There are many factors that contribute to an area of hypoxia. Temperature, salinity, vegetation, and bacteria are just a few of these.
Before these factors are discussed, an understanding of how the oxygen gets into the water is needed. Water holds much less life-giving oxygen than air. Therefore, oxygen levels are extremely important when dealing with aquatic systems. Because oxygen is a byproduct of photosynthesis, plants and algae are the major source of oxygen in aquatic habitats. Another way oxygen gets into the water is through the air-water interface. Since there is more oxygen in the air than the water, the oxygen has a natural tendency to diffuse into water. This process is enhanced by windy conditions, especially during storms. Actively mixing the water to maximize air-to-water contact also helps to add oxygen. This is done, for example, by adding splashing fountains in ponds.
Now that we have established how oxygen can get into the water, lets look at how environmental factors can alter dissolved oxygen concentrations.
Temperature has a very important effect to dissolved oxygen. As the temperature of the water goes up, the water loses the ability to hold the dissolved oxygen and the concentration goes down. When the water cools, it regains the ability to hold higher amounts of oxygen. The chart below simplifies this rule.
Source: Koi Club of San Diego
Knowing this relationship, one can deduce that hypoxia tends to occur in the warmer months of the year, namely during the summer.
Salinity (the amount of dissolved salts in the water) has very interesting affect on the amount of oxygen in the water. First, there
is an inverse relationship between the amount of salt versus the amount of dissolved oxygen in the water. 5 grams of salt in 1000 grams of water (5 ppt) will decrease the oxygen saturation levels about 1 mg/l.
Salinity also has another property that helps to create hypoxic zones. Salt water is more dense than fresh water. This causes two layers of water to form a lighter layer of fresh water on the top of a heavier layer of salt water on the bottom. This prevents adequate mixing of the water column and does not allow oxygenated water to get to the lower depths. Therefore the heavier, saltier layer at the bottom may become oxygen-depleted.
Most ponds and lakes are fresh water and are free from these salinity considerations.
This image shows how salinity can affect hypoxia. The red dots represent the relative concentration of dissolved oxygen during a hypoxia event. Notice that due to inadequate mixing, there is lower concentrations of oxygen in the saltier bottom layer of water.
Vegetation adds oxygen to the water. Plants undergo photosynthesis and release oxygen, so all aquatic plants play an integral role in maintaining healthy levels of oxygen. Also remember that phytoplankton use photosynthesis, too, and add oxygen to the water. Even though most phytoplankton is microscopic, these organisms are extremely important.
This is a simplified schematic of photosynthesis. Note how oxygen is released as a byproduct of the reaction.
Bacteria can have a devastating effect on the amount of dissolved oxygen. Bacteria feed on this decaying material as it sinks to the bottom. These bacteria respire and use oxygen just like fish. So with billions and billions of bacteria respiring, the dissolved oxygen level may dramatically drop to dangerous levels. Bacteria play an integral role in the hypoxia zone that occurs each summer off of the coast of Louisiana.