A raw water storage dam (40Ha) in the North Island of New Zealand was treated over a two year period, using a proprietary liquid product that that contains ten micronutrients that are only bioavailable to diatom algae. Diatom algae are recognised as being a major contributor to primary production for the micro- and macro-faunal ecology within a waterbody. Applying a methodology to solely increase diatom growth was associated with a reduction in Total Nitrogen (TN) and Total Phosphorus (TP), as the important building blocks for algae growth. TN and TP reduced by 40% and 42% respectively by the end of the second season of treatment.
The intention of the treatment was to reduce the Total Nitrogen and Total Phosphorus concentrations and as a natural consequence, led to the reduction of the cyanobacteria density. At the site, an aeration curtain had already been in place for two years to maintain mixing and reduce stratification in an attempt to reduce blue-green algae (BGA), with limited effect. The addition of the product occurred at five points across the dam via automated dosing systems. During the first summer of treatment the BGA cell count was 56% lower than the same time the previous year. The standard seasonal pattern of BGA cell growth, and biovolume, increase through spring and summer was less marked during year one and further reduced in year two.
In season two, the spring BGA count was again lower than previous seasons, but most importantly, the summer count was 98% lower than the average summer cell count, prior to treatment. The winter BGA count (year two) was 90% lower than previous years. The average summer BGA count prior to treatment (70-100,000 cells/mL) reduced with treatment, and by summer of season two was 2,825 cells/mL. The ratio of BGA cell count to diatom cell count reduced from ~9000:1 in the summers prior to treatment to <10:1 in the second summer. This was not due to a marked increase in diatom cell count, but the reduction in BGA cell count.
As has been observed at other sites that have been treated, further reductions would be expected with further treatment.