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Lake Rotoroa (Hamilton Lake)

Lake Rotoroa is a small urban peat lake, formed by a peat dam on top of a levee of the ancestral Waikato River (Green and Lowe, in Clayton and de Winton, eds., 1994). Its catchment comprises parkland and residential housing. The lake is phytoplankton dominated following the collapse of Egeria densa beds in 1989-90. Following this collapse the lake’s trophic state initially improved as nutrients from the decay of the weed beds left the system but it is now in a stable phyto-plankton dominated state with some recovery of charophyte beds which is probably limited by coarse fish browsing and disturbance (de Winton, Safi, Taumoepeau and Burns, 2006). Egeria densa has also returned to some areas of the lake and is the subject of monitoring and control.

Restoration Action

Restoration actions have included the eradication of the exotic Iris pseudacorus (formerly the dominant lake margin vegetation) and its progressive replacement with characteristic native vegetation; the monitoring and control through herbicide application of Egeria densa; the re-introduction of Hyridella menziesi (freshwater mussels, or kakahi); passing stormwater from new developments through vegetation before allowing it to enter the lake; and improvements to edge treatments to minimise mobilisation of sediment into the lake.

Lake Rotoroa Statistics

Area

54 ha

Catchment area

138 ha

Maximum depth

6 m

Mean depth

2.4 m

Volume

1.3 x 106 m-3

Altitude

37 m a.s.l.

Residence time

~2 years

Trophic state

Eutrophic (TLI 4.76 +- .08, 2006)

Catchment

Residential and parkland

Recreation

Yachting, model boats, dragon boating, canoeing etc.

Restoration progress

Marginal pest plant eradication and replacement, level management, water quality monitoring and Egeria densa control, attempted nutrient input management.

Peat influence

Moderate, lightly peat stained

Reserve status

Owned and managed by Hamilton City Council subject to Reserves Act 1977

Submerged vegetation

Nitella sp. aff.cristata, Chara australis beds cover c. 21% of lake area. Isolated beds of Egeria densa.

Harmful algae

Anabaena planktonica, Microcystis aeruginosa, Microcystis flos-aquae,
Limnothrix sp., Phormidium sp., Pseudanabaena galeata all present in low numbers (2006 peak c. 1000 cells/ml.). No blooms recorded.

Invasive fish

Catfish, rudd, perch, tench, goldfish and gambusia.

Images

Lake Rotoroa Lake Rotoroa Aerial

References

De Winton, M.D., Safi, K., Taumoepeau, A., Burns, N. (2006). Lake Rotoroa Monitoring: 2005/06. NIWA Client Report HAM2006-080, National Institute of Water and Atmospheric Research, Hamilton, 2006-11-03.

Clayton, J.S., de Winton, M.D. (eds) (1994). Lake Rotoroa: Change in an Urban Lake. NIWA Ecosystems Publication No. 9, February 1994.