I'd like to add to this statement. One of the things I found most interesting in a class I recently took was the concept that NH4+/NH3 are actually toxic to plants, even though it is less energy intensive to finally use than NO3-. Since NH4+ is toxic, but nutritious, plants act quickly to sequester it in their cellular vacuoles until such times as they can synthesize it into proteins.plantbrain said:NH4+ is worth more and is a good signal for a successful bloom in natural systems than other parameters, it tells the algae spore that no one(no other autotrophs) else is around.
I'm a firm believer in NH4+/NH3 = algae bloom. I just had a load of native plants drop their emersed leaves in one of my tanks while at the same time they didn't grow submersed leaves too well. All that decay = NH4+/NH3 + high light + CO2 => lots of algae
I also did a little experiment outside in some buckets that I'm using for emersed culture. Both containers were right next to eachother so light and temperature equality weren't issues. One had old tank substrate, the other didn't have any, and both had a high biomass. At first I was using my normal aquarium fertilization regimen on the buckets (Stump remover, enema, and the like). I got good growth from this routine. After a month I added a 1/2 tsp of MiracleGro Rose Food (Urea/NH4+ based) to each container which immediately resulted in algae.
The container with the substrate exhibited significantly less algae than did the water only container. This was only a secondary observation but it seems to support the mulm-seeding hypothesis.
For a long time experienced and knowledgable aquarists have said that well growing plants = little/no algae. That's because healthy plants act quickly to remove NH4+/NH3 from the water. Just like Tom asserts, low NH4+/NH3 = "Something else is living there, stay dormant and wait for a better opportunity." = little visible algae.
What about cyanobacteria? At this point I feel it has to do more with O2 content and organic compound concentration with preference for organic compound concentration. 99% of the time when I have a problem with BGA it is in areas where lots of mulm has accumulated and there is little water circulation. That poor/no circulation allows a microclimate that gets hypoxic/anoxic quickly from the respiration of decomposing bacteria. Once that happens anaerobic bacteria such as BGA are able to colonize and thrive. Once the area is aerated again and/or the mulm is removed BGA no longer do well and die off/move to another area.
I've observed a lack of BGA even in dense stands of well growing stemplants where there is little current and decent mulm accumulation, but high available oxygen from healthy growth and photosynthesis at the lower levels. I also saw a lack of BGA in one of my early tanks where the growth wasn't as good, but there was a spraybar along the entire bottom portion of the tank right at substrate level keeping the area oxygenated.
My two-bit hypotheses,