Lake Horowhenua weed harvesting trial completed

Horizons Regional Council and the Lake Horowhenua Trust have completed a weed harvesting trial on Lake Horowhenua to help improve the lake’s water quality.

Horizons chair Rachel Keedwell says the trial aims to provide further information on the effectiveness of the harvesting and enable fine tuning for future operations. 
“A lot of work has gone into preparing to start this trail including having the vessel blessed by Muaūpoko, undertaking initial trips on the water to get crew familiar with the harvesting mapping software, and working with our project partners in the employment of tangata tiaki to ensure the harvesting operation follows local tikanga,” she says.
“The trial included harvesting weed from two twenty-hectare sections of the lake, bringing the weed to shore and taking it away for composting over a period of two weeks. Samples of the weed are also being collected and analysed to help inform future decisions on local uses of the weed.
“At all stages of the harvesting operation tangata tiaki have been present. This has allowed the sharing of western and mātauranga knowledge, working together in a partnership.” 
Cr Keedwell says the trial has gone really well thanks to settled weather and was completed sooner than expected.
“Additional data collection, along with the analysis of the current monitoring results, will be used to check in on the success of the project. Our partners and Horizons will then decide on what the programme will look like for next harvesting season.
“Overall, it’s fantastic to finally get the weed harvesting underway which, alongside other interventions, will help restore pride and mana to Lake Horowhenua and enhance its social, cultural, environmental and economical values.” 
Lake Horowhenua Trust chair Clinton Hemana says he is pleased to see the weed harvester in operation after a long journey to get here.
“On behalf of the owners, we are glad to be involved in programmes to help improve lake water quality and see Muaūpoko owners engaged and working with their taonga, helping to ensure improvements happen in the right way. 
“The science indicates that weed harvesting could reduce lake toxicity and, in conjunction with other projects, we hope to see a lot of improvement over the next few years.”
The lake weed harvesting programme has a phased introduction over its first two years. This harvesting trail is required by resource consent conditions to inform a fuller programme of harvesting in future years if certain resource consent conditions are met.

Harvesting the lake weed has been identified as a key action to reduce toxicity of Lake Horowhenua and to increase the frequency the lake is suitable for recreational use. 
Introduced lake weeds have a significant impact on the chemistry and internal lake processes, including impacting the pH of the lake. The impact on pH can influence the occurrence and frequency of ammonia concentrations in the lake that can be toxic to fish and other aquatic life.
The lake weed also influences the occurrence of cyanobacterial blooms which can impact on both aquatic life as well as suitability of the lake and the downstream estuary and coastal environments for recreational use.
In addition to the sediment trap that was installed in 2017, weed harvesting is viewed as a key restoration measure to reduce toxicity in the lake. It is predicted by NIWA to move four of the five key water quality parameters to above the national environmental bottom lines as contained In the National Policy Statement for Fresh Water for Ammonia, Cyanobacteria, Chlorophyll a, and Total Phosphorus.
Other planned interventions include stormwater improvements that Horowhenua District Council have committed over $6.67 million for in their Long-term Plan.

Horizons and the Ministry for the Environment have also committed $12.5 million to the Lake Horowhenua water quality interventions Jobs for Nature project. The Jobs for Nature project is phase one of a longer term work programme to establish a wetland complex within the Arawhata catchment to reduce sediment and nutrient inputs to the lake.
It is noted that the science indicates that neither the wetland complex project nor stormwater project will address the in-lake processes that the lake weed harvesting project targets.