Lead is a bluish or silvery-grey soft metal. With the exception of the nitrate, chlorate, and, to a much lesser degree, chloride, the salts of lead are poorly soluble in water. Lead also forms stable organic compounds. Tetraethyllead and tetramethyllead are used extensively as fuel additives. Both are volatile and poorly soluble in water. Trialkyllead compounds are formed in the environment by the breakdown of tetraalkylleads. These trialkyl compounds are less volatile and more readily soluble in water. Lead is mined, most usually as the sulfide, "galena".

Hence, entry into the aquatic environment occurs through releases (directly or through atmospheric deposition) from the smelting and refining of lead, the burning of petroleum fuels containing lead additives and, to a lesser extent, the smelting of other metals and the burning of coal and oil. Metallic lead deriving from shotgun cartridges or used as fishing weights is lost in the environment and often remains available to organisms (WHO 1995).

Concentrations of lead have been measured in water, sediments and biota 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 lead in water, sediments and biota for marine sites of nature conservation importance in England.

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

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

Average concentrations (µg l^-1) of dissolved lead in the water column of some English estuaries (from DETR 1998)

Maximum concentrations (µg l^-1) of dissolved lead in the water column of some English estuaries (from DETR 1998)

The fate and behaviour of lead in the marine environment is complex because of the many compounds of lead that can be found and the natural variability of natural systems.

Much of the lead in the marine environment is strongly adsorbed onto sediment and suspended particles, reducing its availability to organisms. The transport of lead in estuaries and coastal waters is therefore closely linked with the movement of particles. The sediments form a sink for lead in the marine environment.

Lead in true solution may be present as the hydrated Pb²⁺ ion or may be complexed. However, in view of the low solubility of most of its salts, lead tends to precipitate out of complex solutions.

An exhaustive literature review on the toxicity of lead 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 (Brown et al 1984, WHO 1995, Grimwood and Dixon 1997). The most sensitive groups of organisms have been identified.

In the form of simple salts, lead is acutely above 2.5 and >500 mg l^-1 for marine organisms (WHO 1995).

Lead salts are poorly soluble in water, with the presence of other salts reducing the availability of lead to organisms because of precipitation. Results of toxicity tests should be treated with caution unless dissolved lead is measured.

In 1984, Brown et al reviewed data on the toxicity of lead to marine organisms and proposed an EQS (for the protection of saltwater life) of 25 µg l^-1, (expressed as a dissolved annual average concentration), a value currently adopted in UK legislation (HMSO 1989). The EQS was proposed at this level as, at the time of writing (1984), adverse effects had not been observed in any saltwater organisms following exposure to concentrations below 100 µg l^-1. However, following a review of more recent toxicity data in 1992, Young (1992) proposed a more stringent EQS of 10 µg l^-1. This value (also expressed a dissolved annual average) was derived by applying an arbitrary safety factor of around 2 to the lowest, most reliable No-Observed Effect Concentration (NOEC) for mysid shrimp.

An additional review by Grimwood and Dixon in 1997 of data on the saltwater toxicity of lead available since the EQS of 10 µg l^-1 was proposed, found that only one study had been reported that perhaps indicated higher toxicity to saltwater organisms. Fernandez-Leborans and Novillo (1992) reported that lead concentrations ranging from 1 to 50 µg l^-1 caused significant effects on the division and biomass of ciliate communities in laboratory microcosms. However, the magnitude of effects at the 1 µg l^-1 exposure level was not distinguished from the effects that occurred following exposure to 50 µg l^-1. Moreover, it is difficult to assess the relevance of these laboratory sub-lethal (growth) data in predicting wider ecosystem effects under field conditions.

Grimwood and Dixon (1997) concluded that no other relevant ecotoxicity data had been reported that suggested a need to revise to the latest proposed EQS value.

The authors recommended that the revised EQS of 10 µg l^-1 (dissolved annual average) proposed to DETR was appropriate for the protection of all saltwater life in the majority of cases. However, where there is concern that the health of communities at sites of nature conservation interest may be compromised as a result of the presence of particularly sensitive algal or ciliate species, a lower value may be used as a Guideline. For instance, a value of 0.5 µg l^-1 may be used where necessary by applying arbitrary factors of 10 and 2 to the lowest algal EC50 and the lowest ciliate effect concentration respectively. In the absence of any reliable supporting data, it was not possible to confirm the precision of this value.

