Dissolved Oxygen
What is Dissolved Oxygen?
Dissolved oxygen (DO) is all of the oxygen gas that is floating around in the water. It is essential for marine fish, mammals, plants, and other organisms to breathe. If the concentration of oxygen drops to between 3 to 5 mg/L, marine organisms are stressed. If the amount of oxygen drops below 3 mg/L, fish and other marine organisms can die.
What Factors Impact the Amount of Dissolved Oxygen?
Dissolved oxygen enters the water through three primary methods: exchange with the air along the water’s surface, the release of oxygen through photosynthesis of marine plants, algae and phytoplankton, and by splashing and waves that mix air with the water.
The temperature of the water, salinity, water clarity, and the amount of organisms using the oxygen can change the amount of DO in the water. Cold water has the ability to hold more gasses than warm water. That is why cold soda is fizzier than warm soda. High water temperatures can result in low DO levels. Salinity has a similar relationship to DO. The higher the salinity, the less oxygen the water is able to hold.
Impacts of Salinity and Temperature on the Amount of Dissolved Oxygen that Water Can Hold
Water clarity (turbidity) relates to the amount of dissolved oxygen due to the impact it has on plants and algae. Clear water allows in the most sunlight, so plants are able to complete more photosynthesis and release more oxygen into the water. Less clear water blocks the sunlight, limiting the amount that plants can photosynthesize.
Organisms can also use up the oxygen in the water. This process is called eutrophication. It typically happens when there is an overpopulation of certain organisms and then a large die off of those organisms. Examples of this include a large fish kill or excessive amounts of algae that die off at the end of the season. These dead organisms then begin to rot. This means that there are millions of bacteria living and breathing as they break down the remains of these organisms. The bacteria use large amounts of oxygen, resulting in unhealthy, low DO levels.
Because of the factors that impact dissolved oxygen, it changes throughout the day. Plants only photosynthesize during the day, so just before dawn dissolved oxygen levels are typically low. No photosynthesis has been happening all night, but organisms have still been breathing and using the oxygen. In the middle of the day, when the sun’s rays are shining most directly on the plants and algae, the most oxygen is being produced.
In warm summer weather, the high amounts of oxygen in the middle of the day are tempered by the high temperatures in the middle of the day. The resulting increases in water temperature decrease the amount of available oxygen.
Testing Dissolved Oxygen
We will be testing dissolved oxygen using the Winkler Titration method.
This Method is a two part process. The first part ‘fixes’ the amount of oxygen in the water, preventing it from escaping or being added from the surrounding air so that the amount you measure is accurate for your sample site. To fix the oxygen, two solutions are added. They contain Mn+2 ions and OH– ions that bond to form a white compound that settles to the bottom of the sample. All the dissolved oxygen in the sample reacts with this white compound to form new compounds that are brown in color. The oxygen is then trapped in those compounds and can’t escape into the air. A strong acid is then added that dissolves all of the compounds. At this point, the amount of oxygen in the sample is stable. As the compounds dissolve and form new chemicals, iodine (I2) is released, two iodine molecules for each dissolved oxygen molecule that was in the water.
The second part of this process measures the amount of iodine (and the corresponding amount of oxygen). The iodine will have turned the sample a yellow/brown color. Into a measured amount of the treated sample (25mL), sodium thiosulfate () is added in measured drops. The sodium thiosulfate bonds with the iodine to form new compounds, two sodium thiosulfates used for each iodine. This removes the iodine from solution and the yellow/brown color lightens. Before the iodine color disappears, a starch is added that is blue when iodine is in the sample, to make the color change easier to observe. When all the iodine is gone, the blue color will disappear. Because you will have measured how much sodium thiosulfate you used to make the iodine disappear, you will be able to figure out exactly how much oxygen was present in the water. The units on the titrator that is used to add the sodium thiosulfate are set up so that 1 unit on the titrator equals 1mg/L of dissolved oxygen.