Pollution's Impact: Dissolved Oxygen Levels And Aquatic Life

Pollution has a significant impact on the amount of dissolved oxygen in water, which is essential for the survival of aquatic organisms. The concentration of dissolved oxygen in water bodies can reveal a lot about their quality. Eutrophication and organic pollution, often caused by agricultural and urban runoff, lead to oxygen deficits in rivers and lakes. This is because bacteria in the water consume oxygen as organic matter decays, resulting in eutrophic conditions that can cause a water body to die.

Additionally, human activities such as industrial waste discharge and the use of chemical fertilisers and pesticides can contaminate water sources, leading to oxygen-depleted dead zones. Climate change, specifically rising temperatures, also plays a role in reducing dissolved oxygen levels as warmer water holds less oxygen.

Therefore, understanding the impact of pollution on dissolved oxygen is crucial for maintaining the health of aquatic ecosystems and ensuring the sustainability of our water resources.

Eutrophication and organic pollution

Eutrophication stimulates excessive growth of aquatic plants and algae, which increases oxygen demand during respiration and decomposition. When these organisms die, bacteria and fungi further consume oxygen during their breakdown. This leads to a decrease in dissolved oxygen, creating an oxygen-deficient environment that can be harmful or even fatal to aquatic life.

Organic pollution also contributes to oxygen depletion. As organic matter decays, bacteria in the water consume oxygen, leading to lower dissolved oxygen levels. This process is particularly significant in stagnant or slow-moving water bodies, where the water is already less oxygenated. The combination of eutrophication and organic pollution can have a synergistic effect, further depleting oxygen levels.

The effects of eutrophication and organic pollution on dissolved oxygen have been observed in various environments, including tropical rivers and estuarine wetlands. In tropical lowland rivers, eutrophication and organic pollution induced oxygen deficits, with the highest oxygen stress found in areas with elevated nutrient concentrations. Similarly, in estuarine wetlands, eutrophication and organic pollution increased soil respiration, leading to higher carbon dioxide emissions and reduced carbon sequestration.

The impact of eutrophication and organic pollution extends beyond oxygen depletion. Eutrophication can lead to algal blooms, which can have toxic effects on aquatic ecosystems. Additionally, the increased nutrient levels associated with eutrophication can alter food resources for fish and invertebrates, impacting their populations. Organic pollution can also introduce toxic compounds and affect the pH and clarity of the water, further influencing aquatic life.

Addressing eutrophication and organic pollution is crucial for maintaining healthy aquatic ecosystems. This can be achieved through a combination of measures, including reducing nutrient runoff from agricultural lands, improving wastewater treatment processes, and restoring natural vegetation along water bodies to act as buffers. By mitigating these issues, we can help improve water quality and support the diverse organisms that rely on sufficient dissolved oxygen for their survival.

Temperature and solubility

The solubility of oxygen in water is influenced by several factors, including temperature, pressure, and salinity. Let's delve into the relationship between temperature and solubility:

Temperature plays a significant role in determining the solubility of oxygen in water. As a general rule, the solubility of oxygen in water decreases with increasing temperature. In other words, cold water can hold more dissolved oxygen than warm water. This relationship is described by Henry's law, which states that the amount of a gas that dissolves in a liquid is directly proportional to the gas's partial pressure above the liquid, provided the temperature remains constant. The higher the temperature, the lower the solubility constant for oxygen, known as Henry's law constant. This relationship has been quantified by scientists, yielding equations that express oxygen solubility as a function of temperature.

The impact of temperature on oxygen solubility has important implications for aquatic ecosystems. Aquatic organisms, such as fish and zooplankton, rely on dissolved oxygen for survival. During summer and fall, when water temperatures are typically higher, dissolved oxygen concentrations tend to be lower. This can create challenging conditions for aquatic life, particularly in stagnant water bodies with high organic matter content. The combination of warm temperatures and low dissolved oxygen can lead to eutrophic conditions, where oxygen deficiency causes a decline in water quality and even the death of the water body.

Additionally, the solubility of oxygen in water is influenced by other factors, such as salinity. Seawater, with a salinity of 30 to 50 parts per thousand, has different oxygen solubility characteristics compared to freshwater. Furthermore, the presence of dissolved materials, such as salts and sugars, can also affect oxygen solubility. These solutes tend to reduce the solubility of oxygen in water, which is an important consideration in various industrial processes, including fermentation.

In summary, temperature plays a critical role in determining the solubility of oxygen in water, with higher temperatures resulting in lower solubility. This relationship has significant implications for aquatic ecosystems and is a key factor in understanding the impact of pollution on dissolved oxygen levels.

Photosynthesis and oxygen production

Photosynthesis is the process by which photosynthetic organisms, such as plants and algae, convert sunlight into chemical energy. At the heart of this process is the light-driven splitting of water into its elemental constituents, releasing molecular oxygen and maintaining an aerobic atmosphere.

