Algal toxins and algae-related fish kills
Entry to the marine environment
Algal toxins do not enter the marine environment
from an external source but are generated during
blooms of particular naturally occurring marine
algal species. Such blooms have been referred to
as toxic algal blooms, harmful algal blooms (HABs)
and red tides. For example Gyrodiunium aureolum
causes a red discolouration of the water (a red
tide) and has been associated with shellfish and
fish mortalities, particularly in marine fish farms.
Chaetoceros, another alga, has spines which
can physically clog and damage fish gills, leading
to the death of cage-reared salmon and other species.
Other algal species, such as Alexandrium
and Dinophysis can cause poisoning through
the food chain when shellfish ingest these algae
(and their toxins) and are then subsequently consumed
by fish, birds and potentially humans (Environment
Agency 1998). The occurrence of blooms of these
and other so-called toxic algae is perfectly natural
but there are concerns that increases in the supply
of essential nutrients (such as nitrogen, phosphorus)
to the marine environment as a result of Man's
activities may be contributing to the increased
frequency and magnitude of these events.
Algal toxins can give rise to a number of different
- NSP - neurotoxic shellfish poisoning;
- PSP - paralytic shellfish poisoning;
- ASP - amnesic shellfish poisoning;
- DSP - diarrhoetic shellfish poisoning;
All are caused by toxins synthesized by dinoflagellates,
except for ASP, which is produced by diatoms of
the genus Pseudonitzschia (WHOI 1995). Some
species of microflagellates may also produce toxins,
e.g. Chrysochromalina sp. A fifth human illness,
ciguatera fish poisoning (CFP), is caused by benthic
dinoflagellate toxins in coral reef communities.
However, this does not represent a problem in UK
The principal concern about the effects of algal
toxins in the environment has been the contamination
of sea food for human consumption and consequently
much of the research and monitoring is directed
at protecting humans from these effects. There must
also be concerns on the effects of these toxins
on natural populations of consumers (fish, birds
and marine mammals).
Recorded levels in the marine
Monitoring for the occurrence of toxic algae and
their effects is carried out routinely by the Centre
for the Environment, Fisheries and Aquatic Sciences
(CEFAS) on behalf of MAFF in England and Wales,
by Fisheries Research Services in Aberdeen on behalf
of SERAD in Scotland and by DANI in Northern Ireland.
The results of this monitoring is reported annually
(e.g. Howard and Kelly 1997 and MAFF 1998).
Monitoring takes the form of the analysis of water
samples for the presence and concentration of toxic
algal species and the measurement of concentrations
of algal toxins in samples of shellfish flesh. Standards
for concentrations of algal toxins in shellfish
flesh (so-called end product standards) have been
set in The Food Safety (Fishery Products and Live
Shellfish) Regulations 1998 as a requirement of
the Shellfish (Hygiene) Directive. Breaches of these
standards can result in the closure of a particular
fishery for a period of time.
It is very difficult with current knowledge to
determine the likelihood of toxic algal bloom occurrence,
since bloom occurrence appears to be only loosely
linked to nutrient levels (if at all), although
it has been suggested by a number of authors that
changes in salinity can stimulate either the growth
or decline of toxic blooms. Other factors that have
been cited for the reported increased occurrence
of harmful algal blooms include increased awareness
and monitoring (especially in relation to the effects
on aquaculture), climate change and the transport
of toxic algal species in the ballast tanks of vessels.
Toxic dinoflagellate species also overwinter by
forming spores which settle on the sea bed. These
germinate and provide the inoculum for bloom development
in future years. Consequently, once a toxic bloom
has occurred for the first time, there is an increased
risk of toxic bloom development at the same site
in future years. Dinoflagellate spores remain viable
for a relatively long period of time (certainly
several, and perhaps tens, of years).
Fate and behaviour in the marine
Algal toxins are naturally occurring compounds
that are released into the environment, either when
algal cells are ingested by filter feeding animals,
or when algal cells are broken down after a bloom
crashes. The fate and behaviour of these toxins
in the marine environment is not well known but
they will undergo microbial biodegradation when
released into the environment.
Some dinoflagellate species of toxic algae form
cysts that can accumulate in the sediment and act
as an inoculum for a new population when conditions
favour germination of the cysts.
Effects on the marine environment
Effects on marine organisms
An exhaustive literature review on the effects
of algal toxins to marine organisms has not been
carried out for the purposes of this profile. The
information provided in this section is taken from
existing general information and selected references.
The direct effects of blooms of toxic algae on
marine organisms include:
- sub-lethal and lethal toxicity, especially to
fish, birds and sea mammals;
- physical damage to fish gills.
Toxic phytoplankton can be filtered from the water
by shellfish, such as clams, mussels, oysters, or
scallops, which then accumulate the algal toxins
to levels which can be lethal to consumers, including
humans (Shumway 1990, Ahmed 1991). Typically, the
shellfish are only marginally affected, even though
a single clam can sometimes contain sufficient toxin
to kill a human. Fish and shellfish can also be
subject to sub-lethal effects, including increased
susceptibility to disease and reduced growth.
Fish can be affected by algal toxins, either by
direct uptake from the water column (planktivorous
fish) or by bioaccumulation through the food chain
(zooplankton and macroinvertebrates). In turn, these
fish can then endanger whales, porpoises, seabirds,
and other animals.
In addition to toxin production, algae have also
been implicated in fishkills by the following direct
- Mechanical damage to gills by algal spines,
notably the serrated spines of Chaetoceros
spp. (e.g. Yang and Albright 1992).
- Irritation of gills resulting in over-production
of mucilage within the gills leading to suffocation
- Physical blocking of the secondary lamellae
of fish gills (Jones and Rhodes 1994).
- Increased water viscosity due to the secretion
of polysaccharides (e.g. Hallegraeff 1992).
The principal indirect effects arise from changes
in the oxygen balance of the water column associated
with the presence of the bloom during its growth
phase (supersaturation with oxygen during the day
and oxygen depletion during the night) and the decay
of the algal cells when the bloom has crashed (oxygen
depletion of parts of the water column and possibly
Algae have been implicated in fishkills by the
following indirect methods:
- Asphyxiation caused by oxygen depletion (e.g.
Brooker et al 1977). This can occur as
a result of the oxygen demand generated by a senescent
bloom, or at night due to extreme diurnal fluctuations
in dissolved oxygen levels which may occur during
- Gas bubble trauma from extreme oxygen supersaturation
Many algal toxins readily bioaccumulate in marine
animals and significantly biomagnify through food
chains posing a hazard to consumers at higher trophic
levels (fish, birds and sea mammals).
Potential effects on interest
features of European marine sites
Potential effects include:
- bioaccumulation and sub-lethal and lethal toxicity
of a range of algal toxins to consumers at higher
trophic levels (fish, birds and sea mammals).
A precautionary approach to determining the scale
of possible impacts of algal toxins should be
adopted if the presence of toxic algal species
or algal toxins is detected in a European marine
- adverse physical effects on fish because of
the presence of harmful algal blooms;
- hazards to all marine organisms resulting from
changes to the oxygen balance of the water column,
and potentially the sediments, both during and
after a harmful algal bloom.