Toxicological Effects of Heavy Metals on Aquatic Life

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Introduction

Heavy metals have assumed a more prominent position as an environmental concern in aquatic systems because of the toxic impact that most of them have on different forms of aquatic biota. This has resulted in pollution of water from lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As), chromium (Cr), nickel (Ni), and all other metals that have been released to water bodies through industrial effluents, agricultural leachates, and growing urbanization. In the aquatic environment, heavy metals can settle in sediments and living organisms; this results in the bioaccumulation and biomagnification of the heavy metals in the food pyramid. The level of toxicity of these metals is, however, different, though the concentration of these metals in water, sediment, and organisms has been observed to hurt fish, invertebrates, and other wildlife and pose risks to humans from the consumption of hazardous seafood. To design appropriate measures to reduce the potential of heavy metals to harm aquatic inhabitants, it is pertinent to determine the toxicological response of the metals to aquatic life.

Bioaccumulation and Biomagnification

Bioaccumulation was among the major ways heavy metals caused impacts on aquatic life. Bioaccumulation, on the other hand, is the process through which the concentration of certain substances, such as heavy metals, is accumulated in the tissues of an organism, and the rate at which they are accumulated is higher than the rate at which they are removed. This in turn results in the accumulation of these metals in the tissues of the organisms that inhabit the water system in question, which exceeds the concentration of the same metals in the water system. Although almost all forms of reproductive tissues may be impacted by bioaccumulation, fish, mollusks, and other water creatures are most affected because they live in water and sediment, which is continually in contact with toxic chemicals.

The other remarkable process is biomagnification, which is the ability of the concentration of heavy metals to accumulate through the food chain. The level of heavy metals in the tissues of marine animals and wildlife increases with the prey consumed by the predators, such as fish, birds, and mammals. There is a sort of magnification effect that may be very disastrous to the higher-ranking species, such as humans and other top predators, who are susceptible to getting into various kinds of ailments when they consume seafood that has been affected by pollution.

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Toxicity in Fish

Fish can be employed in terms of bioindication to assess the presence of heavy metal contaminants in water. Various heavy metals can be deposited in the gills, liver, muscles, and other organs of fish, and the toxicity level of each metal varies. For example, histopathological alteration of the gills, liver, and kidneys of fish at different concentrations of cadmium exposure has been reported. These can affect the respiratory systems, slow down growth, and even enhance mortality among the fish.

Lead is also another heavy metal that is toxic to fish, and its impact is also severe. Lead toxicity affects the brain and reproductive ability and exhibits behavioral changes in fish. Since lead poisoning could also lead to paralysis and death, it is correct to conclude that it is dangerous. In addition, this metallic lead constituent of water affects the normal working of enzymes and other biochemical processes in fish, resulting in the deterioration of the population of fish in water bodies.

Mercury poses a severe threat to aquatic life because it forms methyl mercury, which has the quality of biomagnification in fish. Experimentation with methylmercury in fish raises severe neurological impairment in their motor proficiency, behavioral effect, or reproductive process. Also, when fish embryos and larvae are exposed to methylmercury, the metal may negatively affect human health when fish containing the poison are consumed because methylmercury can accumulate in human tissues and cause severe neurological diseases.

Effects on Invertebrates

Other groups of organisms, including mollusks and crustaceans, which are in the invertebrate class, are also very sensitive to heavy metal pollution. These organisms are generally placed at the base of the food chains and therefore play a key role in transferring heavy metals to species in higher systems. Heavy metals do pose a toxicity problem to invertebrates, as these organisms store these metals in their tissues.

For instance, mollusks that ingest cadmium suffer from oxidative stress damage to their cells and consequent negative impacts on their activities. Cadmium interferes with the usual functioning of enzymes that are involved in the calcium process, shell deposition, and other essential functions. Also echoing the latter, lead can harm the development of mollusk populations in various ways, including a limitation of growth and development.

Fish and those organisms that dwell on the seabed, like crabs and shrimp, are also affected by heavy metal toxicity. These organisms can store heavy metals such as copper, zinc, and cadmium in their external and internal structures. These metals are very toxic to Crustaceans when accumulated, so they elicit developmental disorders, impaired reproductive efficiency, and a high death rate in Crustaceans.

Sediment Contamination and its Impacts

Personally, heavy metals can accumulate in the sediments of aquatic sites as they serve as depots for these compounds. Heavy metals that settle and collect in sediment can have severe impacts on benthos or on organisms that dwell at or near the sediment and are subject to the direct and indirect toxicity of those contaminants. Measures of heavy metals have been found in direct ethnic invertebrates like worms and mollusks, whereby the heavier the metal content in the hydro sediments, the heavier the accumulation in this invertebrate, who in turn biomagnify the metals at higher trophic levels.

Polluted sediments may also influence the general viability of watersheds with aquatic life. The mobilization of heavy metals from sediments to the overlying water can enhance the bioavailability of the toxicants to the aquatic biota rather than protect them; hence, it poses a threat to aquatic life. For instance, although the mercury contained in sediments is largely bound and not very bioavailable, desorption and methylation by microbial action occur, making it more available in the water column and thus more toxic.

