Dissolved Oxygen
an excerpt from our July 2004 Newsletter

What is Dissolved Oxygen?
In water, oxygen is found as a dissolved gas.

Why is it important?

Aquatic organisms depend on dissolved oxygen to live. If there is not enough oxygen in the water, these organisms will move, weaken, or die.

How do we measure Dissolved Oxygen?

Dissolved oxygen can be measured in two ways:
either in milligrams per liter (mg/L) or percent saturation (% sat).

Milligrams per liter is the amount of oxygen in a liter of water. Percent saturation is the amount of oxygen in a liter of water relative to the total amount of oxygen that the water can hold at that temperature. At Stream Team, we use the YSI 55 Dissolved Oxygen meter, which measures both mg/L and % sat.

What factors affect Dissolved Oxygen?

Oxygen is both produced and consumed in a stream, so there are many factors affecting it:
•Because of constant churning, running water dissolves more oxygen in a stream than the still water found in pools.
Therefore, faster flow conditions help increase dissolved oxygen levels.
•During the day, aquatic plants and algae release oxygen underwater during photosynthesis. For this reason, dissolved oxygen is at a maximum in the late afternoon of a sunny day.
•Throughout the night, aquatic plants and algae consume oxygen during respiration. For this reason, dissolved oxygen is at a minimum in the early morning.

What are expected Dissolved Oxygen levels?

Target dissolved oxygen levels vary depending on the organism of concern,
because different types of aquatic life need different levels to survive. As dissolved oxygen levels in water drop below 5 mg/L, most aquatic life is put under stress. The lower the concentration, the higher the stress. Colder water fish (such as trout) need levels above 6 mg/L, and may require levels above 7 mg/L for spawning. Warm water fish can usually tolerate levels as low as 4 mg/L. If oxygen levels dip down to 1-2 mg/L, even only for a few hours, it can result in large fish kills. Because dissolved oxygen is lowest in the early morning hours, this is the best time to determine whether oxygen is declining to dangerous levels.

Stream Team Dissolved Oxygen results

Dissolved oxygen concentrations on the Goleta Creeks and Ventura River from April through June 2004 are shown on the figure found in the respective July 2004 Newsletter. The grey bars indicate the mean (average) of all measurements since June 2002; it’s shown with “error bars” – lines representing the standard deviation of measurements above and below the average. Approximately 2/3 of our monthly measurements would fall within the error bar limits (for example, 2/3 of all the CG1 dissolved oxygen measurements have been between 7 and 12 mg/L and 2/3 of all the VR02 dissolved oxygen measurements have been between 7 and 14 mg/L). standard deviation of measurements above and below the average. for example).Statisticians use two-times the standard deviation as a test to determine if a measurement is either “normal” or “unusual.” A measurement more than twice the standard deviation (2 error bars above and below the average) would normally occur less than 5 % of the time – about once every two years.

Using this yardstick, all of our measurements over the past quarter are “normal.” The next question is how good is “normal”? Fish, like humans, have to breathe, but they have a harder job since their “air” comes from oxygen dissolved in water: oxygen makes up 20 % of the atmosphere, but is rarely found in concentrations above 10 parts in a million in water (10 parts per million, or ppm, is the same as 10 mg/L, the units used in the chart). Small changes in oxygen concentration can make the difference between
life and death. Concentrations above 8 mg/L (the dashed line on the figure) are ideal for salmonid fishes like steelhead and trout; as levels drop below 5 mg/L survival becomes increasingly difficult; below 3 mg/L they're dead. So our results look pretty good. Or
maybe not. Oxygen concentrations do not stay constant during the day. Aquatic plants and algae photosynthesize during daylight hours: removing carbon dioxide from the water and replacing it with oxygen. Unfortunately, at night they respirate (like us and fish), reversing the process and reducing oxygen concentrations. The critical time is just before day break, and in the summer when water levels are low and the stream choked with plants and algae, we would need to be out very early to check if things are really OK.