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An aquatic system lacking dissolved oxygen (0% saturation) is termed anaerobic, reducing, or anoxic. In water, oxygen levels are approximately 7 ppm or 0.0007% in good quality water, but fluctuate. [5] Many organisms require hypoxic conditions. Oxygen is poisonous to anaerobic bacteria for example. [3]
It is a primary cause of eutrophication of surface waters (lakes, rivers and coastal waters), in which excess nutrients, usually nitrogen or phosphorus, stimulate algal growth. [1] Sources of nutrient pollution include surface runoff from farm fields and pastures, discharges from septic tanks and feedlots , and emissions from combustion.
The Streeter–Phelps equation is also known as the DO sag equation. This is due to the shape of the graph of the DO over time. The biological oxygen demand (BOD) and dissolved oxygen (DO) curves in a river flowing right reaching equilibrium after a continuous input of high BOD influent is added into the river at x = 15 m and t = 0 s.
Name Location Dependent population Description Sources of pollution Impact Bharalu River: Assam, India: One of the most polluted rivers in the state of Assam. [19] The biochemical oxygen demand of the river is 52 mg/L in compared to the permissible limit set by the National River Conservation Directorate (NRCD) at 3 mg/L. [20]
Decreased levels of dissolved oxygen (DO) is a major contributor to poor water quality. Not only do fish and most other aquatic animals need oxygen, aerobic bacteria help decompose organic matter. When oxygen concentrations become low, anoxic conditions may develop which can decrease the ability of the water body to support life.
Algal blooms limit the sunlight available to bottom-dwelling organisms and cause wide swings in the amount of dissolved oxygen in the water. Oxygen is required by all aerobically respiring plants and animals and it is replenished in daylight by photosynthesizing plants and algae. Under eutrophic conditions, dissolved oxygen greatly increases ...
Elevated water temperatures decrease oxygen levels (due to lower levels of dissolved oxygen, as gases are less soluble in warmer liquids), which can kill fish (which may then rot) and alter food chain composition, reduce species biodiversity, and foster invasion by new thermophilic species. [48]: 179 [15]: 375
BOD is directly related to the amount of dissolved oxygen available, especially in smaller bodies of water such as rivers and streams. As BOD increases, available oxygen decreases. This causes stress on larger organisms. BOD comes from natural and anthropogenic sources, including: dead organisms, manure, wastewater, and urban runoff. [15]