Hello Johnathon Truong

What were the initial dissolved phosphate and nitrate concentration at the commencement of the supposed bloom?

Blue green algae or Cyanophyceae have an immense propensity for bloom formation in high PO4-P concentrations. They on the other hand are one of the best free living atmospheric nitrogen fixers out there, hence they are also grouped as diazotrophs. If the time when you observed the bloom had lesser sunlight intensities then it automatically favours the BGA as they thrive under diffused sunlight while other autotrophs struggle. You should always keep in mind that at no point in time the water column is solely characterised by one group of algae, others such as green algae are always present but due to their individual adaptability, wax and wane rhythmically based on the ambient environmental conditions.

The sponging off of the phosphates (limiting nutrient in freshwater ecosystems in absence of release from bedrocks) as well as other plant nutrients from the water column combined with the eventual death of the BGA ended the bloom but the release of the trapped phosphate as well as the produced nitrates, at least within the epilimnion (in more hypoxic waters in a thermally stratified setting the oxides might be converted to reduced forms which are also supportive of algal growth) might have triggered the suppressed green algal species to utilize the nutrients and proliferate. This could have also been due to a change in the intensity of sunlight reaching the surface and penetrating the subsurface layers. This is not to convincing say that there were no BGA as Cyanophyceae do possess chlorophyll a.

I hope this speculative answer helps you out.

With regards,

Dr. Abhishek Mukherjee

Very reasonable explanation has been given by Dr Abhishek Mukherjee. Blue Green Algae generally occurs in water containing more dissolved phosphate nutrients in comparison to dissolved nitrate content showing very lower N/P ratio. Most of the algae prefers to uptake dissolved nutrients in a fixed N/P ratio, very near to 8 to 10. But anyway, if there is present excess phosphate, the N/P ratio becomes lower and other algae can not uptake nutrients in that ratio. Blue Green Algae can able to fix aerial nitrogen and increases the N/P ratio so that they can flourish easily.So nutrient levels in water could be highlighted to infer the bloom.

A tremendously important point has been highlighted by Dr. Phani Bhusan Ghosh as he mentioned the N:P ratio. A large number of researchers tend to base their results on the empirical 15:1 ratio but in a dynamic system this surprisingly never ever occurs. As no ecosystem is solely chemical (as it is neither chemogeobiology nor geobiochemistry but Bio-geochemistry) and simply because the inorganic compounds are almost always never stable in their concentration; assimilation and degradation/remineralization are the fabrics which hold biogeochemistry together and a sample taken out of the continuity is merely reflective of the ecosystem at that very moment in time/space and nothing more, it is never a proxy to the entire ecosystem dynamicity.