Water quality of lakes and rivers of Upper Waitaki highlighted

An overview of the water quality of the lakes and rivers in the Upper Waitaki catchment was presented to the Upper Waitaki Water Zone Committee at its October meeting on Friday.

The past year’s annual water quality monitoring results show that five lakes in the zone have not met the Trophic Level Index (TLI) set in the Canterbury Land and Water Regional Plan, although results in several lakes are likely to be impacted by heavy rainfall last summer.

In addition, a number of the rivers in the Upper Waitaki are showing signs of degrading in terms of aquatic life and water quality.

A decrease in water quality compared to last year

The results were presented by Principal Science Advisor Graeme Clarke, who says while the larger lakes are still in very good health, there have been water quality decreases in a number of rivers throughout the zone.

“In general, the large lakes in the Upper Waitaki such as Ōhau, Pūkaki and Tekapo/Takapō are still in very good health with low nutrient levels, but the results from the past year’s monitoring have shown a decrease in water quality compared to last year.

“The change could be linked to heavy rainfall experienced in December 2019, which can wash nutrients and sediment from land into the waterways. It’s important to note that this monitoring is different to swimming and contact recreational water quality – which shows that all sites in the zone are suitable for swimming,” Clarke said.

The TLI is used across New Zealand as a measure of the nutrient status of a lake. TLI limits in the Waitaki lakes are set to help achieve community outcomes agreed by the Upper Waitaki Water Zone Committee, which include providing for a diverse ecosystem of plant and animal life, recreational opportunities and customary use.

Find out more about the Trophic Lake Index and the scores from the last 5 years.

Degrading aquatic life and water quality in rivers

The briefing for the Zone Committee also included information on river water quality in the zone, looking at both aquatic ecology – studying the insects living in the water – as well as water quality. Both sets of results generally showed degrading trends, including the Ahuriri River, Twizel River and Ōtematata River.

“A number of the rivers in the Upper Waitaki are showing signs of degrading in terms of aquatic life and water quality. This is disappointing but not surprising, considering the widespread land use change that has occurred,” said Clarke.

Southern Zone Manager, Chris Eccleston, said the results reaffirmed the importance of measures being taken to reduce the impact of both farming and aquatic pests on the waterways.

“The implementation of on-farm good management practices across the zone and, longer-term, implementation of policies in the Government’s new Essential Freshwater Management framework will address key causes of degradation.

"Over the past five years, we’ve been working closely with both irrigators and farmers in the area to implement tighter environmental rules.”

Water monitoring in Upper Waitaki 

In the Upper Waitaki area, we monitor Lakes Alexandrina, Tekapo, Pūkaki, Ōhau, Benmore and Aviemore, as part of its annual high country lake monitoring programme. Kellands Pond is also regularly monitored.

The Trophic Level Index (TLI) is calculated from annual averages of phosphorus and nitrogen concentrations and phytoplankton biomass (chlorophyll a).

This index allows assessment against objectives set in the Canterbury Land and Water Regional Plan (LWRP).

A separate recreational water monitoring programme, including safety for swimming, is carried out over summer and reported on lawa.org.nz.

Reasons for lake and river water quality deterioration could include the impact of nutrient losses from farming activity, water abstraction, invasive species and climate change.

What is the Upper Waitaki Water Zone Committee?

The Upper Waitaki Water Zone Committee is a community-led committee that recommends actions and tactics to deliver the Canterbury Water Management Strategy in their zone.

It is made up of community members and rūnanga and council representatives. Find out more about the work of the zone committees.

2016-20 Trophic Lake Index (TLI) scores Upper Waitaki

Location 2016 2017 2018 2019 2020 TLI Limit (PC5) 
Lake Alexandrina 2.90 3.33 2.85 2.89 3.30  3.1 
Lake Tekapo 1.32 1.05 0.86 1.26 1.70 1.7
Lake Pūkaki 1.00 1.46 1.25 1.41 1.90 1.7
Lake Ōhau 1.42 1.38 1.31 1.44 1.98  1.7 
Lake Benmore - Haldon 1.69 1.73 1.73 2.12 2.21  2.7 
Lake Benmore- Ahuriri 2.11 2.31 2.29 2.94 2.75 2.9
Lake Benmore - Dam 1.72 1.77 1.89 2.19 2.32  2.7 
Lake Aviemore 1.61 1.74 1.95 1.97 2.24 3.0
Kellands Pond shore 3.41 3.49 3.53 3.33 3.57 3.2 

Description of trophic states and Trophic Level Index (TLI) scores

TLI Tropic state General description
<1 Ultra-microtrophic Practically pure, very clean, often have glacial sources
1-2 Microtrophic Very clean, often have glacial sources, very low nutrient concentrations
2-3 Oligotrophic Clear and blue, with low levels of nutrients and algae
3-4 Mesotrophic Moderate levels of nutrients and algae
4-5 Eutrophic Green and murky, with higher amounts of nutrients and algae
5-6 Supertrophic Very high nutrient enrichment and high algae growth
>6 Hypertrophic Saturated in nutrients, highly fertile, excessive algae growth