Protocol for Monitoring Trophic Levels of New Zealand Lakes and Reservoirs

New Zealand’s lakes and reservoirs are rarely static in their water quality or ecological condition. Trophic levels of lakes are critical indicators of water quality. They provide a measure of the nutrient status of a body of water. This protocol aims to specify a cost-effective method for determining the trophic levels of lakes, and changes in these levels with time.

Executive Summary

The New Zealand Lakes Monitoring Programme was set up to provide a useful database on the trophic condition of some of New Zealand’s more important lakes and to use this database to develop a sensitive, cost-effective Lakes Monitoring Protocol for detecting small changes in the trophic state of lakes. Such data are essential to initiating timely remedial action in the event of deterioration of water quality.

The trophic state of a lake is most simply defined as the life-supporting capacity per unit volume of a lake. Six commonly measured variables are widely accepted as good indicators of the trophic level of a lake: chlorophyll a (Chla), Secchi depth (SD), total phosphorus (TP), total nitrogen (TN), hypolimnetic volumetric oxygen depletion rate (HVOD), and phytoplankton species and biomass. These variables yield considerable relevant information on trophic levels relative to the effort required for their measurement.

The system of lake monitoring and data analysis presented in this manual demonstrates how decisions on the probability of trophic level change can be made on the basis of numerical results, and assigns a numerical value for the trophic level of a lake and shows how a change in this value can be quantified. A decision on the probable change in a lake is made using per cent annual change (PAC) values, while the trophic condition of a lake and the magnitude of change in this condition is assessed using trophic level index (TLI) values.

The protocol explains both the theory and practice necessary to establish a TLI value for a lake. Data sampling and data analysis methods are tailored to fit New Zealand conditions. Case studies are presented illustrating trophic change in New Zealand lakes and a more general reporting of ecological condition.

The manual provides advice on good practice in:

• sampling technique on the lake
• sample handling
• laboratory analysis
• compiling the data into a basic data file
• calculating the monitoring results.

Considerable emphasis is placed on identification of the different thermal layers in a lake and on averaging values only from samples within the same layer.

After baseline monitoring has been carried out on a lake, it is possible to optimise the sampling strategies to reduce unnecessary data collection. A costing study illustrates the choices that may be made to obtain the best possible monitoring data for a given budget.