How high of phosphate levels do you need to induce cyanobacteria?
When I was derping around with dosing, I got to a point of dosing 20ppm of phosphates three times a week. That's 60ppm in a week, not including what is already existing in the water, and what is added with my heavy fish feeding. Not a single speck of cyanobacteria during that time.
Only time I got cyanobacteria was during the startup of my tank last year - this was when I seriously lacking on flow - just an old HOB filter and an old powerhead on a 65-ish gallon tank. Erythromycin, manual removal, and adding of flow (XP3 + 240gph koralia) resolved it the problem.
EDIT: I think I found the (partial) answer.
Box 8.2 Thresholds for phosphorus control of different cyanobacterial ecostrategists
Knowledge of the prevalent ecotypes in a given water body leads to the following consequences for total phosphorus management:
• If scum-forming ecostrategists prevail (such as Microcystis spp. or Anabaena spp.) cell numbers
and biomass are likely to decline if total phosphorus concentrations can be brought well below 50
μg I-1 P. This will also reduce scum formation, because less cells and colonies will be available to
concentrate into scums. Nonetheless, some scums will probably continue to occur until
phosphorus limitation becomes so severe that cell density (and therefore turbidity) decreases to
the point where the depth of light penetration is as deep as the depth of large areas of the water
body (Zeu = Zm). Under these conditions, vertical migration of these taxa is less effective because
their buoyancy regulating mechanism requires some time in the dark (see section 2.2). Therefore,
they lose their competitive advantage over other phytoplankton.
• If dispersed ecostrategists prevail (such as the filamentous species Planktothrix agardhii,
formerly named Oscillatoria agardhii) very pronounced "switches" may be expected. As
phosphorus limitation reduces filament density, and thus turbidity, to the point where the
relationship of the depth of light penetration to the depth of the mixed layer (Zeu/Zm) is greater
than 0.4, these species are likely to disappear quite abruptly, and turbidity will increase even
further, thus stabilising the result.
• If metalimnetic ecostrategists prevail (such as Planktothrix rubescens), the water layer above
these cells is usually quite clear. Very low concentrations of total phosphorus (often below 10 μg
I-1 P) are necessary to decrease turbidity further and thus increase light intensity down to the
depth inhabited by these species. If this can be achieved, metalimnetic ecotypes may disappear.
If not, hydrophysical measures may be more successful in controlling their density.
• If nitrogen fixing ecotypes prevail (such as Anabaena spp.), reduction of total phosphorus down
to concentrations effectively limiting biomass will cause dissolved nitrogen concentrations in
excess of uptake by phytoplankton. Nitrogen fixation is then no longer an advantage in
competition over other cyanobacteria and algae. This may induce disappearance of the nitrogen
It is talking about total phosphorous, and not phosphates, though.
The term total phosphorus is preferable to the term total phosphate, because results are reported
in terms of phosphorus rather than phosphate. This is important because the weight of the PO4
molecule is about three times that of its central P atom, and lack of specification in reporting
results as to whether they refer to μg PO4 or μg P has caused considerable confusion in the
Interesting tidbit on the 2002 article mentioned above:
In treatments where carbon limitation slowed down the growth of heterotrophic bacteria, picophytoplankton became abundant and these organisms showed a positive response to P in combination with a negative response to glucose.
Hmmm, I wonder if low dissolved oxygen + phosphate = cyano proliferation? Meh, I'm sure it's not that simple....LOL! Low dissolved oxygen could easily be had if you have poor water movement, though!