The Banks Peninsula Zone Committee meeting schedule. The committee meets on the third Tuesday of each month.
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Te Roto o Wairewa / Lake Forsyth is still in a state of cyanobacteria bloom; the warning has not been lifted since the summer. Weekly monitoring of cyanobacteria concentrations has shown a large reduction in the amount of cyanobacteria from April, but it has hovered just above the concentration where a warning could be lifted (see note below).
This is unusual; we would normally expect the colder winter weather to have caused the cyanobacteria to have died back to well below warning levels. This is the longest period of warning for the cyanobacterial bloom I can recall but it follows a long period with no, or very short duration, warnings. So I don’t believe it warrants a permanent warning.
The current lake level is 1.43m above sea level. This is reasonably low, but is in response to a lake opening the last week of August. Before the lake was opened it was 2.3m above sea level, which is high. The lake stayed at quite a low level throughout May and some of June, but since then it has been opened twice (including the August opening) and reached 2.55m above sea level in mid June. So it has not been particularly low during the winter.
There is no simple correlation between lake level and cyanobacterial concentrations or water quality. Cyanobacteria respond to a variety of environmental conditions including temperature, concentration of available nutrients (particularly dissolved phosphorus) and salinity. All of these are influenced by water level, but there is no direct correlation as they are also influenced by other factors such as amount of wind, daylight length and river inflows. Trying to predict cyanobacterial blooms is very difficult, so we can’t say whether it is likely to get worse soon.
The Wairewa / Lake Forsyth sub-regional plan (Plan Change 6 to the Land & Water Regional Plan), recommendations on which have recently been accepted by Environment Canterbury, concentrates on reducing phosphorus inputs into Te Roto o Wairewa / Lake Forsyth in order to reduce cyanobacterial blooms. This is a positive step towards improving the water quality of the lake.
Read here for more information
Dr Tim Davie
Surface Water Science Manager
Note: In April the concentration of toxic cyanobacteria was about 3000 mm3/l; recent readings have been between 2 and 10 mm3/l. The threshold for a warning notice is 1.8 mm3/l.
The Banks Peninsula Zone Committee works with community members to develop solutions to local water management challenges. Issues facing Wairewa / Lake Forsyth have been a key focus for the committee, leading to the Wairewa ZIP Addendum.
Environment Canterbury, Lyttelton Port Company Ltd. Te Hapū o Ngāti Wheke, Christchurch City Council and Te Rūnanga o Ngāi Tahu have agreed to work together to develop a catchment plan for the harbour in accordance with the philosophy ki uta ki tai (from the mountains to the sea).
A key objective is to restore the ecological and cultural health of the harbour as a source of mahinga kai. The plan will also address other environmental, cultural and social concerns, including the needs of recreational users and the needs of a working port. Read more
View larger map (jpg 268 kB)
Banks Peninsula Zone Committee Annual Report for 2015
Canterbury Water Management Strategy (pdf 3.28 MB)
Wairewa ZIP Addendum (1.89 MB)
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