Dictionary of Ichthyology  

bloom = a rapid and localised increase in the density of plankton resulting
from a nutrient-rich habitat. The nutrients may come from upwelling, mixing or pollution and the bloom can kill fish populations through toxins or oxygen depletion.

Black Sea blooms, 25 May 2004 from NASA. Wikimedia Commons: http://en.wikipedia.org/wiki/File:Black_Sea_Nasa_May_25_2004.jpg

Canadian Museum of Nature