New Zealand Journal of Marine and Freshwater Research abstracts

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Low-dose alum application trialled as a management tool for internal nutrient loads in Lake Okaro, New Zealand

W. J. Paul
D. P. Hamilton

Centre for Biodiversity and Ecology Research
The University of Waikato
Private Bag 3105
Hamilton 3240, New Zealand

M. M. Gibbs

National Institute of Water and Atmospheric    Research Limited
P. O. Box 11–115
Hamilton, New Zealand

Abstract Aluminium sulfate (alum) was applied to Lake Okaro, a eutrophic New Zealand lake with recurrent cyanobacterial blooms, to evaluate its suitability for reducing trophic status and bloom frequency. The dose yielded 0.6 g aluminium m^–3 in the epilimnion. Before dosing, pH exceeded 8 in epilimnetic waters but was optimal for flocculation (6–8) below 4 m depth. After dosing, there was no significant change in water clarity, hypolimnetic pH decreased to 5.5, and soluble aluminium exceeded recommended guidelines for protection of freshwater organisms. Epilimnetic phosphate concentrations decreased from 40 to 5 mg m^–3 and total nitrogen (TN):total phosphorus (TP) mass ratios increased from 7:1 to 37:1. The dominant phytoplankton species changed from Anabaena spp. before dosing, to Ceratium hirudinella, then Staurastrum sp. after dosing. Detection of effectiveness of dosing may have been limited by sampling duration and design, as well as the low alum dose. The decrease in hypolimnetic pH and epilimnetic TP, and increase in Al3+ and chlorophyll a, are attributed to the low alkalinity lake water and coincidence of alum dosing with a cyanobacterial bloom and high pH.

Keywords flocculation; alkalinity; Anabaena; Ceratium hirudinella;phosphorus

New Zealand Journal of Marine and Freshwater Research, 2008, Vol. 42: 207–217
0028–8330/08/4202–0207     © The Royal Society of New Zealand 2008
M07014; Online publication date 10 June 2008
Received 19 March 2007; accepted 20 January 2008

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