Report: TR 2009/09

Author: Max 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 2007/08 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 0.03 ± 0.03 mg m-3 y-1. The winter 2007 chlorophyll a concentrations, at 2.2 mg m-3, were lower than in the previous winter bloom period. Of interest, the chlorophyll a concentration in September 2008 was 3.0 mg m-3. Regression analysis of maximum winter-spring chlorophyll a concentrations suggests that these have been increasing by 0.12 ± 0.05 mg m-3 y-1 since the start of this monitoring programme in 1994. As noted in the previous report, this trend may not be linear and concentrations may have reached a plateau. Before 2000, the mean of the winter chlorophyll a maximum concentration was 1.46 mg m-3 with an increasing trend of 0.18 mg m-3 y-1. After 2000, the mean of the winter chlorophyll a maximum concentration was 2.5 mg m-3 with no statistically significant trend or substantial change in the concentration data.

Highest phytoplankton biomass occurred in August when the lake had mixed and lowest biomass occurred in the upper water column in early summer when that winter biomass peak had settled from the water column onto the lake bed. Chlorophyll fluorescence profiles show that, each year, a deep chlorophyll maximum (DCM) develops just below the thermocline (40 m) during summer and has up to 70% higher biomass than the upper water column.

The 2007 winter bloom was initially dominated by the diatom Fragilaria crotonensis and then Asterionella formosa became dominant through spring. The colonial green, Botryococcus braunii, and the dinoflagellate, Gymnodinum sp., become dominant in summer and autumn 2008. Cyanobacteria (blue-green algae) were always present in low numbers in the upper water column throughout the 2007/2008 monitoring period, with Anabaena lemmermannii being the most common species but only at around 1% of the total biomass. There was an extended period from October 2007 to the end of February 2008 when algal biomass was very low.
Nutrient concentrations - dissolved reactive phosphorus, ammoniacal nitrogen, and nitrate nitrogen (DRP, NH4-N, and NO3-N) - in the upper water column were comparable with concentrations measured since 2003, although NO3-N concentrations were lower in winter 2007 than in previous years and elevated NH4-N concentrations were present in the upper water column through the summers and autumns of 2007 and 2008. The latter may indicate water column decomposition of the winter-spring bloom, or excretion from a zooplankton bloom.

The total mass of NO3-N in the hypolimnion before winter has increased at a statistically significant rate of about 7.9 t y-1 (P <0.002, r2 = 0.39, n = 21) over the last 33 years. The net accumulation rate of NO3-N in the hypolimnion below 70 m in 2007/2008 was 1.77 t d-1. Regression analysis showed that there has been a weakly significant trend of increase in the net NO3-N accumulation rate of 0.029 t d-1 each year (P <0.1, r2 = 0.15, n = 21) over the last 33 years.
Spring and summer 2007/2008 water clarity was the highest recorded for Lake Taupo, with values greater than 20 m from December 2007 to April 2008, reaching a maximum of 25 m on February 2008. This extreme clear-water phase coincided with a drought (declared officially) and thus may reflect the reduced nutrient input in surface runoff as well as a low input of sediment from erosion.

The 2007/08 net VHOD rate at 14.51 ± 2.94 mg O2 m-3 d-1 (mean ± 95% confidence limit) was almost 4 mg O2 m-3 d-1 higher than the previous year which was 10.74 ± 2.45 mg O2 m-3 d-1.

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. In previous reports, it was also shown that the net accumulation rate of NO3-N in the hypolimnion during the stratified period has increased over the last 33 years although the trend is not strong. These trends are still present in the data to 2008.

Lake Taupo long-term monitoring programme 2007-2008 [PDF, 589 KB]