We present an on-line early warning system that is operational in Scottish coastal waters to minimise the risk to humans and aquaculture businesses of the health and economic impacts of harmful algal blooms (HABs) and their associated biotoxins. The system includes both map and time-series based visualisation tools. A “traffic light” index approach is used to assess risk at highlight locations at elevated HAB/biotoxin risk. A high resolution mathematical modelling of cell advection being used in combination with satellite remote sensing provides early warning of harmful blooms that advect from offshore waters to the coast. Expert interpretation of HAB, biotoxin and environmental data in light of recent and historical trends is used to provide, on a weekly basis, a forecast of the risk from HABs and their biotoxins to aquaculture to allow mitigation measures to be put in place by aquaculture businesses should a HAB