Diversion structure questions
Why is the wall needed?
Most of Lake Rotoiti's problems are caused by nutrients (nitrogen and phosphorus) entering it from Lake Rotorua. High levels of nutrients are the main cause of algal blooms in the lakes. More than 70 percent of Lake Rotoiti's nutrients come from Lake Rotorua through the Ohau Channel.
Why are so many nutrients getting into the lakes?
Some nitrogen and phosphorus has a natural source. For example, phosphorus is naturally present in rock. However, the pressures on the Rotorua lakes are mostly from animal waste from the intensification of agriculture and growth in lakeside communities. Many communities dispose of waste to septic tanks. The nutrients (particularly nitrogen) from this source eventually flow through groundwater into the lake, boosting algae production.
What does the wall look like?
The wall goes from the lake floor to just above the water level. It is 1275 metres long, starting south of the Ohau Channel outlet, which links the two lakes, and extending to Te Akau Point. The wall is located about 75 metres offshore, running parallel to State Highway 33.
Are we able to see it?
Yes, the wall rises above the surface of the water and is marked by lights so that people using the lake, like boaties, can see it.
How much did it cost and who paid for it?
The wall cost approximately $10 million to build and has been funded by both Environment Bay of Plenty and central Government.
What are the other options?
Along with the wall, we are doing other things to improve Lake Rotoiti's water quality. These include sewerage reticulation in some lakeside communities, upgrading septic tank systems and restoration work around lake and stream margins.
How long before we see improvements now that the wall is built?
Scientists are confident that the lake will have fewer algal blooms within five years.
How long did it take to build?
The wall took approximately 12 months to build.
Will it harm the fish or water birds?
We're confident it won't affect the birds but are still researching the potential disruption to fish spawning and migration patterns. It's hard to exactly predict the ecological effects of a project like this, so we will be monitoring things very closely and making adjustments when necessary.
Won't it make the Kaituna River's water quality worse?
Scientists say the diversion wall will have little impact on Kaituna River's ecosystem (e.g. fish and birds), although it will cause a small increase in the level of nutrients down the river. Unlike the lake, river systems are not as affected by an increase of nutrients because the natural flow keeps the water mixed and moving and algae cannot multiply in the short travel time to the ocean.
It's worth remembering that water from Lake Rotoiti already flows into the Kaituna River. In 2004 there were warnings about the levels of cyanobacteria in the river for the first time. So if we don't take any action to improve the water quality of the lake and it gets really bad, the river is likely to be affected even more. To make sure the river stays protected we are developing a Kaituna River Management Strategy.
What will the effect on the Maketu Estuary be?
It's likely to have little effect on the Maketu Estuary. At high tide, just one tenth of the estuary's water volume comes from the Kaituna River.
We already monitor the estuary and Kaituna River for algal toxins and will continue to as part of the annual monitoring programme. When cyanobacterial numbers reach high levels in the Kaituna River, cyanobacterial toxins will be monitored in Maketu shellfish.
Why not fix the source of the problem?
It is extremely important that we improve Lake Rotorua's water quality too. A lot of work is being done towards that, including a multi-million dollar sewerage reticulation programme, upgrades to Rotorua's sewage treatment, and new rules for farming.