Water Pollution's Impact On The Oxygen Cycle Explained

Water pollution is a pressing issue that poses significant threats to aquatic ecosystems and human health. One of the critical aspects of water pollution is its impact on oxygen levels, which can have far-reaching consequences for the organisms that depend on it. Dissolved oxygen (DO) is a crucial indicator of water quality, and changes in its concentration can disrupt the delicate balance of the oxygen cycle, leading to devastating effects on aquatic life. This paragraph aims to delve into the intricate relationship between water pollution and the oxygen cycle, exploring the mechanisms by which pollution influences oxygen levels and the subsequent implications for the health of aquatic organisms.

Eutrophication and organic pollution

Eutrophication is a leading cause of impairment of many freshwater and coastal marine ecosystems worldwide. 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, nitrogen, and phosphorus. Eutrophication occurs naturally over centuries as lakes age and are filled in with sediments. However, human activities such as agriculture, industry, and sewage disposal have accelerated the rate and extent of eutrophication, a process known as cultural eutrophication.

Cultural eutrophication occurs when human water pollution speeds up the aging process by introducing sewage, detergents, fertilisers, and other nutrient sources into aquatic ecosystems. This has had dramatic consequences for drinking water sources, fisheries, and recreational bodies of water. For example, in aquaculture ponds, high concentrations of nutrients due to regular fish feeding result in recurring cyanobacterial blooms and hypoxia.

Eutrophic waters often develop large concentrations of algae and microscopic organisms on their surfaces, preventing the light penetration and oxygen absorption necessary for underwater life. This can cause eutrophic waters to become murky and support fewer large animals, such as fish and birds, than non-eutrophic waters. The dense algal blooms that are characteristic of eutrophication can limit light penetration, reducing the growth and causing the death of plants in littoral zones. High rates of photosynthesis associated with eutrophication can deplete dissolved inorganic carbon and raise pH to extreme levels during the day. Elevated pH can, in turn, impair the chemosensory abilities of organisms that rely on the perception of dissolved chemical cues for their survival.

When these dense algal blooms eventually die, microbial decomposition severely depletes dissolved oxygen, creating a hypoxic or anoxic "dead zone" lacking sufficient oxygen to support most organisms. Dead zones are found in many freshwater lakes, including the Laurentian Great Lakes, and are particularly common in marine coastal environments surrounding large, nutrient-rich rivers. Hypoxia and anoxia as a result of eutrophication continue to threaten lucrative commercial and recreational fisheries worldwide.

In summary, eutrophication and organic pollution induce oxygen deficits in tropical rivers but stimulate decomposition rates, which may further deplete oxygen levels. Eutrophication, particularly when accelerated by human activities, can have severe ecological and economic impacts, including the degradation of water quality, destruction of economically important fisheries, and public health risks.

Oxygen levels and aquatic life

Oxygen levels in water are crucial for the survival of aquatic life, including fish and zooplankton. Aquatic organisms require oxygen to breathe and carry out essential life processes. The amount of oxygen dissolved in water, known as dissolved oxygen (DO), is a key indicator of water quality.

Water temperature plays a significant role in determining the amount of dissolved oxygen it can hold. As a general rule, colder water can hold more oxygen than warmer water. This relationship is inverse, meaning that as water temperatures rise, dissolved oxygen levels decrease. Therefore, during the warmer seasons, such as summer and fall, dissolved oxygen concentrations tend to be lower. Additionally, stagnant or slow-moving water bodies, like lakes, ponds, and rivers, tend to have lower dissolved oxygen levels compared to rapidly moving waters like mountain streams.

The presence of excess organic material, nutrients, and algae in water bodies can also lead to decreased oxygen levels. Bacteria consume oxygen as they decompose organic matter, leading to oxygen-deficient conditions known as eutrophic conditions. This can result in the death of aquatic life, particularly during hot and calm weather. Eutrophication, often caused by agricultural practices and urban runoff, can further deplete oxygen levels in tropical lowland rivers.

Moreover, the oxygen level in water can be affected by its salinity. Freshwater has a higher oxygen-holding capacity compared to saltwater. Therefore, marine life, especially in coastal areas, can be significantly impacted by even small changes in oxygen levels.

The balance of oxygen in aquatic ecosystems is delicate and easily disrupted by human activities. Point source pollution, such as discharges from households, farms, and factories, directly introduces pollutants into natural waters. Nonpoint source pollution, like microplastics and flue gases, is more challenging to trace as it spreads through the air and returns to the water during rainfall. These human-induced disruptions can have devastating effects on aquatic life, with even small changes in oxygen levels leading to fish kills and ecosystem imbalances.

Oxygen, temperature and water quality

Oxygen, Temperature, and Water Quality

Water temperature is a critical factor in determining the amount of dissolved oxygen (DO) in a body of water. DO is one of the most important indicators of water quality and is essential for the survival of aquatic organisms. The amount of DO in a stream or lake can tell us a lot about its health.

