Limnology of Lake Rotokawa and its outlet stream

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Abstract
The water chemistry, flora, and fauna of Lake Rotokawa (38° 37.8’ S, 176° 11.2'E) was studied in 1975–76. The mean pH is 2.1 and thermal inflows may elevate the mean summer temperature of the surface waters 4.2°c above that of nearby cold water Lake Rotongaio (18.9°c). The temperature range of surface water was from 10.1 °c in winter to 23.1°c in summer. The major anions were SO4 2− 679 g.m−3, and Cl‐ 314 g.m−3. Mean concentrations of major cations were Na+ 224 g.m−3, K+ 28.9 g.m−3, Ca2+ 13.3 g.m−3, and Mg2+ 2.6 g.m−3. Two species of flagellate algae were recorded, of which Euglena anabaena was predominant. Only two benthic macroinvertebrates were found, larvae of Chironomus zealandicus, mean density 253 per square metre, and Helobdella sp., 1.3 per square metre. The Parariki Stream was influenced by thermal springs in its upper and lower reaches, being cooler (24–25°c) about halfway along its length than near its source (27.8–39.0°c) or confluence (26.5°‐28.0°c) with the Waikato River. In the cooler stretch of the stream where unidentified benthic algae were not limited by high temperature, chlorophyll and total pigment increased from 3.9 to 377.9 mg.m−3 and from 17.5 to 534.4 mg.m−3 respectively, and nutrient levels fell (NO3‐N, 22–10.5 mg.m−3; NH4‐N, 6440–230 mg.m−3; and PO4‐P, 51–19 mg.m−3).