What are the consequences of excessive algae blooms in water bodies?

Excessive algae blooms, often referred to as harmful algal blooms (HABs), have significant environmental impacts, one of the most severe being oxygen depletion after the algae die off. When algae proliferate excessively, they can cover the surface of the water body, blocking sunlight from reaching submerged aquatic plants, which can hinder their growth and overall ecosystem health.

When the algae eventually die, they are decomposed by bacteria, a process that consumes dissolved oxygen from the water. This depletion of oxygen can lead to hypoxia, conditions where oxygen levels drop significantly, posing a threat to fish and other aquatic organisms that rely on oxygen to survive. Fish populations can decline drastically in such conditions, leading to the phenomenon commonly known as "fish kills."

The other options do not accurately describe the consequences of excessive algae blooms. For instance, algae blooms do not increase fish populations; rather, they can lead to ecosystem collapse and reduced fish populations due to oxygen depletion. Improved water clarity is generally not a result of algal blooms; instead, they often create turbid conditions. Lastly, excessive algal growth does not have a direct impact on water hardness, which is influenced by the concentration of calcium and magnesium salts rather than by algal presence.