In reply to a recent article (“Is our water quality testing of Lake Hawea proactive enough?”), the Guardians of Lake Hawea would like to reassure readers that the change in lake colour this summer is not due to an algal bloom.
The change from clear to bright or milky blue is caused by an influx of sediment from the lake’s major tributary, the Hunter River, at the head of the lake (photos 1 & 2).
Photo 1: Lake Hawea adjacent to the Hunter River delta on March 10 2018. Delta of Big Hopwood Burn visible near top of picture. The water is discoloured with sediment near the river mouth. Trevor Chinn photo.
Photo 2: The lower Hunter River on March 10 2018. The water is milky with sediment. Delta of the High Burn at lower left of the picture. Trevor Chinn photo.
This occurred on and following 1 February with heavy rainfall from ex-tropical cyclone Fehi, and is unlikely to be related to warm summer temperatures in the Queenstown-Lakes district.
Photo 3: Flight over Lake Hawea on March 29 2018. Sediment-laden water flows from the Hunter River. Highburn Delta on left and Big Hopwood on right. John Taylor photo.
Even if the colour does not return to clear in the near future, this is not an issue of poor water quality or the health of the lake. People who have lived near Lake Hawea for many years say that such colour changes after rainstorms have happened in the past.
So the short answer is that there should be no concern that warm weather and algal blooms have been a concern at Lake Hawea.
In the longer answer, below, the Guardians deal with some of the other questions in the article.
Is our water quality testing of Lake Hawea proactive enough?
The water quality testing is carried out by Otago Regional Council (ORC), which is currently 18 months into a “3 years out of every 10” State of the Environment Reporting Exercise. This means that the Regional Council are currently doing substantially more than they would normally be doing to monitor Lake Hawea water quality.
Following is a list of what ORC are currently doing:
On a monthly basis, samples at one site in Lake Hawea (4.5-5kms from the dam) and one site further north on a quarterly basis.
At each of these sites the following is undertaken:
Lake Hawea: Open Water “Surface” Sample
– Sonde profiles – chlorophyll fluorescence, dissolved oxygen, pH, and temperature.
– Secchi depth (shaded side of boat with bathyscope)
– Lake Snow tows
– algal biomass and community composition
– Zooplankton tow (drop net to 150m and slowly retrieve). These samples are being preserved and archived – not being analysed.
• Pooled sample from 0.5, 15, 30 and 45 metres
• Total nutrients, total dissolved nutrients and dissolved nutrients
• Total Organic Carbon / Dissolved Organic Carbon
• Chla (2 litre sample)
• Phytoplankton cell counts
• Suspended and volatile solids
Lake Hawea: Open Water “10m” Sample
• Sample AT 10m
• Total/dissolved nutrients
• Chla (2 litre sample)
• Sonde profiles / Secchi depth
– algal biomass only
Lake Hawea: Open Water “Deep” Sample
• Sample AT 150m
• Total/dissolved nutrients only
These samples are taken by Regional Council staff and are analysed by Watercare Laboratory, who are contracted by the Council to undertake this work.
Otago Regional Council are currently considering an increased monitoring programme for lakes Hawea, Wanaka, Wakatipu and Hayes (Otago Daily Times, April 23 2018).
Is the change in water quality only temporary?
Yes, water clarity will recover when sediments settle out and or flow through the lake. Another event like this has occurred on Lake Wakatipu, where the whole lake (right down to Kingston) changed colour in January 2014 after a massive landslide into the Dart River. The upper part of Lake Wakatipu is still cloudy when the Dart River floods, but much of the lake is clear much of the time
If the lake water clarity does return to normal, will the deterioration be repeated with increased hot weather spells in future summers?
As the cloudiness of the water is caused by sediment, any future hot weather should have no effect. The lake water clarity will be continue to be impacted by heavy rain (as at Lake Wakatipu).
Are our current tolerances of nitrogen levels within the lake too high given the probable increase in mean summer water temperatures resulting in risk to further and perhaps more serious algal blooms?
There have not been any “serious algal blooms”. Lake Hawea nutrients (nitrates and phosphates) are shown by ORC sampling to be very low. Phosphorus is at the limits of detection and total nitrates are only around 0.06mg/litre, with ammonium nitrogen around 0.005mg/litre.
In fact, the small cyanobacteria observed are some of the primary producers in the lake ecosystem – like the grass at the bottom of the food chain. They are not the larger forms such as Phormidium, which occur in warmer and more nutrient-rich water, and can give rise to toxic algal mats. It is incorrect to suggest that serious or toxic algal blooms occurred in Lake Hawea in the past summer.
on behalf of Guardians of Lake Hawea