River Pollution: Harming Animals, Damaging Nature

Water pollution has a devastating impact on the health of aquatic animals and ecosystems. Rivers are particularly vulnerable to pollution from sewage, industrial waste, pesticides, and fertilisers, which can contaminate drinking water sources and harm the wildlife that relies on them for survival. These pollutants reduce oxygen levels in the water, creating 'dead zones' where aquatic life cannot survive and impairing the ability of fish and plants to regenerate and reproduce. Furthermore, the spread of toxic algae and bacteria caused by nutrient pollution can poison aquatic organisms and make the water unsafe for human consumption. With nearly half of rivers and streams and over a third of lakes polluted, the impact of river water pollution on animals is a pressing issue that requires urgent attention.

Reduction in species diversity

The planet's biological diversity is significantly impacted by human activity. Water pollution, caused by sewage, agricultural waste, industrial effluents, and chemical spills, poses a severe threat to aquatic life, leading to a reduction in species diversity.

One of the most prominent causes of water pollution is sewage discharge. Sewage contains industrial, municipal, and domestic waste, including faecal matter, which is often released directly into water bodies without adequate treatment. This contributes to the contamination of rivers and other aquatic ecosystems, leading to a decline in species diversity. For example, the River Nile, from Aswan to Cairo, has experienced a significant reduction in fish species due to severe pollution points resulting from sewage drains, industrial, and agricultural sources.

Agricultural activities also play a significant role in water pollution. Agricultural waste, such as herbicides and pesticides, can contaminate rivers and negatively impact the biodiversity that resides within them. Nutrients from farms and livestock operations, including nitrogen and phosphorus, wash into rivers and streams, fueling the growth of algae. This overgrowth of algae deprives the water of oxygen, creating "dead zones" where marine life cannot survive, leading to a decrease in species diversity.

Additionally, industrial effluents containing toxic heavy metals and other pollutants can find their way into rivers, further exacerbating the problem. Climate change, exotic species invasion, flow modification, and habitat degradation are also contributing factors to the reduction in species diversity in rivers.

The impact of water pollution on species diversity is evident in various studies. For instance, a comparison of data from two separate studies about the Egyptian Nile waters, conducted in 1907 and 1997, revealed a significant reduction in fish species over time, attributed to pollution sources. Similarly, a field study showed that mayfly populations decreased when freshwater oxygen concentration decreased and temperatures increased due to water pollution.

The loss of species diversity in rivers has far-reaching consequences. Common species, often classified as "Least Concern," are crucial drivers of ecosystem processes. Alterations in their population can lead to harmful changes in the ecosystem, affecting the food chain and the overall biodiversity of the riverine ecosystem.

Eutrophication

The growth of dense algae in surface waters can shade the deeper water and reduce the viability of benthic shelter plants, with resultant impacts on the wider ecosystem. As these aquatic plants die, they sink to the bottom, where bacteria that decompose them use up the oxygen in the water, making the water uninhabitable for aquatic animals. While adult fish can usually move to a higher-oxygen environment, many crustaceans and shellfish cannot move and die from a lack of oxygen.

In addition, algal blooms can be harmful to fish because when they feed on algae, toxins accumulate within the fish, and when a predator fish consumes that fish, they ingest higher levels of toxins as well. Eutrophication can also impair the ability of fish and plants to regenerate and reproduce.

Technologies exist to help manage eutrophication, such as the application of the phosphate sorbent to the surface of the water body, which then sinks to the bottom, reducing phosphate levels. Nutrient removal technologies, such as biofiltration, can also be used to capture and biologically degrade pollutants.

Spread of diseases

Water pollution has a significant impact on the spread of diseases that affect animals and humans. It is a worldwide problem affecting one in three people on the planet, according to the United Nations (UN). The World Health Organization (WHO) defines polluted water as water whose composition has been changed to the extent that it is unusable, making it toxic and unsuitable for drinking or essential purposes like agriculture.

Water pollution is caused by natural sources and human activities. Natural sources include mercury filtering from the Earth's crust and polluting oceans, rivers, lakes, and reservoirs. However, the most common cause is human activity, such as rising global temperatures caused by CO2 emissions, which heat the water and reduce its oxygen content. Human activities such as felling forests, industrial waste, and agricultural practices also contribute to water pollution.

