Algal bloom occurs, when an aquatic ecosystem receives lot of limiting nutrients. This cause a rapid increase in the algae amount, so much. Moreover, sometimes algae number can exceed the number of algae consumers. Thus, algae spread all over the water surface.
In addition, algal bloom can disrupt the equilibrium of aquatic ecosystem. Algae are short lived; hence they often die off and produce decaying matter in the water body. Decomposition requires oxygen. The oxygen is taken from the water, thus the environment becomes hypoxic. Such low levels of oxygen do not support the other aquatic life, hence they perish.
Along with these serious complications there are many small effects caused by the algal boom. Thick algal matt can prevent the sunlight from entering into the water body. Algae can also cause hindrance to the fish movement.