New modelling sheds light on lake weed movement in Lake Rotorua

New research has helped identify weed beds that are likely to contribute to weed wash-up events on the Rotorua lakefront after major storm events, providing valuable insights to help inform future control efforts.

The lake weed study, completed by Earth Sciences New Zealand on behalf of Toitū Te Whenua Land Information New Zealand (LINZ), used hydrodynamic modelling of Lake Rotorua to simulate lake weed movement in different conditions.

The study was undertaken in response to weed wash-up events that affect the Rotorua city lakefront area every one to two years.  

Researchers analysed four weed wash-up events between 2020 and 2025. In each case, the wash-ups occurred following prolonged northerly or north-easterly winds.

The modelling identified the large weed bed immediately offshore from Rotorua as a significant contributor to weed washing up on the city waterfront during major storm events. It also found that weeds from the surface down to four metres deep are most vulnerable to wave action during storm events, making them more likely to break free and wash ashore.

LINZ Leader Biosecurity Tracey Burton says the modelling provides a much clearer understanding of how weed wash-ups occur and where management efforts are likely to have the biggest impact.

“We know these events are frustrating for the Rotorua community and can be costly to clean up,” she says.  

"This research helps us understand which weed beds are most likely to contribute to wash-up events and where control efforts could have the greatest benefit."

Lake weed is managed using funding from LINZ, which contributed $350,000 to Rotorua Te Arawa Lakes for the 2025/26 aquatic weed control season.

Decisions on weed control priorities and locations in the Rotorua Te Arawa lakes are made through the Bay of Plenty Aquatic Pest Co-ordination Group (APCG), which includes Te Arawa Lakes Trust, LINZ, Toi Moana Bay of Plenty Regional Council, Rotorua Lakes Council, the Department of Conservation and Eastern Region Fish and Game Council.

The results of the modelling were presented to members of the APCG yesterday.

Burton says the findings highlight both opportunities and challenges for future management.

"The study confirms that the shallow and surface-reaching parts of the weed bed are most likely to be affected by wave action during storms and contribute to weed wash-up events on the lakefront.

“If we can successfully control those shallower areas, there is less opportunity for weeds to break free and wash ashore.

"However, managing these weed beds remains challenging. While the modelling helps identify priorities, the only practical control tool available at this scale is herbicide treatment.

"Herbicide can only be applied when water clarity is suitable and the weeds are free of sediment. Unfortunately, Lake Rotorua is often affected by sediment and poor visibility, which can significantly limit treatment opportunities.

"The weed bed offshore from Rotorua covers a large area. Completely eradicating it would require a substantial long-term investment, and there is no guarantee of success.

"This research won't solve the problem on its own, but it does help us prioritise where our available resources and tools will have the greatest impact," says Burton.

The APCG will use the findings to help inform future aquatic pest management decisions and assess the potential effectiveness of different control approaches.