Effect of Heavy Metals on Aquatic Species

The contamination of aquatic ecosystems with heavy metal has long been recognised as a serious pollution problem (Ayandiran et al., 2009). This is because heavy metals are highly persistent, and research has shown that they are toxic even in trace amounts such that they can potentially induce severe oxidative stress in aquatic organisms (Jiwan and Ajay, 2011).

Aquatic organisms, such as fish, accumulate heavy metals directly from contaminated water and indirectly via the food chain (Khayatzadeh and Abbasi, 2010). In addition, Jennings et al. (2008) also state that fish are exposed indirectly to metals through ingestion of contaminated sediments and food items. Ayandiran et al. (2009) contend that there are in fact five potential routes for a water pollutant to enter a fish, namely, through the food, nonfood particles, gills, oral consumption of water and the skin. However, the presence of metals and their subsequent toxicity vary between fish species

TABLE 5.2

Main Undesirable Effects of Heavy Metals on Plants

Heavy Metal

Effects

Manganese

Not readily remobilised through phloem to other organs after reaching the leave; necrotic brown spotting on leaves, petioles and stems is a common symptom of Mn toxicity; crinkle leaf leading to chlorosis and browning of leaf, stem and petiole

Cadmium

Decreases seed germination, lipid content and plant growth; induces phytochelatins production; impaired aquatic plant growth

Iron

Iron toxicity leads to reduction of plant photosynthesis and yield an increase in oxidative stress and ascorbate peroxidise activity in tobacco, canola, soybean and hydrilla verticillata; secretion of acids from the roots which lowers soil pH

Lead

Reduces chlorophyll production and plant growth; increases superoxide dismutase

Nickel

Reduces seed germination, dry mass accumulation, protein production, chlorophylls and enzymes; increases free amino acids

Mercury

Decreases photosynthetic activity, water uptake and antioxidant enzymes; accumulates phenol and proline

Zinc

Reduces Ni toxicity and seed germination; increases plant growth and АТР/ chlorophyll ratio

Chromuim

Decreases enzyme activity and plant growth; produces membrane damage, chlorosis and root damage

Copper

Inhibits photosynthesis, plant growth and reproductive process; decreases thylakoid surface area

Cobalt

Phytotoxicity study has shown the adverse effect on shoot growth and biomass; excess of Co restricts the concentration of Fe, chlorophyll, protein and catalase activity in leaves of cauliflower; other effects include reduction in shoot length, root length and total leaf area; decrease in chlorophyll content; reduction in plant nutrient content and antioxidant enzyme activity; decrease in plant sugar, amino acid, and protein content has been noticed

Arsenic

Reduces fruit yield, decreases the leaf fresh weight in tomatoes; stunted growth, chlorosis and wilting in canola; reduces seed germination, decrease in seedling height, reduces leaf area and dry matter production in rice

Sources: Loneragan, 1988; Elamin and Wilcox, 1986; Marin et al., 1993; Wu, 1994; Barrachina et al., 1995; Cox et al., 1996; Sinha et al., 1997; Abedin et al., 2002; Gardea-Torresdey et al., 2005; Jayakumar et al., 2007; Solomon, 2008; Li et al., 2009; Akpor and Muchie, 2010; Yadav, 2010; Asati et al., 2016.

and also depend on age, developmental stage and other physiological factors (Khayatzadeh and Abbasi, 2010). There is no doubt also that the toxicity of heavy metals to aquatic species differs depending on a specific metal, concentration of the metal and chemical form of the metal. Heavy metals such as cadmium, copper, lead and zinc are of particular concern because of their severe toxicity to aquatic life (Lewis and Clark, 1997). In fact, acute exposure (short-term, high concentration) of these metals can kill organisms directly,

TABLE 5.3

Aquatic Life Protection Standards

Heavy Metal

Permissible Level (ppb)

Aluminium

5 if pH < 6.5,100 if pH > 6.5

Arsenic

5 (FW), 12.5 (SW)

Cadmium

0.017 (FW), 0.12 (SW)

Lead

1-7 depending on water hardness

Nickel

25-150 depending on water hardness

Manganese

None

Mercury

0.1

Zinc

30 FW

Chromium

Cr'**: 1 (FW), 1.5 (SW); Cr3*: 8.9 (FW), 56 (SW)

Copper

2-4 depending on water hardness

Selenium

1

Source: Solomon, 2008.

FW = freshwater; SW = saltwater.

whereas chronic exposure (long-term, low concentration) can result in either mortality or nonlethal effects such as stunted growth, reduced reproduction, deformities or lesions (Lewis and Clark, 1997). With respect to AMD, Jennings et al. (2008) argue that when fish are exposed directly to metals and H* ions in water through their gills, impaired respiration may result from chronic and acute toxicity.

A major concern is that once heavy metals are accumulated by aquatic organisms, they can be transferred through the upper classes of the food chain (Ayandiran et al., 2009). Carnivores, which are at the top of the food chain including humans, obtain most of their heavy metal load from the aquatic ecosystem through their food, especially where fish are present and so there exists the potential for considerable biomagnification (Cumbie, 1975; Mance, 1987; Ayandiran et al., 2009). However, some studies have indicated that not all the metals taken up by fish are accumulated because fish have the ability, to a certain extent, of regulating the concentration of metals in their bodies (Romanenko et al., 1986; Ayandiran et al., 2009). The regulation of the concentrations of metals in fish may occur through the gills, bile, via feces, kidney and the skin (Heath, 1995; Ayandiran et al., 2009). Table 5.3 shows the levels of metals recommended for the protection of aquatic life (Solomon, 2008).

 
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