Here is a sentence that says: In bodies of water polluted by phosphorus, algae can form large floating mats, called algal blooms.
What substance causes an alfal bloom to form? (I don't understand the sentence)
That's "algal bloom"
See:
http://en.wikipedia.org/wiki/Algal_bloom
In bodies of water that are polluted by phosphorus, algal blooms can occur. An algal bloom is a rapid increase in the growth of algae, resulting in a dense concentration of these microscopic plants in the water. Algae require nutrients to grow, and phosphorus is one of the main nutrients they need. When there is an excess of phosphorus in the water, either due to pollution or natural causes, the algae can take advantage of this abundance and grow rapidly, forming large floating mats known as algal blooms.
To understand this sentence better, you can refer to the Wikipedia page on algal blooms (http://en.wikipedia.org/wiki/Algal_bloom). It provides comprehensive information about what causes algal blooms, their effects on the environment, and how they can be managed or prevented.