What happens when PO4 is removed by Phoslock?
On our fish farm we have learned that if when we apply Phoslock and bring the PO4 down to less than 0.03 mg/l we get some results that are beneficial to our business. These results are:
the blooms are more stable: the types of algae that dominate are the very small in size (less than 5 um) species such as green “chlorella”
this bloom stability means that the risk of bloom “boom and bust” is eliminated: this means that the risk of bloom crashes is eliminated as well
the diurnal oxygen and pH max and min are held in a much narrower range, leading to significant savings in electricity consumption
with stable blooms and flatter diurnal pH and oxygen variation, the fish are less stressed
We have also learned that removing PO4 does not mean we remove blooms. The blooms stay with us but they have slowed down. By this I mean that the photosynthetic activity of the blooms is reduced. This is measured by the DO max that is reached when the radiation from the sun is at its maximum and this is about 14:00 - 15:00 hrs. When PO4 is over about 0.1 mg per litre, this maximum can easily reach 300% saturation. When PO4 is less than 0.03 mg per litre, 150% saturation is rarely exceeded.
Some observers of the research that I am doing on my farm are surprised that removal of all the PO4 does not have the effect of clearing the pond and removing the bloom altogether. Is it not to be expected that removal of such an important nutrient would stop the algae from growing? The answer is yes, if ALL the sources of PO4 were removed then the algae would not be able to grow. But on the farm we are adding feed to the ponds every day. And the PO4 can come from many sources. Bacteria and algae in the pond have the ability to produce enzymes called phosphatases. In aquaculture ponds these phosphatases are usually alkaline phosphatases, because the pH of the ponds is usually well over 7.0 (ie alkaline). The alkaline phosphatases begin to work when free PO4 drops under about 0.04 mg/l. These enzymes are able to break up larger molecules containing PO4, thereby releasing the PO4 which is then available to the algae. The PO4 that is released by the action of the alkaline phosphatases is almost impossible to measure. The reason for this is that very little if any of the PO4 from these reactions is released into the water. There is a very close coupling between the alkaline phosphatase mediated reactions which release PO4 and the algae which take up the PO4.
The work of Jurgen Overbeck and his colleagues highlighted the importance of enzymes in aqautic systems. A good review of their work and others can be found in Microbial Enzymes in Aquatic Environments, Ryszard J. Chrost (ed) Brock/Springer Series is Contemporary Bioscience, 1991.
Another more recent paper is of great interest as it helps in understanding what is happening in our ponds when we limit the availability of PO4. In summary the paper describes how removing PO4 reduces primary productivity (as we see in our ponds with reduced daytime DO max) and how this has profound flow on effects within the whole microbial ecosystem. The reference for this paper is : Chiara Larato et al , Production and utilization of organic matter in different P-availability conditions: A mesocosm experiment in the Northern Adriatic Sea, Journal of Experimental Marine Biology and Ecology 391 (2010) 131-142