This master thesis is composed at the Department of Built Environment at Aalborg University. The aim of the project is to develop a system for forecasting blooms of blue-green algae in bathing waters. The lake Knudsø in the municipality of Skanderborg is the project site and has a total of three bathing waters that are frequently visited, two of which have Blue Flag.

Based on remote sensing it is documented that blooms of blue-green algae occur every year in Knudsø. Toxic species are dominating and therefore, blue-green algae may pose a health risk to bathers and it has been assessed that there is a potential for establishment of a forecasting system at this lake.

An analysis of the parameters affecting the blue-green algae in Knudsø has been composed. Based on this analysis, a forecasting system has been set up on the basis of an observation strategy and flow modelling. The areas of the lake in which the observations should be made is analyzed based on the risk that blooms in the lake affect the bathing waters. This analysis is based on remote sensing of blue-green algae and flow modelling.

Based on pre-simulated flow patterns and real time observations of blue-green algae in Knudsø, a forecasting system is set up to inform bathers if the concentration of blue-green algae in a bathing water area exceeds concentrations that can pose health risks. Further research can usefully be carried out to improve the forecasting system in Knudsø and to gain increased knowledge about the blooms of blue-green algae within this lake.