New algal species in Rotorua lakes not toxic

A new algal species occurring in the Rotorua lakes, including the Okere Arm, is not toxic, according to test results.

Environment Bay of Plenty Environmental Scientist Matt Bloxham said that toxins from Microcystis wesenbergii, which has appeared in the Rotorua lakes for the first time, were either absent or below detection limits.

“It has been quite dominant this year in Lake Rotorua, in Okere Arm (Lake Rotoiti) and in the Kaituna River, yet hasn’t occurred in high levels in the main body of Lake Rotoiti,” Mr Bloxham said.

“Indications are that where M. wesenbergii vastly outnumbers other cyanobacteria species, blooms are less likely to be producing toxins. However, monitoring of the lakes since 1997 has shown us how much the species makeup of blooms can change. What might test as non-toxic one week can change the next if the species composition changes, so it is important to continue heeding health warnings,” Mr Bloxham said.

Environment Bay of Plenty environmental scientists and the Environment Bay of Plenty Chair in Lakes Management and Restoration, Professor David Hamilton, met with residents in the Okere Arm area last month to listen to their concerns and talk to them about the bloom activity.

Algal blooms are symptomatic of a catchment problem, caused by nutrients that have entered the lake over time and new nutrients entering the lakes from ground and surface water.

While there have been no sustained blooms in Lake Rotorua over the past two seasons, this all changed in March when Environment Bay of Plenty began recording prolonged bloom activity in the lake. “Unfortunately if there is a period of intense bloom activity in Lake Rotorua, blooms may also register downstream. Entrainment of blooms leaving Rotorua via the Ohau Channel diversion has led to elevated bloom activity in the Okere Arm and in the Kaituna River.”

By channelling water from Lake Rotorua, the Ohau Channel Diversion Wall has helped prevent bloom activity from occurring in the main body of Lake Rotoiti this season. “While this is something to celebrate, it has certainly drawn attention to the blooms in Okere Arm,” Mr Bloxham said.

However, the blooms do appear to be subsiding in Lake Rotorua, in the Okere Arm and in the Kaituna River. “Based on current trends and cooling lake temperatures this looks set to continue,” Mr Bloxham said.

“Lake dynamics are complex, and it is very difficult to tell from year to year which species will dominate. In the past three years we haven’t seen as much bloom activity across both lakes as we did say six years ago. This current bloom activity is not necessarily a sign of things to come; we may have a clear season next year. If Lake Rotorua and Lake Rotoiti both have clear years, then it is likely so too will Okere Arm and the Kaituna River,” he said.

Environment Bay of Plenty is working with farmers and landowners through its Regional Water and Land Plan, Ten Year Plan and the Resource Management Act to reduce the amount of nutrients entering the lakes. The regional council works in tandem with Toi Te Ora – Public Health and the Rotorua District Council in running the cyanobacteria monitoring programme for the lakes.

ENDS

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