The spread of diseases through polluted river water is a significant concern. Waterborne pathogens, including bacteria and viruses from human and animal waste, are a major cause of illness. These pathogens can lead to diseases such as cholera, dysentery, typhoid, poliomyelitis, and bilharzia. In a 2023 assessment, the World Health Organization identified between 21,000 and 143,000 cholera-related deaths worldwide, with an estimated 1.3 to 4.0 million cases of cholera annually.

In addition to gastrointestinal diseases, water pollution can also cause skin diseases. A study in Bangladesh found a correlation between high arsenic levels in drinking water and an increased incidence of skin conditions like melanosis and keratosis. Water pollution from industry has also been linked to skin cancer.

Furthermore, nutrient pollution caused by excess nitrogen and phosphorus in water or air can lead to algal blooms, which produce toxins harmful to both wildlife and humans. These toxins can poison aquatic organisms such as seabirds, fish, sea turtles, and aquatic mammals like dolphins, manatees, and sea lions.

The impact of water pollution on the spread of diseases is a global health issue that requires attention and intervention to improve water quality and reduce its impact on human and animal health.

Oxygen depletion

One of the main causes of oxygen depletion in rivers is the decomposition of organic matter. When organic matter such as leaves, algae, or dead organisms accumulate in the water, they undergo microbial decay, which consumes oxygen. This process is further exacerbated by increased temperatures, as warmer water has a reduced capacity to contain oxygen. Additionally, human activities such as wastewater discharge, industrial effluents, and agricultural runoff introduce pollutants and organic materials that increase oxygen demand due to their decay.

The presence of excessive nutrients, such as nitrogen and phosphorus from fertilisers, can also contribute to oxygen depletion. These nutrients promote the growth of algae and water plants, leading to algal blooms. As these plants decay, they further reduce oxygen levels in the water. Eutrophication, caused by high levels of nutrients in the water, can create eutrophic dead zones where aquatic life cannot survive due to a lack of oxygen.

The impact of oxygen depletion on aquatic life can be severe. Fish and other aquatic organisms may die due to a lack of oxygen. Some species, such as adult fish, may be able to move to higher-oxygen environments, but many crustaceans and shellfish cannot and perish. Additionally, the accumulation of toxins in fish that consume harmful algae can further affect the health and survival of aquatic organisms.

Impaired reproduction

Environmental pollution is an ever-growing problem for wildlife across the globe. Animals are exposed to various forms of pollution, including chemicals, light, noise, and heat, which can interfere with critical biological processes such as reproduction. The impact of pollution on reproductive processes can significantly reduce the number and quality of offspring, leading to potential repercussions on the ecology and evolution of affected populations.

One of the primary ways pollution impairs reproduction in river animals is by disrupting the cueing systems for sex determination and sexual differentiation, resulting in skewed sex ratios. This is particularly evident in species with temperature-dependent sex determination, including fish, amphibians, and reptiles. For example, sea turtles exhibit a higher proportion of females when exposed to higher nest temperatures, and this imbalance is predicted to become more pronounced as global temperatures continue to rise.

Pollution can also directly impact the hormones and sexual development of river animals, affecting fertility and gamete function. For instance, male fish exposed to oestrogenic pollutants may experience reduced sperm quality due to feminization. Similarly, female brown trout exposed to bisphenol A (BPA) prior to and during spawning displayed lower ovulation levels and a delayed onset of ovulation. The males of the same species showed reductions in sperm density, motility, and swimming velocity, likely due to the oestrogenic properties of BPA.

In addition to the direct effects on sexual development and gamete function, pollution can also impair reproduction by disrupting the production, transmission, or detection of sexual communication signals. This disruption can hinder mating opportunities and lead to reduced fertilization success. For example, in a whole-lake experiment, chronic exposure of fathead minnows to low concentrations of 17α-ethinyloestradiol (EE2) resulted in the feminization of males and altered oogenesis in females, driving the species nearly to extinction within the lake.

River pollution can also have indirect effects on reproduction by impacting the food sources that river animals rely on. For example, mayflies, which are a crucial food source for many fish and birds, are sensitive to water quality and can be negatively affected by sewage pollution. When mayfly numbers decline, it can have ripple effects up the food chain, further impacting the reproductive success of predators that depend on them as a food source.

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