- Excessive and rapid growth of algae and other aquatic plants when they are stimulated to grow too quickly by pollution. It takes place when there are too many nutrients in the water and is aggravated when accompanied by a rise in temperature. Although the algae grow quickly they soon die because they have swallowed up all the water's nutrients. As they decompose they tend to rise to the surface and form a green slime. Algal bloom have increased because higher levels of nitrogen and phosphates from agricultural areas have leached from the fields into water courses.