KEY POINTS:
A $10 million wall designed to stop nutrients polluting Rotorua's Lake Rotoiti is successfully diverting water away from the lakebed.
But Environment Bay of Plenty says it is too early to say whether the wall has lowered levels of harmful algae in the lake.
The Ohau Channel Diversion Wall was finished in September and officially opened the following month. Jointly funded by the regional council and the Government, the 1275m wall runs along a portion of Lake Rotoiti to divert nutrient-rich water from Lake Rotorua down the Kaituna River.
The council's Rotorua lakes programme manager Andy Bruere said since the wall's completion, flow monitoring had been undertaken in three locations around the wall.
"The results of that are showing the wall is performing as predicted," he said. "A lot of boaties are [also] noticing it's diverting the water from [Lake] Rotorua quite clearly down the Okere arm [of Lake Rotoiti] and into the Kaituna."
Mr Bruere said the response from lake residents and the wider public about the wall's effectiveness was also good, but it was too early to say whether the structure was having an effect on algae. That would be determined at the end of an intensive five-year monitoring programme covering water quality, algal concentrations, and the effect of the wall on fisheries and bird life.
The wall is part of a wider project aimed at restoring Rotorua's degraded lakes, which also involves sewage reticulation and controls on dairying in the catchment area. The structure extends from the lakebed to 500mm above the water, and Mr Bruere said it took advantage of the geography of Lake Rotoiti.
He said the wall was only feasible because the inlet and outlet of the lake were so close, and it was revolutionary in that respect.
Before it was built, opponents argued that it would simply transfer pollution downstream. Iwi from Maketu, on the coast, said increased nutrients in the water would affect the mauri [life force] of the Kaituna River and create adverse health effects for locals. Others argued it would pollute the coast with additional nitrogen and phosphorous.
Since the wall was finished, fishermen have also voiced concern about the possible effect on fish stocks in Lake Rotoiti, but Mr Bruere said there was no evidence either way yet.
The water in the Rotorua lakes has become degraded over decades, and the high nutrient content can cause toxic algal blooms, particularly in summer.
The blooms are unsightly, and can be harmful to animals and humans.
