What would the best approach be to model the movement of a dissolved species (here oxygen) in a solvent (water) in Fluent? The mixture is very dilute (of the range 0-100% of sat. conc. 0.008 kg O2/m3) and therefore would have virtually no effect on momentum trasport; unless bubbles form which can be ignored at this stage. Is there anything easier than using the analogous heat transfer case (i.e. particle tracing, injectionts)? Source and sink terms will later be added by way of reactions with other dissolved species (nitrites and nitrates).

If the dissolved species doesn't affect the flow field, You may also add a simple advection-diffusion equation for a concentration c. This feature of FLUENT is described in the subsection "User-defined scalar transport modeling".