I have been asked whether removal of all measurable PO4 using Phoslock will clear up ponds, lakes and lagoons. The answer to this question is maybe and then again maybe not. Here are some reasons why the answer can not be more specific.

Phoslock is a wonderful tool for removing free PO4 from the water. However, algae and bacteria are great survivors. They have numerous mechanisms that allow them to survive and even grow when there is no free PO4 available to them.

One of these mechanisms is the ability of many algae and cyanobacteria to store excess PO4 inside their cells. When the times are good, they can store many times more PO4 than they need. This extra PO4 is then passed on to the next generations of cells. Sooner or later the store of PO4 will run out, and the algae will not be able to reproduce. The amount of stored PO4 can be measured quite easily and the method is described here . In summary, the sample with algae is boiled in distilled water for 60 minutes, the PO4 is then measured, and any increase in PO4 over the original sample is the PO4 that was contained as stored PO4 inside the algal cells. This can then be related back to the original TP to gain valuable insights into the future dynamics of the bloom that is under investigation.

Another mechanism that both algae and bacteria employ to gain PO4 is the use of phosphatase enzymes. These can be either acid or alkaline phosphatases depending on the environment of the water. These enzymes are produced by bacteria and algae when the PO4 runs out. The enzymes are released into the outer membrane of the cells or in some cases into the water, where they are able to break down the molecular structure of otherwise non-bioavailable molecules, and release PO4 which they can then take into themselves so they can continue to grow. In aquaculture ponds I strongly suspect that these enzymes are busy in our ponds where we have effectively removed free PO4. In these ponds we are always adding feed, and the P from this is bound up in complex molecules which would require phosphatase to break them down. The dynamics of these interactions and the effect of pond parameters such as pH is quite intriguing and I suspect worthy of further study.

Another method that algae and bacteria employ when there is simply no source of P left in the water is to move to the mud. Here they can use their phosphatase enzymes to help release PO4 from the soil or organic material on the pond bottom. Some of these algae can then move back up into the water column with their renewed stores of PO4 to carry on their life cycle with the assistance of the sunlight they require for photosynthesis.

It is pretty clear from this that the role that enzymes (phosphatases) play is very important. The ability of phosphatases to perform can be enhanced or reduced by the water and its properties. For example, Mg is fundamental to the activity of alkaline phosphatase so one may imagine that a pond with very soft water with low Mg would be easier to control in terms of removing nuisance blooms than a pond with hard water. Humic acids are known as repressors of phosphatase activity. pH may effect the activity of phosphatases. There are many unknowns and simply removing free PO4 will not necessarily result in completely clearing up a nuisance algal bloom, though it will definitely help. The reason it will help is that all these processes that algae and bacteria have to employ to get their PO4 when it is not freely available are not free of charge. It takes energy to produce enzymes so the rates of their growth has to be slowed down considerably. This is what we are seeing in our aquaculture ponds where photosynthesis is slowed down and this is manifested in the halving of DO maxima and the improved DO minima leading to much reduced risk of bloom crash and stressful low DO's.