Comparing past and present trophic states of seven Central Volcanic Plateau lakes, New Zealand

Data on seven lakes in the Rotorua District were examined to determine trophic state changes in these lakes. The method of data analysis was specifically designed to find small changes in the trophic state of lakes. For each variable, the P-value from 12 monthly paired sample Student t-test comparisons was calculated and a change of trophic state value (CTS) was assigned, based on the P-value. The CTS values for all the variables examined for a single lake were averaged (C) with a standard error (CV). The resulting Change/Confidence Value index (C/CV) indicated the nature of the trophic change observed and the degree of confidence that could be placed in the stated change. This study finds that in recent years Lakes Rotorua and Rotoiti have become less eutrophic; Lakes Okareka and Rotoma may have become less eutrophic; Lakes Okataina and Rotokakahi have not changed in trophic state in the last 22 years, and Lake Tikitapu may have become more eutrophic.