Brian Barton
Water pollution can cause oxygen depletion, or hypoxia, in aquatic environments. This occurs when excess biodegradable material is added to the water, leading to an increase in microorganisms that consume the available oxygen. As a result, relatively harmless aerobic microorganisms die off, and anaerobic microorganisms, which can produce toxins harmful to people, animals, and the environment, begin to thrive. Eutrophication, or nutrient pollution, is a significant contributor to oxygen depletion in water bodies, as it leads to excessive growth and decomposition of certain species, such as algae, which consume oxygen during this process. Human activities, such as agricultural runoff, fossil fuel burning, and wastewater treatment, are major sources of nutrient pollution, impacting water bodies like the Baltic Sea and the Gulf of America.
| Characteristics | Values |
|---|---|
| Water pollution causing oxygen depletion | Eutrophication and organic pollution induce oxygen deficits in tropical rivers but stimulate decomposition rates, which may further deplete oxygen levels |
| Excessive nutrient flow into the sea, primarily from agricultural fertilizers, can trigger large phytoplankton blooms, increasing primary production—a process known as eutrophication | |
| Hypoxia refers to low or depleted oxygen in a water body, which often leads to "dead zones" — regions where life cannot be sustained | |
| Hypoxia occurs most often as a consequence of human-induced factors, especially nutrient pollution | |
| Oxygen depletion occurs when too much biodegradable material is added to water, increasing the number of microorganisms that use up the available oxygen | |
| Anaerobic microorganisms that thrive due to oxygen depletion produce harmful toxins such as ammonia and sulfides | |
| Oxygen-depleted zones are caused by restricted vertical mixing of water layers, intensified by eutrophication and ocean warming | |
| Weather conditions fostering oxygen depletion include long periods of calm sunny weather that promote extensive algal growth |
What You'll Learn
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, and nutrient fertilisers. While eutrophication occurs naturally over centuries as lakes age and are filled in with sediments, human activities have accelerated the rate and extent of eutrophication through both point-source discharges and non-point loadings of limiting nutrients, such as nitrogen and phosphorus, into aquatic ecosystems.
One of the most conspicuous effects of cultural eutrophication is the creation of dense blooms of noxious, foul-smelling phytoplankton that reduce water clarity and harm water quality. These algal blooms limit light penetration, reducing growth and causing die-offs of plants in littoral zones while also lowering the success of predators that need light to pursue and catch prey. Furthermore, the high rates of photosynthesis associated with eutrophication can deplete dissolved inorganic carbon and raise pH to extreme levels during the day. 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.
Eutrophication is also associated with major changes in aquatic community structure. During cyanobacterial blooms, small-bodied zooplankton tend to dominate plankton communities, which has been attributed to the anti-herbivore traits of cyanobacteria, such as toxicity, morphology, and poor food quality. The biomass of planktivorous fish is often positively related to nutrient levels and ecosystem productivity. For example, in nutrient-poor, oligotrophic lakes, piscivorous fishes tend to dominate, while planktivorous fishes become increasingly dominant with nutrient enrichment.
Estuarine wetlands, such as the Jiuduansha Wetland in the Yangtze River estuary, are subject to organic pollutants and eutrophication. A study comparing two sampling zones in this wetland, an upstream area (Shang shoal) and a downstream area (Xia shoal), found that eutrophication and organic pollution levels were higher in the upstream area due to the presence of a large sewage treatment plant. This pollution can have significant impacts on the soil microbial community and carbon sequestration in these wetlands.
In summary, eutrophication and organic pollution can induce oxygen deficits in aquatic ecosystems, particularly in tropical rivers, by stimulating decomposition rates and reducing the availability of oxygen for organisms. This can lead to the creation of "dead zones" where most life cannot be sustained due to insufficient oxygen levels.
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Anaerobic microorganisms
Anaerobic organisms, or anaerobes, are organisms that do not require oxygen to grow. In fact, anaerobes can react negatively or die if free oxygen is present. They are the opposite of aerobic organisms, which require oxygenated environments to survive. Anaerobic bacteria are part of the normal flora of human skin and mucosal membranes. They can also be found in injured human tissue that does not have oxygen-rich blood flowing to it. Infections like tetanus and gangrene are caused by anaerobic bacteria.
There are three types of anaerobes: obligate anaerobes, facultative anaerobes, and aerotolerant organisms. Obligate anaerobes are harmed by the presence of oxygen. Clostridium botulinum and the bacteria that live near hydrothermal vents on the deep-sea ocean floor are examples of obligate anaerobes. Obligate anaerobes can be further divided into two types based on the percentage of oxygen that can prove toxic. Strict obligate anaerobes cannot survive in environments with more than 0.5% oxygen, while moderate obligate anaerobes can grow in environments with 2-8% oxygen. Facultative anaerobes can grow with or without oxygen. In the presence of oxygen, facultative anaerobes use aerobic respiration, while in its absence, they may use anaerobic respiration or fermentation. Aerotolerant organisms are strictly fermentative.
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Climate change
Water pollution and climate change are closely interconnected issues. Water pollution is largely caused by human activity, with over 80% of the world's sewage ending up in seas and rivers untreated. This includes chemical dumping, which is a major contributor to the eutrophication of water, and plastic pollution from fishing boats, tankers, and cargo ships. Oil and its derivatives are also common pollutants, as their transportation and storage are subject to leakage. Climate change, meanwhile, is altering nearly every stage of the water cycle, with rising temperatures causing glaciers and ice sheets to melt, leading to rising sea levels. This, in turn, jeopardizes coastal properties and infrastructure worldwide.
