Arsenic is a ubiquitous element with metalloid properties. Its chemistry is complex and there are many different compounds of both inorganic and organic arsenic. In nature, it is widely distributed in a number of minerals, mainly as the arsenides of copper, nickel, and iron, or as arsenic sulfide or oxide. In water, arsenic is usually found in the form of arsenate or arsenite. Methylated arsenic compounds occur naturally in the environment as the result of biological activity. The most important commercial compound, arsenic (III) oxide, is produced as a by-product in the smelting of copper and lead ores. Arsenic compounds are also used in wood preservatives.

Arsenic enters the marine environment from natural diffuse sources and from anthropogenic point and diffuse sources.

The ambient level of arsenic in the marine environment is generally accepted as being in the range 2 - 3 mg l^-1 (Mance et al 1984).

Concentrations of arsenic have been measured in sediments and fish muscle as part of the National Monitoring Programme at sites throughout the UK in estuaries and coastal waters (MPMMG 1998). The results of the National Monitoring Programme are summarised in Appendix D. MPMMG (1998) should be consulted for further details.

Grimwood and Dixon (1997) compiled monitoring data for arsenic in water, sediments and biota for marine sites of nature conservation importance in England.

As an example of the recorded levels of dissolved arsenic in the marine environment, the following concentrations have been reported by DETR (1998) for some English estuaries (see tables below).

Minimum concentration (mg l^-1) of dissolved arsenic in the water column of some English estuaries (from DETR 1998)

Average concentration (mg l^-1) of dissolved arsenic in the water column of some English estuaries (from DETR 1998)

Maximum concentration (mg l^-1) of dissolved arsenic in the water column of some English estuaries (from DETR 1998)

Sedimentation of arsenic in association with iron and aluminium may sometimes be considerable. In oxygenated water, arsenic usually occurs as arsenate, but under reducing conditions, for instance, in deep well waters, arsenite predominates. Methylation of inorganic arsenic to methyl- and dimethylarsenic acids is associated with biological activity in water. Some marine organisms have been shown to transform inorganic arsenic into more complex organic compounds, such as arsenobetaine, arsenocholine, and arsoniumphospholipids.

An exhaustive literature review on the toxicity of arsenic 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 et al 1984, Smith and Edwards 1992 and Grimwood and Dixon 1997). The most sensitive groups of organisms have been identified.

Mance et al (1984) reviewed information on the aquatic toxicity of arsenic to saltwater organisms, in order to derive an EQS for the protection of marine organisms. A value of 25 mg l^-1, (expressed as a dissolved annual average concentration) was proposed and this is currently adopted in UK legislation (HMSO 1989).

Mance et al (1984) found limited data on the toxicity of arsenic to marine organisms. However, the authors concluded that invertebrate species appeared more sensitive than vertebrate species and it is likely that larval stages may be more susceptible. The limited data on algae suggested that they may exhibit a sensitivity similar to that exhibited by the more sensitive invertebrates species. The EQS was established by applying an arbitrary factor of 20 to the lowest 96 hour LC50 of 508 mg l^-1 reported at that time for the copepod (Arcatia clarsi).

Following a review of more recent toxicity data, Smith and Edwards (1992) proposed that the EQS should remain unchanged. The EQS of 25mg l^-1 was confirmed by applying a safety factor of around 10 to the lowest, most reliable 96 hour LC50 of 232 mg l^-1, reported for zoae of the Dungeness crab Cancer magister. Given the high sensitivity of this life-stage and the low acute-to-chronic ratios for arsenic, a reduced safety factor of 10 was considered suitable.

Smith and Edwards (1992) reported that concentrations as low as 7 mg l^-1 caused significant inhibition of growth of the alga Fucus vesiculosus. However, the data were considered insufficiently reliable to influence the EQS. Moreover, the corresponding concentration causing complete inhibition was much higher at 75 mg l^-1. Nevertheless, Smith and Edwards concluded that where sensitive algal species such as F.vesiculosus are important primary producers in a saltwater ecosystem, a more stringent EQS may be required. They recommended that further research into algal sensitivity be undertaken.

In 1997, a further review of available data on the saltwater toxicity of arsenic was carried out by Grimwood and Dixon (1997). They found no reliable toxicity data that indicated higher sensitivity of saltwater organisms had been reported for arsenic. They recommended that the EQS of 25 mg l^-1 (dissolved annual average) was appropriate for the protection of all saltwater life in the majority of cases. However, as suggested by Smith and Edwards (1992), where there was concern that the health of communities in sites of nature conservation importance may be compromised as a result of the presence of particularly sensitive algal species, a lower value may be used as a guideline. For instance, a value of 7 mg l^-1 may be used where necessary by taking into account potential effects on growth of the sensitive species Fucus vesiculosus. In the absence of any reliable supporting data, it was not possible to confirm the precision of this value.

Arsenic is found in sediments and can pose a hazard to sediment dwelling organisms at concentrations above 7.24 mg kg^-1 according to Canadian interim marine sediment quality guidelines.

A range of marine organisms have been found to accumulate arsenic from sediments and the water column, including the bivalve molluscs Scrobicularia plana, Cerastoderma spp. and Mytilus edulis, the flatworm Planaria and the algae Fucus vesiculosus, Ectocarpus siliculosus, Cladophora glomerata and Enteromorpha intestinalis. S. plana and M. edulis were considered to take up sorbed arsenic from suspended or surficial sediments and F. vesiculosus from dissolved arsenic from the water column. While these species appear to accumulate arsenic to quite high levels, a large proportion may be present as arsenobetaine which is a water soluble compound that poses little hazard to the organism or its consumer (Smith and Edwards 1992). Arsenic is bioconcentrated in organisms but is not biomagnified in food chains and so bioaccumulation is unlikely to be a problem in marine organisms.

Potential effects include:

• acute toxicity to algae and invertebrates at concentrations above the EQS of 25mg l^-1 (annual average) of dissolved arsenic in the water column;
• sediment concentrations above 7.24 mg kg^-1 according to Canadian interim marine sediment quality guidelines can pose a hazard to sediment dwelling organisms.

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