2. Dissolved Oxygen (DO)
Fish breathe oxygen through their gills and if there is no oxygen in the water they die. The amount of oxygen dissolved in the water can readily be measured while one is taking the temperature. The GLA has such a meter that could be made available to others who want to try their hand at monitoring the oxygen content in Garner Lake. The data for the three years 1994-1996 look as follows:
These three plots show that the oxygen is sucked out of the water at the depth of the thermocline. The technical term for this phenomenon is a metalimnetic minimum. I haven’t checked whether it is as severe now as it used to be. The cause is well known. Rotting material sinks in the lake but because the density of the water is greater where the temperature is less it means it sinks much slower there and begin to accumulate and suck in lots of oxygen. At that level decomposition uses up oxygen to the point where it is almost totally removed.
Meanwhile decomposition of organic stuff at the lake bottom begins to use up the oxygen there as well and in the above plots the red area (no oxygen) moves up over time to meet the thermocline that descends ever deeper as the summer wears on.
When the lake turns over again, at the end of November usually, the lake waters again become fully oxygenated (blue color).
Fish do not like DO levels below about 2 ppm. Nor do they like very warm water. Some years when the weather is hot for long periods the surface layers get too hot for fish comfort and the lower levels are depleted of oxygen. That doesn’t leave the fish with much lake volume to flourish, they become stressed and may die. That creates a fish kill, which we have seen a few times in the 19 years I have lived here. Again, that is Nature in action.