Lake Taupo Long-term Monitoring Programme 2009-2010

Report: TR 2011/12

Author: M Gibbs (NIWA)

Abstract

 A long term monitoring programme of Lake Taupo’s water quality was commissioned by Waikato Regional Council in October 1994 in the expectation that the trophic state of the lake would change to reflect changes in land use within the lake's catchment.

This programme is conducted by NIWA with field assistance from the Department of Internal Affairs, Taupo Harbourmaster’s Office. Various additions and improvements to the monitoring methodology have occurred with advances in available technology but the core monitoring parameters remain unchanged. This report presents data from the routine mid-lake monitoring station from August 2009 to July 2010. Additional information for water clarity, temperature, and chlorophyll a collected between August 2010 and the time of writing this report has also been included in the data sets in the appendices.

The monitoring programme has 3 components: bottom water oxygen depletion, upper water column water quality, and whole water column water quality. Bottom water oxygen depletion is estimated as the volumetric hypolimnetic oxygen depletion (VHOD) rate, which is sensitive to changes in trophic state of lakes that thermally stratify for part of the year. VHOD is considered a good indicator to detect changes in the water quality of Lake Taupo.

Estimates of VHOD are made from dissolved oxygen and temperature profiles measured at 2-3 week intervals during the stratified period. However, the VHOD rate can only indicate changes that may occur in water quality but not identify their underlying causes. In order to enable understanding of contributing processes, the upper water column (0-10 m depth) is sampled for nutrients, chlorophyll a, phytoplankton species composition and water clarity at 2-3 weekly intervals, and full depth profiles are carried out twice during the stratified period. The first profile is taken in spring, when thermal stratification has become established and is stable, the second profile in autumn the following year before thermal stratification begins to break down, as the thermocline deepens.

This report presents the results from the 2009/10 stratified period plus the winter 2010 mixing, and refers to data in previous annual monitoring reports from 1995 to 2009 for inter-annual comparisons, and archived historical data since 1974 held by NIWA.