and state attribute bands for more information. 

While this summary provides information on how our lakes are tracking, it is important to note it only relates to lakes where monitoring has been conducted and where there are enough data to calculate TLI or determine state.  Monitored lakes 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 are not necessarily representative of all New Zealand lakes. 

 

Lake health current state

The Trophic Level Index (TLI) provides an integrated measure of lake condition.  TLI uses water quality measures for chlorophyll a (indicating phytoplankton (algae) growth), for total nitrogen and total phosphorus (key nutrients), as well as a measure of water clarity (Secchi disc depth).  Here we present TLI results and compare upland lakes with lowland lakes, and lakes shallower than 10 m and deeper than 10 m.  

Overall, the condition of 52 monitored lakes (33%) can be categorised as either 'very good', 'good' or 'fair', whereas 108 monitored lakes (67%) can be categorised as either 'very poor' or 'poor' (Figure 1). 

 

LAWA National Lake Trophic Level Index (2023)

Figure 1. Trophic Level Index (TLI) scores for the 160 monitored lakes with suitable data to determine this index in the latest available hydrological year (1 July 2022 – 30 June 2023).  The location of these monitored lakes and their TLI score is shown on the map. 

 

A lake’s health can be affected by its location in the landscape. In New Zealand, lowland lakes are often shallower 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 (e.g. urbanisation, farming) on their water quality.  Currently around half of the lakes monitored are lowland shallow lakes. 

In general, our upland lakes are in better condition than our lowland lakes.  Similarly, our deep lakes are generally in better condition than our shallow lakes (Figure 2).  This means that the biggest challenges with lake health are in our lowland shallow lakes where 62 monitored lakes (94%) were in 'very poor' or 'poor' health.  In contrast, 35 monitored upland deep lakes (68%) are in 'very good', 'good' or 'fair' health. 

 

LAWA Lake Trophic Level Index (2023) by Altitude and Depth

Upland Lakes (Altitude >170 m)

Lowland Lakes (Altitude <170 m)

 

Figure 2. Comparison of Trophic Level Index (TLI) scores by lake altitude and depth.  TLI scores were calculated from data collected in the latest available hydrological year (1 July 2022 – 30 June 2023).  The number of monitored lakes is shown by altitude (upland > 170 m altitude or lowland < 170 m altitude) and depth category (lakes shallower or deeper than 10 metres).  The location of these monitored lakes and their TLI score is shown on the maps.

 

The current state for 2023 is further assessed by four indicators for aquatic life and water quality: chlorophyll a, total phosphorus, total nitrogen, and ammonia toxicity (Figure 3).

 

LAWA National Lake Health Summary of State (2023)

* 90 monitored lakes with ammonia (toxicity) results available 

Figure 3.  Attribute bands for the four indicators discussed in this national picture summary.  Bands were calculated over a five-year hydrological period (1 July 2018 – 30 June 2023).  The number of sites with suitable data to determine an attribute band is shown for each indicator, and their location is shown on the map.

 

Those lakes with state attributes 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.  Attribute band C suggests lake ecological communities are moderately impacted by algal and plant growth arising from elevated nutrient concentrations, with subsequent reduced water clarity. Band D indicates lakes in a degraded state due to impacts of elevated nutrients, excessive algal growth and potential loss of oxygen from water at the bottom of lakes.

Chlorophyll a is the green pigment in algae and plants 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 phytoplankton (algae) in a lake.  Over 65% of the monitored lakes were classified in C and D bands (69 lakes), indicating that their ecological communities are impacted by algal and plant growth arising from elevated nutrient concentrations, with subsequent reduced water clarity.  Lakes in 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 aquatic plant cover.  Of monitored lakes whose state could be assessed for total phosphorus and total nitrogen, around 65% (68 and 69 lakes, respectively) were classified in the C and D bands, suggesting these lakes are impacted by nutrient enrichment, with lakes in the D band already in a degraded state.

Ammonia is one of several forms of nitrogen that exist in aquatic environments.  It is a nutrient and can have enriching effects in the ecosystem, but at high concentrations ammonia is toxic to aquatic life.  The NPS-FM 2020 attribute bands only assess direct toxicity effects. More than 90% of monitored lakes (82 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 the monitored lakes in C band (8 lakes).  Note that for ammonia toxicity the national bottom line is between the B and C band (not the C and D band, as for other attributes), so lakes in the C band do not meet the national bottom line.

 

Lake health state over time

The above plots illustrate the current lake state or condition, but how has this changed over time?  TLI scores are presented annually for the last decade, for the 89 lakes that have this sampling history available.  At the national level, TLI showed relatively consistent scores over the last 10 years, with a slight increase in the proportion of ‘very poor’ and ‘poor' lakes, but overall roughly half of the monitored lakes are in a 'very good', 'good' or 'fair' condition (Figure 4).  Information on changes over time for individual lakes can be found on their pages on LAWA.

