Zinc :: all ecosystems

Key Concerns

Symptoms of zinc toxicity in plants include reduced root growth, and inhibition of various physiological processes including transpiration, respiration and photosynthesis. Stunted growth, leaf epinasty and chlorosis of younger leaves are characteristic of zinc toxicity (WHO 2001). Reduced yield, and chlorosis have been reported in various crop species, including fescues, blue grass, barley, soybean, lettuce and cyperus grown in soil with high zinc concentrations (Chaney 1993). 

However, large variations in inter-species sensitivity and bioavailability of the zinc must be taken into account when assessing possible effects. The concentration of total zinc is not necessarily indicative of the bioavailble fraction. The bioavailability of zinc is determined by various factors, including the chemical speciation of the zinc, and the characteristics of the soil, including its pH and the concentrations of inorganic and organic ligands, and competition with other chemical species (WHO 2001).

Ecosystem specific information

Arable habitats – high levels of zinc have been reported in wheat (Triticum aestivum) and corn (Zea mays) grown near non-ferrous mining and smelting bases (Nan et al. 2002). Some plants have been shown to tolerate soils with high zinc concentrations; the grasses Agrostis capillaries and Festuca rubra which were sown near a zinc smelter could tolerate soil zinc concentrations up to 16000 mg/kg (Bouwman et al. 2001). Laboratory studies have demonstrated effects on nematode communities indigenous to agricultural soil. Exposure to zinc concentrations exceeding 50 mg/kg soil caused the disappearance of some nematode taxa (Korthals et al. 2000).

Bogs, wetland and heath – effects on plant biomass, density and diversity have been reported for various species of grass growing on floodplains in the vicinity of sites previously used for mining. The zinc threshold concentration for the decrease in diversity of the floodplain grass communities was 170 mg/kg soil (Stoughton and Marcus 2000). Some wetland plants such as Typha latifolia and Phragmites australis contain the ability to tolerate soils with high zinc concentrations,(McCabe et al. 2001).

Coastal and rocky habitat – zinc has been shown to exert adverse reproductive, biochemical, physiological and behavioural effects on a variety of saltwater organisms at concentrations exceeding 50 µg/l (WHO 2001). Large variations in inter-species sensitivity and bioavailability of the zinc must be taken into account when assessing possible effects. The concentration of total zinc is not necessarily indicative of the bioavailable fraction. The bioavailability of zinc is determined by various factors, including the chemical speciation of the zinc, and the characteristics of the water, including its pH, salinity and the concentrations of inorganic and organic ligands, and competition with other chemical species (WHO 2001).

Minimum thresholds for adverse effects of dissolved zinc on aquatic organisms are as follows (WHO 2001):

• 50-100 µg/l: acute effects on mysids
• 100-200 µg/l: acute effects on fish
• 200-1000 µg/l: acute effects on amphipods and decapods
• 1000-10000 µg/l: acute effects on polychaetes and molluscs

Toxicity data is also available for marine algae, but this was not considered of sufficient quality for inclusion in the threshold concentrations listed above (WHO 2001). EC50 values, for culture growth, based upon acute exposure periods of either 72 or 96 hours, ranged from 58 to 271 µg/l (Rosko and Rachlin 1975; Fisher and Jones 1981; Stauber and Florence 1990).

Freshwater – zinc has been shown to exert adverse reproductive, biochemical, physiological and behavioural effects on a variety of freshwater organisms at concentrations exceeding 20 µgl (WHO 2001). Large variations in inter-species sensitivity and bioavailability of the zinc must be taken into account when assessing possible effects. The concentration of total zinc is not necessarily indicative of the bioavailable fraction. The bioavailability of zinc is determined by various factors, including the chemical speciation of the zinc, and the characteristics of the soil, including its pH and the concentrations of inorganic and organic ligands, and competition with other chemical species (WHO 2001).

Minimum thresholds for adverse effects of dissolved zinc on aquatic organisms are as follows (WHO 2001):

• 20-50 µg/l chronic effects on cladocerans in soft water (water with hardness <100 mg CaCO3 / litre).
• 50-100 µg/l chronic effects on cladocerans in hard water (water with hardness >100 mg CaCO3 / litre), acute effects of cladocerans in soft water, acute and chronic effects on fish in soft water and chronic effects on freshwater insects.
• 1000-10000 µg/l acute effects on molluscs, copepods and amphipods, and acute effects on fish in hard water.

Toxicity data is also available for freshwater algae and plants, but this was not considered of sufficient quality for inclusion in the threshold concentrations listed above (WHO 2001). 72 Hour-EC50 values for growth of freshwater algae (Scenedesmus quadricauda) range from 150 to 170 µg dissolved Zn/l, with corresponding No Effect Concentrations of 30 to 50 µg/l (Van Ginneken 1994; Van Woensel 1994). Toxicity in aquatic plants has been reported following exposure to zinc concentrations ranging from 8100 to 67700 µg/l (Brown and Rattigan 1979; Dirilgen and Inel 1994).

Grassland – symptoms of zinc toxicity in plants include reduced root growth, and inhibition of various physiological processes including transpiration, respiration and photosynthesis. Stunted growth, leaf epinasty and chlorosis of younger leaves are characteristic of zinc toxicity (WHO 2001). Some plants have been shown to tolerate soils with high zinc concentrations; the grasses Agrostis capillaries and Festuca rubra which were sown near a zinc smelter could tolerate soil zinc concentrations up to 16000 mg/kg (Bouwman et al. 2001). However, other plants grown in the vicinity of zinc smelters demonstrated phtotoxicity, leading to the disappearance of most species of vegetation, with the exception of some zinc resistant grasses and ragweed (Ambrosia artemisiifolia) (Li et al. 2000).

Woodland and hedgerow - symptoms of zinc toxicity in plants include reduced root growth, and inhibition of various physiological processes including transpiration, respiration and photosynthesis. Stunted growth, leaf epinasty and chlorosis of younger leaves are characteristic of zinc toxicity (WHO 2001). High zinc concentrations have been reported in the leaves of trees growing near sites where zinc ore is mined or processed (Pugh et al. 2002). Adverse effects, including reductions in body growth, biomass and diversity have also been reported in enchytraeid worms from coniferous forests polluted with zinc (Salminen et al. 2001b), with complete extinction at zinc soil concentrations exceeding 2393 mg/kg dry weight (Salminen et al. 2001a). Similar effects have also been found in earthworms, resulting in a reduction in the incorporation of organic matter into woodland soils contaminated with zinc (Beyer 2001). Woodland habitats, containing scots pine and birch and oak have been used as shelterbelts beside major roads, with entrapment of the metals, including zinc, released from vehicles, and subsequ ent soil enrichment (Heath et al. 1999).

Large variations in inter-species sensitivity and bioavailability of the zinc must be taken into account when assessing possible effects. The concentration of total zinc is not necessarily indicative of the bioavailable fraction. The bioavailability of zinc is determined by various factors, including the chemical speciation of the zinc, and the characteristics of the soil, including its pH and the concentrations of inorganic and organic ligands, and competition with other chemical species (WHO 2001).

Additional Comments

Zinc is an essential element for all biota, therefore any adverse effects must be balanced against its essentiality. This means that for all organisms, there will be range of optimal zinc concentrations. Exposure to zinc concentrations outwith this range, will result in adverse effects, due to either zinc deficiency, or zinc toxicity.