Q&A: LAWA Lakes Water Quality National Picture

What does the LAWA Lakes Water Quality National Picture Summary tell us about lake water quality?

The summary shows us, for the 124 monitored lakes, that New Zealand has some pristine lakes with very good water quality, however, many of our lakes are considerably affected by human activities and development, especially in the lowland.  There is little evidence of change in lake condition over time at the national scale over the last 10 years.  Overall, only a small proportion (124 lakes) of the approximately 3800 lakes (>1 ha) in New Zealand are currently monitored regularly for the indicators required to calculate Trophic Level Index, and are not necessarily representative of all New Zealand lakes. 

What do the different coloured bars in the national summary graphs mean?

Each bar is divided into five colours to show the proportions of lakes in each of five conditions states – very good through to very poor.

What is TLI in a nutshell?

The Trophic Level Index (TLI) is a summary index of the trophic condition of a lake and is frequently used as an indicator for overall life-supporting capacity of a lake It combines four separate measures of various aspects of lake trophic condition. These measures are the concentrations of key nutrients (total nitrogen and total phosphorus), an indicator of algae biomass (chlorophyll a - the photosynthetic pigment present in all plants), as well as a measure of water clarity. For more about the TLI, see our factsheet.

Why have you focused on TLI?

The components of the TLI have been monitored for a long time in New Zealand and it is a good summary measure that reflects several aspects of lake condition.

What other lake info can I find on LAWA?

The several component datasets that are used to generate TLI are all available, along with other lake water quality and ecology indicators that do not contribute to TLI calculation for lakes throughout the country.  These indicators, and monitoring histories, can be found on the lake individual site pages on the LAWA website. 

What does TLI mean for swimming water quality?

There is not necessarily a direct relationship between TLI and swimming water quality, but a higher TLI usually means more algae growth and a higher risk of toxic cyanobacteria blooms. Swimmers should not enter the water during toxic algae blooms. Over summer, regional councils and unitary authorities monitor lakes for E. coli and cyanobacteria to help would-be-swimmers decide where it is safe to swim. This information is available on the ‘Can I swim here?’ topic on the LAWA website.

What causes lake condition to degrade?

Changes in the catchment that drains to the lake can lead to changes in the water quality and ecological condition of the lake.  Such changes might be natural, such as landslides caused by earthquakes, but are often caused by human activities such as urban or agricultural development, transport or other infrastructure. 

Does the condition of our lakes vary by their elevation?

Yes, the upland lakes tend to exhibit better condition than lowland lakes.   Human developments tend to be more concentrated in the lowland areas, where climate, gradient and geology are more suited to our activities. Lowland lakes affected by these activities are often in poor condition. Lowland lakes are also often smaller and shallower than upland lakes, which generally renders those lakes more vulnerable to human pressures.

How can the data presented on LAWA help inform lake improvement efforts?

Lake monitoring data helps identify current lake condition relative to where the community would like it to be. It can also indicate if a lake is getting better or worse. Signs of improving lake condition can provide encouragement to people and agencies involved with undertaking actions to make things better.

How was the national picture generated?

Data routinely collected by the regional councils and unitary authorities around the country were collated.  Where multiple sites were present on some lakes, these were combined to represent that lake.  For each lake, the annual TLI value was calculated using the available data from that year.  TLI grades were generated based on the numeric score. The latest year’s TLI grades were graphed as a complete set, and also separated by lake elevation.  Changes in TLI over the last decade were plotted for those lakes with a full dataset over 10 years.

How is lake water quality monitored in New Zealand?

Regional council and unitary authorities have a set of lake sites that they regularly visit to collect water samples and take measurements within the lake.  The water samples are analysed at accredited independent laboratories for levels of various chemical compounds (for example, phosphorus and nitrogen concentrations).  The results are stored on the council’s databases and are accessible via LAWA and each council. 

What is Land, Air, Water Aotearoa (LAWA)?

LAWA aims to connect New Zealanders with their environment through sharing scientific data online. It’s world-leading in its approach to making data collected by authorities freely available to the public. This is made possible by a collaboration of NZ’s 16 regional and unitary councils, the Ministry for the Environment, and Cawthron Institute, with support from the Department of Conservation and Statistics New Zealand. LAWA is committed to sharing the best available information, so New Zealanders can make informed decisions.