Report: TR06/30

Author: MM 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 2004/05 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.

While there is still a long-term trend of increasing phytoplankton biomass (chlorophyll a) over the monitoring period of 0.054 ± 0.036 mg m-3 y^-1 the 2005 year was unusual in that chlorophyll a concentrations were lower than in the previous five years in both winter and summer. The lower winter peak in chlorophyll a may be attributed to the incomplete mixing of the lake in winter 2005 and a retention of about 50 per cent of the accumulated mass of NO₃N 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 2004 winter bloom was dominated by the diatom Fragilaria crotonensis but included Asterionella formosa. In winter 2004 the dominant species were Asterionella formosa and Aulacoseira granulate. In summer, Oocystis sp. and Botryococcus braunii were co-dominant and were replaced with Cyclotella stelligera and Ceratium hirundinella through late summer and autumn.

Blue-green algae (Anabaena flos-aquae and Aphanizomenon sp.) were present throughout the lake in increasing abundance from about November 2004 but rarely exceeded third level order of dominance at the mid lake site. Anabaena flos-aquae became the dominant species briefly in May 2005 but had disappeared by June.

Nutrient concentrations (DRP, NH₄N, and NO₃N) in the upper water column were comparable with concentrations since 2003. DRP and NO₃N concentrations rose during initial winter mixing and then were depleted as phytoplankton grew although DRP was much slower to be depleted than NO₃N. The peak concentration of DRP at winter mixing 2005 was the highest ever recorded at 3.9 mg m^-3. The summer concentrations of NH₄N and NO₃N were below detection level on most occasions. Bottom water (150 m) NO₃N and DRP concentrations increased at similar rates to the previous year. The accumulated mass of NO₃N and DRP in the hypolimnion before winter mixing has been similar for each of the last three years but is presently about twice the accumulated mass recorded in the hypolimnion at the beginning of the present monitoring programme in 1994/95. From historic data, the accumulated mass of NO₃N in the hypolimnion by autumn each year since 1975 is increasing at a statistically significant rate of about 11 t y^-1 (P <0.001, r² = 0.59, n = 17).

During the 2004/05 monitoring period, summer water clarity was high for an extended period reaching a Secchi disc depth of 20.7 m in February 2005 and remaining above 19 m from January to late March 2005. Water clarity in winter 2005 was higher than previously recorded, remaining above 13 m. This can also be attributed to incomplete mixing and the concomitant low winter algal biomass. Since January 2003, there has been a strong inverse relationship between chlorophyll a and water clarity (P <<0.0001, r² = 0.55, n = 47).

The 2004/05 net VHOD rate at 11.30 ± 1.13 mg m³ d¹ (mean ± 95 per cent confidence limit) was about same as the previous year, 11.50 ± 2.80 mg m³ d¹. The present VHOD rates are more than 2 mg m³ d¹ higher than in 1994/95 and 1995/96.

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.

Lake Taupo Long-Term Monitoring Programme 2004-2005 [PDF, 1.1 MB]