Final answer:
An algal bloom can drastically affect an aquatic ecosystem by reducing light penetration, depleting dissolved oxygen leading to 'dead zones,' and potentially releasing toxins, making it a cascading effect.
Step-by-step explanation:
When an algal bloom occurs, it is typically driven by an excess of nutrients such as nitrogen and phosphorus, which can come from sources like fertilizer runoff or sewage. These algal overgrowths can have multiple impacts on an aquatic ecosystem. Firstly, they can reduce light penetration in the water, which hinders the growth of underwater photosynthetic plants. Additionally, when the algae die, they decompose and significantly deplete the dissolved oxygen in the water, creating 'dead zones' where fish and other aerobic aquatic organisms cannot survive due to the lack of oxygen. Furthermore, certain types of algae, like Cyanobacteria and those that cause red tides, can release harmful toxins into the water which can lead to allergic reactions, liver damage, or even neurological issues if ingested by humans or wildlife.