A large algae bloom that can create low-oxygen dead zones in ocean water is being monitored by state authorities in Casco Bay. Karenia mikimotoi, the phytoplankton causing the widespread bloom, is not hazardous to humans, but fish kills and have been reported during blooms of the species in Asian and European countries, according to the Department of Marine Resources. The toxicity of the algae is not fully understood, but it is believed that die-offs are caused by the creation of a low-oxygen environment created by the bloom itself and a toxin carried by the phytoplankton.