When monitoring freshwater aquatic environments (rivers, waterbodies), it may be necessary to know how to detect and char- acterize the presence of a proliferation on the surface or in the water column. The proliferation can be benthic or planktonic, be algal or composed of cyanobacteria or be of another nature. Its correct characterization will influence its consideration and the actions to be implemented to monitor or manage it. The determination key below, adapted and simple, is recommended for use in the field, without precise taxonomic determination, in order to qualify the proliferation encountered. Summary sheets with the main characteristics are provided at the end for cyanobacterial blooms (benthic and planktonic), macroscopic algae and fila- mentous heterotrophs.