Chemical levels in lake high
Extremely high levels of phosphorus in the sediments and lower waters of Takaka’s Lake Killarney at may be driving worrisome algal blooms.
Tasman District Council resource scientist Trevor James on Thursday told councillors NIWA was working on a report about the lake after samples of water and sediment were taken and tested.
‘‘The early indications and interpretations they’ve provided on the data so far is that they’ve never seen phosphorus levels in the sediments of a lake or the bottom waters quite this high – extremely high phosphorus levels – and that’s going to be one big reason for the driving of these algal blooms,’’ James said.
In a report, James says algal blooms were recorded in Lake Killarney in 2014 for the first time in living memory.
‘‘Over successive summers, these blooms appear to cover more of the lake, are more dense and occur for longer periods,’’ he says.
‘‘The council has received a few complaints since that time and some owners of property around the lake claim that their property values have declined as a result of the unsightly blooms.’’
Takaka residents bordering the reserve, which sits behind Commercial St near the town centre, are increasingly concerned about the discolouration of the lake and potential effects of the algal blooms.
In his report, James says a ‘‘major source of nutrient contamination’’ to the lake was discovered in August when two discharges of untreated stormwater from a council system were observed within three days.
The stormwater entered a sump on neighbouring farmland and discharged to the lake.
Samples were taken and sent to a laboratory for analysis.
‘‘From these data, loading of nitrogen and phosphorus to the lake was estimated to be 700g of total nitrogen and 320g of total phosphorus for the two events sampled,’’ James says in his report.
It was estimated similar dis- charge events flowed into the stormwater system from the sump in the farmland eight to 10 times a year.
‘‘Taking an average concentration of nutrients over these events, then the total annual load to the lake could be 2.8-3.5kg of total nitrogen and 1.3-1.6kg of total phosphorus,’’ he says.
‘‘This is more than enough to cause algal blooms in a lake of this size after a period of nutrient accumulation over about 10 years.’’
After the meeting, James said the completed report by NIWA was expected before Christmas and it would recommend treatment options for the lake.
The solutions might include adding chemicals to lock up the phosphorus so it was not available to fuel algal growth.
Meanwhile, the council’s engineering department was investigating how to divert ponded water in the dairy farmland away from the lake.
‘‘There was an option to reroute that runoff from the pasture in a different direction,’’ James told councillors. ‘‘Lakes are very, very sensitive receiving environments, especially lakes like this that don’t have any outlet at all so they don’t get benefits of any flushing; any contaminants will just stay in there.’’
Golden Bay Ward councillor Paul Sangster said he believed runoff from a slip on a nearby hill and the road was ‘‘all going into Lake Killarney’’. ‘‘The lake was never like that,’’ Sangster said.
James said the exact solution would be determined once the analysis of the data was complete.