Zinc is used in coating to protect iron and steel, in alloys for die casting, in brass, in strips for dry batteries, in roofing and in some print processes. It may enter the aquatic environment through natural or anthropogenic sources, including sewage and industrial discharges.

Concentrations of zinc 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 should be consulted for further details.

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

As an example of the recorded levels of dissolved zinc 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 zinc in the water column of some English estuaries (from DETR 1998)

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

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

Zinc is one of the most ubiquitous and mobile of the heavy metals and is transported in natural waters in both dissolved forms and associated with suspended particles (Mance and Yates 1984) In river water, zinc is predominantly present in the dissolved form. In estuaries, where concentrations of suspended particles are greater, a greater proportion of the zinc is adsorbed to suspended particles (CCREM 1987). In low salinity areas of estuaries, zinc can be mobilised from particles by microbial degradation of organic matter and displacement by calcium and magnesium. In the turbidity maximum, zinc associated with suspended sediment will be deposited with flocculated particles where it can accumulate particularly in anaerobic sediments. In seawater, much of the zinc is found is dissolved form as inorganic and organic complexes.

An exhaustive literature review on the toxicity of zinc 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 Yates 1984, Hunt and Hedgecott 1992 and Grimwood and Dixon 1997). The most sensitive groups of organisms have been identified.

Mance and Yates (1984) reviewed data on the toxicity of zinc to marine organisms. The authors found invertebrates to be generally more sensitive than the fish species studies, while, effects on marine macro and microalgae were noted at concentrations slightly lower than reported for invertebrates. They also reported a complicating factor was the apparent development of increased tolerance. Hunt and Hedgecott (1992) reported the toxicity and bioaccumulation of zinc to be greater at lower salinity.

Mance and Yates (1984) proposed an EQS (for the protection of saltwater life) of 40 µg l^-1 (expressed as a dissolved annual average concentration), which is currently adopted in UK legislation (HMSO 1989). The EQS was established by applying an arbitrary factor of 4 to a 96 hour LC50 of 166 µg l^-1 reported at that time for the mysid shrimp Mysidopsis bahia. However, following a review of more recent toxicity data, Hunt and Hedgecott (1992) proposed a more stringent EQS to DoE of 10 µg l^-1. This value (also expressed a dissolved annual average) was based on the lowest, most reliable NOECs reported for a range of organisms (7 - 20 µg l^-1).

Grimwood and Dixon (1997) reviewed data on the saltwater toxicity of zinc following the Hunt and Hedgecott review and found only one study had been reported that perhaps indicated higher toxicity to saltwater organisms. Exposing the calanoid copepod Temora stylifera to zinc chloride, Nipper et al (1993) reported 48 hour LC50s ranging from 30 - 40 µg l^-1, following exposure in saltwater of salinity 28 - 32 ppt. In addition, an LC50 as low as 4 µg l^-1 was reported on exposure in saltwater of 23 ppt salinity. However, the authors concluded that this value should be treated with caution as there was also an unacceptable level of mortality in the control organisms. NOECs were not determined, although since the LC50s are lower than those reported for any other copepod species, it is conceivable that the NOECs may also be lower.

While the above data gave cause for concern, Grimwood and Dixon concluded that in the absence of a measured NOEC (the above are nominal concentrations), it was difficult to assess the implications of these values for the existing EQS. Furthermore, Temora stylifera is not indigenous to the UK. Moreover, any decrease in the revised EQS of 10 µg l^-1 would lead to a value below Abackground@ levels of zinc in saltwaters.

Grimwood and Dixon recommended that the revised EQS of 10 µg l^-1 (dissolved annual average) proposed to DoE is appropriate for the protection of all saltwater life, although 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 species (e.g. mollusc communities and sensitive copepods), a lower value may be used as a guideline. However, in the absence of a suitable toxicity dataset, it was not possible to make any recommendations on such a value. This is particularly pertinent considering that if the EQS is lowered any further, the value would be at a level below background concentrations.

Zinc accumulates in sediments and can pose a hazard to sediment dwelling organisms at concentrations above 124 mg kg^-1, according to Canadian interim marine sediment quality guidelines.

Zinc is an essential element for many marine organisms and, as such, is readily bioaccumulated. Several species of crustacean are able to regulate the uptake of zinc but, at higher concentrations, this process appears to breakdown leading to an influx of zinc. These issues complicate the calculation of bioconcentration factors which can be misleading. Organisms can take up zinc which is reflected in the BCF but the concentrations in the tissues are of no toxicological significance. Highest concentrations of zinc reported by Hunt and Hedgecott (1992) were: 300 - 9700 Fg g^-1 (dry weight) in Fucus vesiculosus; 605 - 619 µg g^-1 in Littorina littorea; 16460 µg g^-1 in Elminius modestus and 2800 µg g^-1 in dogfish.

Potential effects include:

• acute toxicity to algae, invertebrates and fish above the proposed EQS of 10 µg l^-1 (annual average) for dissolved zinc;
• accumulation in sediments and can pose a hazard to sediment-dwelling organisms at concentrations above 124 mg kg^-1, according to Canadian interim marine sediment quality guidelines;
• bioaccumulation in marine organisms posing a potential threat to fish, birds and Annex II sea mammals.

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