Marine microalgae (phytoplankton) are the dominant primary producers in marine waters and, in the open ocean, they are the only source of primary production. Consequently, all marine productivity and all marine fisheries (wild caught and aquaculture) are dependent on algae.
The marine environment has historically been a receiving basin for a multitude of pollutants (chemicals, nutrients etc.) from human activities, all of which may impact the marine environment and marine life directly, but which can also impact these indirectly via the food web by having a direct impact on the algae.
Local, state and national agencies and authorities responsible for marine waters, therefore, take great care to ensure that activities which impact on marine ecosystems (wastewater discharge, dredging, port and harbour facilities, offshore oil and gas facilities, aquaculture activities etc.) do not adversely impact phytoplankton assemblages.
As is the case for freshwater algal blooms, marine algal blooms (usually called “red tides”) are a natural phenomenon. The Red Sea gets its name due to the frequent algal blooms which occur there and, during his voyages along the Australian coastline in 1770, Captain James Cook described extensive marine algal blooms off the coast of Queensland.
However, human activities, particularly the nutrient enrichment of coastal waters, have increased the occurrence of marine algal blooms. Large-scale international shipping has also, historically, spread the geographic range of many bloom-forming marine algal species via the ballast water carried by these ships.
Marine algal blooms are generally caused by cyanobacteria, dinoflagellates or raphidophytes. The ecological implications of these blooms are similar to the ecological implications of freshwater algal blooms.
Marine algal blooms can cause fish kills and mortality of other marine life as a result of deoxygenation of the water column when the algal biomass is particularly high and/or when this large biomass decomposes.
However, many marine algal species produce powerful toxins. This is particularly the case for the cyanobacteria, dinoflagellates, diatoms and raphidophytes. Some of these toxins are toxic to other marine life, but it is those which are responsible for fish kills, due to the presence of ichthyotoxins and those which are highly toxic to mammals (humans in particular), that are of the greatest concern.
Although humans may come into direct contact with toxic marine algae whilst undertaking recreational activities and may suffer a range of allergic reactions. As a result, direct ingestion of marine waters is unlikely.
The impact on humans of marine algal toxins is generally via the consumption of shellfish, crustaceans and fish. Shellfish (mussels, oysters etc.) are filter feeders, they filter large volumes of water, removing and feeding on, the phytoplankton. Toxins contained within the phytoplankton cells (generally referred to as “shellfish toxins”) are not toxic to the shellfish and accumulate in their tissues via a process known as “bio-magnification”.
These toxins are extremely toxic to mammals, and when humans consume affected shellfish they can become ill with symptoms ranging from mild to severe and, in extreme cases, consumption can be fatal.
There are several types of shellfish toxins, and they are generally characterised according to the symptomatic responses when consumed – Paralytic Shellfish Toxins (PST), Diarrhetic Shellfish Toxins (DST), Neurotoxic Shellfish Toxins (NST) and Amnesic Shellfish Toxins (AST). Consumption of PST and AST can be fatal.
Some tropical reef fish also can also accumulate algal toxins known as ciguatoxin and maitotoxin causing ciguatera fish poisoning, a condition which can be fatal, when these fish are consumed.
Some examples of marine algae known to produce toxins.
These factors can each be solely responsible for fish mortality or can have a cumulative impact, or they can lower fish health substantially such that other factors, especially disease, ultimately kill the fish.
Some examples of marine algae known to be harmful to fish.
Click here for more information harmful marine algae and shellfish toxins.
Climate change is having an increasingly significant impact on the occurrence of harmful algae, and harmful algae blooms worldwide. The consensus among algal scientists is the frequency, duration and intensity of harmful algal blooms will all increase globally as a result of climate change.
With respect to those species of marine phytoplankton which produce shellfish toxins and those species which are harmful, for these species to be potentially problematic the biomass (cell density) of algae required does not need to be such that it constitutes an algal bloom. In fact, toxin concentrations can be at harmful levels when cell density is relatively low. Many of these species are potentially problematic at cell densities as low as 100 cells per litre – a typical marine algal bloom, or red tide, would have cell densities well in excess of several million cells per litre.
As such, detection of these species, and especially detection of changes in concentration at such low cell densities, is best carried out by skilled algal taxonomists using microscopy.