Harmful algal bloom

Lore & Background

Harmful algal blooms have been occurring since before humans began affecting the environment, though in some locations they appear entirely natural while in others they are linked to human activities. They are induced by eutrophication—an overabundance of nutrients—often from agriculture, industrial pollution, fertilizer use, and urban runoff. Higher water temperature and low circulation also contribute. Blooms can last from days to many months; after the bloom dies, microbes decomposing the algae consume oxygen, generating dead zones that can cause fish die-offs.

Cyanobacteria blooms in freshwater appear as foam, scum, or mats, often bright green with surface streaks like floating paint. Not all blooms produce toxins; some only discolor water or produce odors. It is impossible to tell if a bloom is harmful from appearance alone—sampling and microscopic examination are required, and sometimes toxin-level testing or gene detection is needed. The term 'red tide' is being replaced by more precise terminology like 'harmful algal bloom' for harmful species and 'algal bloom' for benign ones.

Many HAB-forming species have a dual-stage life cycle, alternating between a benthic resting stage near the ocean floor and a pelagic vegetative state near the surface where rapid reproduction occurs. This transition can rapidly terminate a bloom as cells return to the benthic stage. HABs have been increasing in size and frequency worldwide, a trend many experts attribute to global climate change.

Reader's Guide

Harmful algal blooms are significant because they negatively impact human health, aquatic systems, and economies. The article notes that HABs can cause fish die-offs, create dead zones, and produce toxins that water purification plants may be unable to remove, leading to advisories against drinking tap water. The definition of HABs is disputed: some restrict it to toxin-producing blooms, while others include any bloom causing hypoxia or anoxia. The U.S. Environmental Protection Agency adopted a broad definition in 2008, covering potentially toxic species and high-biomass producers that cause oxygen depletion and marine life mortalities. HABs have been increasing in size and frequency, with NOAA predicting more blooms in the Pacific Ocean. Potential remedies include chemical treatment, reducing nutrient runoff, sensors and monitoring devices, research, and management. The article emphasizes that HABs are a societal concept rather than a purely scientific definition, and their causes can be natural or human-induced, with links to nutrient pollution, coastal development, and aquaculture.

Did You Know?

• HABs can last from a few days to many months.
• Cyanobacteria blooms in freshwater may appear bright green with surface streaks like floating paint.
• The mixotrophic ciliate Mesodinium rubrum produces non-toxic blooms colored deep red.
• Some cyanobacteria produce microcystins, which are hepatotoxins that harm the liver of mammals.