Hi Hans-Werner!The cited text above says, that with 1 - 2 µmol/l the N uptake is saturated, which means the coral just can't take up more N than at this concentration, further increase doesn´t make any sense. 2 µmol is 36 µg or 0.036 mg of NH4+ or 62 µg or 0.062 mg of Nitrate. Nitrate levels in coral reefs are not much higher than Phosphate levels, in average about 2.5 times higher by weight, this means around 0.04 ppm, and corals show net nitrate uptake.
Everything else you see at higher nitrate concentrations is not a positiv effect of nitrate uptake but rather a sign of stress.
David here, from Gothenburg. Thank you for a great talk at MACE
In the ocean corals get some of their N from feeding on different types of plankton. That part is probably lower in most aquariums. Could it be possible they need to take up more N in an aquarium than on the reef because of that?
Also, isn't a risk to get too low when aiming for natural N levels? How do we keep a steady level of NH4 in an aquarium?