- Oxygen Depletion: Algal blooms can rapidly grow and multiply, forming dense mats on the water's surface. As these algae die and decompose, they consume large amounts of oxygen. This process, known as eutrophication, depletes the oxygen levels in the water, creating a hypoxic or anoxic environment. Fish and other aquatic organisms, unable to breathe, may suffocate and die.
- Toxins: Certain types of algae produce harmful toxins as part of their natural defense mechanism. These toxins can be released into the water during algal blooms. Fish that ingest or come into contact with these toxins can experience various health problems, including damage to gills, liver, kidneys, and nervous system. The toxins can cause organ failure, disrupt vital bodily functions, and eventually lead to fish mortality.
- Habitat Disruption: Dense algal blooms can block sunlight from reaching underwater plants and disrupt the food chain. Submerged aquatic plants that rely on sunlight for photosynthesis may die off, reducing food sources for herbivorous fish. These disruptions in the food web can indirectly impact fish populations, leading to malnutrition, reduced growth, and increased vulnerability to diseases.
- Gill Damage: High concentrations of algae can clog fish gills, making it difficult for them to breathe. The physical irritation caused by the algae can damage the delicate gill filaments, impairing the fish's ability to extract oxygen from the water. This can result in respiratory distress and eventually death.
- Disease Spread: Algal blooms can also contribute to the spread of diseases among fish. The decomposing algae release organic matter that can serve as a breeding ground for bacteria and parasites. These pathogens can cause infections and diseases, further stressing and weakening fish populations.
The severity of the impact on fish populations depends on the extent and duration of the algal bloom, the types of algae involved, and the vulnerability of the fish species present in the affected water body.