How can pollution cause an algal bloom?
Water pollution causes algae blooms due to the agricultural and land use practices humans have in place.
Large-scale agriculture uses many chemicals to better grow their crops. They use pesticides to keep insects and other critters away and they use fertilizers to grow their crops faster and larger.
These nutrients from farms and lawns and so forth are carried through streams and runoff when it rains to lakes and other bodies of water. These excessive amounts of nutrients (phosphorus and nitrogen) are good for growing our food, but they're too excessive in these natural ecosystems.
This increase in nutrients allows more algae and other plants to grow than would occur naturally.
When the algae die, they are consumed by bacteria, and the amount of bacteria in the water body increases. Bacteria rely on dissolved oxygen in the water, and they use more of it up. The result is less oxygen available for every other living organism in the water body, which is a problem. This process is called eutrophication and can be seen in the image below.