September 3, 2019

003 – Harmful Algal Blooms (HABs)

What is a Harmful Algae Bloom (HAB)?

The National Oceanic and Atmospheric Administration (NOAA) defines a harmful algal bloom as “Harmful algal blooms, or HABs, occur when colonies of algae — simple plants that live in the sea and freshwater — grow out of control and produce toxic or harmful effects on people, fish, shellfish, marine mammals and birds.”

This is certainly an adequate definition, but it does not take into account that, for the presence of some types of algae to be considered potentially harmful, they do not have to be present in large numbers or as “blooms”.

The broader explanation and definition of HABs  preferred by Dalcon Environmental is that of the  IOC Intergovernmental Panel on Harmful Algal Blooms (IPHAB) which is:

“Phytoplankton blooms, micro-algal blooms, toxic algae, red tides, or harmful algae, are all terms for naturally occurring phenomena.  About 300 hundred species of micro algae are reported at times to form mass occurrence, so-called blooms. Nearly one-fourth of these species are known to produce toxins.  The scientific community refers to these events with a generic term, ‘Harmful Algal Bloom’ (HAB), recognising that, because a wide range of organisms is involved and some species have toxic effects at low cell densities, not all HABs are ‘algal’ and not all occur as ‘blooms’.”

Although this Intergovernmental Oceanographic Commission (IOC) is, as its name suggests, focussed on marine systems, this definition also suits freshwater systems.  It is the highlighted (bold) part of the last sentence in the above definition, which is the major difference between this and other definitions of what constitutes a harmful algal bloom.

For convenience, whenever algal blooms, harmful algae, toxic algae etc. are referred to herein, this includes both the cyanobacteria and the algae.

Three “groups” of organisim responsible for HABs

Based on the mechanism(s) by which they cause harm, the three main “groups” of HAB organisms are:

1) Toxin producers – which can cause harm to aquatic life and organisims coming into contact with or consuming the water containing them as a result of direct toxicity;

2) Irritant producers – which cause physical harm due to either physical irritation caused by needle-shaped cells or the presence of sharp spines or protruberances, or to the algae producing large amounts of mucilage, both of which can irritate fish gills or sensitive mucous membranes of other organisms; and

2) High biomass producers – which can cause anoxia (oxygen depletion) that can indiscriminantly kill fish and other aquatic organisms and/or release toxic compounds as their large biomass decomposes when the “bloom” has run its course.

In the latter instance, decomposing biomass can also cause a proliferation of bacteria which may be harmful to some aquatic life.  An example of this is the bacterium Clostridium botulinum which thrives in the conditions created by a decaying algal biomass (low oygen environment with an ample high-protein food supply).  The toxin released by this bacterium is called Botulinum and it is a powerful neurotoxin which often impacts waterbirds, a condition which is referred to as avian botulism.

Some HABs are a result of more than one of these causes.



Freshwater vs Marine systems

In freshwater systems, toxin producers are exclusively the cyanobacteria which are aslo very commonly high biomass producers.  However, in order to be considered harmful, these toxic cyanobacteria do not have to be present in high concentrations.  This is certainly the case for potable waters where, if present, the toxic algal cells are consumed directly with the water.  Several other types of algae, although not toxic, are considered to be potentially harmful in freshwater systems due to them being potential high biomass producers.  Freshwater algal toxins, cyanotoxins, and the algae that produce them will be discussed in more detail in a future blog post.

In marine systems, several types of algae are capable of producing toxins.  Some of these toxins are directly toxic to marine life, e.g. ichtyotoxins which can kill fish, whilst others are toxic to marine life via the accumulation of toxins in the food chain.  In the latter instance, these toxins can also be harful to non-marine life which consumes seafood in which the toxins have accumulated (e.g. humans eating shellfish).  With respect to the accumulation of algal toxins in the food chain, particularly the accumulation of toxins by filter-feeding shellfish, the actual concentration of algae in the water can be relatively quite low and still be considered potentially harmful.

There are also several types of algae that can form high biomass blooms in marine systems.

Marine algae responsible for HABs will be discussed in detail in upcoming blog posts.

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