Supersaturation of DO when modelling multiple algal groups

I am currently modelling a dam subject to climate change to verify algal succession for diatoms, greens and cyanobacteria. The algal growth constants are all literature or measured values and will within limits and the model is calibrated and validated. The problem I have encountered is that the surface layer of dam goes supersaturated for DO for extended periods with peaks in excess of 40mg/l . I have found that wind fetch correction and wind speed measuring heights do not significantly affect the surface DO concentrations. What i have found is that the algal growth rates affect photosynthesis and subsequently the DO concentration. Lower growth rates produce more normal and acceptable DO levels but then the algal growth is not modeled accurately. I could in effect say greater AG means greater DO concentrations and/or persistent supersaturation on the surface. Is this a bug in the model ? How could i overcome this and keep the algal growth rates realistic ?