As NH4 is a nice easy to use form of N, often a limiting nutrient, and NO3 is common in ground water, what might a good ion to monitored to make sure no one lese is there?
High O2 levels is one, means something like algae or plants or both are growing......
NH4, since plants remove it directly, as do algae from the water column.
If the levels are low, someone else is already there, and there's not enough to get going to germinate yet.
So the spore waits until there's a spike in NH4, then it's got enough to make it to adulthood(hopefully),
I do not know how much NH4 is required because there are other factors, light being the biggest, and CO2, which influences NH4.
Few aquarists measure either light or CO2 accurately.
Measurement of NH4 needs to be very accurate and at lower levels.
Say 0.005 to 0.5 ppm. Or 5 ppb to 500ppb NH4.
Not many test methods can do that I've seen.
The other issue would be that the NH4 levels are virtually always higher around feces, decaying organic matter, the sediment layers etc, vs the open water or near the surface.
The NH4 is also cycled in the system really fast, this is due to both bacteria in the filter and on virtually every surface, as well as plant and algae uptake.
You have plenty algae, mostly diatoms and some BGA's growing on leaves etc, but you really cannot see them.
So visible algae vs non visible algae is yet another issue.
This is why I've been reluctant to give hard data concentrations on NH4 for inducing algae, rather, use it as model to explain how, why and what we can dpo to prevent algae.
Thus far, that works really really well.
Also, few will disagree with if the plants are growing very well etc, then algae will not. So what about their growth reduces algae presence?
We can add pure O2 and see, no effect even at high light.
We can add NH4 and high light and see it rather fast for some species (GW), and longer for others like Staghorn. CO2 can regulate NH4 uptake and demand, likely by 5-20X, or 500-2000% decrease from say 30ppm to 0.5ppm CO2.
Generally high light means 4w/gal or higher and PC/T5/MH type lighting, about 350- or higher micromols/m^2/sec.
why might a tiny little old alga spore need a more available type of N and need a good signal to know when best to germinate while a huge multi billion celled plant cutting with vast reserves of N, not?
The plant is not a seed/spore here, it's already there and growing.
It'll take NO3 and NH4.
The algae spore response to NH4, the presence of NH4 often signals seasonal turnovers in lakes, ponds, and streams, as influx of organic materials enters them during wet spring, or fall etc.
If you go to where the system is stable in terms of CO2, and inputs, say a Fw spring, there have been plants in such systems for 100's if not thousands of years with little algae domination.
Seasonal scouring due to flooding removes many plants and algae.
In order to past their genes along and to exist, they need something that will keep them there and coming back. Algae have different mechanisms and niches than plants do.
Plants do very well with clonal vegetative reproduction.
That's why we can garden easily with our "weeds".
They can grow well where they find good conditions.
No need to put energy into a lot of seed and flower production.
Algae sort of do this, but when they sense tough times coming, they produce spores. Plants just stop growing (a few algae will also, BBA etc) and wait.
It's either this or they die.
Some plants measure the day length and produce tubers that sprout in spring(Hydrilla, Aponogetons, Lilies, Lotus etc).
Our tank day cycle can be modified.
If you run light 14-16 hours at high light, you are more likely to have algae issues than at 10 hours. Winter lighting is about 10 hours for many locations.
Algae does not really do this, plants certainly do.
Hydrilla at the lab is all melting back the vegetative structures and really making tubers like mad right now. If I added FL lighting and keep the the daylength at 14 hours, I can stop this.
This light detection is signaled by the Phytochrome system.
Algae use a similar system but have different demands, signals and niches.