Protecting our Lake Environment

Update on Protecting our lake Environment

Water Quality & Diversion wall

At a recent meeting called by the lakes water quality society, we saw the results of modelling and research that has been done by the University of Waikato team, presented by Deniz Ozkundakci. The modelling concluded that leakage from the Ohau channel wall did not currently contribute significantly to Rotoiti surface water quality due to the strong draw current of water down the Okere Kaituna river. However some small leakage of denser water from holes in the wall and outflow appears to flow along the bottom strata down to the deepest bottom of the lake. This water is the densest, most polluted water and will be damaging to lake water quality in the long term. The conclusion was that the wall has a significant long term role to play in reducing the amount of water ingress into Rotoiti from lake Rotorua, beyond it’s expected 50 years lifespan – 2057.

The second trial of HDPE panels to cover holes has concluded as being successful and the BOPRC will now be consulting with industry and the technical panel to work out a workable long term repair solution, costings and timing.

Weed.

The twice yearly Diquat applications to reduce Elodea, Egeria, Hornwort and Lagrosiphon weed infestations have the effect of knocking back the weeds but needs to be regularly re-applied. TALT and BOPRC are hoping the herbicide Endothall, still undergoing consenting, will be a new tool, which kills weeds and their roots outright, so has a longer effect and does not need biannual applications.

However, neither spray will work in cloudy water, as we saw recently in Okawa bay, so it’s important that we lakeside residents undertake our part to reduce nutrient flows – deciduous leaves, fertilisers, stormwater, grass, garden waste, silt etc, into the lake and avoid stirring up bottom silt during boating activities. There is a list of suggestions in past newsletters and on the LRCA website detailing how we can reduce the nutrient load from our properties into the lake and better boating behaviours.

Gold clam (Corbicula) infestation

Over the last 12 months, a significant amount of surveillance has been carried out within the Rotorua Te Arawa Lakes to detect the freshwater clam. Surveillance efforts include:

Bi-annual dive surveillance at 23 boat ramps and other key vector points across 10 lakes.

Over 250 eDNA samples collected at high-risk sites across 10 lakes.

Increased community engagement through media, schools, and public meetings.

Increased biosecurity officer presence at boat ramps carrying out boat and trailer hygiene inspections. Over 1000 vessels and trailers inspected at boat ramps with all but 26 passing inspections. None of the failed vessels contained freshwater clams, but they did contain other unwanted pests such as lake weeds.

At present, the freshwater clam has not been detected in any of the Rotorua Te Arawa Lakes. With the summer fast approaching MPI Biosecurity, TALT and BOPRC are looking to repeat last year’s surveillance efforts.

It’s important to understand how devastating an infestation could be in our lake. Water drawn from the lake for domestic use, permanently moored boats and swimming and wading in the shallows will all be either impossible or severely compromised. The high bacteria load left by dead and decomposing clams will pollute the water and make safe water activities a thing of the past.

Here is the link to the Biosecurity website, with information on the clam and details of what you can do to prevent the spread.

Koura re-population study.

This is a study being carried out by Dr Ian Kusabs for TALT to determine why there has been a reduction of 96% of the biomass of Koura in Lake Rotoiti and to determine how remaining Koura can be helped to re-populate. As part of his PhD, Olivier Raven is conducting this work. He'll be trialling rocky reefs around the shoreline of Lake Rotoiti (including lakeshore habitat walls). So, he would love to get a list of interested shoreline property owners to contact and visit to see if their properties are suitable.

Köura monitoring

Did you know our köura (freshwater crayfish) populations in the Rotorua Te Arawa Lakes are facing challenges? Koura are currently under pressure from various stressors, such as the introduction of exotic fish species that prey upon them, invasive plant species hindering their movement and contributing to organic detritus accumulation and reduced dissolved oxygen levels in lake bottoms (due to eutrophication).

Recent monitoring efforts have revealed significant declines in koura abundances, particularly in Lake Rotoiti (-96%) and Lake Rotorua (-87%), following the establishment of brown bullhead catfish (Ameiurus nebulosus), In Lake Rotoiti, koura primarily inhabit areas offering adequate cover, such as rocky substrates, woody debriis or water depths greater than 15m.

One effective method to protect and enhance koura numbers in Lake Rotoiti is by constructing lakeshore habitat walls, specifically designed to provide suitable habitats:

Currently, we are monitoring various wall designs, including those composed of large rocks, gabion baskets with timber frames and timber structures, such as planks or garden edging. However, we require additional monitoring sites to further evaluate their effectiveness.

If you are considering upgrading your lakeshore habitat wall or wish to participate in this research programme, we encourage you to contact Dr. lan Kusabs at ian.kusabs@outlook.com. Your involvement could play a crucial role in safeguarding our local koura populations.