There is mounting evidence that coastal systems are experiencing an increasing and disturbing trend in the incidence of problems associated with harmful algae, including human illness from contaminated shellfish or fish and the closure of shellfish beds. Certain marine organisms produce toxins, which are poisonous to humans. The causative organism is often a species of dinoflagellate with the ability to bloom (e.g. red tides). The most common types of occurrences are:

• Paralytic Shellfish Poisoning: It is caused by saxotoxin and other toxins, which accumulate within the tissues of shellfish filtering dinoflagellates containing these toxins. In humans, the disease causes respiratory paralysis, which sometimes can lead to death due to asphyxia.
• Ciguatera: caused by ciguatoxins that are produced by dinoflagellates living in macro algal beds. Herbivorous fish ingests the algae and toxin, which is concentrated as it moves up the food chain. See section on ciguatera.
• Plankton blooms: can range from benign to hazardous depending on the organisms implicated. Seasonal blooms such as the visible plankton blooms - red tides and brown tides - can cause fish kills and odour discomfort. Sewage effluent from hotel and resorts often contribute to the large amounts of nutrients required for these organisms to 'bloom'.

Circumstantial evidence seems to suggest that outbreaks of biotoxic organisms are due to coastal pollution and/or construction. Thus reducing pollution, sewage input and construction could help in keeping such occurrences at bay.