- Harmful algal blooms tend to form through the activities of plants, as well as the accumulation of cells that don't contain chlorophyll. When the accumulation grows to an extent that it dominates the area, it is called a harmful algal bloom. The proliferation of these algal blooms could come about as a result of some positive stimulus, such as a change in the water temperature.
- Harmful algae tend to produce toxins that are detrimental to aquatic life, such as fish and shellfish. When the fish and shellfish eat the harmful algae, they accumulate the toxins. Humans and marine animals that eat these poisoned fish and shellfish also tend to get poisoned.
- When one organism feeds on another, the former is said to be predatory. Some of these harmful algae produce toxins that help them capture prey, such as fish. When the algae are exposed to secretions from fish, they produce these toxins. The toxins affect the fish's neurological system so that it becomes lethargic. When there is enough of the toxin present, the fish dies. The toxins cause the fish's external layers to peel off and make it vulnerable to infections. The algae can then actually ingest fish tissue.
- When harmful algae take over a water body, they also tend to create oxygen deprivation. As the algae take in oxygen, the local water loses its oxygen supply so that the local area is low on oxygen, or does not have any at all. This affects the marine life forms that also need oxygen to survive, causing fish and shellfish to die. Some larger crustaceans could also die as these algae get into their gills, causing their respiratory systems to malfunction.