Entry into the marine environment
Iron is the fourth most abundant element in the
crust. While it is naturally released into the environment
from weathering, it may also be released into the
aquatic environment through human activities, such
as burning of coke and coal, acid mine drainage,
mineral processing, sewage, iron related industries
and the corrosion of iron and steel (CCREM 1987).
Recorded levels in the marine
Concentrations of iron were not measured as part
of the National Monitoring Programme (MPMMG 1998).
Grimwood and Dixon (1997) compiled available monitoring
data for iron in water, sediments and biota for
marine sites of nature conservation importance in
In unpolluted oceanic seawater, concentrations
of iron between 2.8-29 ng l-1 and 224-1,228 ng l-1
have been reported, although higher concentrations
may be found in estuarine waters (Whitehouse et
Iron concentrations in saltmarsh sediments are
frequently much higher than those occurring in the
overlying waters. Mean concentrations as high as
20,800 mg kg-1 appear to be tolerated
in coastal saltmarshes which are designated as 'healthy'. It has been shown that iron
concentrations are higher in the immediate vicinity
of saltmarsh plant roots and in the burrow walls
produced by organisms, such as Arenicola.
Fate and behaviour in the marine
On reaching saltwater, suspended iron oxyhydroxides
are rapidly precipitated such that at salinities
of 10 ppt or greater, the vast majority of the iron
present occurs in particulate form and is effectively
removed from solution. In anoxic marine waters,
ferrous iron is mobilised from sediments and diffuses
into the water column.
Effects on the marine environment
Toxicity to marine organisms
An exhaustive literature review on the toxicity
of iron 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 (Mance and Campbell 1988, Grimwood
and Dixon 1997 and Whitehouse et al 1998).
The most sensitive groups of organisms have been
Mance and Campbell (1988) reviewed data on the
toxicity of iron to saltwater species and proposed
EQSs (for the protection of saltwater life) of 10,000
and 25,000 Fg l-1 for total iron in less turbulent seawaters
and more turbulent estuaries with high suspended
solids content, respectively (as annual averages).
These are adopted in UK legislation (HMSO 1989).
In addition, an EQS of 1,000 mg l-1 expressed as a dissolved
annual average concentration has also been adopted.
Due to a lack of data, the derivation of an objective
EQS based on iron toxicity was not considered possible.
Therefore, the above values are based on observations
of general water quality at various estuarine and
marine sites. A review of these EQSs was recommended
once direct observations of biological quality associated
with these concentrations became available.
Grimwood and Dixon (1997) reviewed data on the
saltwater toxicity of iron and found no reliable
toxicity data that indicate higher sensitivity of
saltwater organisms had been reported for iron.
The authors recommended that the EQSs of 1,000,
10,000 and 25,000 mg l-1, expressed as dissolved, total
and total annual averages, respectively, were appropriate
for the protection of all saltwater life. The latter
value should only be adopted in estuaries of considerable
tidal energy and containing a high level of suspended
A further review in 1998 (Whitehouse et al
) also found that, against a background of a very
wide range of sensitivities for different species
and a relatively sparse dataset for saltwater organisms,
that there was little evidence to suggest that saltwater
organisms were more or less sensitive than freshwater
organisms and that toxicity data reported since
Mance et al (1988) do not indicate any greater
sensitivity. Based on their findings, Whitehouse
et al proposed no change to the current annual
average of 1,000 mg
Marine organisms accumulate iron but also rapidly
excrete iron in clean water conditions. Normally,
tissue concentrations of iron are related to the
water and sediment concentrations, but there is
considerable variability. Tissue concentrations
vary seasonally, being lower in winter and spring
than in summer and autumn and furthermore tissue
and shell concentrations increase with increasing
salinity (Mance and Campbell 1988). The bioaccumulation
of iron by marine organisms does not appear to pose
a hazard to higher trophic levels.
Potential effects on interest
features of European marine sites
Potential effects include:
- acute toxicity to marine organisms at concentrations
above the EQS of 1,000, 10,000 and 25,000 mg
l-1, expressed as dissolved, total
and total annual averages, respectively, in the