Nelson’s Blue Lake – The clearest freshwater ever reported

6 June 2017

Recent research by NIWA scientists reveal that Blue Lake, in Nelson Lakes National Park, has extreme visual clarity, perhaps only exceeded worldwide by certain ocean waters, such as those in the SE Pacific near Easter Island. The visibility of Blue Lake even surpasses that of the renowned Te Waikoropupu ("Pupu") Springs in Golden Bay.

Nelson-based NIWA hydrologist Rob Merrilees first recognised that Blue Lake might be optically outstanding, having observed on tramping trips that this water body appeared broadly similar to Te Waikoropupu, in which a visibility of 63 metres has been reported. He mentioned his suspicions to NIWA aquatic optics specialist, Dr Rob Davies-Colley, who had led the original work on Te Waikoropupu Springs. On a preliminary tramping visit to Blue Lake in March 2009, the two Robs were surprised to find that the visibility of Blue Lake exceeded that of Te Waikoropupu.

They subsequently organised a scientific study of the lake with involvement of NIWA scientist Mark Gall, an expert in ocean optics instrumentation. Several visits by helicopter (six in all) established that the horizontal visibility in the lake typically ranges from 70–80 metres.

"The theoretical visibility in distilled water is about 80 metres, as estimated from the best available instrumental measurements in the laboratory," says Dr Davies-Colley. "So Blue Lake is a close approach to optically pure water".

Blue Lake is characterised by blue-violet hues seen only in the very clearest natural waters. It is highly unusual in its geological and hydrological setting, being apparently spring-fed from neighbouring glacial Lake Constance. Almost all suspended particles appear to be filtered from the water as it passes through landslide debris that forms a dam between the two lakes - which probably accounts for its extreme visual clarity.

Water clarity is measured either as spectral light penetration in the vertical direction, or as visual range ("visibility") in either the vertical or horizontal direction. In shallow Blue Lake (7m), vertical measurement of visibility was not an option so visual observations were made horizontally.

The research on Blue Lake was conducted from a small (oar-powered) inflatable boat, dropped onto the lake shore by helicopter together with the scientific team. "We suspended a one metre diameter black disc under a buoy and anchored that to the shore. Then we paddled away while viewing the disc. The distance at which it disappears completely gives the visibility," says Mark Gall.

Water visibility is a familiar concept to divers for whom visual range (such as in very clear seawater) is considered "outstanding" when it exceeds around 40m. A black target (e.g., a black disc) is used for scientific measurements because its visibility is not affected by lighting conditions (overcast, cloudy or sunny) or solar altitude. The black target is observed as a dark 'hole' contrasting with the background light field within the water.

"It is highly significant that Blue Lake varies only very slightly from pure water. The absorption of light by pure water is not well known in some parts of the spectrum, including the ultraviolet. That means the best data may actually come from the clearest natural waters like Blue Lake and Te Waikoropupu Springs, and extremely clear sea water," says Dr Davies-Colley.

"It's also 'refreshing' to see that, while water quality has generally declined over recent years in most developed parts of New Zealand, in conservation land we have a freshwater that is truly outstanding in terms of optical purity."

On rare occasions, after heavy rain, Blue Lake can become temporarily murky, when particles washed in by surface runoff have been dispersed in the water. For a few days visibility may be temporarily reduced to perhaps one metre. "The water still stays blue, but appears bright and milky coloured," says Dr Davies-Colley.

Because of the heavy rain in the Nelson region over the last few days, the lake is likely to be murky right now, but may be expected to clear within a few days.

The spring-fed source of the water, and its altitude above sea level (1200m), just below the tree line of stunted mountain beech, ensures that Blue Lake is always cold, ranging between 5 and 8°C.

This research was funded by NIWA, and supported by the Department of Conservation. NIWA is currently planning future surveys with DOC approval to more thoroughly investigate the biodiversity of Blue Lake, the limnology (freshwater science) of Lake Constance and the connections between the two.

Background:

Blue Lake is a side trip off the popular Travers Sabine Circuit multi day tramp.
The Blue Lake Hut receives in excess of 700 trampers a year, mainly through the summer months.
Numbers will be monitored to see if visitation increases.
The clarity of the lake will remain monitored to establish if there are any changes in the next year or two.
The waters of Blue Lake/ Rangimairewhenua are sacred to Ngāti Apa ki te Rā Tō iwi. The significance to them was acknowledged in their Treaty of Waitangi settlement signed in 2010.
The Department of Conservation would ask that the waters are respected and that people don't wash themselves or their clothes or dishes in the lake.