The dissolved oxygen levels and quality of water play an important role in supporting aquatic life.  Ten water locations were sampled for both dissolved oxygen levels and water clarity, using a dissolved oxygen testing kit and Secchi depth meter, respectively.  These samples were taken to determine a possible correlation between the two variables.  The data in this experiment do not greatly support a positive relationship between amount of dissolved oxygen present and the clarity of water.  Cloudy, windy weather could greatly affect both oxygen production and water clarity.   

Key words: dissolved oxygen, Secchi depth meter, water clarity

Introduction

            Dissolved oxygen levels are considered the most important and commonly employed measurement of water quality and an indicator of a body of water’s ability to support aquatic life (NALMS 2002).  Similarly, aquatic life is sensitive to water clarity, in terms of how far light is able to penetrate the water (EPA 2002).  With these ideas in mind, this experiment was performed to determine if there is a correlation between the dissolved oxygen levels of a body of water and its clarity measurement.  It was hypothesized that if the water had poor clarity then this might result in depleted dissolved oxygen levels.  Dissolved oxygen refers to oxygen gas that is dissolved in the water (Francis-Floyd, 1992).  Concentrations of 5 parts per million (ppm) is recommended for fish and vegetation health (Francis-Floyd, 1992).  When the dissolved oxygen concentrations fall below 2 ppm, aquatic life becomes stressed, and below 1 ppm aquatic life begins to die (Francis-Floyd, 1992).

Methods and Materials

Ten water samples and ten Secchi depth meter measurements were taken from various areas around Lake Michigan on October 26, 2002.  The first sample was taken near the Yacht Club.  The second sample was taken from the pond located south of the Yacht Club.  The third sample was taken from the Milwaukee River across from Rock Bottom Cafe.  The fourth sample was taken from the Menomonee River. The fifth sample was taken from South Shore Yacht Club.  The sixth sample was taken from Grant Park.  The seventh sample was taken from the Wastewater Treatment Plant area.  The eighth sample was taken near the Pieces of Eight Restaurant.  The ninth sample was taken from Port Washington outside of Smith Brothers Restaurant.  The tenth sample was taken from Cedarburg behind the winery from the Milwaukee River.

            All samples were placed in an airtight bottle and were labeled to prevent any confusion. After all samples were taken they were tested for their dissolved oxygen content. The test method we followed was taken from the Lamotte dissolved water and oxygen test kit.  This kit essentially measures the ppm of dissolved oxygen in the sample through a series of titrating steps.  The end result is the ppm in the sample.

            A Secchi depth measurement was also taken at each site the water sample was taken from. A Secchi disc is used to measure water clarity or turbidity (MERP 2002). The Secchi disk is a weighted white plastic plate measuring 30 cm in diameter and is attached to a line marked off in meter intervals (MERP 2002). The disc is lowered into the water by hand until no longer visible and the depth of disappearance is recorded. The disc is then raised and the depth of first reappearance is recorded as well. The two recorded depths are then averaged to determine the Secchi depth measurement (MERP 2002).

Materials

Meter stick (Westcott)                                Water Sample

10 each Plastic Bottles

Dissolved Oxygen Test Kit (Lamotte)

Secchi Disc and Rope

Results

Table 1. Measurements of Water Clarity & Dissolved Oxygen

Figure 1. Correlation graph for water clarity and dissolved oxygen levels of water

                Samples.

Table 1 shows the measurements taken at each location of water clarity and ppm of dissolved oxygen.  The water clarity column shows the already averaged measurement.  The dissolved oxygen column shows the results from using the test kit.  Figure 1 shows the water clarity measurements versus the dissolved oxygen levels.  A trend line was added to determine R² value, which was 0.0147.  This means there is barely a correlation, although there is an obvious outlier.

Discussion/Conclusion

            We found that our results did not support our hypothesis. We had thought the clearer the sample of water is, the more dissolved oxygen will be present.  We had expected to get more diverse results when it came to the dissolved oxygen present at each of the sites. Perhaps, the fact that it was a very windy, cloudy day played a role in the disruption of water clarity as well as dissolved oxygen content.  During cloudy weather, the intensity of light reaching surface waters is greatly diminished, resulting in a marked decrease in oxygen production (Francis-Floyd, 1992).  In addition, wind mixing in water resuspends sediments and decreases water clarity (EPA, 2002).  If the outlier on our graph (yacht club) could be removed, the data would have a higher R² value, thus indicating more of a correlation between the experiment variables.   

            If we were to perform this experiment again, we would change our sampling method.  First, test the sights several times over a period of 2 months. The changes in weather would provide us with sunny, cloudy, and windy days, which might affect water clarity and oxygen depletion.  Second, we would take the water temperature of the water each time we sampled.  This would tell us if the temperature of the water plays a role in oxygen depletion.

Literature Cited

Environmental Protection Agency (EPA). (2002). More about dissolved oxygen.  EPA Empact UCONN. Retrieved October 20, 2002 from the World Wide Web: http://www.mysound.uconn.edu/tut_disso.html

Environmental Protection Agency (EPA) (2002). Water clarity in mid-atlantic estuaries. U.S. Environmental Protection Agency. Retrieved October 20, 2002 from the World Wide Web: http://www.epa.gov/maia/html/clarity.html

Francis-Floyd, Ruth. (1992). Dissolved oxygen for fish production. University of Florida Cooperative Extension Service. Retrieved October 20, 2002 from the World Wide Web: http://edis.ifas.ufl.edu/BODY_FA002

North American Lake Management Society (NALMS). (2002). Lake and water word glossary. Retrieved October 20, 2002 from the World Wide Web: http://www.nalms.org/glossary/glossary.htm