Polluted Water: Deprived Of Oxygen?

Water pollution is a pressing issue, with less than half of all rivers and lakes in a good ecological state. One of the consequences of polluted water is a decrease in oxygen levels, which can have detrimental effects on aquatic life and ecosystems. The presence of certain pollutants, such as nutrients and organic matter, can lead to oxygen depletion, creating conditions that are inhospitable for fish and other organisms. This introduction will explore the relationship between water pollution and oxygen levels, discussing the causes and impacts of low oxygen in aquatic environments.

High water temperatures reduce oxygen solubility

Water pollution is a pressing issue, with less than half of all rivers and lakes in a good state. Polluted water can cause harm to humans, aquatic plants, and animals. One of the consequences of polluted water is low oxygen levels, which can lead to fish kills.

One of the factors that influence the oxygen levels in water is temperature. High water temperatures reduce oxygen solubility due to the inverse relationship between dissolved oxygen and temperature. As the water temperature increases, the solubility of oxygen decreases. This means that warm water can hold less dissolved oxygen (DO) than cold water.

The amount of dissolved oxygen in water is crucial for aquatic life, as it indicates the availability of oxygen for aquatic organisms. When water temperatures rise, the increased energy breaks the weak molecular interactions between water and oxygen molecules, causing the oxygen to escape. This decrease in dissolved oxygen can negatively affect aquatic habitats and organisms.

In addition to temperature, other factors such as nutrients, sediments, and ammonia can also impact dissolved oxygen levels. High nutrients can lead to excessive plant growth, resulting in increased respiration and decomposition, which in turn, can deplete dissolved oxygen levels. Similarly, embedded sediments can prevent dissolved oxygen from permeating all areas, and low oxygen levels can inhibit nitrification, leading to increased ammonia levels.

The presence of plants and algae can also influence dissolved oxygen levels. High plant abundance, especially during the night, can result in high respiration and high oxygen demand for the decomposition of plant detritus, leading to a decrease in dissolved oxygen. Slow-moving or still water is more susceptible to low dissolved oxygen levels due to a lack of turbulent aeration and reduced water volume.

Eutrophic conditions caused by excess organic material

Eutrophication is a process that occurs when an aquatic ecosystem experiences an increase in nutrients, which causes a population increase in algae, known as an algal bloom. Phosphorus and nitrogen are the two main nutrients that cause eutrophication as they enrich the water, allowing for some aquatic plants, especially algae, to grow rapidly and bloom in high densities. Eutrophication can also be caused by an increase in carbon, which can lead to ocean acidification, slowing the growth of fish and shellfish and preventing shell formation in bivalve mollusks.

Eutrophication sets off a chain reaction in the ecosystem, as the excess algae and plant matter eventually decompose, producing large amounts of carbon dioxide. This lowers the pH of seawater, a process known as ocean acidification. The dead algae are then broken down by bacteria, which removes oxygen from the water, creating anoxic conditions that can be fatal to aquatic life such as fish and invertebrates. This process can also affect terrestrial animals, as they are restricted from accessing the water as a drinking source.

The increase in nutrients that causes eutrophication can come from a variety of sources, including runoff from fertilized fields, lawns, and golf courses, as well as untreated sewage and detergents. In recent years, efforts have been made to enlist the help of indigenous estuarine residents, such as bivalve mollusks, to slow or reverse eutrophication. Bivalve mollusks, such as oysters, clams, and scallops, can naturally reduce nutrients through their filter-feeding activities. For example, a modeling project in Long Island Sound showed that the oyster aquaculture industry in Connecticut provided significant nutrient reduction benefits, comparable to traditional nutrient-reduction measures such as wastewater treatment improvements.

To prevent and reverse eutrophication, it is crucial to minimize point source pollution from sewage and agriculture, as well as other nonpoint pollution sources. Introducing bacteria and algae-inhibiting organisms, such as shellfish and seaweed, can help reduce nitrogen pollution and control the growth of harmful algae blooms. Additionally, nutrient removal technologies, such as biofiltration, can be employed to capture and biologically degrade pollutants. However, it is important to note that the recovery of eutrophicated lakes can be a slow process, often requiring several decades.

Algal blooms and bacterial respiration deplete oxygen

Algal blooms, or the excessive growth of algae, are a frequent cause of oxygen depletion in water bodies. While algae do produce oxygen during the day through photosynthesis, they consume oxygen at night, like all plants. Dense blooms can block sunlight from reaching the bottom of the water body, reducing photosynthesis and dissolved oxygen levels. This can lead to a buildup of toxic compounds, even in aerated ponds, causing physiological stress and reduced fish growth.

Additionally, algal blooms can respond to changes in the weather, and certain weather conditions can exacerbate the problem of oxygen depletion. For example, calm days can prevent phytoplankton from mixing near the brighter surface, reducing oxygen production. In summer, the warmer the water, the less dissolved oxygen it can hold, and any surplus of oxygen produced by the blooms will escape into the atmosphere.