During photosynthesis, the oxygen-evolving complex splits water to provide protons and electrons to the chloroplastic electron chain, generating ATP and NADPH. These are the energy source and reducing power for plant metabolism. The majority of this chemical energy is used to drive photosynthetic carbon metabolism, which consists of ribulose-1,5-bisphosphate carboxylation (photosynthetic carbon reduction cycle) and oxygenation (photosynthetic carbon oxidation cycle).

The amount of dissolved oxygen in a stream or lake can tell us a lot about its water quality. Aquatic life requires dissolved oxygen to survive, and most are dependent on the oxygen dissolved in the water column. Photosynthesis is the primary process affecting the dissolved-oxygen/temperature relation; water clarity and the strength and duration of sunlight, in turn, affect the rate of photosynthesis.

Although it is challenging to calculate the exact percentage of oxygen produced by the ocean, scientists estimate that about half of the oxygen production on Earth comes from the ocean. The majority of this production is from oceanic plankton — drifting plants, algae, and some bacteria that can photosynthesize. One particular species, Prochlorococcus, is the smallest photosynthetic organism on Earth but produces up to 20% of the oxygen in our biosphere.

Sources of pollution

Eutrophication and Organic Pollution

Eutrophication and organic pollution are common in tropical lowland rivers due to anthropogenic inputs of nutrients and organic matter. This can induce oxygen deficits in rivers, further stimulating decomposition rates and depleting oxygen levels.

Municipal Waste Treatment Plants

Municipal waste treatment plants process municipal wastewater and operate under permit limits to protect receiving water bodies from excess inputs of nutrients and organic matter. However, during storms, excess flow may be diverted into combined sewer overflows (CSOs), depositing untreated waste directly into streams.

Septic Seepage and Failed Package Plants

Septic seepage from failed septic tanks and emissions from poorly functioning package sewage treatment plants can contribute significant amounts of nutrients and organic matter, creating biological oxygen demand (BOD).

Industrial Point Sources

Some industries release organic chemicals that require oxygen for decomposition. End-of-pipe discharges are regulated under permit limits to protect water bodies. However, inadequate system design or operational problems can lead to inadequately treated discharges, resulting in oxygen depletion.

Agricultural and Urban Runoff

Nutrient runoff from agricultural or residential fertilizer applications can increase the amount of algae and macrophytes in water. This can lead to higher oxygen inputs during the day and increased oxygen demands at night. When plants die, bacteria and fungi consume oxygen during decomposition. Organic matter washed into water bodies from animal wastes or landfills can also increase oxygen demand.

Devegetated Riparian Areas

Removing vegetation from the banks of water bodies increases surface runoff, decreases shading, and alters in-stream physical characteristics. Decreased shading increases water temperatures and plant production, with higher temperatures decreasing the solubility of oxygen in water. Plant decomposition can deplete dissolved oxygen, and reduced turbulence from less woody debris may decrease aeration.

Channel Alteration

Stream channel straightening often reduces turbulence, alters curves and riffles, and deepens the channel, reducing the surface-to-volume ratio and thus diffusion and aeration. While natural inflows of groundwater usually have low dissolved oxygen concentrations, changes to local hydrology and surface water temperatures may shift their effect on dissolved oxygen levels.

Effects on aquatic life

Dissolved oxygen (DO) is a measure of how much oxygen is dissolved in the water and is essential for the survival of aquatic life. The amount of dissolved oxygen in a stream or lake is indicative of its water quality.

The effects of pollution on dissolved oxygen levels can have a range of impacts on aquatic life. Firstly, low DO levels can cause changes in the types and numbers of aquatic macroinvertebrates in surface waters. Certain species of mayflies, stoneflies, caddisflies, and beetles are intolerant of low DO and may be replaced by worms and fly larvae. Fish communities can also be impacted, with large fish of a given species dying before smaller fish. Species with higher oxygen requirements may perish while others are less affected.

Aquatic organisms may exhibit characteristic body movements in response to low DO conditions. Some stonefly larvae may perform "push-ups," while caddisfly larvae may undulate to increase water flow across their respiratory structures. Fish may gulp air at the water surface and stay in shallow waters.

Low DO levels can also lead to respiratory distress in aquatic biota, especially those with higher oxygen requirements, such as salmonids and riffle invertebrates. This can result in widespread fish kills, particularly during early morning or on cloudy days preceded by warm, sunny weather.

In addition, low DO can impact the oxidation-reduction reactions that determine the bioavailability of important compounds like nitrogen and sulfur. This can have cascading effects on the food resources available for fish and invertebrate assemblages.

On the other hand, extremely high DO levels can also pose problems. Supersaturation can occur due to high levels of oxygen-generating photosynthesis or turbulence downstream of impoundments. This can lead to oxygen-related gas bubble disease in fish, causing death.

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