Harmful algae are types of algae that can produce toxins, which are harmful to marine life, humans, and the environment. These algae blooms often occur in nutrient-rich waters, where excess nutrients from sources like agriculture and sewage promote their growth. When harmful algae bloom, they can deplete oxygen in the water, leading to dead zones where fish and other aquatic organisms cannot survive. Some species, such as Karenia brevis, can cause red tides, which can affect seafood safety and public health through contaminated shellfish and respiratory issues in nearby communities.