Fighting Algae with Borax

[chem geek]

I may be wrong about this, but I believe that nitrogen and phosphorous are both required nutrients -- it's not an "or" relationship. This link describes some of the nutrient requirements and this link talks more about the N:P ratio and how a lower ratio can lead to algal blooms and this link refers to N-limited vs. P-limited algal growth with the strong implication that both nutrients are required (i.e. both nitrates and orthophosphates). I've read other info that's consistent with this. I think the only either/or situation was between phosphorous and silicon for some species of algae that could or do use silicon instead of phosphorous. You'll also find C:N:P ratio data that adds carbonates to the mix, but again it's not an "or" relationship but an "and" relationship (i.e. all are required nutrients so eliminating any of them will eliminate algal growth).

We can't obviously eliminate carbonates from the water since it's exposed to carbon dioxide in the air and we want a pH buffer and calcium carbonate saturation. Ammonia may be present as a source of nitrogen, but only if chlorine is not present (since monochloramine occurs very, very quickly), but monochloramine would be taken up as algae food and would then kill the algae. So realistically it is nitrates that would be a source of nitrogen for algae. Typically the nitrates and phosphorous would both get into pools from fertilizer (i.e. blown soil), but nitrates can also come from the oxidation of organics (where nitrates are produced instead of nitrogen gas) and nitrogen gas itself can combine with oxygen and water to form nitrates (this is actually slightly thermodynamically favored and were it not for living organisms doing denitrification, biosynthesis and nitrogen fixation, the oceans would be 0.1 Molar HNO3).

Richard