Organic Pollutants: Stealing Oxygen, Harming Life

Organic pollutants can cause oxygen levels to drop, leading to severe implications for aquatic life and human health. Aquatic organisms rely on dissolved oxygen in water to survive, and when organic waste is introduced, oxygen levels can deplete rapidly due to the increased oxygen consumption during the decomposition process. This is particularly evident in eutrophication, where excess nutrients fuel algae blooms, and subsequent decomposition further depletes oxygen, creating hypoxic environments. Similarly, air pollution from fine particles and gaseous pollutants has been associated with reduced oxygen saturation in the elderly, leading to adverse cardiovascular and respiratory outcomes. Understanding the impact of organic pollutants on oxygen levels is crucial for maintaining the health of both aquatic ecosystems and human populations.

Eutrophication and organic pollution

Eutrophication is a leading cause of impairment of many freshwater and coastal marine ecosystems globally. It is characterised by excessive plant and algal growth due to the increased availability of one or more limiting growth factors needed for photosynthesis, such as sunlight, carbon dioxide, and nutrient fertilisers. While eutrophication occurs naturally over centuries as lakes age and are filled with sediments, human activities have accelerated the rate and extent of eutrophication through the discharge of limiting nutrients, such as nitrogen and phosphorus, into aquatic ecosystems.

The presence of dense blooms of phytoplankton and cyanobacteria associated with eutrophication can further contribute to oxygen depletion. These blooms reduce water clarity and harm water quality. When the blooms eventually die, their microbial decomposition severely depletes dissolved oxygen, creating hypoxic or anoxic "dead zones" that lack sufficient oxygen to support most aquatic organisms.

Additionally, eutrophication can lead to changes in aquatic community structure, with small-bodied zooplankton often dominating plankton communities during cyanobacterial blooms due to their anti-herbivore traits. The dominance of certain species can impact the oxygen dynamics within the ecosystem. For example, the biomass of planktivorous fish is positively related to nutrient levels, with planktivorous fishes becoming more dominant with increased nutrient enrichment. These complex interactions between eutrophication, organic pollution, and aquatic community dynamics contribute to the overall oxygen deficits observed in affected ecosystems.

Aquatic life and tolerance

Aquatic ecosystems are the ultimate sinks for contaminants, and organic pollutants are one of the leading causes of pollution in aquatic environments. Organic pollutants, such as biodegradable organic materials, contribute to the reduction in oxygen concentration in water. This is due to the increased demand for dissolved oxygen by bacteria and other decomposing organisms that break down these materials. As the population of these decomposers increases, the demand for oxygen rises, leading to a decrease in dissolved oxygen levels.

The presence of organic pollutants in water triggers a sequence of events that impact aquatic life. Initially, there is an increase in organic material, followed by a rise in the number of decomposers. This leads to a higher respiratory oxygen demand, resulting in oxygen depletion. The tolerance of different aquatic species to low oxygen levels varies, and some may be eliminated or severely affected by these conditions. For example, fish and other organisms start to show signs of distress, such as lethargy and "piping" (swimming to the surface and gasping for air), when oxygen levels drop below certain thresholds.

The impact of organic pollutants on aquatic life is influenced by various factors, including the initial oxygen levels, water temperature, and the ability of the water body to recover. Water with higher initial oxygen saturation levels can buffer the decrease in oxygen concentration to an extent. Additionally, colder water can hold more oxygen than warmer water, which is why DO concentrations are usually higher in winter than in summer. Slow-moving or still water is also more susceptible to low oxygen levels due to reduced turbulent aeration and slower delivery of oxygen to organisms.

The tolerance of aquatic life to low oxygen conditions varies among species. Some organisms, such as certain bacteria and algae, can continue to survive and consume oxygen even at low levels. However, many aquatic organisms, especially those with higher oxygen demands, struggle to survive in hypoxic conditions. For example, fish may exhibit distress behaviours and even die-off as oxygen levels drop below their critical threshold. This can have cascading effects on the entire aquatic ecosystem, as the balance of species is disrupted.

To protect aquatic life, organizations like the US Environmental Protection Agency (EPA) have established criteria and guidelines for assessing and mitigating the impact of pollutants. These guidelines consider both short-term and long-term exposure risks and aim to safeguard aquatic organisms and their habitats. By setting standards for water quality and implementing measures to reduce pollution, efforts are made to ensure that aquatic life can tolerate and recover from the presence of organic pollutants, thus maintaining the delicate balance of aquatic ecosystems.

Weather conditions

Calm, sunny weather that promotes algal growth can contribute to oxygen depletion. When followed by cloudy days and nights, respiring plants, including algae, consume more oxygen than they produce, leading to a further decrease in oxygen levels. Additionally, weather events and seasonal changes can impact water volume. Reduced water volume can concentrate aquatic life into confined spaces, leading to increased respiration and exceeding oxygen renewal capacity.

