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IMA > News  > Harmful Algal Blooms: Why the need for concern?
Macroalgae within a Coral Reef Tobago

Harmful Algal Blooms: Why the need for concern?

Prepared by Aleisha Dennie, Laboratory Technician, Institute of Marine Affairs

Have you ever heard of a ‘red tide’ that is followed by a fish kill? This phenomenon is caused by algal blooms which occur along coastal regions, making these tides a lot more noticeable.  Harmful algal blooms or HABs as they are commonly referred to, occur when photosynthetic microscopic algae, or phytoplankton grow uncontrollably within an aquatic ecosystem. These organisms pose many threats to marine habitats where they are found, but consequentially have the potential to harm humans as well. In fact, some are even capable of producing toxins that can bio-accumulate as they move up the food chain. It is for this reason HABs deserve a closer look.

Red Tide captured off Mexico. Photo credit: NOAA

Contributing Factors:

Let us examine the factors that lead to these blooms or overgrowth. One of the major drivers behind HABs is an influx of nutrients; nitrogen and phosphorus in particular. When runoff from agricultural or industrial processes find its way into natural water courses and then out to sea, it can give rise to these overgrowth events. Upwelling from deep ocean water can also increase the nutrient levels at the surface and cause blooms. Even the use of excessive fertilizers in our home gardens, if released into the ocean, can cause algal bloom events. 

The physical chemistry of the water has to be ideal for blooms also. Warm temperatures and slow water movement, as is often the case during the dry season or a drought, are ideal.  Climate change is said to worsen both the frequency and severity of HAB events globally, resulting in a higher occurrence of toxin-producing algae.  

Ecological Impacts and Human Health Concerns:

HABs are considered a “one health” issue globally. This is because it has the potential to affect an entire ecosystem (including aquatic animals and human health).  Of particular concern are the toxins produced by the cells of microalgae. Cyanobacteria, which are often mistaken for green algae, produce a highly potent toxin known as cyanotoxin. Cyanotoxins are usually released upon cell lysis or death, but in some rare cases they can be excreted extracellularly into the water whilst the organism is still alive. 

Dinoflagellates and Diatoms are forms of microalgae responsible for the algal blooms found in marine and brackish water.   Climatic conditions working in harmony with the pigments of these algal cells can give rise to a brick red (or some variance of it) appearance at the water surface. Of even greater concern are the toxic benthic (at the bottom of the water) dinoflagellates which can adhere to macroalgae particularly in coral reefs. Reef fish such as commercial species of barracuda and grouper, graze on the macroalgae containing the toxic benthic dinoflagellates. According to the Centers for Disease Control and Prevention (CDC), the year 2021 has seen an increase in HABs events across sixteen (16) states in the United States during the summer months. The CDC reported that over one hundred (100) human illnesses associated with HABs were documented and over twenty (20) times that amount for animal related infections. In this report the CDC warns that HABs are expected to increase in frequency, scale and geographical coverage as a result of climate change and nutrient pollution acceleration. 

Humans are normally exposed to these toxins by physical contact, swallowing contaminated water, or eating contaminated food, and even breathing-in the toxins.  It is only on rare occasions, however, that some persons may experience health crises by breathing-in the toxins found in mist or sea spray or consuming water droplets from contaminated water.  Symptoms can range from skin irritation to respiratory problems and other extreme ailments. On the other hand, fish and other aquatic organisms that come into contact with these toxins may or may not be themselves impacted.   Further ecological damage occurs when blooms become so dense that they block out all sunlight needed by other primary producers. Likewise, when they die, the oxygen demand for their decomposition is so high, the available dissolved oxygen in the water required to sustain the life of other marine or aquatic organisms becomes dangerously low. 

HABs in the Caribbean:

The presence and impact of HABs in Latin America and the Caribbean have been well documented in the past fifty (50) years. Gymnodinium, Catenatum, Pyrodinium and Gambierdiscus spp are the most prevalent species in the Region. However, Margalefidinium polykrikoides, a dinoflagellate, has become more common, often resulting in mass mortalities of fish and invertebrates, negatively affecting tourism activities in some Caribbean islands. The impact of HAB events on the economy and ecosystems of the region, however, is not yet sufficiently investigated.  

While several international organisations such as the UNESCO Intergovernmental Oceanographic Commission (IOC) have conducted several research studies on HABs events in the region, theses tend to be focus largely on increasing awareness of toxic species, detecting new toxins, examining the geographical expansion of the known species, and their impact holistically. However, the impact of HAB events on the economy and marine ecosystems within the region is not sufficiently investigated nor studied.

Mitigation Practices and Policies:

Since we are aware that nitrogen and phosphorus are the main contributing nutrients to HABs, the prevention of these events are centred around controlling the release of these nutrients into waterways. To achieve this goal, appropriate agencies have to be employed to implement programs that continuously monitor water quality. The Institute of Marine of Affairs (IMA), a multidisciplinary research organisation has been involved in nutrient monitoring exercises through its Environmental Quality Programme.  Additionally, close attention must be placed on ensuring that industrial and municipal bodies reduce their output of nutrients. Policy makers and environmental governing organisations would be the key players in this process. Community-based programs can be implemented to manage practices that reduce nutrient loading to streams and rivers. These practices can be as simple as ensuring fertilizer and livestock waste are diverted away from major watercourses and treated appropriately, as well as increasing the number of suitable shoreline plants so excess nutrients can be absorbed and runoff entering watercourses can be buffered. At the IMA, readiness is being undertaken. Through collaboration with the International Atomic Energy Agency, the IMA is becoming equipped to investigate HAB events, in particular, those that have the potential for toxin production. In the near future, Radioligand Receptor Binding technology (equipment/method used to identify toxin produced by harmful microalgae) will be available to assist in this process and no doubt offer invaluable support to the fishing industry.


Algae, whether phytoplankton or seaweed, are a valuable resource for the oceans and humans. It is estimated that algae are responsible for producing half of the oxygen needed to sustain life on earth. Care must be taken however, to avoid proliferation of these organisms in an unsustainable way. Human activities can be altered to both prevent these HAB events from occurring and manage their negative effects. At the helm of our response should be education, followed closely by policy reform and policy implementation that aims to reduce nutrient outputs into water courses. 


Sunesen, Inés, Silvia M. Méndez, José Ernesto Mancera-Pineda, Marie-Yasmine Dechraoui Bottein, and Henrik Enevoldsen. “The Latin America and Caribbean HAB status report based on OBIS and HAEDAT maps and databases.” Harmful Algae 102 (2021): 101920.

Yaakob, Maizatul Azrina, Radin Maya Saphira Radin Mohamed, Adel Al-Gheethi, Ravishankar Aswathnarayana Gokare, and Ranga Rao Ambati. “Influence of nitrogen and phosphorus on microalgal growth, biomass, lipid, and fatty acid production: an overview.” Cells 10, no. 2 (2021): 393.