Development delivering undesirables
In the first in a series on water issues, former Guardians of Lake Wanaka member Laurel Teirney recalls the beginning of her water focused career in the 1970s.
At age 12, Lake Kaniere fostered my love of lakes. and I’ve indulged my passion for studying waterways over 45 years.
Since being a Guardian of Lake Wanaka a decade ago, I believe our lovely lake is not getting the attention it deserves, especially monitoring trends in water quality. Regional council policy of intensively sampling a single lake site once a decade was entirely inadequate. Thankfully, monthly monitoring started in 2010, but still at just a single site.
Since tenure review, farming the valley floors has intensified, with nutrients from fertiliser and livestock carried by streams and rivers into the lake. Last time I visited the Rob Roy Glacier, aerial topdressing planes were working the Matukituki Valley and on my return to the car, cattle were freely grazing the banks and relieving themselves in the river.
In Wanaka, development is delivering a range of undesirables to the lake. The most obvious is sediment from huge new subdivisions and slips. Intense rainstorms, now an accepted part of climate change, can deliver tonnes of sediment, and whatever it is carrying, into the lake.
This happened via Stoney Creek in 2004 and Bullock Creek last year. And when I was told that most of a lawnmower’s clients fertilise and irrigate their lawns it struck me that a ‘mosaic of mini farm paddocks’ is being created in town. Stormwater drains also carry every type of chemical, from road surfaces and town facilities, into the lake.
Why be so concerned? Because of Lake Tutira. In 1973, I began working on a lake manipulation project at the popular recreational lake, 50km north of Napier. Lakes react to a one-way input of nutrients in a reasonably predictable way.
There’s usually an excessive growth of introduced plants, such as Hydrilla in Tutira and Lagarosiphon. Microscopic green algae flourish, as nutrient concentrations increase. Dying blooms sink to the bottom and are decomposed, releasing nutrients back into the water.
Lake Tutira’s situation was so extreme the deep, cold water lost all oxygen within three weeks. With plant nutrients trapped in the deeper water, blue-green algae that fix nitrogen from the air flourish and create thick lake scum. This can kill small animals. In my case, ear infections and rancid abrasions were the legacy of diving during monthly sampling over four years.
The major cause of the incredibly rapid 10 year deterioration in Lake Tutira was the advent of aerial topdressing. Despite a huge effort, our attempts to lift deoxygenated water to the surface to absorb oxygen made little difference.
Last summer, 43 years and many other futile attempts later, Lake Tutira lost all oxygen, killing most lake inhabitants - a terrible first for a lake of this size in NZ.
Wanaka aquatic scientist Laurel Teirney.