A near real-time algal bloom detection service has been developed for European waters. Daily chlorophyll a data from Envisat/MERIS and Aqua/MODIS are compared to a predefined threshold map to determine whether an algal bloom has occurred. The design of the threshold map takes account of two factors. Firstly, over European waters regional differences in typical and extreme levels of chlorophyll a span two orders of magnitude. A concentration, e.g. 2 μg/1, that would be considered as a bloom concentration in one region could correspond to a relatively low concentration in another region. Secondly, the errors in satellite chlorophyll a can be significant in coastal waters effectively giving an artificial background level in satellite images. To account for these two factors a threshold map has been designed using satellite chlorophyll a data from a previous year, with separate threshold maps for each sensor to take account of sensor-specific bias. This threshold is defined here as the top 10\% level of a previous year's chlorophyll a data. The results of algal bloom detection in various regions are analysed in terms of bloom timing, duration and strength. Differences between the performance for the MERIS and Aqua sensors are also discussed.