Algal blooms

Every summer in New England beaches close due to high levels of microbes in the water.  Sometimes it is due to bacteria and sometimes it is due to a small organism called algae.  Algal blooms can be so big that they are observed from space.

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The phenomenon of a algal bloom - or any microbial bloom - has to do with the basic ecology of the amount of food in an environment and the size of the population.  When blooms occur, it is largely due to sudden increase in the amount of food for the algae - leading to a sudden increase in algae growth.  Since alga are green due to the chlorophyll they use for photosynthesis, this is easily visible in the water.