In one laboratory study, a 12 day EC50 (growth) as low as 5 µg l^-1 was reported for the diatom Skeletonema costatum although the reliability of this result is questionable due to uncertainties in measurements and apparent medium-dependent effects. All other algal data were higher than the NOEC reported for the mysid shrimp. Nevertheless, Young (1992) still concluded that, on the basis of the potential higher sensitivity of some algae, a more stringent standard may be required where lead-sensitive algal species were important primary producers in a saltwater ecosystem. Further research into algal sensitivity was recommended.

In aquatic invertebrates communities, some populations are more sensitive than others and community structure may be adversely affected by lead contamination. However, invertebrate populations from polluted areas can show more tolerance to lead than those from non-polluted areas. In other aquatic invertebrates, adaptation to hypoxic conditions can be hindered by high lead concentrations (WHO 1995).

Young (1992) reported a No Observed Effect Concentration (NOEC) of 17 µg l^-1, for the mysid shrimp Mysidopsis bahia following 44 days exposure.

Young stages of fish are more susceptible to lead than adults or eggs. Typical symptoms of lead toxicity include spinal deformity and blackening of the caudal region. The maximum acceptable toxicant limit (MATC) for inorganic lead has been determined for several species under different conditions and results range from 0.04 mg l^-1 to 0.198 mg l^-1. The acute toxicity of lead is highly dependent on the presence of other ions in solution, and the measurement of dissolved lead in toxicity tests is essential for a realistic result. Organic compounds of lead are more toxic to fish than inorganic lead salts (WHO 1995).

Lead accumulates in sediments and can pose a hazard to sediment-dwelling organisms at concentrations above 30.2 mg kg^-1, according to Canadian interim marine sediment quality guidelines.

In aquatic ecosystems, uptake by primary producers and consumers seems to be determined by the bioavailability of the lead. The uptake and accumulation of lead by aquatic organisms from water and sediment are influenced by various environmental factors, such as temperature, salinity, and pH, as well as humic and alginic acid content.

In many organisms, it is unclear whether lead is adsorbed onto the organism or actually taken up. Consumers take up lead from their contaminated food, often to high concentrations, but without biomagnification (WHO 1995).

Lead uptake by fish reaches equilibrium only after a number of weeks of exposure. Lead is accumulated mostly in gill, liver, kidney, and bone. Fish eggs show increasing lead levels with increased exposure concentration, and there are indications that lead is present on the egg surface but not accumulated in the embryo. In contrast to inorganic lead compounds, tetraalkyllead is rapidly taken up by fish and rapidly eliminated after the end of the exposure (WHO 1995). Alkyllead was identified as the cause of a major bird kill in the Mersey estuary in 1979/80 (NRA 1995). Approximately 2,500 birds from 20 species were killed and alkyl lead was found in analyses of bird tissue and in a common bivalve Macoma balthica. The source of the pollution was traced to an industrial discharge from a manufacturer of tetraalkyllead.

In shellfish, lead concentrations are higher in the calcium-rich shell than in the soft tissue; they relate to the concentrations in sediment. Lead concentrations in some marine fish are higher in gills and skin than in other tissues, but this may be largely due to adsorption. Liver levels increase significantly with age (WHO 1995).

In dolphins, lead is transferred from mothers to offspring during fetal development and lactation. This might be related to the calcium metabolism. In studies on the common porpoise Phocoena phocoena from the east coast of Scotland, Falconer et al (1983) found that lead residues were below detectable limits (0.5 mg kg^-1). The sampled animals had died after becoming entangled in cod nets. The tissues analysed were the brain, liver, kidney, heart, and spleen. Honda et al (1986) sampled striped dolphin Stenella coeruleoalba and found significant accumulation of lead in the bone of offspring during the suckling period. Significantly more lead was found in adult males than females. The authors suggested that lead was removed from the mother via the milk and as the result of parturition. Lead levels ranged between 0.09 and 0.74 mg kg^-1 wet weight.

The potential effects include:

• acute toxicity to algae, invertebrates and fish at concentrations of dissolved lead above the proposed EQS of 10 µg l^-1 (annual average) in the water column. A lower guideline value of 0.5µg l^-1 of dissolved lead has been suggested by Grimwood and Dixon (1997) for sites of nature conservation importance where particularly sensitive algal species are to be protected;
• accumulation in sediments and can pose a hazard to sediment-dwelling organisms at concentrations above 30.2 mg kg^-1, according to Canadian interim marine sediment quality guidelines;
• bioaccumulation in the food chain posing a hazard to fish, birds and Annex II sea mammals.

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