Water molecules contain an oxygen atom, but this is not the form of oxygen that aquatic organisms need to survive. Oxygen enters a body of water mainly from the atmosphere and, in areas where groundwater discharge is significant, from groundwater. The amount of oxygen that can dissolve in water depends on its temperature, with colder water able to hold more oxygen than warmer water. Therefore, in winter and early spring, when water temperatures are lower, DO concentrations are higher. Conversely, in summer and autumn, when water temperatures are higher, DO concentrations tend to be lower.

Water temperature also influences the rate of photosynthesis in aquatic plants, which affects DO levels. Photosynthesis is the primary process that affects the relationship between DO and temperature. Water clarity and sunlight strength and duration also influence the rate of photosynthesis.

In addition to temperature, other factors such as salinity and turbulence affect DO levels. Fresh water can absorb more oxygen than salt water. Turbulence, caused by factors such as wind and waves, increases the aeration of water, enhancing the incorporation of oxygen.

High temperatures can reduce DO levels in water bodies, creating stressful conditions for aquatic life. This is further exacerbated by the presence of excess organic material, which can lead to eutrophic conditions, an oxygen-deficient state that can cause a water body to "die". Stagnant water with high temperatures and excessive organic matter can result in aquatic life struggling to survive, especially during summer when DO levels are naturally lower.

The presence of pollutants in water can also negatively impact DO levels. Pollution from households, farms, or factories can introduce harmful substances such as metals, plastics, chemicals, and bacteria into water bodies. These pollutants can cause excessive algae growth, known as algal blooms, which can lead to oxygen depletion and sudden fish kills.

Oxygen and algal blooms

Algal blooms are a direct result of eutrophication, or the enrichment of water bodies with nutrients and minerals. This is often caused by human activity, such as agricultural and urban runoff, which introduces excess nitrogen and phosphorus into water bodies. These excess nutrients cause an overgrowth of algae, which blocks sunlight from reaching underwater plants and consumes oxygen as it grows. When the algae eventually die, they are decomposed by bacteria, which further depletes the oxygen in the water. This depletion of oxygen can force fish and other organisms to leave the area or die.

The largest dead zone in the United States, which recurs every summer in the Gulf of Mexico, is caused by a harmful algal bloom resulting from nutrient pollution in the Mississippi River Basin. This algal bloom not only depletes oxygen but also releases toxins that contaminate drinking water, causing illnesses in animals and humans.

The oxygen cycle is affected by algal blooms in a few ways. Firstly, the growth of the algae itself consumes oxygen. Secondly, when the algae die, they are decomposed by bacteria, which further depletes oxygen from the water. This can lead to hypoxic or anoxic conditions, where oxygen levels are too low to support most life. Additionally, the algae can block sunlight from reaching underwater plants, which can affect photosynthesis and, consequently, oxygen production in the water.

The presence of excess nutrients, particularly nitrogen and phosphorus, is the main driver of algal blooms and the resulting oxygen depletion. These nutrients can come from a variety of sources, including agricultural runoff, urban runoff, and septic seepage. The decomposition of organic matter by bacteria is another key factor in oxygen depletion.

Oxygen and water salinity

Oxygen solubility in water is influenced by several factors, including water temperature, atmospheric pressure, and salinity. Salinity is an important factor in determining the amount of oxygen dissolved in a body of water. As salinity increases, dissolved oxygen decreases exponentially. This is because when ionic salts enter the water, their ions attract water molecules, leaving fewer oxygen ions available. This leads to a decrease in the solubility of oxygen molecules.

Freshwater absorbs more oxygen than saltwater. For example, saltwater holds about 20% less dissolved oxygen than freshwater. The solubility of oxygen in seawater is approximately 20% less than in freshwater at 32°F and 17% less at 100°F. The solubility of oxygen in freshwater decreases from 14.6 mg/L to 8.24 mg/L as the temperature rises from 32°F to 100°F, a 46.3% decrease. In contrast, oxygen solubility in seawater decreases from 11.5 mg/L to 6.75 mg/L over the same temperature range, a 41.3% decrease.

The diffusion rate of oxygen in seawater is influenced by salinity and temperature. While the activity of oxygen in water remains constant when partial pressure and temperature are constant, the diffusion rate of oxygen varies proportionally with the solubility of oxygen, which decreases as salinity increases. The respiratory rate of oxygen-dependent marine invertebrates increases with decreasing salinity when measured at constant partial pressure. However, the respiratory rate is independent of salinity when measured at equal oxygen concentrations in water of different salinities.

Dissolved oxygen levels are critical for aquatic life. According to the Environmental Protection Agency (EPA), dissolved oxygen levels below 3 mg/L are in the danger zone for supporting common aquatic life, and levels below 1 mg/L cannot support any aquatic life. Therefore, increasing salinity can negatively impact aquatic life by decreasing dissolved oxygen levels.

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