When trace minerals or nutrients needed by the algae are used up, it can cause the bloom to die back temporarily. The resulting bacterial decomposition and loss of normal oxygen production can lead to further oxygen depletion, causing fish kills. This process can cause pond water to change colour, turning black, grey, brown, or clear after a phytoplankton die-off.

Overall, algal blooms and bacterial respiration can deplete oxygen levels in water bodies, leading to negative consequences for aquatic ecosystems, including fish kills and reduced fish growth.

Pollution from farms, factories and households

Water is a "'universal solvent", able to dissolve more substances than any other liquid on Earth. This makes it very vulnerable to pollution. Water pollution can come from a variety of sources, including farms, factories, and households. These sources can discharge polluted water directly into natural waters, or pollution can enter waterways through stormwater runoff or debris blown into the water.

Farms are a major contributor to water pollution, especially in OECD countries, where the environmental and social costs of agricultural water pollution are estimated to exceed billions of dollars annually. Farms discharge large quantities of agrochemicals, organic matter, drug residues, sediments, and saline drainage into water bodies. For example, fertilizers, pesticides, and animal waste from farms and livestock operations can wash nutrients and pathogens, such as bacteria and viruses, into waterways. In addition, a new class of agricultural pollutants has emerged in the form of veterinary medicines (antibiotics, vaccines, and growth promoters) that move from farms through water to ecosystems and drinking water sources.

Factories also contribute to water pollution by discharging contaminated water into natural waters. This water can contain chemicals, nutrients, and heavy metals that are carried by streams and rivers into bays and estuaries and eventually out to sea. Marine debris, particularly plastic, is also a significant contributor to ocean pollution, blown in by the wind or washed in via storm drains and sewers. Oil spills and leaks, big and small, can also contaminate marine environments.

Household pollution contributes to water pollution through the use of sinks, showers, and toilets, which produce sewage. This sewage can contain metals, solvents, and toxic sludge that can contaminate waterways. In addition, stormwater runoff from impermeable surfaces, such as roads, can carry road salts, oil, grease, chemicals, and debris into our rivers and streams.

The effects of water pollution are far-reaching and can cause severe problems for both people and the environment. Polluted water can harm human health, with contaminated water causing approximately 1.8 million deaths in 2015, according to a study published in The Lancet. In addition, aquatic plants and animals are also endangered by even small disturbances in their ecosystems. Low oxygen levels caused by excessive algae growth, for example, can lead to sudden fish kills. Therefore, it is important to address water pollution and implement measures to reduce the discharge of polluted water from farms, factories, and households into natural waters.

High nutrient levels cause excessive plant growth

Water pollution is a pressing issue, with only 40% of European surface waters in a good ecological state. Water pollution can come from households, farms, or factories that discharge polluted water into natural waters. These are the point sources of polluted water. However, we also have non-point (diffuse) source pollution, which includes microplastics and flue gases spread in the air and returned to the water during rainfalls. These sources can also be a result of human activities such as traffic exhaust fumes.

Water is essential for plants, helping them take up nutrients from the soil and carry sugars and other elements required by flowers or fruit. However, the quality and quantity of water play a significant role in plant health. For instance, too much water can cause issues such as mould, and when the soil is too damp, roots have difficulty absorbing oxygen. On the other hand, too little water can make it impossible for plants to absorb necessary nutrients, leading to root damage and, eventually, plant death.

High nutrient levels in water, particularly nitrogen and phosphorus, can cause excessive plant growth through a process called eutrophication. While nutrients are essential for plant growth, an overabundance can have adverse health and ecological effects. Eutrophication is a natural process where nutrients accumulate in lakes or other bodies of water, leading to the growth of algae that feed on these nutrients. This excessive algae growth, known as algal blooms, can clog water-intake pipes, produce unpleasant tastes and odours, and, most critically, deplete oxygen levels in the water. The decay of dead algae by bacteria consumes the oxygen dissolved in the water, which is essential for fish and other aquatic life to "breathe". If enough oxygen is removed, the water becomes hypoxic, creating a "dead zone" where life cannot be sustained.

Human activities, such as fertiliser use, can accelerate eutrophication by increasing the rate at which nutrients enter water sources. High EC (electrical conductivity) water, indicative of high levels of natural salinity and salinity from fertiliser residues, can be found in containment ponds rich in fertiliser residues and certain wastewaters used for irrigation. Excess soluble salts, including calcium and magnesium, can impair root function and reduce water uptake in plants, impacting their growth. Additionally, certain ions in water, such as Na and Cl, can be directly toxic to plants, increase soluble salt levels, and inhibit water uptake. Therefore, it is crucial to test water quality and manage fertiliser practices to prevent contamination and excessive plant growth.

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