SIT-FUSE is a self-supervised machine learning framework that leverages multi-sensor satellite data to detect and map the severity and species of harmful algal blooms (HABs). It fuses reflectance data from operational instruments such as VIIRS, MODIS, Sentinel-3, and PACE with solar-induced fluorescence (SIF) from TROPOMI. This framework generates HAB severity and species analyses without instrument-specific labeled datasets, and classifies phytoplankton abundances and species into interpretable classes using self-supervised representation learning and hierarchical deep clustering. It has been validated using field data from the Gulf of Mexico and Southern California, demonstrating high agreement with measurements of total phytoplankton, Karenia brevis, Alexandrium spp., and Pseudo-nitzschia spp.