Oxygen - Understanding Lake Data

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• DISSOLVED GASES - Page 12

• Oxygen

Oxygen (O2) is undoubtedly the most important of the gases, since most aquatic organisms need it to survive. The solubility of oxygen and other gases depends on water temperature. The colder the water, the more gases it can hold. Boiling water removes all gases. Table 5 shows this effect for oxygen in typical lake water temperatures.

Temperature       Oxygen solubility
     Deg.C   Deg.F    (mg/l)
	 0	32	15
	 5	41	13
	10	50	11
	15	59	10
	20	68	 9
	25	77	 8

The values in Table 5 are found in lakes where continuous mixing occurs, allowing free oxygen exchange between water and the atmosphere. (The atmosphere contains about 21 % oxygen.) However, the levels often differ greatly from the values found in Table 5 because mixing is seldom complete. Ice cover dramatically reduces mixing. In addition, biological reactions in the lake consume or release oxygen.

Oxygen is produced whenever green plants grow. Plants use carbon dioxide and water to produce simple sugars and oxygen, using sunlight as the energy source. Chlorophyll, the green pigment in plants, absorbs sunlight and serves as the oxygen production site. This process is called photosynthesis (Equation 1).

EQUATION 1. Photosynthesis AND EQUATION 2. Respiration

Photosynthesis occurs only during daylight hours and only to the depths where sunlight penetrates. The amount of photosynthesis depends on the quantity of plants, nutrient availability, and water temperature. Higher temperatures speed up the process. Plants and animals also constantly use oxygen to break down sugar and obtain energy by a process called respiration, basically the reverse of the photosynthetic reaction as shown in Equation 2. Burning fossil fuels or other organic matter produces the same chemical reactions shown for respiration, releasing more carbon dioxide (CO2) to the atmosphere.

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The combination of these two reactions largely determines the amount of oxygen and carbon dioxide present in lakes at different times of day and at different depths. During daylight hours, it is not uncommon to find oxygen values in surface waters that exceed those listed in Table 5 (supersaturation), while at night or early morning before photosynthesis begins they may fall below those values. At lake depths below the reach of sunlight, the only reaction that occurs is oxygen-consuming respiration. The deep hypolimnic waters of productive lakes often experience oxygen depletion. Lakes with high biological activity undergo greater fluctuations than lakes with few plants and animals.

Typical oxygen levels in a productive lake following summer stratification are shown in Figure 9. Low oxygen levels in the hypolimnion mean that fish must live in the epilimnion and metalimnion. Fish (trout) that need high oxygen levels and cool water disappear from such lakes.

FIGURE 9. Typical oxygen and nutrient status of mesotrophic and eutrophic lakes after summer stratification.

Winter oxygen depletion (winterkill) is a common problem in many shallow Wisconsin lakes. It happens in years when at least four inches of snow cover the lake, which prevents sunlight from reaching the water. All photosynthesis stops and plants begin to die and decompose. The extent of oxygen loss depends on the total amount of plant, algae and animal matter that decays. Drought increases the chance of winterkill by reducing the volume of water in the lake.

The water quality standard for oxygen in "warm water" lakes and streams is 5 mg/l. This is the minimum amount of oxygen needed for fish to survive and grow. The standard for trout waters is 7 mg/l. A smart angler would know that the lake in Figure 9 contains no trout and that it would be silly to fish for walleye in the deep holes in late summer. (See Equation 3.)

EQUATION 3. Carbon Dioxide Reactions.

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For more information on this topic, contact:

James Vennie
Watershed Management
(608) 266-2212

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