Yep, an order of magnitude is a factor of 10. two orders of magnitude is a factor of a hundred.
A jump of 0.1 to 1.0 is just as bad as 1 to 10. They are both an order of magnitude difference and the plants response in manufacturing the Rubisco enzyme is just as disrupted.
Most are not aware of how important Rubisco is and how critical it's production is to Carbon uptake. This enzyme dependency is what make CO2 shortfalls so much more complicated than other nutrient shortfalls. It's a HUGE complicated molecule. It has binding sites, kind of like electromagnets, which attract and captures other specific molecules. On one side, a binding site captures either a CO2 or an O2 molecule. On the other side, a binding site captures a molecule known as Ribulose biphosphate (aka RuBP which is basically a sugar) Rubisco "ferries" these two captured molecules together during photosynthesis in order to carry out the process of "carbon fixation".
Plants sense the level of CO2 in the water and make specific adjustments to the Rubisco production. There must be a certain ratio of RuBP, CO2 and Rubisco for this ferry system and subsequent carbon fixation and food production to occur efficiently. It is a finely tuned system. When CO2 levels change, RuBP and Rubisco synthesis must also change. The complexity of the enzyme makes it very difficult for higher plants to adjust to rapidly changing CO2 concentration levels.
Algae have much less complicated systems so they adjust easily to changing or unstable conditions in the environment. This is the reason we see so much hair, BBA and staghorn. The more transient and unstable the system, the more these types proliferate. This is not to say that NH4 isn't a contributing factor, only that the dominant factor in these species reflect the difficulty we have with stabilizing adequate levels of CO2. Most species can adapt to low, long term CO2 shortages or to medium or high long term levels but fluctuating levels are a real problem.
Cheers,