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The zooplankton communities of seven Rotorua, New Zealand, lakes of different trophic status were studied in 1977-78. They were generally dominated by the calanoid copepod, Calamoecia lucasi. Bosmina meridionalis occurred in all the lakes and Ceriodaphnia duhia in most. Only small numbers of Macroc~~clops alhidus ever occurred. Rotifers were not studied in detail. Community composition was similar to that in other northern New Zealand lakes. No well defined patterns of seasonal change in abundance were found and the timing of changes, which were of low magnitude, was different in each lake. Clutch sizes in all species were small. Calamoecia population parameters were analysed using multivariate methods and shown to be related to lake trophic level. Population densities were higher in more productive lakes whereas breeding levels were inversely related to indices of trophic status and population abundance. It is suggested that the populations, as in other northern New Zealand lakes, are food-limited, probably as a consequence of a lack of marked climatic seasonality and the absence of major predation pressures. Groupings of the lakes based on the Calamoecia data are in general agreement with those derived from parallel studies of water chemistry, phytoplankton and macrobenthos.

Lake Ototoa is a warm monomictic lake at 36° 31' S, 174° 14'E. During a year's study (March 1969-March 1970), the lake became thermally stratified in November, the metalimnion being between depths of 12 m and 16 m. Surface temperatures ranged between 10.2°c (in August) and 25.2°c (in late January), and bottom temperatures between 9.7°c and 17.5°c. The annual heat budget was calculated to be 642 354 KJ.m-2 (15 500 cal.cm-2) and the work of the wind in distributing the heat income 1.730 KJ.m-2 (1766 g.cm.cm-2). Secchi disc transparencies ranged between 5 m and 9.2 m (mean 7.07 m) and were greatest in the summer. Light transmission per metre was also high, ranging between 61% and 87%. Surface waters were normally supersaturated with oxygen, but during summer stratification oxygen concentrations in the bottom waters dropped to a minimum of 2.3 mg.litre-2 and a positive heterograde distribution of oxygen with depth was found. The oxygen deficit was 0.015 mg.cm-2 .day-1 and showed the lake to be oligotrophic. Mean surface pH was 7.82, and the ionic composition of the waters was similar to that of other small New Zealand and Australian lakes located near the sea. Compared with other New Zealand lakes PO 4-P concentrations (range 1.00-10.20 µg.litre-1) were low and NO 3-N concentrations (range 0.12-0.60 mg.litre-1) high.

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