Maximum primary productivity

The maximum primary productivity of algae is about 4000 mg C/sq m per day. This is how much carbon (in CO₂) that algae can use up. This maximum figure is directly related to the amount of light incident on the water, so the actual figure for each farm changes depending on season, day length, cloud cover, algal species and so on. From this figure we can calculate the maximum amount of N the algae can theoretically consume per day.

• 4000 mg C is 4000/12 moles C = 333 moles C

• The molar C:N ratio of algae is 6.625:1, so 333 moles C is 50 moles N

• 50 moles N is 700 mg N /sq m or (assuming the pond is 1 m deep) 0.7 mg N/L

These figures show that the maximum amount of N that can be processed per day by algae is 0.7 mg/L. This figure is important to pond managers as it helps us to understand the fate of the nitrogen content of the feed we are putting into our ponds.

Of course a pond cannot continue to process the maximum amount of N per day. Sooner or later a peak will be reached where the bloom is so dense that it cannot continue to grow. At this stage it may die off quickly (or “crash”) with subsequent rapid proliferation of bacteria which can use up oxygen very quickly and cause very major problems in the pond. This is where Phoslock is so useful as it can limit the availability of the other essential algal nutrient, PO₄ , and so stop the bloom from becoming too dense.