Aquatic Weeds

Update March 2024

Weed Spraying & Water Quality Issues

It’s been a frustrating year of poor water quality for some residents on our lake. The weed spraying in some areas was cancelled because the water was too murky for it to be effective and although the weed harvester was deployed, it has some significant limits that didn’t help. This has all seemed to add up to worsening water quality and weed growth.

We are well aware of the concerns of residents in Okawa bay, Te Weta bay, Okere inlet and Otaramarae inlet and that some parts of the lake are more sensitive to these pressures than others.

The LRCA will continue to support spraying as one of a number of tools for keeping invasive weed at bay in our lake, however we have seen that sometimes it cannot be deployed and it’s not effective on its own.

Unfortunately, actions over the last 100 years or so have pushed us to this point and so if we want it to improve, we can’t keep doing the same things and expecting things to get better. We can’t blame just the geographical circumstances and so it’s clear that without change, the lake water will continue to degrade.

Our actions have introduced ever more and varied nutrients into the lake and these help promote algal blooms, murky water and increased weed growth.

So we are suggesting that, in addition to continuing to push for weed spraying and harvesting, there are some other things that we can all do to help improve Lake Rotoiti’s water quality by reducing the amount of these nutrients we put into the water column.

We know that water quality, algae and weed growth are all directly affected by excessive nutrient supply and these suggestions are all about reducing the nutrient supply into the lake.

Nutrients in our lakes are caused by things like:

Fertiliser runoff

Grass clippings, leaves and plant debris blown or thrown in

Floating clumps of weed dislodged by boating activity and storms

House gutters flowing into the lake

Silt runoff

Sewerage seepage

Water runoff from the road

Contaminated domestic stormwater - leaves, oils, chemicals, paints etc

Rubbish and food waste.

Boat propellers stirring up nutrient laden bottom sediment

Here’s some ideas of things you can do to help keep our lake as clean as possible:

Stormwater - if possible, direct your stormwater away from the lake. Consider installing a detention tank to settle out contaminants before sending stormwater into the lake.

Lawns - Don't allow grass clippings to get into the lake. DON'T fertilise your lawn.

Create a barrier zone - If your property is lawn to lakefront, plant native filter plants like Harakeke (flax), native grasses, small shrubs etc to form a 1-metre-deep barrier across the lakefront. This will also help stop erosion.

Garden - Remove exotic/deciduous trees and bushes that drop leaves and flowers and replace with evergreen natives.

General maintenance - Keep your gutters clean and free of leaves and twigs.

Earthworks - Ensure no runoff gets into the stormwater and the lake.

Floating weed clumps – bring these on to dry land and dispose of them in your garden compost. If left in the lake, they provide the perfect nutrients for new weed growth and murky water. Don’t drive through them as your propellor will just cut them up, increasing and spreading the nutrient supply.

Boating - Propellers stirring up the mud on the bottom of the lake release nutrients so lift your propellor in shallow waters.

Sterilise your boat and trailer before launching if it has been used in any other lake or river to rid it of unwanted organisms. Propellers and wake also introduce large amounts of air/oxygen into the lake which feeds weed and algae growth so please go slow in shallower water.

Update March 2023

Lake Weed Control Trial Planned

Concern was expressed at the AGM about lakeweed, particularly in Okawa Bay, and the question why is the weed harvester not used? This was obviously passed onto BoPRC Lakes Management and TALT, resulting in member Don Atkinson and Deputy Chair Roy Duffy being invited to join William Anaru (TALT), Andy Bruere & Roly Bagshaw (BoPRC), and the weed harvester operator to inspect Okawa Bay weed in the patrol boat. Don’s extensive knowledge of Okawa Bay, and his years on the Water Quality Society was helpful. We debated the fact that with the weed spraying traditionally being conducted in November, there is inevitably a great deal of dead weed biomass in the bay when the heat of summer warms the water, and is a probable trigger for algal blooms in the bay.

We discussed:

The viability of trialing the Okawa bay spray programme earlier in spring at the beginning of the growth period, that would probably create less dead biomass, and reduce algal blooms.

Utilisation of the weed harvester. As was stated at the AGM by Greg Corbert (Biodiversity Manager at BoPRC), the harvester can only reach 1.8m down and leaves the roots in place for those weeds that adhere to the lakebed. Its use depends on funding being obtained.

