November 9, 2016 temperature, dissolved oxygen and % saturation profiles, Geneva Lake WI.

The November 9, 2016 temperature and dissolved oxygen readings show the lake has a very thin thermocline's of only is only 6.5 ft. (2 m.) thick located at approximately 68 ft ( 21 m). The thermocline separates the lake's upper circulating warm water from the lake's deep cold waters of the hypolimnion. During the fall, it becomes eroded from the cooling surface water and the action of the fall winds. Within the next couple of weeks it is suspected that it will be completely gone and the lake will be in its fall turnover mixing water from top to bottom. Cold weather which settled in the end of the third week of November along with fall winds could cool off the epilimnion and erode it even faster.

At the time of this sampling the surface temperature at the deepest hole was recorded as 56.5 °F. (13.6 °C.) with the bottom of the epilimnion being 54.7 °F. (12.6 °C). Within the thermocline's 6.5 ft. (2 m.) thickness, water temperature dropped 2.6 °F. (1.4 °C.). The thermocline is defined as that part of the water column where the water temperature decreases at a rate of 1 °C per meter or greater.

Below the thermocline, the temperature dropped from 50.0 °F. (10.0 °C.) at 71.5 ft. (22 m.) to 45.0 °F. (7.2 °C) at the lake's bottom, 138 ft. (42.0 m.).

At the lake's surface, the dissolved oxygen concentration was measured at 9.4 mg/L and decreased to 5.5 mg/L at the bottom of the epilimnion, The relatively consistent dissolved oxygen levels for the top 52.0 ft. (16.0 m.) is most likely from good mixing caused by the fall winds. At the very bottom of the epilimnion, dissolved oxygen levels dropped significantly. As organic matter settles in the lake and hangs at this level because of the colder water being more dense, the organic material is broken down and decomposed. This decomposition process and the "critters" that tend to hang at the layer because of the availability of food, all are using the limited dissolved oxygen in the water. This causes the oxygen level to drop rapidly.

Oxygen concentration then quickly decreased in the thermocline's 6.5 ft. (2 m.) from 5.5 mg/L to 3.0 mg/L at the top of the hypolimnion. With a strong but thin thermocline the deep cold stagnant bottom waters of the hypolimnion have not mixed with the upper oxygen rich water of the surface. Thus dissolved oxygen decreased with depth. Dissolved oxygen levels dropped to less than 2 mg/L. at 100.7 ft. (31.0 m.). Dissolved oxygen continued to drop to practically zero (0.2 mg/L) at the bottom. Fish experience higher levels of stress when dissolved oxygen levels drop below 5 mg/L. Most fish species cannot survive for any period of time where dissolved oxygen is less than 2 mg/L.



Percent saturation of dissolved oxygen refers to the amount of oxygen the water is holding at a given temperature relative to what the water can actually hold at that same temperature. The colder the water the more oxygen it can hold. From a lake management and fisheries perspective the actual concentration of oxygen may not be as important as the percent saturation. Percent saturation helps understand what is happening in the water from an oxygen supply vs. oxygen demand perspective.

During the November 9, 2016 sampling, good mixing and strong winds resulted in the top 52 ft. (16 m) of water was saturated in the low 90%, high 80's. Once below the thermocline the per cent saturation dropped rapidly as the concentration of oxygen dropped. The actual oxygen level decreased due to use by organisms and other bio-chemical processes. This resulted in the percent saturation of oxygen becoming lower with depth. The percent saturation at the bottom of the lake was 1.0%, not uncommon for deep lakes in this part of the county during this time of the year.