Energy balance (methane emission and water evaporation), pH and salinity of water, oxygenation of substrate (e.g. peat), soil and water chemicals (e.g. calcium), carbon production, etc. :)

I have these parameters to calibrate. See the link below...

http://www.hydro.washington.edu/Lettenmaier/Models/VIC/Documentation/LakeParam.shtml

Model calibration is the process of adjustment of the model parameters and forcing within the margins of the uncertainties to obtain a model representation of the processes of interest that satisfies pre-agreed criteria – you only have 6 parameters, so you should incorporate all of them imo. Also, do a sensitivity analysis :)