New footage has revealed the spectacular clarity of one of New Zealand's largest freshwater springs - now at the centre of an environmental row.
Te Waikoropupū Springs, near Takaka, has some of the clearest waters ever measured, along with largest cold water springs in the Southern Hemisphere.
Local iwi and other groups have been fighting for legal protection for the springs, through a Water Conservation Order now being considered by the Environmental Protection Authority.
Those arguing for the order were worried about water being taken for dairy farming essential upstream in the catchment, and the impact of nitrates.
Niwa recently finished a three-month monitoring project at the springs measuring the clarity of the water for Tasman District Council (TDC).
Scientist Mark Gall used a GoPro camera to film the deployment of the monitoring instruments and to check they were in the right place.
The underwater footage he captured also provided him with some insight into how the springs work, and how the water clarity varies for short periods when water pushes out through layers of sand.
Niwa estimated the visual clarity of the springs to be about 76m, although at times it approached 81m which is just short of the theoretical maximum of pure water at about 83m.
That was similar to the globally renowned Blue Lake, in Nelson Lakes National Park.
The last time water clarity measurements were made at Te Waikoropupū was in 1993 by the Department of Scientific and Industrial Research.
At that time the visual clarity was measured by divers at 63m.
Since then, technology has advanced, theoretical maximum clarity measurements have been revised and divers were no longer permitted in the spring.
That prompted Niwa to use measuring instruments hung from a submerged line near one of the main vents.
This novel deployment method, similar to a zip line, was used to position the instruments with the line then lowered into place below the surface.
The main instrument used is a beam transmissometer which captured 60 readings in one-minute bursts, every 10 minutes, resulting in nearly a million water clarity estimates over the deployment period.
This provided a comprehensive data set to assess how visual clarity in the springs varies over different time scales, from minutes to months.
"What this study shows us is that the Te Waikoropupū Springs have exceptional visual clarity, at times close to that of pure water, and broadly comparable to that of Blue Lake which has a visual clarity of 70 to 80m," Gall said.
The high frequency monitoring detected small daily variations corresponding to about 1-2m in visual clarity, with the highest visibility at midnight, and lowest around midday.
"This is likely to be due to plants in the spring basin releasing light-scattering oxygen bubbles as they photosynthesize during the day," Gall said.
Underwater video demonstrated that "dancing" white marble sands on the floor of the springs coincided with some short-term episodes of reduced visual clarity, lasting between a few hours and several days.
The exceptional visual clarity appears to result from extremely efficient natural filtering removing particles within the springs aquifer before re-emergence of the water.
Maintaining the existing clarity of Te Waikoropupū was a priority identified through TDC's collaborative planning process with the Takaka community under the National Policy Statement for Freshwater Management.