strange algae growth

No algae growth at the upstream stations! The reservoir I work on has 4 branches and 2 tributaries. I compare the simulated chla value with observed data and find that the algae growth at two stations in the main branch close to dam is normal. But, there are few algae production by the model at another two stations at the upstream of the two tributaries. We normally do not observe algae in the inflow. So,I set the algae inflow from the two tributaries to be zero and let the initial algae concentration in the waterbody drive the algae growth. By the way, I simulate three algae growths including blue-green, green and diatom. After running the model, I find blue-green alage dominate all the time. There are green algae growth sometime but no diatom at all. This is not true for the real world. The model indicates the limiting factor for blue-green and green is light and diatom for Si. I have tried changing kinetic parameters for three algae groups and initial algae concentrations but didn't solve the problem. Would you please let you know your opinion about the two questions? Thanks.