An algal bloom refers to the rapid increase or accumulation of microscopic algal species in marine or freshwater ecosystems, which can result in significant environmental, economic, and public health impacts. These phenomena are also known in scientific literature as "phytoplankton blooms," "microalgal blooms," or "red tides." Due to the ecological and toxicological diversity of the causative species, such events are commonly studied under the term harmful algal blooms (HABs). While algal blooms are natural occurrences, their frequency and severity have been notably amplified by anthropogenic factors such as eutrophication, climate change, and increased nutrient loading. Consequently, HABs are presently understood as the result of combined natural and human-induced influences.

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Classification and Impact

Harmful algal blooms are primarily categorized into two types based on their biological and ecological effects:

• Toxin-Producing Species: Certain algal species synthesize toxins that bioaccumulate in marine organisms, particularly shellfish, and can cause various poisoning syndromes in humans upon consumption. Additionally, some non-toxic species may induce mortality in aquatic fauna through physiological stress, including irritation of gill tissues, enhanced mucus secretion, and respiratory impairment.
• High Biomass-Producing Species: Species characterized by substantial biomass proliferation may cause hypoxic or anoxic conditions by depleting dissolved oxygen in the water column, leading to mass die-offs of aquatic organisms.

Some harmful algal blooms exhibit both toxic and high biomass effects concurrently. For instance, Gymnodinium cf. mikimotoi has been reported to cause both aquatic animal mortality and respiratory and dermal irritation in humans through aerosolized particles.

Ecological and Economic Consequences

Harmful algal blooms have profound impacts on fisheries and aquaculture, especially affecting shellfish and cage-cultured fish species. Toxins produced by certain microalgae accumulate in the tissues of filter feeders such as mussels and oysters. These toxins are heat-stable and do not alter the sensory properties (taste or odor) of the seafood, thereby posing challenges for detection by consumers. Routine monitoring through biological and chemical analyses is essential for ensuring food safety.

Historically, HABs have caused considerable economic damage. For example, in 1994, a bloom dominated by Ceratium furca and Prorocentrum micans in St. Helena Bay, South Africa, generated hydrogen sulfide gas and led to mass mortalities of approximately 1,500 tons of fish and 60 tons of lobsters. In Japan, a bloom by Chattonella antiqua resulted in cage fish losses valued at approximately 500 million USD.

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Human Health Implications

Toxins from harmful algae can affect human health through the consumption of contaminated seafood or inhalation of aerosolized toxins. The major clinically recognized poisoning syndromes associated with HABs include:

• Amnesic Shellfish Poisoning (ASP): Caused by domoic acid, leading to neurological effects such as memory loss.
• Ciguatera Fish Poisoning (CFP): Linked to consumption of tropical reef fish, associated with neurological and cardiovascular symptoms.
• Diarrhetic Shellfish Poisoning (DSP): Induces gastrointestinal disturbances and may have tumor-promoting properties.
• Neurotoxic Shellfish Poisoning (NSP): Results from inhalation of toxic aerosols, causing respiratory symptoms resembling asthma.
• Paralytic Shellfish Poisoning (PSP): A neurotoxic syndrome with potentially fatal outcomes; global annual cases approximate 2,000 with a mortality rate of around 15%.
• Azaspiracid Shellfish Poisoning (AZP): A recently identified syndrome resembling DSP clinically but generally non-lethal.

Furthermore, certain cyanobacteria (blue-green algae) species found in freshwater systems produce toxins that pose significant public health risks, including tumorigenic potential.

Taxonomic Groups of Concern

Harmful algal species are taxonomically distributed among five principal groups:

• Dinophyceae (Dinoflagellates): Includes genera such as Gambierdiscus, Alexandrium, Dinophysis, and Karenia, which are major contributors to human poisoning syndromes.
• Prymnesiophyceae (Haptophyceae): Comprising genera such as Prymnesium and Chrysochromulina, known for inducing fish kills.
• Raphidophyceae: Genera like Chattonella cause fish mortalities via toxic and non-toxic pathways.
• Bacillariophyceae (Diatoms): Notably Pseudo-nitzschia species produce domoic acid responsible for ASP.
• Cyanophyceae (Cyanobacteria): Important in freshwater toxic events; in marine settings, genera such as Trichodesmium contribute to nitrogen fixation, while Nodularia causes blooms in brackish environments (e.g., Baltic Sea).

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Monitoring and Management

Accurate identification of harmful algal species is critical for monitoring and managing HABs. This requires specialized taxonomic expertise and microscopic methods, including electron microscopy in some cases. Limitations in such capacities, particularly in developing regions, hinder effective detection and mitigation. Recommended preventive measures to protect public health include:

• Avoiding fishing in visibly discolored waters and refraining from consuming dead fish.
• Retaining samples of seafood in suspected poisoning cases and seeking medical evaluation.
• Observing warning signs and restricting swimming activities, especially for children, in affected waters.

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Dispersal of Harmful Algae as Invasive Species

Several harmful algal species have been reported to spread as invasive organisms to different geographic regions. Mechanisms facilitating this dispersal include ballast water transport, aquaculture practices, and attachment to ship hulls. The primary criteria considered when classifying a species as invasive are:

• Toxic potential
• Ability to form resting cysts
• Broad tolerance to temperature and salinity
• Previous history of invasiveness

In this context, species such as Gymnodinium catenatum, Alexandrium tamarense/catenella, and Alexandrium minutum have been identified as invasive in certain regions. Additionally, species like Pfiesteria piscicida have been shown to be transported via ballast water, which is taken on board ships for stability during voyages and discharged at ports.