Question

causes of low dissolved oxygen in water

Answer 1

Low dissolved oxygen (DO) primarily

results from excessive algae growth caused

by phosphorus. Nitrogen is another nutrient

that can contribute to algae growth. As the

algae die and decompose, the process

consumes dissolved oxygen. This can

result in insufficient amounts of dissolved

oxygen available for fish and other aquatic

life. Die-off and decomposition of

submerged plants also contributes to low

dissolved oxygen. The process of

decomposition is called Carbonaceous

Biochemical Oxygen Demand (CBOD).

Sources of phosphorus include discharges

from municipal and private wastewater

treatment, cropland and urban storm water

runoff, and natural decay of vegetation.

Direct discharge of pollutants from point

source and nonpoint sources into a river

segment add to its CBOD loadings,

creating an oxygen demand that may

D

Low Dissolved Oxygen in Water • Water Quality/Impaired Waters 3.24 • February 2009

page 2

depress DO below acceptable concentrations. Nutrient

levels can in certain rivers occasionally cause sufficient

eutrophication to generate CBOD loads from decaying

algae. This may not occur locally, but instead farther

downstream in pools where the current slows and algae

collect.

Answer 2

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Dissolved oxygen levels below 3 ppm are stressful to most aquatic organisms. Low dissolved oxygen levels can be the result of elevated temperature and thus the inability of the water to hold the available oxygen. Low dissolved oxygen levels can also indicate an excessive demand on the oxygen in the system.

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