Background Info

Oxygen is necessary for all living creatures whether aquatic or terrestrial. It is essential for respiration as well as photosynthesis, however it is crucial that a stable level is maintained due to the fact that very high or low concentrations can be fatal. Due to the fact that water cannot hold as much oxygen as the air it is extremely important that an aquatic environment have a healthy level of dissolved oxygen in order to maintain life. With increases in temperature the ability of water to hold oxygen decreases, so aquatic environments must also rely on wind and turbulence to mix oxygen into the water. Without these forces oxygen depletion can be fatal.
The Primary Productivity of an aquatic ecosystem is measured by the energy that is both used and stored by plants. The total amount of photosynthesis that occurs is referred to as gross primary productivity. Plant respiration, however, is equally important because it provides free energy that is used for production, maintenance, and reproduction. Net Primary Productivity, or plant growth, refers to the energy that is left after respiration and storage.
The objective of this lab was to measure the dissolved oxygen concentration of various water samples as well as calculate gross productivity, primary productivity, and net productivity of each sample. Through various experimental setups the effects of biological factors such as light were also recorded and measured in relation to their affect of the solubility of gases in an aquatic environment. In order to ensure that the levels remained relatively protected is was important to cap the bottles as soon as a sample was taken or after chemicals were added to it.
To determine the dissolved oxygen levels the water samples were collected in small sampling bottles and capped underwater to ensure that oxygen from the air was not affecting the samples. The temperature was measured and then eight drops of manganous sulfate was added, followed by eight drops alkaline iodide. After this the sample was inverted several times, allowing it to mix. Once the precipitate had settled acid was added and the sample was inverted again several times, turning the sample a clear yellow. This indicated the formation of free iodine. Next 20 ml of the sample was measured out and eight drops of starch indicator was added to the 20 ml sample. This changed the sample to purple. Next sodium thiosulfate was titrated into the sample, one drop at a time until the sample turned clear. At this point the free iodine had been converted into sodium iodide. To determine the dissolved oxygen concentration we observed how much sodium thiosulfate had been used to convert the free iodine.
The Primary Productivity was measured in various samples by reducing the levels of natural light to 65%, 25%, 10% or 2% from 100%. They were then left overnight and the next morning the dissolved oxygen concentration was measured by repeating the experimental steps used in part A. Through that the Gross Primary Productivity, Net Primary Productivity, and Respiration rates were calculated. The Gross Productivity levels were then converted to determine the levels of carbon productivity. Using a nomograph the percent saturation was measured, using water temperature and dissolved oxygen levels.
Through each of these measurements we were able to determine the health of each sample of water taken from an aquatic ecosystem. Because the health of the ecosystems relies on dissolved oxygen it was important to document the levels. Through the measurements of primary, net, and gross productivity we were able to determine how and ecosystem was functioning, or if there was simply not enough dissolved oxygen to support the life within the samples.