There is a reference to algal blooms in the Bible, ‘ . . . all the waters that were in the river were turned to blood. And the fish that was in the river died; and the river stank, and the Egyptians could not drink of the water of the river’ (Exodus 7: 20–1). The scenario described here fits the description of a nontoxic bloom which created anoxic conditions that led to a fish kill. Read more here.