 

LAWA Lake Trophic Level Index Change Over Time (2014 - 2023)

Figure 4.  Changes in Trophic Level Index (TLI) scores from 2014 - 2023 at 89 lakes where there were enough data to determine the TLI score each year.  The location of these lakes and their current TLI score (2023) are shown on the map.

 

We also identified lakes that have enough data to calculate attribute bands for each five-year period over the last decade.  Each state assessment is based on the previous five years of information, so the 2014 grades, for example, are based on a five-year dataset from 1 July 2009 – 30 June 2014.  The following figures show results only for those lakes that have this long and comprehensive sampling history.

For chlorophyll a there has been an increase in the number of lakes in bands C and D over the last 10 years (Figure 5).  Chlorophyll a responds to increases in nutrients in the ecosystem (nitrogen and phosphorus), and improvements in this indicator usually require improvements in the nutrient indicators.

 

LAWA National Lake Health State Change Over Time (2014 - 2023)

Chlorophyll a

Figure 5.  Changes in attribute bands for chlorophyll a, from 2014 - 2023 at the 46 monitored lakes where there were enough data to determine a band each year.  The location of these lakes and their current attribute band (2023) are shown on the map.

 

For total phosphorus there has been an increase in the number of monitored lakes in band A and in band C, over the last 10 years (Figure 6).  Phosphorus is often bound within the lakebed sediment, from where it can be released at a later stage in response to changes in ecosystem conditions and weather events.  For this reason, improvements in this indicator may show ups and downs over time.  Phosphorus is also often delivered to lakes attached to sediment, so high rainfall events can drive phosphorus inputs to lakes.  Climatic cycles, such as ENSO (El Niño-Southern Oscillation) are drivers of phosphorus dynamics, particularly in sub-alpine, low nutrient lakes.  Total phosphorus can persist in lake sediments for decades after reductions in inflow concentrations have occurred, therefore lake restoration can be a slow process.

 

LAWA National Lake Health State Change Over Time (2014 - 2023)

Total Phosphorus

Figure 6.  Changes in attribute bands for total phosphorus, from 2014 - 2023 at the 51 monitored lakes where there were enough data to determine a band each year.  The location of these lakes and their current attribute band (2023) are shown on the map.

 

For total nitrogen the number of lakes in the C and D bands has been relatively consistent over the last 10 years (Figure 7).  However, the number of sites in band A has decreased.  That means our low-nutrient lakes are becoming more enriched with nitrogen.  Total nitrogen can contribute to the excessive growth of phytoplankton (algae), reducing water clarity.  Improvements in this indicator can contribute to improvements in chlorophyll a and, to a lesser extent, ammonia.

 

LAWA National Lake Health State Change Over Time (2014 - 2023)

Total Nitrogen

Figure 7. Changes in attribute bands for total nitrogen, from 2014 - 2023 at the 50 monitored lakes where there were enough data to determine a band each year.  The location of these lakes and their current attribute band (2023) are shown on the map.

 

Analysis of lake health state change over time for ammonia (toxicity) showed only 39 lakes with results available over 10 years, not enough for a national summary.  Weather events, COVID-19 lockdowns, changes in resource allocation and availability of monitoring staff can make collection of long-term monitoring data challenging.  In some cases, these challenges have created gaps in our long-term data which we openly acknowledge and do our best to minimise the impact of.

 

 

Lake health monitoring and reporting

To present a national picture, this summary uses lake information that is freely available on the LAWA website for communities to explore.  Looking at monitoring site data from individual lakes provides further context alongside information on lake type, size, depth, mixing pattern, and ecological condition.

We expect that information from more lakes will become available for reporting over time as sampling records grow.  There will be a broader range of indicators available to better understand the health of our lakes as councils introduce changes to their monitoring network to better respond to up-and-coming legislative requirements, such as being able to report on ecosystem health, human contact, mahinga kai and threatened species.

Figure 8. An infographic showing an example of a lake with good ecosystem health (left panel) versus a lake under pressure with poor ecosystem health (right panel).

 

Healthy lakes have high ecological value.  They support a thriving community of plants and animals. Healthy lakes also have high social value.  They can be used for mahinga kai and provide recreational activities opportunities (walking, swimming, sailing, and places for connection).

There are numerous initiatives underway to improve water quality in New Zealand lakes.  Gathering data regarding when and why changes have occurred is essential to set goals for restoration projects and to identify which lakes are in most need of protection.

We are working to align our reporting with national legislation and include information from a wider variety of indicators as long-term monitoring data becomes increasingly available over time.  As more indicators become available, we will be able to provide a holistic ‘health check’, and integrate assessments of rivers, lakes, groundwater, and estuaries.  

LAWA Project partners are working hard to improve the consistency of monitoring across New Zealand through the development and use of National Environmental Monitoring Standards (NEMS).  As these standards are implemented, we expect to see further improvements in the availability of high-quality data on our environment.

 

Explore more on LAWA

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.