Water Temperature

How is water temperature measured on the vessels?

Usually water temperature is measured on the vessels using thermometers that are mounted on the sampling bottles. The thermometers on the sampling bottles must be read immediately after the bottles are retrieved. Temperature is recorded in degrees Celsius (°C). A bathythermograph is sometimes used on advanced trips (see Appendix D). The bathythermograph records a continuous temperature profile of the water column.

On the day of a cruise, one can get an indication of the surface water temperature on Lake Michigan by querying the Michigan Sea Grant Coast Watch. An example of the surface temperature data is found in Figure 8, which is based on information from the Coast Watch Internet Site.

What is the significance of temperature data?

Several interesting studies can be made from the data obtained from the temperature readings. Most prominent is the identification of temperature stratification at various depths and the relationship to the seasons of the year. Stratification refers to distinct layers of water that form in the lake. The stratification, or lack of it, is associated with vertical motion of water in a lake.

The temperature readings also give an indication of whether conditions are favorable for cold water fishes. The rates of metabolism of cold-blooded animals, as well as rates of photosynthesis, are temperature sensitive. The migration of fish and their spawning behavior are associated with temperature changes. Temperature and dissolved oxygen are related in that warmer water holds less oxygen than cooler water.

How does the lake temperature vary throughout the year?

In Lake Michigan, the temperature of the lake is essentially uniform from top to bottom two times a year, spring and fall. When this occurs, wave action on the surface will mix oxygen in the air with the water and the oxygen-rich water is driven down to lower depths. The bottom oxygen-poor water will be brought to the surface where it can be replenished with oxygen. Since this occurs twice a year in spring and fall, Lake Michigan is called a dimictic lake; "di" meaning two and "mictic" meaning "to mix". Turnover is not uniform in a lake and strong winds can cause upwellings where nutrient rich deep water moves towards the surface.

At other times of the year, the water temperature of a large body of water like Lake Michigan can be understood by looking at how the density of water is related to temperature. Water freezes at 0°C (32°F) but it has its greatest density at approximately 4°C (39.2°F). Less dense water will float on top of dense water, that is, water colder or warmer than 4°C will float.

During winter, Lake Michigan achieves a relatively uniform temperature from surface to bottom with slightly colder water near the surface. If covered with ice, the water just beneath the ice would be slightly above the freezing point, and would increase to no more than 4°C (the temperature of maximum density) towards the bottom of the lake. It is because of the direct relationship between water temperature and density that ice floats on the surface of a lake. The ice layer provides protection from mixing of water by wind action, inhibits diffusion of oxygen and transmission of light to the water below, and prevents lakes from freezing from the surface to the bottom.

The relationship between water temperature and density also plays a determining role in lake conditions during the summer. Remember that above 4¿ C, the water density decreases and this warmer, lighter water will tend to float on colder, heavier water. Hence, as heat from the sun increases during the summer, the upper waters of the lake become warmer and lighter than deeper waters. The energy of the wind may be inadequate to mix the upper water with the colder, more dense, deeper water. This situation leads to a summer stratification period with the warmer water separated from the deeper, much colder, bottom water (Figure 9).

In the summer, lakes show thermal stratification, which means that there are distinct layers defined by temperature. When stratification occurs, the different layers are given names to identify their location. Warm water is found in the top layer (epilimnion) followed by a deeper layer where there is a temperature drop (thermocline or metalimnion). Colder, heavier water is found in the bottom layer (hypolimnion).

One very important consequence of summer stratification of a lake is that circulation due to wind action is largely confined to the upper water mass known as the epilimnion. Because the lower water mass is isolated from the atmosphere and receives little, if any, sunlight, dissolved oxygen is not replenished in this water mass. The dissolved oxygen may diminish to such a level that it limits aquatic life. However, oxygen will be replenished at the end of summer when water temperatures become more uniform and the wind circulates all of the water in the lake basin.

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