This article gives some information about the interesting biology of the blue algae, which are no algae in sensu strictu but do belong to the bacteriae.

You may also have microscopical looks at some of the freshwater-forms, which can be found in the Wakenitz

In summer these specialistic bacteriae, which are able to practice photosynthesis, reach very high population densities and cover the water.
Then swimming in the river is dangerous and the town authorities recommend not to bathe.
The reasons why these algae grow up in masses were found out by chemical investigations in 1998 and 2001. These investigations were lead out by two teachers and some students.