Lake Mangahia
General Information
Lake Mangahia was regarded as one of the least modified lakes in the Waipa District. However, the surface area of Lake Mangahia has shrunk markedly since 1943 when the area of the lake was 15.3 ha. In 1992 the open water area was 11.3 (Thompson & Champion, 1993) and in 2002 the area had declined to 10 ha. Environment Waikato has installed a concrete weir and an automatic water level recorder on the outlet of Mangahia to help prevent further shrinkage of the lake area and the surrounding peat.
It is disappointing to see the recent decline in trophic state of this lake so well documented in the literature. In 1993 Thompson & Champion recommended Mangahia as an excellent choice for restoration as it is a good example of a minimally modified peat lake (dystrophic) system. Both Thompson & Champion (1993) and Champion et al. (1993) realised this lake was an excellent target for restoration, but was at risk due to the clearing of native riparian vegetation and the leaching of nutrients from nearby dairy effluent ponds. In a recent report Lake Mangahia was classified as hypereutrophic, and suffering from high nutrient and suspended sediment concentrations (Barnes, 2002).
Restoration Action
Currently the lake is fenced to exclude stock and there is some marginal vegetation, although this appears to be dominated by Salix cinerea (Pussy willlow). Maintaining the lake level using a weir to prevent peat shrinkage is in action. Fencing and establishing riparian margins on streams flowing into Lake Mangahia to exclude stock and help decrease sediment loads would be highly desirable. Thompson and Champion (1993) recommended at least a 50m buffer riparian margin should be established to prevent further peat shrinkage and associated lake surface area decline, with a further 50m buffer zone with limited drainage allowed. Species recommended for riparian plantings are Leptospermum (manuka), Phormium (flax), Dacrycarpus (kahikatea), Baumea spp. and Eleocharis sphacelata.
Lake Mangahia is privately owned, therefore it is not administered by the Waipa district council, Environment Waikato or the Department of Conservation. This has probably severely limited restoration actions to date.
Lake Mangahia Statistics
| Area | 10 ha |
| Maximum depth | 3.2 m |
| Trophic state | Hypereutrophic |
| Recreation | Pastorally developed peat bog |
| Restoration progress | Game bird hunting |
| Peat influence | Concrete weir to control water levels. Fenced to exclude stock. |
| Reserve status | High, edge of Rukuhia peat bog |
| Submerged vegetation | Privately owned land, with no public access. |
| Harmful algae | |
| Invasive fish |
Images
Lake Mangahia. Photo taken by Brendan Hicks
References
Barnes G. (2002). Water quality trends in selected shallow lakes in the Waikato region: 1995-2001. Environment Waikato Technical Report No. 2002/11.
Champion et al. (1993). The Vegetation of the Lower Waikato Lakes. Volume 2: Vegetation of thirty-eight lakes in the lower Waikato. NIWA Ecosystems Publication No.8 August 1993.
Thompson K. & Champion P. (1993). Esplanade reserve recommendations for Lakes Serpentine, Mangahia, Rotomanuka, Ruatuna and Cameron (Waipa District). Conservation Advisory Science Notes No. 47. Department of Conservation, Wellington.