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Ecological Risk Assessment

The objective of evaluating the likelihood of heavy metal pollution and its impacts on water ecosystems can help us predict possible effects on species and the services they provide. Some of the indices and models widely used in the assessment of heavy metals include metal concentration, availability, and toxicity levels, among others.

PLI is the Pollution Load Index; it gives a general picture of the degree of pollution by heavy metals in the sediment. This means that whatever impacts are being assessed, a high value for PLI will show a strong relationship with pollution and other negative effects on water organisms. Further, the Enrichment Factor (EF) biochemistry is applied to evaluate the level of anthropogenic influence on the heavy metal content in the sediments to establish the source of pollution.

Ecological risk assessments also factor in the likely bioaccumulation and biomagnification of heavy metals in the aquatic food web. To this effect, the present study sought to establish the bioaccumulation trends, transport routes, and consequences of heavy metal pollution to promote the formulation of intervention measures for the protection of aquatic life.

Mitigation and Remediation Strategies

In response to the impacts of heavy metals on the toxicity of aquatic life, there are distinctive ways of implementing mitigation and remedial measures. Such strategies are aimed at reducing the input of heavy metals into water systems, the removal of contaminants from water and/or sediments, and the rehabilitation of damaged ecosystems.

One of the measures that need to be taken is tighter control over the emissions from industries and agricultural practices that are known for causing heavy metal dispersal. With the reduction of the employment of heavy metals in production procedures and the application of metals containing fertilizers or pesticides, the introduction of these pollutants into water bodies can be controlled to a great extent.

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Another main strategy is the remediation of contaminated sediments. Irascible aspects for the removal or stabilization of heavy metal-contaminated sediments include dredging, capping, or in-situ treatment that significantly minimize the availability and toxic effects of the metals on benthic organisms. Phytoremediation is the process in which plants with the capacity to recover such metals are used to uptake and bioaccumulation of heavy metals from the sediments of water bodies is another suitable approach to restoration.

However, vigilance programs are crucial for evaluating the concentrations of heavy metals in the water systems and the results of remedial activities. This way, signs of contamination not previously detected can be noticed, and the necessary measures can be taken to preserve aquatic bioresources.

Conclusion

Among the water pollutants, heavy metals rank highly as pollutants due to their toxicity, ability to accumulate, and concentrated effects in food chains due to biological magnification. Aquatic animals, including fish, invertebrates, and other water organisms, are very sensitive to the toxic impacts of several metals. Cadmium, lead, and mercury inhibit physiological processes affecting reproduction and the density of species. These problems are further compounded when the sediments are contaminated because they then act as a source of heavy metals that can adversely affect benthic organisms and other animals.

Measures to prevent the input of heavy metals and methods for the remediation of affected ecosystems are equally crucial for preserving endangered species. Only by determining the general process and impacts of heavy metal pollution can researchers and other officials better work toward protecting the environment and people who rely on water sources.

References

  1. Ali, H. and Khan, E., 2019. Trophic transfer, bioaccumulation, and biomagnification of non-essential hazardous heavy metals and metalloids in food chains/webs—Concepts and implications for wildlife and human health. Human and Ecological Risk Assessment: An International Journal, 25(6), pp.1353-1376.
  2. Ali Azadi, N., Mansouri, B., Spada, L., Sinkakarimi, M.H., Hamesadeghi, Y. and Mansouri, A., 2018. Contamination of lead (Pb) in the coastal sediments of north and south of Iran: a review study. Chemistry and Ecology, 34(9), pp.884-900.
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  4. Islam, M.S., Proshad, R. and Ahmed, S., 2018. Ecological risk of heavy metals in sediment of an urban river in Bangladesh. Human and ecological risk assessment: an international journal, 24(3), pp.699-720.
  5. Ali, H. and Khan, E., 2018. Assessment of potentially toxic heavy metals and health risk in water, sediments, and different fish species of River Kabul, Pakistan. Human and Ecological Risk Assessment: An International Journal, 24(8), pp.2101-2118.
  6. Yousafzai, A.M., Ullah, F., Bari, F., Raziq, S., Riaz, M., Khan, K., Nishan, U., Sthanadar, I.A., Shaheen, B., Shaheen, M. and Ahmad, H., 2017. Bioaccumulation of some heavy metals: analysis and comparison of Cyprinus carpio and Labeo rohita from Sardaryab, Khyber Pakhtunkhwa. BioMed Research International, 2017(1), p.5801432.
  7. Ahmed, M.K., Parvin, E., Islam, M.M., Akter, M.S., Khan, S. and Al-Mamun, M.H., 2014. Lead-and cadmium-induced histopathological changes in gill, kidney and liver tissue of freshwater climbing perch Anabas testudineus (Bloch, 1792). Chemistry and Ecology, 30(6), pp.532-540.
  8. Malik, D.S. and Maurya, P.K., 2014. Heavy metal concentration in water, sediment, and tissues of fish species (Heteropneustis fossilis and Puntius ticto) from Kali River, India. Toxicological & Environmental Chemistry, 96(8), pp.1195-1206.

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