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Exploration of Primary Productivity and Dissolved Oxygen

We investigated dissolved oxygen, primary productivity in relation to depth and enrichment by nitrogen and phosphorous. We read the lab manual and were supplied results. The results, not surprisingly, affirmed all of the hypotheses insinuated by the lab manual. Further study is required to verify the results and determine to what extent the results hold.


The lab has been divided into three major parts. They explore dissolved oxygen, primary productivity in various enriched solutions, and primary productivity versus a simulation in depth.

Dissolved oxygen is the oxygen dissolved in water. Since fish and other animals a dependent on dissolved oxygen, it is an important measure of the habitability of a particular environment. Dissolved oxygen varies according to dissolved solids, pH, salinity, and temperatures. To measure dissolved oxygen, one can use a electronic probe or one can use a kit version. The specific version mentioned in the book uses manganous hydroxide, alkaline iodide, sodium thiosulfate, and starch solution. The sample being tested is mixed with manganous hydroxide and alkaline iodide. Free iodine is proportional to the initial dissolved oxygen. Titrating with sodium thiosulfate, the iodine is consumed. The starch serves as an indicator when all the iodine has been used. Thus we can measure the amount of dissolved oxygen.

Productivity is how much biomass is produced. An easy way to measure this is the production of dissolved oxygen. By having a dark bottle, we are able to see how much is lost through respiration and then calculate the gross productivity of algae. Nitrogen and phosphorous are components of various biomass and thus should increase productivity.

By simulating various depths by attenuating light, examination of the various changes in productivity by depths are possible. Also by adding nitrogen or phosphorous supplements, effects of those elements on productivity is also possible.

Materials and Methods

Using great care to avoid splashing and otherwise adding more oxygen to the solution, the procedure recommends using the stopper to displace excess water. After taking its temperature, it suggests adding 2 mL of manganous sulfate with the pipette tip below the surface of the water. Then it suggests adding 2 mL of alkaline iodide in the same manner. After shaking and waiting for the precipitate to drain, the manual recommends that the instructor, wearing gloves and goggles add 2 mL highly concentrated sulfuric acid to the bottle. After some more inversion of the bottle, titration with standardized sodium thiosulfate is performed with starch as an indicator.

Several hazards go not mentioned in the lab book. Alkaline iodide seems to be a basic solution and thus should be treated with more care. Also material data safety sheets should be consulted on the sodium thiosulfate before proceeding. The lab manual is equally vague about the concentrations of the solutions involved. The solutions should be clearly labeled so as to aid any cleanup of a hazardous spill. Use of lab aprons and goggles for this lab would most likely be a good precaution.

Using plastic window screens, we can simulate the attenuation of light. Temperature must be kept constant during the experiment. The algae cultures are treated with nitrogen, phosphorous, or should be treated with 1mL of distilled water. After a period of a day, the bottles were stopped by adding manganous sulfate, alkaline iodide, and mixing them by inversion. Then each bottle is titrated. Bottles were either treated to darkness, nitrogen, or phosphorous. The plain bottle in the light, dark, and without the algae sample are used as benchmarks for the other samples.


The results were merely provided to us. We have no way of verifying that these numbers are accurate other than performing the experiment ourselves. No margins of error were given on the numbers. Furthermore, the conversion formula between the mL titrated to the mg of dissolved oxygen was not provided and is not needed as the supplied data does not require the student to perform the conversion. Furthermore, the determination of the percent saturation of dissolved oxygen was also performed for us, even though a nomograph was provided without formulaic justification.

Temperature (°C)  Mean DO         % DO Saturation   

5                 9.3             >0                

20                8.9             95                

30                8.5             110               

Table 1: DO Concentration Data Sheet

The net productivity is the light bottle minus the initial bottle, the one without the algae sample, divided by the time elapsed. The gross productivity is the light bottle minus the dark bottle divided by the time elapsed. The respiration rate is the initial bottle minus the dark bottle divided by the time elapsed. The units for the measurements are . Once again, all the calculations have been precomputed in the supplied data.

% Light                   Gross                               Net                 

            N             P          -           N            P           -           

100         0.19          0.16       0.15        0.10         0.07        0.06        

65          0.18          0.15       0.15        0.09         0.06        0.06        

25          0.14          0.13       0.13        0.06         0.04        0.04        

10          0.10          0.40       0.10        0.01         0.01        0.01        

2           0.07          0.06       0.06        -0.03        -0.03       -0.03       

Table 2: Productivity at different light intensities for different nutrient solutions

Respiration is given as 0.09 .

Graph 1: Net productivity versus light intensity

For the comparison of the hypothetical ponds, converting the units: = 261.75. The graphs are appended in the back with depth as a negative number to kludge the computer to plot properly. Please note the differing scales.

            Tiger Paw Lake                         Bulldog Pond              

Depth (m)       Gross Productivity      Depth (m)      Gross Productivity      

              0                 39.2625              0                 39.2625 

            0.5                 39.2625            1.5                 39.2625 

            1.5                 34.0275              4                 34.0275 

            2.5                  26.175              7                  26.175 

              4                  15.705             11                  15.705 

Table 3: Gross productivity with depths of hypothetical ponds - converted values

Respiration is 23.56 .


As implied by the introduction, we expected the temperature to lower the dissolve oxygen. The data affirms this observation. Further data is needed to quantify and determine the exact relationship.

  • 1. Temperature affects the solubility of oxygen inversely. An increase in temperature makes oxygen less soluble.
  • 2. Salinity affects the solubility of oxygen inversely. An increase in the salinity makes oxygen less soluble. This was not observed by the data but is merely a regurgitation of the graph found in the lab manual.
  • By the Graph 1: Net productivity versus light intensity, it seems that the control sample was nutrient limited at the highest illumination levels. All the samples show the effects of limited light at all levels of light. It must be shown by another experiment that the 100% light intensity used here is not the lighting for the maximum level of productivity.

    1. The Bulldog Pond is more productive as it produces more per column of water because its columns are just plain longer than those of Tiger Paw Lake. The light penetrates more allowing more algae to produces good organic products.
    2. The Bulldog Pond's compensation depth, where respiration equals productivity is approximately 9 m. The Tiger Paw Lake compensation depth is approximately 3 m.
    3. Turbidity lowers productivity given the hypothetical example of the two bodies of water.