Water quality in the lower catchments of the Manuherikia and Clutha/Mata Au rivers continues to be poor, according to the latest State of the Environment water monitoring report.

The report Water Quality and Ecosystem Health in Otago Lakes and Rivers 2024 was considered by Otago Regional councillors at the Environmental Science and Policy Committee yesterday.

The report, from scientist - water quality Helen Trotter was for the period July 2023- June 2024, and required by the National Policy Statement - Freshwater Management.

In recent years, the programmes have been expanded to include surface water quality, and biological and physical habitat assessment data.

The report showed water quality was generally good, particularly in headwater and upper catchment areas, which comprised a significant proportion of the region. 

But in lowland areas and catchments where urban and intensive agricultural land was dominant, water quality was poor; phosphorus, sediment and E. coli were among the poorest performing attributes. 

“Components of ecosystem health can be influenced by both natural processes and conditions as well as human activities,” Helen said.

A snapshot of river and lake water quality across Otago. PHOTO: ORC

The environmental monitoring team completed monthly sampling for a suite of physio-chemical and microbiological water quality variables at 106 sites across 84 rivers and 14 sites across eight lakes. 

Electrofishing surveys were conducted at 16 river sites and macro-invertebrate samples were collected at 100 river sites, during the summer months. 

Lake Submerged Plant Indicators (LakeSPI) surveys were also undertaken across monitored lakes on a rotating basis.

 In the 2023-24 monitoring period NIWA divers completed surveys at three lakes.

Swimming spots across Central Otago/Queenstown Lakes monitored by LAWA over summer. PHOTO: SUPPLIED

The report showed the poorest water quality was found in urban streams and in the lowland rivers and stream of catchments.

During summer the regional council also monitored 17 freshwater swimming sites weekly across Otago in conjunction with Land Air Water Aotearoa (LAWA).

Based on data for the last five summers, 67 per cent of sites were graded excellent or good. But 33 per cent of sites were graded poor (below the national bottom line).

Helen said the eight lakes across Otago – the large lakes in the Upper Lakes and Dunstan Rohe (Lakes Whakatipu, Hāwea, Wānaka and Dunstan) all have excellent water quality and low nutrient levels.

“There is no defined national bottom line for dissolved reactive phosphorus, however about 13 per cent of sites are in the D band indicating the levels are substantially elevated compared to reference conditions (poor water quality, significantly impacted/degraded = D/E band).’’

Water quality results for monitored lakes showed about 75 per cent of sites were in the A band for all water quality attributes. But 37 per cent of river sites were in the D/E band.

LAWA will start monitoring swimming sites across Otago this week, and regularly over the summer period until March 15.

The Manuherikia River at Shaky Bridge in Alexandra has had the highest E.Coli rating per 100ml value since monitoring began five years ago, with a reading of 2420 recorded (levels over 540 were considered high).

From the samples last summer, 72 per cent of the time the river was suitable for swimming, 20 per cent of the time caution was advised and 7 per cent it was unsuitable.

LAWA also checks Lake Dunstan at Alpha Street in Cromwell, the Clyde Rowing Club, the Clutha River/Mata-Au at Dunorling Street, Pinders Pond and the Taieri River at Waipiata.

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