This paper states that algal bloom are a sign of anthropogenic disturbance. They actually can occur from natural eutrophication as well. The thesis of this paper is not quite clear from the abstract (rather eutrophic so to speak).
Lee J, Rai PK, Jeon YJ, Kim KH, Kwon EE. The role of algae and cyanobacteria in the production and release of odorants in water. Environmental pollution 2017 May 2;227:252-262. doi: 10.1016/j.envpol.2017.04.058.
This review covers literatures pertaining to algal and cyanobacterial odor problems that have been published over the last five decades. Proper evaluation of algal and cyanobacterial odors may help establish removal strategies for hazardous metabolites while enhancing the recyclability of water. A bloom of microalgae is a sign of an anthropogenic disturbance in aquatic systems and can lead to diverse changes in ecosystems along with increased production of odorants. In general, because algal and cyanobacterial odors vary in chemistry and intensity according to blooming pattern, it is necessary to learn more about the related factors and processes (e.g., changes due to differences in taxa). This necessitates systematic and transdisciplinary approaches that require the cooperation of chemists, biologists, engineers, and policy makers.