New Zealand Journal of Marine and Freshwater Research abstracts

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LIMNOLOGY OF LAKE NGAHEWA, NORTH ISLAND, NEW ZEALAND

D. I. Forsyth
R. H. S. McColl*

Ecology Division, Department of Scientific and Industrial Research, P.O. Box 415, Taupo, New Zealand

Abstract The water chemistry, flora, ana fauna of Lake Ngahewa are described. Its waters are coloured by humic material from a flax swamp and the mean carbon: nitrogen ratio of the sediments lies in the range for dystrophic lakes. Thermal stratification is very weak or absent during summer, although deoxygenation occurs at depths below 3 m in late summer. The lake is highly productive and has a diverse phytoplankton and zooplankton population, which is different in species composition from other nearby eutrophic lakes; some alga] species common in nearby eutrophic lakes are absent, and there is a high ratio of rotifers to cladocerans and copepods. The high levels of dissolved phosphorus (42.2 mg.rrr³) and nitrogen (up to 36.1 mg.ni-³ NOr-N) in the lake waters, and the differences in the flora and fauna compared with neighbouring lakes give only weak support to its tenuous classification as a dystrophic lake on the basis of water colour and sediment carbon values. The concept of dystrophy is discussed.

N.Z. Journal of Marine and Freshwater Research 9 (3): 311-32
(Received 16 January 1975)

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