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Lake Rotomanuka

Lake Rotomanuka is made up of two open water areas - Rotomanuka North and Rotomanuka South, which are remnants of a once larger single lake. It is located about 12 kilometres north of Te Awamutu. Water monitoring shows that the water quality of the lake is probably degrading over time.

Photograph of Lake RotomanukaOn this page: A remnant of a larger lake, Water quality in Lake Rotomanuka North, Changes in water quality over time, Why water quality declined, Can the lake’s water quality improve?

Lake Rotomanuka is located about 12 kilometres north of Te Awamutu. Rotomanuka North has 12.3 hectares of open water and a maximum depth of 8.7 metres, while Rotomanuka South has five hectares of open water and a maximum depth of 4.8 metres.

The Rotomanuka catchment is pastoral, surrounded on all sides by intensive dairy farming.

Lake Rotomanuka is administered by the Department of Conservation as a wildlife management reserve. The reserve lies on the edge of the Moanatuatua peat bog, most of which has been drained.

Find out more about the extent of wetlands in the Waikato region in 1840, and what we’ve got today.

A remnant of a larger lake

Lake Rotomanuka North is a remnant of a once larger lake. Several factors lead to the shrinkage of the original lake into two distinct lakes:

  • drainage in the eastern and northern catchments
  • diversion of water from the Lake Rotopiko (Serpentine) catchment during the 1950s and 1960s
  • lowering of an outlet channel in 1973
  • subsequent lowering of lake water levels.

Rotomanuka North is now isolated from Rotomanuka South. However, the two lakes are linked by 10 hectares of marginal wetland that formed following the lowering of the lake.

Water quality in Lake Rotomanuka North

Results from water quality monitoring carried out by Waikato Regional Council show that Lake Rotomanuka North is eutrophic, and that Lake Rotomanuka South is hypertrophic.

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

Changes in water quality over time

Environment Waikato collected monthly water quality samples from Rotomanuka North between October 1995 and December 2004. Results shows a decline in water clarity prior to 1998, but little change since then. Levels of Chlorophyll a and nutrients were reasonably stable.

Rotomanuka South was monitored from 1995 to 2001 and showed increasing levels of chlorophyll a and decreasing water clarity. Nutrient levels showed little change.

Why water quality declined

Lake Rotomanuka North used to have large areas of the submerged aquatic plant Egeria densa (‘oxygen weed’). However populations of Egeria densa died off during the summer of 1996/97. The following decrease in water clarity was probably due to large amounts of decomposing plant material. Levels of nutrients and chlorophyll a did not increase.

The more degraded state of Rotomanuka South is probably a result of contaminants entering the lake from neighbouring pastoral land. A major drain that flows into the lake used to receive treated effluent from neighbouring dairy farms.

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

Can the lake’s water quality improve?

The poor water quality of Lake Rotomanuka South is probably a result of contaminants entering the lake from neighbouring pastoral land. Drains that are not fenced and protected from stock can act as nutrient ‘highways’, channelling nutrients into the lake.

To stop the lake’s water quality from deteriorating further, there needs to be better management of drains and waterways feeding into the lake.

Find out more about fencing farm drains.