An algal bloom with Secchi transparency in the 30 – 40 cm range will have a chlorophyll-a content in the range 50 -200 ug per L . Chlorophyll-a is about 1.5% of the dry weight of algae (further info here ) so the dry weight of algae in a pond with Secchi 30 – 40 cm is in the range of 3350 – 13400 ug/L. Algae can be described by the following equation: C₁₀₆H₂₆₃O₁₁₀N₁₆P₁ (see here) indicating that 1 mole of algae weighs 3550 g, and that P is about 0.9% of the dry weight of algae. Thus we can estimate that the quantity of P in a 30 - 40 cm bloom is in the range 30 – 120 ug per litre, or as PO₄, 90 – 360 ug per litre or 0.09 – 0.36 mg PO₄ per L.

Bacterial organic carbon in aquaculture ponds falls in the range 3 – 20 g C m^-3. Bacterial biomass can be characterized as C₆₀H₈₇O₂₃N₁₂P₁ indicating that a mole of bacteria is about 1374 grams, and that 3 – 20 grams C equates to a bacterial biomass of approximately 6 – 40 grams because C accounts for about half the biomass of bacteria. P is about 2.3% of the weight of the bacterial biomass so 0.13 – 0.9 mg P per L or 0.4 – 2.7 mg PO₄ per L.

From these figures we can assess that aquaculture pond water contains about 0.5 – 3 mg PO₄ per L. when the Secchi transparency is in the range 30 – 40 cm.

In reality, a water body may contain a much higher level of PO₄ than these figures suggest. The reason for this includes the fact that many algae species will store PO₄ excess to their requirements. Therefore, in a bloom with Secchi of 35 cm, although the PO₄ measured may be close to 0, the bloom can continue to grow due to the PO₄ that the cells have stored. So in practice, when monitoring PO₄, it is necessary to test it once or twice a week and apply Phoslock as required. However the general rule of maintaining PO₄ at less than 0.03 mg per litre when the Secchi transparency is at the level required by the pond manager works well in practice.

The pond soil also can contain a very large amount of PO₄, and can act as both a source and a sink for PO₄ . Each farm and indeed each pond will behave differently and each farm manager will understand best about his ponds. By monitoring PO₄ regularly the farm managers will be able to build up a deeper more thorough understanding of the dynamics of each of their ponds and as a result get more consistent results.

Some farmers have asked “will removing the excess PO₄ inhibit the ability of the bloom to absorb ammonia?” The fact is that once the bloom reaches a certain level (around 20 - 30 cm Secchi) it can no longer act to absorb ammonia, simply because the bloom can not get any thicker. Blooms begin to become very unstable when Secchi is less than 30 cm. Holding the bloom in the 35- 40 cm range is optimal under most circumstances. Phoslock provides pond managers with the ability to hold blooms at the desired density, while reducing the need to change water. This allows the pond to build up nitrifying and denitrifying bacterial populations. It also allows managers extra options for example use of organic carbon such as molasses to manage their ponds.