Nutrient water budget for Lake Tarawera

Abstract

A nutrient budget has been constructed for Lake Tarawera using two techniques. One technique involved analysis of incoming stream discharges and concentrations of nutrient species (total nitrogen (TN), ammonium, nitrate, total phosphorus (TP) and phosphate) based on measurements taken at weekly to fortnightly intervals in the summer of 2005/06. The second technique involved an analysis based on the predominant land use together assigned coefficients for areal rates of nutrient export. Both techniques produced very similar nutrient load estimates of approximately 12-13 tonnes TP per year and 95-100 tonnes TN per year. On the basis of comparisons of incoming nutrient loads, discharge and nutrient concentrations in the Tarawera River outflow, and total lake volume, it is estimated that incoming water will reside in the lake for an average duration of 10 years, and that 86 % of TP and 72 % of TN will be trapped within the lake; either buried in the lake sediments or lost as nitrogen gas in the process of denitrification. A water budget constructed from stream discharges, rainfall and evaporation revealed that nearly 80 % of the water entering the lake (not including rainfall) was from sources that were not gauged as part of the summer (2005/06) stream survey; groundwater appears to be the dominant source of water for Lake Tarawera.