BIOLA isa biogeochemical lake model within the V ASTRA research programme. The model's main purpose is to predict the ecological responses to changed nutrient loads. The phosphorus simulations were not satisfactory and the sediment was thought to be the critical part. The aim of this work was to improve the phosphorus simulations. Therefore a new sediment approach has been developed. Three main changes of the sediment processes were carried through:
• Resuspension of sediments from erosion and transportation bottoms was added
• The sediments were divided into an upper, aerobic, and a lower, anaerobic, layer
• The relation between sorbed and dissolved phosphorus in the sediments was described by Langmuir isotherms, with a sorption 2.5 times higher at aerobic compared to anaerobic conditions
The modelling of total phosphorus and blue-green algae was improved. Especially the timing of high concentration peaks was much better. One problem still lingering is the modelled oxygen levels, which were much higher than the observed levels. Therefore the model has not been tested for anaerobic conditions. A verage release rates from the sediments were 2.5 mg P a-1 <luring the summer, which is reasonable. A 20-year simulation of step-response was run with increased and decreased nutrient loads. Roughly four years after the change, the model had reached a new equilibrium.