How do I measure PO₄ and what will be the benchmark before we use Phoslock?

PO₄ that can be used by algae for growth is quite easy to measure (see footnotes below on PO₄ ). On our farm we use the Palintest photometer and the LR (Low Range) PO₄ reagents (for quotes and delivery : palinaus@ozemail.com.au). The key to getting good reliable results with any testing method is to filter the sample before doing the actual test. We filter the 10 ml sample to be tested through a 1 micron glass fibre filter. The filter paper sits in a reusable 13 mm stainless steel syringe filter holder (Millipore Holder 13 mm SST Swinney Syringe). Each test costs about one dollar, and takes 10 minutes.

What will be the benchmark before we use Phoslock?

The first thing is to check that excess PO₄ is indeed a problem. The following indications suggest that PO₄ is likely a problem: off flavour in the fish or prawns, filamentous blue green blooms such as Oscillatoria, potentially dangerous blue greens such as Microcystis Nodularia or Anabaena, other toxic algae including Prymnesium parvum, consistently dense blooms less than 25 cm transparency. If PO₄ is suspected, then check how much PO₄ is present by doing the tests.

As far as advising what level of PO₄ is optimum for each farm, this is very difficult because all farms are different. So settling on a figure and saying “more than this is too much and less than this is not enough” is impossible. For example a barramundi farm like ours uses fully formulated feed that provides all the nutrition to the fish, so we keep our PO₄ levels down to less than 0.03 mg/L, once the bloom is established. A tilapia farm will rely on the bloom for all or some of the nutrition of the fish, so it is essential that the bloom has a balanced N:P.

A tilapia farmer will be adding N and P to stimulate the bloom. It is very probable that the N:P ratio will drift from the optimum and begin to favor development of cyanobacteria (blue green algae) or other problematic species as P accumulates. This is why it is important to monitor PO_4. If a problem is identified and it is determined that elevated PO₄ is likely the problem then the best course of action would be to remove the free PO₄ from the pond to allow development of a favorable bloom. Once the favorable bloom is re-established, then maintain PO₄ monitoring to ensure that the N:P ratios remain in the safe zone. Have a look at the information accessible through this Table of Contents to get more information on N and P in ponds.

The keys to finding a suitable result for each farm is to 1) establish that there is a problem 2) do the tests to find the PO₄ level .

Foot notes:

Algae use free PO₄. This is called SRP (soluble reactive phophate) or FRP ( filterable reactive phosphate) or free orthophosphate.

Algae also use PO₄ that is stored inside their cells. This is not measured by the normal test. To measure this the water sample needs to be boiled in order to liberate this stored up PO_4. ( see here)

For a very good review of methods for measuring PO4 see here