Phytoplankton blooms are sporadic events in time and isolated in space. This complex phenomenon is produced by a variety of both natural and anthropogenic causes. Early detection of this phenomenon, as well as the classification of a water body under conditions of bloom or non-bloom, remains an unresolved problem. This research proposes the use of Inherent Optical Properties (IOP) in optically complex waters to detect the bloom or non-bloom state of the phytoplankton community, as well as it is an active or a decaying bloom. An IOP index is calculated from the absorption coefficients of the colored dissolved organic matter (CDOM), the phytoplankton (φ) and the detritus (d), using the wavelength (λ) 443 nm. The effectiveness of this index is tested in five bloom events in different places and with different characteristics: 1. Dzilam de Bravo (Caribbean Sea, Atlantic Ocean, Mexico) a diatom bloom (Rhizosolenia cf hebetata), 2. Holbox (Caribbean Sea, Atlantic Ocean, Mexico) a mixed bloom of dinoflagellates (Scrippsiella sp.) and diatoms (Chaetoceros sp.), 3. Campeche in the Gulf of Mexico (Atlantic Ocean, Mexico) a bloom of dinoflagellates (Karenia brevis), 4. Upper Gulf of California (Pacific Ocean) a diatoms bloom (species to be determined) and 5. Ensenada (Pacific Ocean) a dinoflagellate bloom (species to be determined). The results show that the use of the IOP index is a suitable method to determine the bloom conditions in coastal stations.