Report: TR 2007/21

Author: M.M. Gibbs (NIWA)

Abstract

With the expectation that the trophic status of Lake Taupo will slowly change to reflect changes in land use within the lake's catchments, a long term programme monitoring the lake's water quality was commissioned by Environment Waikato. This programme commenced in October 1994 and is conducted by NIWA with field assistance from the Department of Internal Affairs, Taupo Harbourmaster’s Office.

The monitoring programme was designed to detect change through assessment of the rate of consumption of oxygen from the bottom waters of the lake (volumetric hypolimnetic oxygen depletion – VHOD) as an integration of all biological processes occurring in Lake Taupo. Additional parameters are measured to provide a more comprehensive picture of water quality. Recently it has become apparent that VHOD may be too coarse to determine trophic change in a lake the size and complexity of Lake Taupo. Consequently, more emphasis is now focused on the parameters ‘phytoplankton biomass’, ‘water clarity’, and nutrient (particularly nitrate) accumulation in the lake. This report presents the results from the 2005/06 monitoring period at the mid-lake site, Site A. Monitoring of additional sites in the Kuratau Basin (Site B) and the Western Bays (Site C) between January 2002 and December 2004 determined that spatial variability of water quality across Lake Taupo is minimal and that it is valid to use the mid-lake site as representative of the open water quality of the lake.

There is a long-term trend of increasing phytoplankton biomass (chlorophyll a) in the upper 10 m of water column over the monitoring period of 0.050 ± 0.030 mg m^-3 y^-1 with winter 2006 chlorophyll a concentrations being higher than in most previous winter bloom periods. The low maximum chlorophyll a in winter/spring 2005 was attributed to the incomplete mixing of the lake in winter 2005 and a retention of about 50% of the accumulated mass of NO₃N and DRP in the bottom waters that would normally have been mixed up into the surface waters.

Highest biomass occurred in August when the lake had mixed and lowest biomass occurred in early summer when that winter biomass peak had sedimented from the water column. The 2005 winter bloom was dominated by the diatoms Aulacoseira granulata, Asterionella formosa and Fragilaria crotonensis and these species were present throughout most of the year. These 3 algal species also dominated the algal assemblage in the winter 2006 bloom. However, they were replaced in dominance by Dinobryons and Botryococcus braunii through late spring 2005 and summer 2006 before resuming dominance through late summer and autumn 2006. While blue-green algae (Anabaena flos-aquae) were present throughout the lake in spring, these did not exceeded 4^th level order of dominance at the mid lake site at any time during the 2005/2006 monitoring period.

Nutrient concentrations (DRP, NH₄N, and NO₃N) in the upper water column were comparable with concentrations since 2003, although NO₃N concentrations were very high during initial winter mixing in 2006. Incomplete mixing in winter 2005 left elevated NO₃N and DRP concentrations in the bottom waters (150 m). However, contrary to expectations of subsequent nutrient accumulation adding to these concentrations as the new baseline, the concentrations of both DRP and NO₃N were lower in autumn 2006 than in previous years and there was a net loss of NO₃N from the hypolimnion. As a similar drop in nutrient accumulation in the hypolimnion followed incomplete mixing in winter 1998 it is likely that this is an anomaly which occurs in years when winter mixing was incomplete.

The total mass of NO₃N in the hypolimnion before winter mixing in 2006 was 270 t, which was lower than in previous years. Regression analysis showed that the total mass of NO₃N in the hypolimnion at this time of year had increased at a statistically significant rate of about 9.5 t y^-1 (P <0.001, r² = 0.52, n = 21) over the last 30 years.

In past reports, only the total mass of NO₃N in the hypolimnion has been reported as a “standing stock” before winter mixing. In this report, it is recognised that the standing stock is a function of the mass of NO₃N in the hypolimnion at the beginning of the stratified period plus the net mass that was released from the sediments and accumulated in the hypolimnion during the stratified period. While the standing stock is the mass present at one time, the mass released from the sediments over the stratified period can be expressed as a rate which may give a more reliable estimate of change. Consequently, in this report the net accumulation rate of NO₃N in the hypolimnion is also reported. Net values were estimated as the increase that occurred between the spring and autumn samplings in each period of stratification divided by the time in days between these two samplings. Converting all standing stock data to net accumulation rates showed that the net accumulation rate of NO₃N in the hypolimnion ranged from about 0.5 t d^-1 in 1975 to 2.5 t d^-1 in 2002. A regression analysis showed that there has been a weakly significant trend of increase in the net NO₃N accumulation rate of 0.03 t d^-1 yr¹ (P = 0.11, r² = 0.14, n = 19) over the last 30 years. However, these data also showed that, while there was a net NO₃N accumulation rate of around 2 t d^-1 in the hypolimnion below 70 m in 2004/2005, there was a net loss of around 0.25 t d^-1 in the 2005/2006 stratified period. The reason for this loss is not known, but may be associated with the incomplete mixing in winter 2005.

During the 2005/06 monitoring period, summer water clarity was lower than in the previous few summers with Secchi depth values of 16 to 19 m and this level of clarity continued through most of autumn 2006. Water clarity in winter 2005 was higher than previously recorded, remaining above 13 m. This was attributed to incomplete mixing and the concomitant low winter algal biomass.

The 2005/06 net VHOD rate at 9.56 ± 2.24 mg m³ d¹ (mean ± 95% confidence limit) was 2 mg m³ d¹ lower than the previous year which was 11.30 ± 1.13 mg m³ d¹. The present VHOD rate is around the level of 9 mg m³ d¹ suggested in an earlier report as a possible natural VHOD level for Lake Taupo.

In the 2002 review of the long-term monitoring programme data, 3 trends in the data were identified — increasing phytoplankton biomass in the upper 10 m, increasing NO₃-N mass in the hypolimnion prior to winter mixing, and an increasing range in the variability of water clarity — that were of concern with respect to the water quality of Lake Taupo. These trends are still present in the data. In this report, it has also been shown that the net accumulation rate of NO₃-N in the hypolimnion during the stratified period has increased over the last 30 years although the trend is not strong.

Lake Taupo Long-Term Monitoring Programme 2005-2006 [PDF, 558 KB]