Algae are simple plants. What is referred to as "blue-green algae" are not algae but a type of bacteria called cyanobacteria. These bacteria are a normal presence in many lakes, however they tend to thrive in warm, nutrient rich water.
During the right conditions, the bacteria can begin growing at a rapid rate and can form an "algae bloom." Sometimes these blooms are described as "looking like spilled green paint." Blue-green algae prefer warm, calm, sunny weather and warm water temperatures above 75 degrees. These bacteria are harmful if swallowed and can cause a variety of symptoms. Dogs especially may be affected.
To reduce this type of dangerous algae in our waters, the nutrient rich environment must be reduced. This is best accomplished by limiting the use of man-made phosphorus and nitrogen rich fertilizers.
Source: pca.state.mn.us Minnesota Pollution Control Agency