Lake Taupo long-term monitoring programme 2008-2009

Report: TR 2010/27
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.

The long-term monitoring programme uses the historical 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. Further validation of the use of a single mid-lake monitoring site was obtained from a comparison of upper water column nutrient and chlorophyll a concentrations and algal enumeration between Site A and near-shore sites in Whangamata Bay (Kinloch) and Whakaipo Bay, over a two-year period from February 2007 up to June 2009. That study determined that “the near-shore water quality was very similar to the mid-lake water quality” and that “within this similarity in the measured data was much variability which may be due to short period time lags between the near-shore and mid-lake sites with respect to nutrient sources, and the zones of algal growth”. This report presents the results from the 2008/09 monitoring period at the mid-lake site, Site A.

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 0.025 ± 0.017 mg m^-3 y^-1. The winter 2008 chlorophyll a concentrations, at 3.0 mg m^-3, were the highest on record for Lake Taupo.

As the long-term data accumulates, it has become apparent that the increase in chlorophyll a data may not be a linear trend and could be part of a long-term cycle. The annual mean chlorophyll a data from 1994 to 2003 was increasing at a statistically significant rate of 0.087 ± 0.029 mg m^-3 y^-1 (P < 0.001, r² = 0.857, n = 10), but since 2000 there has been a statistically significant trend of decline at a rate of 0.033 ± 0.031 mg m^-3 y^-1 (P <0.05, r² = 0.423, n = 10).

Highest phytoplankton biomass occurred in August 2008 when the lake had mixed and lowest biomass occurred in the upper water column in early summer 2009 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 2008 winter bloom was initially dominated by the diatoms Asterionella formosa and Aulacoseira granulata with Fragilaria crotonensis and a green algae, Monoraphidium sp., becoming dominant through spring. The colonial green, Botryococcus braunii, and some large dinoflagellates, Gymnodinium sp. and Peridinium sp., become dominant in summer and autumn 2009. Cyanobacteria (blue-green algae) were always present in low numbers in the upper water column throughout the 2008/09 monitoring period, with Anabaena lemmermannii being the most common species. There was an ever present background of small (<5 µm) unicellular flagellates throughout the year.

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. NO₃-N concentrations were lower and NH₄-N concentrations were elevated in the upper water column since 2007. The elevated NH₄-N concentrations 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 <0.001, r² = 0.41, n = 22) over the last 34 years. This value is the same as the previous year but includes an increase of around 90 t of NO₃-N in the hypolimnion in autumn compared with autumn the previous year. The total mass of NO₃-N in the hypolimnion in autumn 2009 was 470 t. The net accumulation rate of NO₃-N in the hypolimnion below 70 m for the 2008/09 stratified period was 1.93 t d^-1, which is a 20% increase over the previous year. However, because of high variability in the data, regression analysis indicates that the trend of increase in the net hypolimnetic NO₃-N accumulation rate during the stratified period is only weakly significant at 0.028 t d^-1 (P = 0.07, r² = 0.154, n = 22) over the last 34 years.

Spring and summer 2008/09 water clarity was high at 22 m but was not as high as the previous summer which at 25 m in February 2008, was the highest clarity on record for Lake Taupo. These extremely clear-water phases both coincided with periods of drought (declared officially summer 2008) and thus may reflect the reduced nutrient input in surface runoff as well as a low input of sediment from erosion.

Lowest water clarity in winter did not coincide with the peak of chlorophyll production in August 2008, rather, lowest water clarity occurred in November 2008 during a wet and windy phase and clarity remained low until the weather calmed down again in February 2009.

The 2008/09 net VHOD rate at 17.50 ± 3.64 mg O₂ m^-3 d^-1 (mean ± 95% confidence limit) was almost 3 mg O₂ m^-3 d^-1 higher than the previous year which was 14.51 ± 2.94 mg O₂ m^-3 d^-1. Evaluation of the VHOD data shows that there has been a statistically significant (P <0.002. r² = 0.69, n = 11) increase of around 1.04 mg m^-3 d^-1 in the VHOD rate each year since the low of 1999. The period of this regression is selected from lowest to the present value, which is the highest measured during the monitoring programme, and thus does not reflect a long term trend in Lake Taupo. The persistent increase in hypolimnetic oxygen demand over the past 11 years does, however, imply a change in the carbon load on the lake.

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 34 years. These trends are still present in the data to 2009.

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