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Vol. 9  No. 6



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Many toxicity studies have found that dissolved oxygen is the most important parameter for protecting fish and aquatic biota. Regulatory levels can be set for fish and acquatic biota protection at three different levels, for three different purposes. The least stringent would just prevent death, the second level would allow the fish and organisms to be active, and the third allows for proper growth and reproduction. Most standards allow for the third, and most stringent, level of protection.

Clearly dissolved oxygen is an important parameter in water quality; this newsletter will introduce and discuss dissolved oxygen.

1) What is dissolved oxygen?

2) What are typical sources of dissolved oxygen?

3) What processes deplete dissolved oxygen?

4) How do you measure dissolved oxygen?


1) What is dissolved oxygen?

Dissolved oxygen is a measurement of the amount of gaseous oxygen dissolved into the water or aqueous solution. DO is typically expressed in units of milligrams per litre, percent saturation or parts per million.

2) What are typical sources of dissolved oxygen?

There are many different sources of dissolved oxygen, however there are two major sources. They are:

  • Atmospheric reaeration; where oxygen is transported from the air into the water at the air-water interface

  • Photosynthesis; where organisms such as algae and aquatic plants covert CO2 to organic matter, producing oxygen as a by-product of the reaction

3) What processes deplete dissolved oxygen?

There are many processes that deplete the level of dissolved oxygen. Some of these include:

  • Decomposition of organics; bacteria and fungi use oxygen while they decompose biodegradable organics in the water

  • Sediment oxygen demand (SOD); oxygen is used up by the uppermost level of sediment in the stream/river/lake bed.

  • Nitrification; oxygen is used up during the oxidation of ammonia and organic nitrogen to nitrates

  • Respiration; algae and aquatic vascular plants use oxygen during the night to sustain themselves.

4) How do you measure dissolved oxygen?

Dissolved oxygen is a commonly measured in surface waters; in the interest of ecosystem health.  There are a variety of probes and measurement devices available for field measurements. One of these methods is the use of the YSI Multiparameter System.


Drever, J.I. (2002). The Geochemistry of Natural Waters (Third Edition). Published by Prentice Hall, Inc., New Jersey, USA.

Novotny, V. and Olem, H. (1994). Water Quality Prevention, Identification and Management of Diffuse Pollution. Published by International Thomson Publishing, Inc., New York, USA.


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The YSI 556 Multiparameter System is rugged and reliable. It combines the versatility of an easy-to-use, easy-to-read handheld unit with all the functionality of a multiparameter system. The main features of this unit include:

  • Simultaneous measurement of dissolved oxygen, pH, conductivity, temperature, and ORP

  • Field replacement electrodes

  • Compatible with EcoWatch for Windows data analysis software

  • Stores over 49,000 datasets, time and date stamped, interval or manual logging

  • Three-year warranty on the instrument; one-year on the probes

  • GLP assisting, records calibration data in memory

  • Available with 4,10 and 20-m cable lengths

  • IP-67, impact-resistant, waterproof case

  • Easy-to-use, screw-on cap DO membranes

  • RS-232 interface for PC connection

For more info click here:  YSI 556 Multiparameter System
To purchase click here: Order Online

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