The relationship between water pollution and climate change is bidirectional, with water pollution contributing to climate change and climate change exacerbating water pollution. Rising global temperatures caused by CO2 emissions heat water bodies, reducing their oxygen content and making them more susceptible to pollution. Warmer water has a lower capacity to hold dissolved oxygen, which is essential for the survival of aquatic life, including fish. This decrease in oxygen levels can be further exacerbated by eutrophication and organic pollution, which induce oxygen deficits in rivers and other water bodies.
The impact of climate change on water pollution is particularly evident in the increased frequency and severity of droughts and wildfires. Drought conditions can lead to water scarcity, making it difficult to dilute or naturally flush away pollutants. Wildfires, intensified by drier conditions, can contaminate water sources with ash and other combustion by-products. Additionally, the destruction of vegetation and tree cover associated with wildfires exacerbates soil erosion, further degrading water quality.
To address these interconnected challenges, a range of strategies must be implemented. Efforts to reduce water pollution should focus on properly treating and safely reusing wastewater, restricting the use of single-use plastics, and encouraging sustainable fishing practices. Climate change mitigation measures, such as adopting climate-smart agriculture, exploring and protecting groundwater sources, and limiting global warming, are also crucial. By integrating these approaches, we can work towards preserving water quality, maintaining aquatic ecosystems, and adapting to the changing climate.
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Algal blooms
The overgrowth of algae consumes oxygen and blocks sunlight from reaching underwater plants. When the algae die, they are consumed by bacteria, which further deplete the oxygen levels in the water as they decompose the organic matter. This depletion of oxygen, or hypoxia, can lead to the formation of dead zones, where aquatic life cannot survive due to the lack of oxygen. These dead zones can force organisms such as fish to leave the area or die.
The impact of algal blooms on oxygen levels in water can be significant. As the algae grow and multiply, they compete with other aquatic organisms for oxygen, leading to a decrease in overall oxygen availability. Additionally, the presence of excess organic matter in the water can stimulate the growth of bacteria, which further contributes to oxygen consumption during the decomposition process.
Moreover, algal blooms can have indirect effects on oxygen levels by affecting the water's circulation and movement. For example, when large amounts of algae are present, they can form thick mats or scums on the water surface, impeding the exchange of gases between the water and the atmosphere, including the replenishment of oxygen.
It is important to note that not all algal blooms are harmful. Some types of algae can provide benefits to the ecosystem, such as serving as a food source for certain organisms or improving water quality by absorbing excess nutrients. However, when algal blooms occur due to excessive nutrient pollution, they can have detrimental effects on aquatic ecosystems, including the depletion of oxygen levels.
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Aquatic life
One of the primary causes of oxygen depletion in water is excessive algae and phytoplankton growth, which is driven by high levels of phosphorus and nitrogen. These nutrients come from human-induced factors, such as agricultural runoff, fossil-fuel burning, and wastewater treatment. When these photosynthetic organisms die, their decomposition requires significant amounts of oxygen, which further depletes the oxygen levels in the water.
Water temperature also plays a role in oxygen depletion. High temperatures (86°F or higher) increase the metabolic rate of fish, causing them to consume more oxygen. Additionally, still and windless conditions can limit the circulation of water and the diffusion of atmospheric oxygen into the water. During summer, deeper ponds can experience stratification, where the water near the surface is warm and less dense, while the water near the bottom is cool, stagnant, and depleted of oxygen. This can result in the loss of many fish, especially during periods of hot, calm weather.
The effects of oxygen depletion on aquatic life can be mitigated through proper management strategies. For example, aeration systems can be used to mix water with diffused aeration from bottom to top, preventing stratification and maintaining oxygen levels. Additionally, beneficial bacteria can be applied to out-compete algae for available nutrients, reducing aggressive blooms that contribute to oxygen depletion.
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Frequently asked questions
Hypoxia is a term used to refer to low or depleted oxygen in a water body. It often leads to "dead zones", where life cannot be sustained.
Hypoxia is caused by a combination of natural and human-induced factors. Natural causes include stratification in the water column, which restricts the supply of oxygen from surface waters to bottom waters. Human-induced causes include nutrient pollution, agricultural runoff, fossil fuel burning, and wastewater treatment.
Water pollution, specifically organic pollution and eutrophication, can induce oxygen deficits. When too much biodegradable material is added to the water, the number of microorganisms increases, and they use up the available oxygen. This process is called oxygen depletion.
Oxygen depletion can lead to the death of relatively harmless aerobic microorganisms and the growth of harmful anaerobic microorganisms that produce toxins such as ammonia and sulfides. It can also cause changes in aquatic communities, with species that are intolerant of low oxygen levels being replaced by tolerant organisms.
Addressing oxygen depletion in water requires a combination of strategies, including reducing nutrient inputs into coastal waters, improving water treatment processes, and implementing policies to reduce pollution. The EU, for example, has introduced targets for reducing nutrient use in agriculture and losses into the environment through initiatives such as the EU Biodiversity Strategy 2030 and the Zero Pollution Action Plan.