The impact of organic pollutants on oxygen levels is also influenced by nutrient-rich runoff from land, which is more common during rainy seasons. Increased rainfall can wash nutrients such as nitrogen and phosphorus into water bodies, leading to eutrophication and oxygen deficits. This is particularly true for tropical lowland rivers, where high temperatures and elevated nutrient concentrations can result in oxygen stress.

Furthermore, weather conditions can affect the circulation of air and water, influencing oxygen levels. Turbulent aeration, caused by wind or water currents, enhances the absorption of oxygen from the atmosphere into water. Slow-moving or still water, often associated with specific weather patterns, may have lower oxygen levels due to a lack of turbulent aeration.

Air pollution, including fine particles (PM2.5), sulfate (SO42-), and elemental carbon (EC), can also lead to decreased oxygen saturation in the atmosphere. Increased exposure to these pollutants has been associated with negative impacts on respiratory health, particularly in senior adults. Thus, weather conditions that trap pollutants, such as calm or windless days, can indirectly contribute to oxygen depletion by allowing pollutant concentrations to build up.

Organic decomposition

The presence of organic waste in a water body suggests low dissolved oxygen (DO) levels as organic decomposition consumes oxygen. Organic decomposition that takes place in the absence of oxygen or with a limited supply of oxygen is called anaerobic composting. This process involves the collection of organic materials in pits, accompanied by layering with a thick layer of soil. The mixture is left undisturbed for about 6 to 8 months to ensure complete degradation. The prevailing anaerobic conditions promote the growth of anaerobic microorganisms that decompose the organic matter, forming compounds like hydrogen sulfide, methane, and organic acids. These compounds continue to accumulate and are not further metabolized.

Anaerobic digestion by microorganisms produces methane gas and a small amount of heat. This procedure takes longer and does not kill pathogens and bacteria. It generates sludge-like material that is even harder to break down. In contrast, aerobic digestion produces heat, water, and carbon dioxide as byproducts. The heat generated during this process kills pathogens and bacteria.

The decomposition of organic matter by microorganisms occurs in the presence of oxygen. Oxygen is essential for many forms of life, and for aquatic organisms, it is dissolved in the water. Aquatic fauna obtain oxygen by actively moving water across their respiratory structures or passively allowing currents to deliver oxygen to them. Some organisms require nearly saturated levels of oxygen, while others can tolerate very low DO levels. Decreases in DO levels can cause changes in the types and numbers of aquatic macroinvertebrates in surface waters.

Fungi also play a vital role in decomposing organic matter in the soil. They break down complex organic compounds, releasing essential nutrients in the process. This decomposition process helps in nutrient cycling, making nutrients readily available to plants. Additionally, fungi contribute to the creation of stable soil aggregates, improving soil structure, water-holding capacity, and aeration.

Pollution and human health

Pollution of air, water, and soil is a significant threat to human health. The World Health Organization (WHO) has concluded that pollution is responsible for approximately 9 million deaths annually, with economic losses in the trillions. More than 60% of pollution-related deaths are caused by cardiovascular disease, and children, the elderly, and pregnant women are more susceptible to pollution-related diseases.

Air pollution is the leading cause of pollution-related deaths, particularly in older age groups. It is the presence of one or more contaminants in the atmosphere, such as dust, fumes, gas, mist, odour, smoke, or vapour, in quantities that can be harmful to human health. The main pathway of exposure is through the respiratory tract, causing inflammation, oxidative stress, immunosuppression, and mutagenicity in cells, impacting the lungs, heart, and brain, among other organs. Fine particulate matter, such as PM, carbon monoxide (CO), and nitrogen dioxide (NO2), can penetrate deep into the lungs, enter the bloodstream, and cause systemic damage to tissues and cells. Short-term exposure to high levels of particulate matter can lead to reduced lung function, respiratory infections, and aggravated asthma, while long-term exposure increases the risk of non-communicable diseases such as stroke, heart disease, and cancer.

Water pollution also poses significant health risks, especially in infancy. Eutrophication and organic pollution in tropical rivers, for example, induce oxygen deficits, which can lead to aquatic life kills. Additionally, soil pollution is a growing concern, as it reduces the soil's ability to yield food, resulting in food crop contamination and disease. Soil pollutants can also wash into rivers, causing water pollution. Healthy soil is essential for human health as it supports diverse ecosystems, provides food, sustains populations, and captures carbon to slow climate change.

Soil may be polluted by heavy metals, organic chemicals such as pesticides, biological pathogens, and micro/nanoplastic particles. While the health effects of microplastics are not yet fully understood, it is recommended to reduce exposure by avoiding plastic bottles and food packaging, opting for hard floors instead of carpets, and regular vacuuming. PFAS (Per- and polyfluoroalkyl substances) are another group of persistent chemicals found in water and soil, although the levels typically found in the environment are likely to pose low health risks.

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