The detection of dense harmful algal blooms (HABs) by satellite remote sensing is usually based on
analysis of chlorophyll-a as a proxy. However, this approach does not provide information about the
potential harm of bloom, nor can it identify the dominant species. The developed HAB risk classification
method employs a fully automatic data-driven approach to identify key characteristics of water leaving
radiances and derived quantities, and to classify pixels into ‘‘harmful’’, ‘‘non-harmful’’ and ‘‘no bloom’’
categories using Linear Discriminant Analysis (LDA). Discrimination accuracy is increased through the
use of spectral ratios of water leaving radiances, absorption and backscattering. To reduce the false alarm
rate the data that cannot be reliably classified are automatically labelled as ‘‘unknown’’. This method can
be trained on different HAB species or extended to new sensors and then applied to generate
independent HAB risk maps; these can be fused with other sensors to fill gaps or improve spatial or
temporal resolution. The HAB discrimination technique has obtained accurate results on MODIS and
MERIS data, correctly identifying 89% of Phaeocystis globosa HABs in the southern North Sea and 88% of
Karenia mikimotoi blooms in the Western English Channel. A linear transformation of the ocean colour
discriminants is used to estimate harmful cell counts, demonstrating greater accuracy than if based on
chlorophyll-a; this will facilitate its integration into a HAB early warning system operating in the
southern North Sea.