Our Introduction to Phoslock.

In June 2009 our farm lost 20 tonnes of 6 month old barramundi to what we subsequently discovered was a toxic algae – Prymesium parvum. The loss from the affected pond was total: not one fish survived. This was the first record of a Prymnesium parvum fish kill in tropical Australia. This event was very stressful for us: could our farm survive with this, one of the most toxic of the toxic algae, in the environment ? Our local Aquaculture department quickly convened a workshop with Dr Gustaaf Hallegraef as the invited speaker. Gustaaf is an expert on toxic algae. His advice was that P. parvum toxicity might be minimised by providing it with N:P in an optimum ratio as close as possible to the Redfield ratio. In December 2009, the opportunity came up to try Gustaaf's approach. Another pond had begun to develop a population of P parvum. The fish were off their feed and microscopic examination of algae showed P parvum was present. On measuring the PO₄ in the pond, it was found to be around 0.6 mg per litre, which meant we needed to add about 4 mg per litre of urea to achieve the correct N:P ratio. This is a big hit of urea, and the pond already had a significant bloom with Secchi around 30 cm, so I thought about removing the PO₄, but the only method I had was to pump water through the pond, which I commenced immediately. At the same time I got on the computer and googled “phosphate removal in ponds” and found Phoslock in Sydney. I rang and ordered 1 tonne for immediate delivery to Darwin. The Phoslock arrived 3 days later and I immediately applied 200 kg to the pond. Within hours the PO₄ level had dropped and N:P ratio was getting closer to the molar optimum of 16:1. It took about two weeks to finally rid the pond of all P parvum, but by maintaining the N:P ratio we got through the crisis without losing a single fish. The relief I felt from this was enormous. Although I have to be extra careful now with gold colored blooms, I know that I can manage and survive and not lose fish.

The lessons I learned from this episode led me to consider using Phoslock for normal pond management. Could I achieve more stable blooms by keeping PO₄ at levels more close to an optimum? Was high PO₄ having an impact on my ponds that was detrimental? As soon as I started applying the Phoslock I began to notice the effect on the ponds. They became more stable, with less pH and DO fluctuation through the daily cycle and higher DO minimums during the night. It became a lot easier to maintain my blooms at the optimum 35cm Secchi transparency, and I noticed that the blooms were more diverse with more species of algae plus rotifers, copepods and other zooplankton.