The European satellite sensor MERIS (Medium Resolution Imaging Spectrometer) can be used to detect a peak in the optical spectrum of water-leaving radiance at about 705 nm which provides a signature of intense plankton blooms. We present examples of this signature in images and spectra derived from MERIS data, and demonstrate how this signature arises using model results. The MERIS band at 709 nm, which is an essential part of the detection scheme, is not present in either Seawifs or MODIS. In many cases the higher spatial resolution (300 m) provided in the FR mode of MERIS data is important for detecting small area events. The combination of wide area coverage, 300m spatial resolution, and appropriate spectral bands makes MERIS a unique and potentially important tool for global, intense plankton bloom monitoring. Here we show bloom examples and describe two types of "false alarm" signals, one due to benthic vegetation and one that appears to be due to cosmic ray impacts on the MERIS CCD detectors