PAHs (in general)
Entry into the marine environment
Polycyclic aromatic hydrocarbons (PAHs) are a diverse
group of aromatic compounds containing two or more
fused arenes structures. Commonly occurring PAHs
They are formed by the incomplete/inefficient combustion
of organic material, diagenesis and biosynthesis.
PAHs are usually coloured crystalline solids with
high melting and boiling points, low vapour pressures
and low water solubilities. In general, it can be
concluded that low molecular weight PAHs are more
soluble and volatile and have less affinity for
surfaces than do high molecular weight PAHs.
PAHs are ubiquitous in the environment, with natural
background levels resulting from forest fires, volcanoes
and possibly production by some plants. However,
a significant fraction of PAHs resulting in the
environment are due to anthropogenic sources (e.g.
burning of fuel, internal combustion engines etc.).
Their widespread occurrence results largely from
formation and release during the incomplete combustion
of coal, oil, petrol and wood, but they are also
components of petroleum and its products. PAHs reach
the marine environment via sewage discharges, surface
run-off, industrial discharges, oil spillages and
deposition from the atmosphere (CCME 1992).
Recorded levels in the marine
PAHs are ubiquitous environmental contaminants.
Although they can be formed naturally (e.g. forest
fires), their predominant source is anthropogenic
emissions, and the highest concentrations of PAH
are generally found around urban centres.
Concentrations of PAHs in the aquatic environment
are generally highest in sediment, intermediate
in biota and lowest in the water column (CCME 1992).
Monitoring data for water and sediments from the
National Rivers Authority and the National Monitoring
Programme Survey of the Quality of UK Coastal Waters
are presented in Appendix D.
The available data suggest that significant concentrations
of PAH can be found in some major estuaries. However,
PAH concentrations at offshore sites were generally
low or undetectable.
For sediments, while PAH concentrations are generally
low or undetectable at most intermediate and offshore
sites, further work should be concentrated on fine
sediments and depositional areas. Significant concentrations
of PAHs were found in a number of estuaries. However,
the bioavailability of sediment associated PAHs
and potential uptake still need to be further evaluated.
Fate and behaviour in the marine
PAHs have a low water solubility and hydrophobic
nature and so they will tend to be associated with
inorganic and organic material in suspended solids
and sediments. In general, most PAHs (with the exception
of some low-molecular weight compounds, such as
naphthalene) will be strongly sorbed by particulate
matter and biota in the aquatic environment (CCME
Effects on the marine environment
Toxicity to marine organisms
An exhaustive literature review on the toxicity
of PAHs to marine organisms has not been carried
out for the purposes of this profile. The information
provided in this section is taken from existing
review documents (CCME 1992). The most sensitive
groups of organisms have been identified.
The lower molecular weight PAHs can be acutely
toxic to aquatic organisms, but the major concern
is that some PAHs form carcinogenically-active metabolites
(benzo[a]pyrene is the prime example) and PAH concentrations
in sediments have been linked with liver neoplasms
and other abnormalities in bottom-dwelling fish
(Malins et al., 1988, Vethaak and ap Rheinallt,
1992). Elevated PAH concentrations may therefore
pose a threat to aquatic organisms and potentially
also to human consumers of fish and shellfish (MPMMG
Environment Canada has issued interim marine sediment
quality guidelines which include data for a number
of individual PAHs (see table below).
Interim marine sediment quality guidelines (ISQCs)
and probable effect levels (PELs; dry weight) for
individual PAHs (from CCME 1999)
ISQG (µg kg-1)
PEL (µg kg-1)
1 ISQGs and PELs presented
here have been calculated using a modification of
the NSTP approach.
These guidelines have been derived for use in Canada
and should only be used as an indication of the
concentrations of individual PAHs that may affect
sediment-dwelling organisms. MPMMG (1998) reported
highest recorded concentrations of individual PAHs
and National Monitoring Programme sites two orders
of magnitude greater than the Canadian guidelines
for these substances. If such concentrations were
reported from a European marine site, they should
be a serious cause for concern.
Aquatic organisms may accumulate PAHs from water,
sediments and food. In general, PAHs dissolved in
pore water are accumulated from sediment and digestion
of sediment may play an important role in the uptake
of PAH by some species. The relative importance
of the uptake routes from food and sediment is not
The bioconcentration factors of PAH in different
species vary greatly (WHO 1998). Species that do
not metabolise PAH at all or to only a limited extent,
such as algae, oligochaetes and molluscs, and the
more primitive invertebrates (protozoans, porifers
and cnidaria) accumulate high concentrations of
PAHs, as would be expected from their high log Kow
values. However, organisms that metabolise PAHs
such as fish and higher invertebrates, such as arthropods,
echinoderms and annelids accumulate little or no
Biomagnification of PAHs through the food chain
has been shown to occur to some degree (from annelids
to fish for example) but the greater capacity of
higher organisms to metabolise PAHs reduces the
efficiency of the transfer.
Potential effects on the interest
features of European marine sites
Potential effects include:
- toxicity of low molecular weight PAHs to aquatic
organisms in the water column;
- accumulation in sediments and potential hazard
to sediment-dwelling organisms at concentrations
between 6 and 150 µg kg-1
(dry weight), depending on the individual PAH;
- bioaccumulation of PAHs, especially in algae
and lower invertebrates and some biomagnification
to higher trophic levels.