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Water is considered freshwater when it contains less than 1,000 milligrams of dissolved solids. But water is considered undesirable for consumption and industrial usage when it contains more than 500 milligrams per liter.  If the percent concentration of total dissolved solids is greater than the percent concentration of dissolved oxygen, eutrophication may begin to occur. The proportions difference between each is very important to maintaing a healthy ecosystem. Algal blooms, as well as fertilizer pollution increase total dissolved solids, in turn decreasing oxygen concentration significantly
Source: https://water.usgs.gov/edu/dictionary.html

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Unsafe concentrations of dissolved oxygen can be extremely dangerous and lead to severe environmental change. Without dissolved oxygen in water, it is almost impossible for an aquatic ecosystem to exist. Therefore, as dissolved oxygen concentrations begin to dip, aquatic life begins to become unable to sustain itself and fish and others creatures will begin to die. This also leads to large amounts of decomposition occurring within water sources leading to further dissolved oxygen decreases.
Link:
https://water.usgs.gov/edu/dissolvedoxygen.html

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Dissolved oxygen and water temperature have an inverse relationship. As the water gets colder (Winter) the dissolved oxygen content goes up, and as the water gets warmer (Summer) the dissolved oxygen content goes down. Source: https://water.usgs.gov/edu/dissolvedoxygen.html

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There are many old methods of measuring dissolved oxygen, but the most modern ones are small, mostly electronic, and are called DO2 meters. They use a probe that goes into the water and, since dissolved oxygen and temperature have an inverse relationship, needs to be recalibrated after every use. Dissolved oxygen is also measured in terms of mg/L or ppm.
Source:
https://water.usgs.gov/edu/dissolvedoxygen.html

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A healthy concentration of dissolved oxygen would be approximately 5-10 mg/L. However, classification for water bodies based on oxygen content is based on a 0-60 mg/L. This concentration of oxygen allows organisms such as zooplankton to survive, which in turn supplies fish and other creatures with a sustainable food source. Without dissolved oxygen, the entire food chain is disrupted within a body of water, making it impossible for life to exist.
Link:
https://water.usgs.gov/edu/dissolvedoxygen.html

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Point source pollution is defined as  “any single identifiable source of pollution from which pollutants are discharged, such as a pipe, ditch, ship or factory smokestack”. This type of pollution lowers the dissolved oxygen content since the water source is filled with bacteria from pollution, which feeds on the dissolved oxygen.
Link:
https://oceanservice.noaa.gov/education/kits/pollution/03pointsource.html

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Algal Bloom is a rapid increase in algae in fresh water or marine water systems. Algal booms are produce by large amounts Cyanobacteria. Algal blooms can be toxic to both fish and humans, by contaminating the water with bacteria.  Algal blooms also reduce the amount of oxygen in the water, as algae itself consumes oxygen. Not to mention an overabundance of algae would consume extraordinary amounts of algae and be unhealthy towards the environment. The majority of algal blooms are directly linked to the pollution of fertilizers from lawns and/or farms. The fertilizer significantly increases the growth of algae and its ability to excessively grow in water bodies.
Link:
https://water.usgs.gov/edu/qa-chemical-fishkills.html

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Eutrophication is the process by which a body a water can die. Death, in this context, means it becomes uninhabitable. Life is no longer able to exist within the body of water. This is a result of the lack of dissolved oxygen within the water. Without this dissolved oxygen, aquatic organisms are unable to breathe or generally survive. Eutrophication can be caused by pollution, specifically fertilizer pollution as algal blooms decrease the oxygen concentrations of water.
Link:
https://water.usgs.gov/edu/dissolvedoxygen.html

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Dissolved oxygen is the oxygen concentration existing within a water solution. Oxygen in this case is the solute, or what is dissolved into water.

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