How much oxygen in a water sample is dissolved oxygen (O2). Oxygen can be dissolved into the water in two ways: from plants in the water that produce oxygen through photosynthesis and the amount of movement of the water. Because of these factors, dissolved oxygen tends to differ drastically from each body of water. For example, water with more biodiversity is expected to have more dissolved oxygen (DO) than bodies of water with less biodiversity because they lack organisms that can perform photosynthesis, Scientists must take this into consideration when doing tests. In general though, 0-2 mg/L is too little, 4-7 mg/L is on the lower side of average, 7-11 mg/L is the ideal for unpolluted water, and above that is too much. Though the dissolved oxygen of a sample may be low, some species can tolerate it, especially fish that live in water with higher temperature levels. Depending on various conditions such as movement of water and temperature of water, dissolved oxygen changes on an individual basis. Low levels of dissolved oxygen force fish who cannot tolerate levels that are below 7 mg/L to swim to other bodies of water with higher levels of oxygen, thus decreasing populations of fish within a community and indirectly affecting other species who depended on those fish to survive. Despite this, low levels of oxygen can affect bodily functions of fish in response to stresses like low levels of dissolved oxygen. For example, in order to preserve oxygen, the heartrate of fish slows down and more water goes through their gills to obtain as much oxygen possible. With high levels of dissolved oxygen, fish could acquire “gas bubble syndrome” which is when oxygen bubbles slow down the cardiovascular system by blocking blood vessels of fish, causing death. Because dissolved oxygen is important to fish communities, an increase or decrease to dissolved oxygen can wipe out key species in ecosystems that other fish rely on, affecting the whole ecological chain. In order to measure dissolved oxygen, the Winkler Method is used. It takes advantage of the fact that iodide negatively charged ions oxidize to iodine and that the resulting amount of iodine determines how much oxygen is in the water with a measuring process called titration. The procedure to do the Winkler Method first requires that the scientist fix a sample of water by adding reagents like Manganese sulfate, alkali-iodide-azide, and concentrated sulfuric acid. These reagents essentially form a chemical reaction where an acid is produced. In order to create a color change as to measure the dissolved oxygen, another reagent (sodium thiosulfate) must be put into the resulting acid, all part of titration. Lastly, the scientist then calculates how much titrant it takes for the water to transition from a blue color to clear using the measurement that 1 mg/L of oxygen is the same as 1 mL of titrant put into the sample.