Hello Gunnar,
If you give a tracer with name TEMPERATURE for the keyword 'NAMES OF TRACERS', the O2 module should take into account this temperature field rather than a constant temperature.
With the name TEMPERATURE, the corresponding tracer will have the tracer number IND_T in TELEMAC-3D and in the CALCS3D_O2 subroutine, the array storing temperature values will be used rather than a constant temperature given by the keyword WATER TEMPERATURE.
It should work like this, please tell me if not.
Hope this helps,
Chi-Tuan