Can humans cause algal bloom in a lake?

Yes. But generally inadvertently, or by accident.

There is another class of causation - the willfully ignorant who refuse to accept responsibility for their actions.

An excess of phosphate or nitrogen may cause an algal bloom - an overproduction of algae. This algal mat will block light from the water body, and when it eventually dies, it will create an anoxic mat on the lake floor.