We have recently been advised the following:

The Aquatic Pest Management Coordinating Group met last week and one of the agenda items was the upcoming weed spraying programme. As discussed with you recently the proposal to change the spraying programme in Okawa Bay was discussed. The Group agreed to trial a change to the spraying programme. To clarify, this means Okawa Bay will not be sprayed in the April spray programme. Instead, Te Arawa Lakes Trust will attempt to programme the spraying of invasive weed earlier than usual, targeting a September application. As you will understand this is targeting an earlier period when the weed is expected to be emerging but the biomass will not be so great, reducing the potential to release a large mass of nutrients. The time of spraying will be dependent on the monitoring that is routinely undertaken for the programme. As a result of the earlier spraying, we believe there may need to be a second spraying in about November to ensure minimal weed impacts over the summer period. Again, this will be determined by monitoring. In addition, BOPRC is prepared to use the weed harvester if there is sufficient surface reaching weed to make setting up the operation worthwhile. This will also be dependent on monitoring observations and may be appropriate in November. I think you will appreciate this is a trial situation and the outcome of this change to the spraying will be considered in recommending future spray applications in the bay.

We are very pleased that Council and Te Arawa Lakes Trust responded to our concerns, but it remains to be seen if this trial will be successful in Okawa Bay.

Following that, Don and Roy attended a Lakeweed hui arranged by TALT with presentations from TALT, LINZ management and NIWA scientists. This of course was considering lakeweed across all Rotorua lakes, not just Rotoiti. We confirmed our concerns and that our Association and other bodies would support any campaign to seek more funding to improve the effectiveness of lake weed management.

We mentioned that LRCA is impressed with the very good communications and organisation of the spray programme by William and his team at TALT, since they took over its management.

In addition to the above, LRCA will continue to provide active input to Linz and TALT with weed observation feedback we collate from our members in each of the key bays and communities around the lake. We welcome feedback from members in September and April each year to identify hot spots and lobby for them to be included in the twice yearly spraying.

Update April 22


BoPRC has advised residents that they would not take action on the weed wash up at Okawa Bay on a public amenity basis because it was really a private amenity one which they would need to address themselves if they wanted to have the weed removed commenting as follows:

“While I agree with you that rotting weed isn’t pleasant, I don’t consider it to be toxic. Although it certainly creates an unpleasant amenity effect for those private dwellings. Our scientist confirmed with me that it’s unlikely to be a driver of the algae blooms that have been present. However, once any weed dies and breaks down to the bed of the lake it does become available as nutrient, which is a driver in algae blooms. The presence of lake weed may actually be an indicator of good water quality (clarity), i.e. if there was an algae bloom there wouldn’t be sufficient light for the weed to grow.”

They also clarify responsibilities around weed on the Te Arawa Lakes - including weed harvesting, spraying, spraying limits and managing new incursions of weed on lakes that do not already have invasive weeds present.

Emails on the subject from BoPRC and Okawa Bay representative Richard Amery can be read here.

Update December 21


LRCA scrutinises the twice yearly notification by Boffa Miskell (who manage the spray programme on behalf of LINZ), to gain an understanding of the areas that have been approved for spraying in the Spring and Autumn spraying, and the areas that have been excluded.

There are two prime reasons for excluded areas. The first is that there is little weed, and the second is ‘dirty weed’ identified by the the diver inspection.

We have asked for an explanation of why dirty weed causes areas not to be sprayed.

Some Good News

The pre spray inspection found that Lake Rotoiti had a significant amount of clean weed. Therefore the largest control programme ever is being undertaken with just over 75 hectares of weed to be targeted.

Regarding Dirty Weed

This is an issue experienced across all New Zealand lakes. There are generally a few reasons as to why an application of herbicide is ineffective.

* The weed is too dirty, and organic particles in the water and on the weed deactivate the active ingredient in the chemical Diquat before it can attach itself to the plant material.

* The herbicide has been applied but has drifted off target and settled outside of where the plants are. This can be common and can be caused by currents, and the thermal stratifications of the different temperatures in the water body. This is very hard to avoid as we cannot monitor water currents at every location across all the lakes we work in.

* The plants need to be actively photosynthesizing to absorb the chemical. If its early in the morning or a really overcast day, then this can also affect results. The plants will decrease how much energy they are taking in and therefore how much chemical they absorb.

