Wednesday, May 7, 2014


Harmful Algal Blooms (HABs) occur when usually harmless phytoplankton--microscopic or very small, photosynthetic marine organisms--reach high densities. Their numbers rapidly increase in a given area to the point where they go from harmless to hazardous.

The hazard they present can come from their anatomy, physiology or metabolism.

Some diatoms, like those pictured below, have sharp spines that can cause fish to suffocate by getting lodged in their gills and causing acute inflammation. Some microscopic algae produce mucus, clogging gills and leading to fish's respiratory failure.

Fish with spiny diatoms in its gills.
(Image from
The spiny diatom Corethron pennatum 
(image from

Some phytoplankton affect theorganisms that prey on them by interfering with feeding. The mucus the phytoplankton produce might make them unpalatable or indigestible. Others have such low nutritional value that the organisms that eat them actually starve to death!

Perhaps the most well-known HABs are those that produce toxic blooms. We commonly call those "red tides" although the water color isn't necessarily red, but can be yellow, green, or brown due to the sheer numbers of phytoplankton. Some of the algae that cause the color changes are capable of producing powerful toxins that are harmful or deadly to other species. The toxins may kill fish, injure marine invertebrates, and cause human illness or death from ingesting shellfish that have accumulated the toxin in their tissue. Some of the human illnesses from ingesting contaminated seafood include Amnesic Shellfish Poisoning (ASP), Diarrhetic Shellfish Poisoning (DSP), Neurotoxic Shellfish Poisoning (NSP), Paralytic Shellfish Poisoning (PSP), and Ciguaterra Fish Poisoning (CFP).

A "red tide" bloom.
          From The Baltic Sea Portal (Itameriportaali).

While phytoplankton use photosynthesis to create their food source and release oxygen into the environment, their high productivity and short lifespan means a high turnover of biomass---lots of dead microalgae. As those decompose, they quickly deplete all the oxygen in an area, resulting anoxia---low or no oxygen in the surrounding water. Their decomposition also results in the formation of toxic sulfides in the water. The lack of oxygen and production of sulfides creates a deadly scenario for most marine life that can lead to massive fish die-offs.

Fish die-off due to anoxic conditions from a HAB
(photo from G. Pitcher, via

At least one species of phytoplankton, Pfiesteria piscicida, is actually a predator, killing and then feeding on fish in the waters they inhabit.


We all should. HABs affect everyone! They're a global threat to living resources, fishing, tourism, and human health because the number and intensity of these events appear to be increasing in many countries. In the last two decades, HABs are estimated to have caused as much as $1 billion in losses to coastal resources and communities (NOAA). Check HERE for more information on the socioeconomic impacts of HABs (WHOI).

Preventing and eliminating harmful algal blooms is no easy tasks. There are numerous factors that influence the formation, distribution and duration of blooms. It’s difficult to control one factor in the environment, let alone all factors for all algae in all locations. In addition, any action taken to prevent or to remove a bloom has consequences for the other organisms in the environment.

A combination of factors contribute to the development of HABs such as the presence of  nutrients, warm temperatures and lots of light. Rising ocean temperatures from climate change, increased nutrient run-off from land due to poor land-development practices and a loss of wetlands, and the break down of marine food chains due to overfishing all play roles and are all increasing, so we can most likely expect to see more and more HABs in the future. 

Researchers are constantly trying to gain a better understanding of HABs to help find ways to control their development and reduce their impacts. Early detection of their formation is critical to prevent impacts to human health by issuing fishery and beach closures in impacted areas. 

If that makes you feel helpless to do anything about HABs, you're NOT! You can help by asking state and local lawmakers to implement better land-use planning and wetland preservation to protect coastal water quality, learning the facts about climate change, and demanding good, research-based science, not hype, someone's political- or financial-gain, or fear, drive government decision-making.



  1. Whoa! This I did not know--and now I'll never look at algae the same way again. Thanks for the education, Lynne!

  2. Ciguaterra is probably the biggest concern where you are, Guillie. Fascinating-frightening symptoms, like hot/cold sensation reversal, increased sensitivity to alcohol, and eating fish later can cause a relapse of symptoms, even if the fish doesn't carry the toxin. Oh, and different people have different sensitivity--what makes one person violently ill might not have any effect on another! It's crazy and interesting.