A Monitoring and Classification System for New Zealand Lakes and Reservoirs

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Abstract
Five variables gave strong indications of change in the trophic state of New Zealand lakes, namely; concentrations of chlorophyll a, total phosphorus and total nitrogen, as well as Secchi depth and dissolved oxygen depletion rate. The New Zealand Lake Monitoring Program, investigated 17 lakes for 3 to 4 years to develop a method for detection of small changes in these five variables and to develop a cost-effective monitoring system for lakes. Two different methods were developed which enabled trends observed separately for variables to be combined into an indicator of change (PAC, Percent Annual Change) and an index of trophic state (TLI, Trophic Level Index). The PAC is a relative indicator that gives the average percent annual change of the trophic state variables, and is an indicator of the magnitude of the change observed. It is calculated from the 5 variables listed above. The TLI is a numerical indicator of lake trophic level, which typically varies from 2 (oligotrophic) to 7 (supertrophic). The TLI can be examined for time trends that generally reflect the changes detected by the PAC relative indicator. These two indicators, when used together, give a good estimate of the probability and magnitude of trophic level change in a lake as well as the trophic level of the lake. Results for all the monitored lakes are given. The PAC and TLI indicators also gave useful results when used to study reservoir monitoring data.