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Lake Rotokauri

Lake Rotokauri is located north-west of Hamilton city. Water quality in Lake Rotokauri declined significantly during the 1990s. This was probably due to large numbers of submerged aquatic plants dying off. Work is now being done within the catchment to improve water quality for the future.

On this page: Water quality in Lake Rotokauri, Changes in water quality over time, Why water quality declined, Can the lake’s water quality improve?

Photograph of Lake Rotokauri

Lake Rotokauri is located seven kilometres north-west of Hamilton city. It has a surface area of 77 hectares and a maximum depth of four metres.

Lake Rotokauri is a peat lake and is part of the Waipa peat lake complex. The catchment area surrounding the lake is a mixture of three main land uses:

  • residential
  • industrial
  • pastoral.

Lake Rotokauri drains to the Waipa River through the Ohote Stream. A weir at the outlet controls the lake level.

Water quality in Lake Rotokauri

During 1993 and 1994 Lake Rotokauri was classed as eutrophic. Although nutrient enriched, Lake Rotokauri's water clarity was good when compared to other peat lakes in the Waikato region.

However, between then and 2001 water quality declined significantly. Water clarity became much worse and the lake became much more nutrient enriched. The lake is now classed as hypertrophic (very nutrient enriched).

Check out our Nutrient Enrichment of Shallow Lakes indicator and its data for Lake Rotokauri.

Changes in water quality over time

Waikato Regional Council collected water quality samples monthly from Lake Rotokauri, during:

  • February 1993 to June 1994
  • August 1997 to December 2001.

The average annual water quality results are summarised in the table below. They show a significant decline in water quality between 1993 and 2001.

Measurements 1993 2001
Secchi depth (metres) 1.7 0.6
Chlorophyll a
(milligrams per cubic metre)
9.5 79
Total phosphorus
(milligrams per cubic metre)
39 118
Total nitrogen
(milligrams per cubic metre)
742 1900

Find out more about typical levels for secchi depth, chlorophyll a, total phosphorus and total nitrogen in lakes of different nutrient (trophic) state in New Zealand.

Why water quality declined

Photograph of oxygen weed

The decline of Lake Rotokauri’s water quality through the 1990s was probably related to large areas of the submerged aquatic plant ‘oxygen weed’ (Egeria densa) dying off.

The waters of Lake Rotokauri were once dominated by the introduced Egeria densa. In 1979 Egeria densa covered 60 per cent of the water surface, and by 1989 this figure had risen to nearly 100 per cent. However, in the summer of 1996/97, neighbouring residents noted that large areas of Egeria densa were dying off. Observations by Waipa District Council confirmed that populations of Egeria densa had collapsed and are currently still in a collapsed state.

Find out more about problem aquatic plants in the Waikato region.

Can the lake’s water quality improve?

Microscopic algae (phytoplankton) are now the dominant plant life in Lake Rotokauri. While water clarity is low and there is little light penetrating the water, it’s unlikely that the lake will go back to having healthy populations of large aquatic plants (macrophytes).

However, there has been work done within the catchment to improve Lake Rotokauri’s water quality. This includes:

  • reconstructing a weir - to control water levels within the lake
  • installing a sediment trap on the main inflow to reduce the amount of sediment entering the lake
  • fencing the lake from stock and maintaining a planted buffer area between the lake and farm activities
  • restoring wetland areas next to the lake – which will help to ‘filter’ out pollutants and excess nutrients before they enter the lake.

Find out more about how land use affects the water quality in our region’s lakes.