In my last column, I examined the structure of a lake and how in some years some lakes can suffer a winter or spring kill. I want to expand on this theme of lake stratification and describe how a lake can suffer a summer kill as well.
I have been in contact with Ron Bowron, the local fisheries technician, who works for the Ministry of Water, Lands and Natural Resources in Williams Lake. Ron is responsible for, among other things; installation, operation & maintenance of aerators on Simon, Dewar, Skulow and Irish Lakes. This includes taking oxygen samples periodically to monitor oxygen levels in these and other lakes in the Cariboo Region.
Ron is also instrumental in capturing mature Horsefly Strain Rainbows for broodstock in hatcheries. As I mentioned in my column dealing with rainbow trout strains, the Horsefly Strain Rainbows are fast-growing as well as strong fighters but are not known for jumping. They do best in multi-species lakes feeding mostly on other fish (known as being piscivores) where they can reach a large size.
Ron said “when submitting anything to do with aeration my title is ‘Head Aeration Technician’ - British Columbia Conservation Foundation.”
He also went on to say that “anything to do with Population Dynamics/Broodstock Development my title is ‘Fisheries Field Technician’ – FLNRO” (Ministry of Forests, Lands and Natural Resource Operations).
Ron is quite knowledgeable when it comes to lakes and lake environments. I have included his explanation of lake stratification as a refresher to my last column. He states “thermal stratification, resulting from heating from the sun, is the most important physical event in a lake's annual cycle and dominates most aspects of a lake's structure. Heating decreases the density of the upper water, which, in combination with the wind, results in a three- layered system. If the lake is deep enough, summer stratification produces an upper warm, lighter layer - the epilimnion; a cool denser layer - the hypolimnion; and a transitional zone between them - the metalimnion. The region of greatest change in temperature is called the thermocline. In many temperate lakes the seasonal thermocline spans a difference of 10 or even 15 degrees Celsius. Shallow lakes, due to the unique temperature-density relationship of water, have stable stratification with a thermocline spanning only one to three degrees Celsius.”
He further explains that “the parent, or seasonal thermocline, is not fixed in depth. It gradually descends during the summer until the lake turns over in autumn. The seasonal thermocline occurs in the metalimnion.
"During the summer the volume of the epilimnion increases at the expense of the hypolimnion. Finally de-stratification occurs in autumn as heat loss from conduction and evaporation exceed the supply of solar heat. The difference in temperature and density between the overlying water and that beneath is so slight that a strong wind overcomes the remaining resistance to mixing and the lake undergoes the fall overturn. The stratified structure is lost, and the lake becomes homothermous; that is, it has a uniform temperature from surface to bottom.
:In cold climates mixing and further cooling continue until the surface of the lake reaches zero degrees Celsius and freezes. Dimictic lakes (lakes that mix twice, once in the fall and once in the spring) are covered in ice in the winter and exhibit what is called inverse stratification. At this point, the epilimnion is zero degrees Celsius and the hypolimnion is warmer in relation to the amount of decay occurrence (to a maximum of four degrees Celsius). Water is most dense at four degrees Celsius, therefore, in the spring, once the ice melts the wind mixes the cold water until the water temperature increases sufficiently to reestablish thermal stratification, the spring turnover begins, and the cycle repeats itself.”
The condition that occurs in the late summer that causes a fish kill in shallow lakes results from the epilimnion descending to a greater than normal depth. Oxygen transfer. if you recall, takes place from the interface between the air and lake surface, and the cooler the water the more oxygen is introduced into the lake. When the surface temperature rises, very little oxygen transfer takes place and at times it is low enough to no longer provide enough to sustain fish. This can result in a “summer kill”.
Please refer to Ron’s graph showing temperature relationships to lake stratification.
