Lake health in the Ahuriri catchment and the Trophic Level Index (TLI)

What is the Trophic Level Index?

The Trophic Level Index, or TLI, is a nationally used measure of lake health in Aotearoa New Zealand. It helps show how much nutrient enrichment and algae are present in a lake.

Environment Canterbury measures nutrient and algae concentrations to monitor long-term water quality trends in lakes across Waitaha Canterbury, including Lake Benmore and the Ahuriri Arm.

Why lake health and water quality in the Ahuriri catchment matters

The Ahuriri catchment includes highly valued recreational lakes, nationally significant wetlands and important freshwater habitats. These places support biodiversity, cultural values and community wellbeing.

Monitoring shows there has been a long-term decline in lake water quality in the Ahuriri Arm of Lake Benmore, including changes in nutrient concentrations and algal biomass.

We are working with the local community on practical ways to respond to these pressures and support better outcomes for the catchment over time.

How we monitor Lake Benmore

For high-country lakes such as Lake Benmore, water samples are collected by helicopter from the same monitoring site each month between December and April. This monitoring programme has been running since 2005. 

The TLI calculation uses three key measures: 

• Total nitrogen: a nutrient that can move through soil, groundwater and streams into the lake. 
• Total phosphorus: a nutrient often carried by overland runoff into streams and lakes. 
• Chlorophyll A: a pigment found in algae, used as an indicator of algae levels in the lake. 

Additional monthly sampling of the Ahuriri Arm of Lake Benmore by boat began in May 2025 to provide a 12-month dataset and improve understanding of seasonal nutrient patterns. These extra samples do not change the standard TLI assessment, which continues to use December to April samples.

2025/26 TLI score for the Ahuriri Arm

The 2025/26 TLI score for the Ahuriri Arm of Lake Benmore is 2.77. This is unchanged from the 2024/25 score.

A TLI score of 2.77 is above the early warning trigger of 2.75 that applies to some older consents. This means some consent holders will be required to reduce their nutrient discharges.

What the TLI score means for consents

Most farming or water take consents in the Ahuriri catchment include a TLI limit. If the relevant limit or trigger is exceeded, consent holders need to take action to reduce their impact on the Ahuriri Arm of Lake Benmore. 

Since 2015, new or renewed consents in the Ahuriri catchment have generally included a TLI limit of less than or equal to 2.9, following Plan Change 5 to the Canterbury Land and Water Regional Plan. If the TLI reaches 2.91, affected consent holders must take measures as detailed in their consent conditions. 

Some older consents include an early warning trigger of 2.75. When this trigger is exceeded, those consent holders are required to reduce nutrient discharges by a prescribed percentage.

How Environment Canterbury works with consent holders

After the annual TLI score is confirmed, Environment Canterbury contacts affected consent holders. Staff explain the score, what it means for consent requirements and what actions may be needed next.

Where a consent condition is triggered, staff work directly with consent holders, so they understand their obligations and response timeframes. This may include preparing a plan to reduce nutrient losses and minimise impacts on Lake Benmore.