Chemistry and trophic status of seven New Zealand lakes

Abstract

The chemistry of the surface and bottom waters of seven Rotorua lakes (Okataina, Rotoma, Tikitapu, Okareka, Rotokakahi, Ngapouri, and Okaro) was examined monthly between April 1970 and April 1971 in relation to the biology of the lakes. Twenty‐four parameters were measured, including dissolved 0₂, major ions, silica, phosphorus, nitrogen, algal pigments, and some trace elements. All lakes were warm monomictic, second‐class lakes. All lakes contained “bicarbonate waters” except Rotoma which had “chloride waters”. The waters were soft and variations between lakes in composition of major cations and anions appeared to be related to catchment differences. The major cations and CT varied little with season. Sulphate varied seasonally in the hypolimnia of the most productive lakes (Okaro, Ngapouri, and Rotokakahi). Silica was abundant in all lakes, except Tikitapu where diatoms were poorly represented. Depletion of silica by diatom populations was only observed in the two most productive lakes (Okaro and Ngapouri). All lakes except Ngapouri showed anomalous silica distribution, with lower concentrations in the bottom waters at all times. Copper and zinc varied little between lakes and depths, but showed marked peaks during winter circulation. The combination of the difference in alkalinity between surface and bottom waters during summer stratification, mean concentrations of total phosphorous, phosphate, nitrate, hypolimnetic ammonia, chlorophyll, hypolimnetic dissolved oxygen, hypolimnetic iron and manganese, and water transparency was used to classify Okaro and Ngapouri as eutrophic, Rotokakahi and Okareka as mesotro‐phic, and Rotoma, Tikitapu, and Okataina as oligotrophic. Total P, chlorophyll, and bottom oxygen concentrations could be correlated with land use in the lake catchment. A linear multiple regression equation was derived which related hypolimnetic oxygen concentration to a land‐use index and lake depth. Algal growth was most vigorous during winter circulation and in some lakes a related phosphate “depletion phase” occurred. Nitrate concentrations were low by world standards, except in the most productive lakes during winter circulation. Nitrite occurred irregularly in the surface waters of some lakes and in trace amounts in the hypolimnia of all lakes in late summer.