Environment Canterbury takes action on Ahuriri Arm water quality

Environment Canterbury is taking action to investigate what may have caused a decline in water quality at the Ahuriri Arm of Lake Benmore.

This year’s annual water quality monitoring resulted in the Trophic Level Index (TLI) for Lake Benmore’s Ahuriri Arm increasing to 2.9, from the previous year’s TLI of 2.3.

The TLI is used across New Zealand as a measure of the nutrient status of a lake. A higher TLI results from higher nutrient concentrations and higher algal biomass (phytoplankton).

TLI is not used as an indicator of safety for swimming; Land and Water Aotearoa reports that the sites monitored in Lake Benmore are generally suitable for swimming, based on three years of summer-time E.coli monitoring.

Environment Canterbury will report to the Upper Waitaki Water Zone Committee on Friday 19 July to summarise the action it is taking due to the Ahuriri Arm TLI increase, including examining nearby irrigation activity.

The report to the Zone Committee explains that the increase has triggered a regulatory process to determine if 10 consent holders are either partly or wholly responsible for the TLI trigger exceedance of 2.75.

A further group of five consent holders, with a TLI trigger of 2.9, will also be contacted by Environment Canterbury, as they are part of the same Ahuriri Arm catchment area. However, that TLI limit of 2.9 has not been exceeded.

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

Expert panel to be formed

Environment Canterbury now requires an expert panel to review each consent to establish if its irrigation is contributing to the increased TLI.

Environment Canterbury Southern Zone Manager, Chris Eccleston, said: “The health of our waterways is paramount, and we are taking prompt action.

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.

“We have been in touch with the consent holders impacted to begin the process. This is a collective issue, so we are also undertaking actions with other landholders within the Ahuriri catchment.

This will include establishing a new catchment group for this area to further progress implementation of good management practices across all farms.”

The two-person expert scientist panel for the Ahuriri Arm will consist of one expert nominated by Environment Canterbury and one expert nominated by the consent holders.

The role of the panel will be to determine if the breach was ‘unlikely’ or ‘not unlikely’ to have been caused in whole or in part by nutrient losses associated with the irrigation authorised by the consents.

This will involve consideration of nutrient losses, irrigation use, compliance history and Farm Environment Plans.

The timeframe for establishing the panel and consideration of the consents is expected to be completed before October, which is when the new irrigation season begins.

If a contribution to the increased TLI is indicated, then the nutrient discharge allowances on the water permits for irrigation will be reduced, in accordance with a formula set out in the consents.

Other additional measures Environment Canterbury is taking include compliance follow-up with farms identified as a high priority due to their compliance history and working one-on-one with farms that require farming land use consents to ensure these are being obtained.

Water monitoring in Upper Waitaki

In the Upper Waitaki area, Environment Canterbury monitors Lakes Alexandrina, Tekapo, Pūkaki, Ōhau, Benmore and Aviemore, as part of its annual lake monitoring programme.

The 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).

The two other monitored sites at Lake Benmore have also shown an increased TLI but sit at lower levels that do not require action beyond usual compliance and consent processes. A separate recreational water monitoring programme, including safety for swimming, is carried out over summer and reported on www.lawa.org.nz.

Lake Benmore was artificially created in the 1960s by the construction of Benmore Dam. The lake covers an area of approximately 75 km². Parts of it lie in the Mackenzie, Waimate, and Waitaki districts within the southern portion of Canterbury.

Come along to the Zone Committee meeting

The Upper Waitaki Water Zone Committee meeting on 19 July at 9.30am will be held at the Mackenzie Country Inn in Twizel and is open to members of the public.

A report from Environment Canterbury compliance and science will be presented to update the water zone committee members and is available within the agenda.

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.

2015-19 Trophic Lake Index (TLI) scores Upper Waitaki

When 2015 2016 2017 2018 2019 TLI Limit (PC5)
Lake Alexandrina 3.1 2.9 3.3 2.9 2.9 3.1
Lake Tekapo 1.2 1.3 1.1 0.9 1.3 1.7
Lake Pūkaki 1.8 1.0 1.5 1.3 1.4 1.7
Lake Ohau 1.1 1.4 1.4 1.3 1.4 1.7
Lake Benmore - Haldon 1.8 1.7 1.7 1.7 2.1 2.7
Lake Benmore- Ahuriri 2.6 2.1 2.3 2.3 2.9 2.9
Lake Benmore - Dam 1.8 1.7 1.8 1.9 2.2 2.7
Lake Aviemore 1.9 1.6 1.7 2.0 2.0 3.0

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