The weed can still be healthy and covered in sediment. It might also seem clean once you have swum around as the sediment has been disturbed and will be suspended in the water column and no longer on the weed.

If you see the article and photos here from divers at Lake Rotoiti this shows weed at a 4 on the dirtiness weed scale. This weed will not be getting treated this round.

When spraying you need the weed to be in condition 1 or 2 or you will get poor results.

Therefore, the main way to implement an effective spray programme is to have clean weed. How do we get clean weed? That’s essentially up to mother nature.  If we had a chemical that we could apply and get 100% kill rate every time, then we would not have a weed issue. Unfortunately, we don’t have that luxury at present. As mentioned on the phone, the EPA is working on getting a new chemical registered for use in New Zealand that could control dirty weed. However, this will more than likely need a consent to be used in large public water bodies which is another hurdle before we can effectively use it.

If you are concerned about weed in your bay, please notify LRCA, as we can discuss with LINZ.

Update November 21


Lake weed is a problem in shallower water around the lake. Weed like Hornwart enjoys the cleaner water due to the wall in Okere channel that directs Rotorua water into the Kaituna river.

LRCA has regular discussions with LINZ (which has responsibility under Settlement Act), Boffa Miskell (manages the programme), and BoPRC (Biodiversity responsibilities), particularly to highlight the problem areas brought to LRCA attention by members, and to query when some areas are not sprayed.

Below is the email received this week confirming the spring lakeweed spraying plan, which is the largest area to be sprayed.

You can open the Rotoiti map here and zoom in on your bay. In each location with historical weed, there is a red circled zone. The yellow shaded areas identify the areas to be sprayed this time.

You will see there are some areas with no yellow shading, meaning no spray this time, and other areas where only part of the red circle are being sprayed.

We have spoken to LINZ (and Boffa Miskell) who are engaged by LINZ to manage the spray programme, and record below the reason they provided for minimal OR no spraying in some locations.

Hot Springs - 0 hectare – weed evident at drop off only – outside zone

Manupiura Bay 0 ha - minimal weed

Tapuakura (Curtis Road) - 0 ha - Weed identified at each end of bay, but evidence of sediment that would prevent effective attachment of diquat gel to the weed, so no spraying.

Hauparu Bay – 1 ha - weed evident at eastern end only

Whangamoa Road (west end) 0 ha – hornwort around rocks, dirty with sediment

Kuharua (Lees Pt) – 0 ha - patches of relatively dense native underwater plants – outside scope of spray programme

We understand that diquat will only adhere and be effective on healthy weed that is free of sediment. Before each spray plan is determined, divers inspect every location to determine evidence and condition and weed. We are seeking more information about this to share with our membership.

You are welcome to advise LRCA of concerns about the spray programme and we can discuss with LINZ and Boffa Miskell.

From: BML Biosecurity

Sent: Sunday, 31 October 2021 12:53 PM

Subject: Aquatic Weed Spraying - Te Arawa/Rotorua Lakes

Kia ora koutou,

Aquatic weed spraying is scheduled to be carried out in the Te Arawa/Rotorua Lakes from Monday 8th November and will continue intermittently until Friday 10th December 2021.


This work will be carried out via boat-based boom spraying using the herbicide diquat in gel form at the following sites at Rotoiti:

Wairau Bay

Ōkahutoroa Bay/Cherry Bay

Ōkawa Bay

Teal Bay

Te Weta Bay

Ōtaramarae Boat Ramp

Te Ārero Bay

Gisborne Point

Ōkere Inlet

Te Akau Peninsula

Te Waiiti Point

Te Ruato Bay

Hauparu Bay

Here is a map showing where work will be carried out.

All work is dependent on suitable weed, water and weather conditions and is subject to change without notice.


Advertisements will been placed in the Rotorua Daily Post to inform lake users of the control work.


Warning signage will also be erected at official boat ramps 24 hours before work is taking place.


If you have any questions in relation to this work, please feel free to call us on 0800 638 943 or keep up to date with treatment progress on the LINZ website.


Further updates on projected works will be provided on Sunday 7th November once we have a better idea of the weather forecast.


Ngā mihi,


 Boffa Miskell.



Review of Aquatic Weed Control for Amenity Purposes on Rotorua Lakes

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