Sediment swirls around the banks of Lake Horowhenua. Photo / Alexander Robertson
Horizons regional councillors want the weed harvesting in Lake Horowhenua to go ahead, despite having to stump up more money. A trial of the machine is expected to start this spring.
Horizons is prepared to spend another $223,600 on the project (including $72,000 for security), which has met strong opposition, including court action costing more than $500,000, over the years. So far the project has cost the regional council $2.843m.
Lake Horowhenua Trust chair Clinton Hemana said the weed harvesting project is an important step forward in a very long journey.
"On behalf of the Lake owners, the Lake Trust is very pleased that Horizons councillors have voted to continue the Lake Horowhenua weed harvesting project this year," he said.
"As the Tangata Whenua we are looking forward to gaining the knowledge that will come from this phase of the project and to continuing the impetus towards the return of the mauri and mana of the Lake that we are working towards under the partnership of the Lake Horowhenua Accord with Horizons, Horowhenua District Council and the Muaūpoko Tribal Authority."
Horizons chair Rachel Keedwell said harvesting the lake weed has been identified as a key action to reduce the 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 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," she said.
"The lake weed also influences the occurrence of cyanobacterial blooms, which can also affect aquatic life as well as suitability of the lake and the downstream estuary and coastal environments for recreational use."
She said the money approved is for costs that have been encountered for the trial year and include one-off set-up costs, costs to meet consent conditions, costs related to disposal of the weed, security costs and costs for Tangata Tiaki involvement for the Lake Horowhenua Trust and Muaūpoko Tribal Authority, potential legal costs, compliance monitoring costs and the costs of the harvesting itself.
Weed harvesting is viewed as a key restoration measure to reduce toxicity in the lake and is predicted by NIWA to move four of the five key water quality parameters to above the national bottom lines (Ammonia, Cyanobacteria, Chlorophyll a, and Total Phosphorus).
Weed harvesting was first mooted in 2011 as one way to improve water quality. It was included in the Lake Accord Action Plan in 2014. The weed harvester has been available since 2014. Ongoing court action stalled the project and has cost the regional council $506,000, with only some of the money recovered after the courts dismissed the objections to the resource consents.
The aim of using a weed harvester is to reduce the weeds' ability to change the pH of the lake (less weed means less photosynthesis), thereby creating conditions in the lake that are more favourable for fish and other aquatic life, including the native lake weeds.
Harvesting the weed and managing the removal of the cut weed aims to reduce or eliminate the development of the high pH that can lead to phosphorus release from the sediment and ammonia toxicity. It also aims to reduce or eliminate the development of cyanobacteria blooms, which will remove part of the nutrient load from the lake.
Other measures to help improve the water quality in the lake include: sediment traps, installed a few years ago, stormwater improvements, installing a fish trap as well as the ambitious wetland project, which deal with water quality issues of water coming into the lake.
Use of the harvester will be phased in over two years, with a trial being a condition for the resource consent. The trial will help decide what to do next.
The harvester will deal with issues within the lake only. Other measures are dealing with run-off coming into the lake, which is contributing to its current state.
Lake Horowhenua is a hypertrophic lake that had a mean trophic level index (TLI) of 6.4 for the year 2013-14. The lake develops high ammonia concentrations in spring/summer and cyanobacteria blooms in summer.
The lake has two main species of aquatic macrophytes (weeds) – Potamogeton crispus (curly-leaf pondweed) and Elodea canadensis (Canadian pond weed). Although both weeds are exotic invasive species, they have very different life/growth cycles. Elodea is a perennial plant with a clumping growth form. It develops a dense weed bed that eventually reaches the surface.
Although the plants flower, they do set seed and propagation is entirely from small fragments broken off the stems. In contrast, Potamogeton is an annual plant that grows from propagules (turions) shed by the mature plants before they die in summer.
The propagules germinate in autumn (April –May) and over winter as low-growing plants. In spring these plantlets grow rapidly to reach the water surface.
In summer, they flower and produce turions which fall to the lake bed to produce the next year's plants. In mid-summer the mature plants die and collapse onto the lake bed where their decomposition causes anoxia and the release of phosphorus (P) from the sediment beneath the decomposing plant matter.
The trial will establish where the weed is at its most prolific, how much can be collected, how much downtime should be allowed for (this is weather-related), and at what depth to harvest to. The harvested weed will be composted. The trial year is targeting 40ha at an estimated cost of $160,000.
Horizons have consulted with the Lake Horowhenua Trust, Muaūpoko Tribal Authority, and Horowhenua District Council. The weed-harvesting activity has been consulted on in Annual Plan and Long-term Plan processes and been subjected to a publicly notified resource consent processes, Horizons said.
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