Iron is the fourth most abundant element in the Earth's 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).

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 England.

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 al 1998).

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.

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.

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 identified.

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 solids.

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 l^-1.

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 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 water column.

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