Water Temperatures and Turnover
Typically, my Musky 360 articles are about scientific principles that can be used to better understand muskie behavior. I normally don’t chime in about current conditions and patterns: I leave that to the experts (muskie guides) who spend long hours on the water! But this article is about current water conditions, and the reason I am bringing this up now is because I feel there is a significant misconception about the water-cooling process and turnover.
The misconception that many people have about turnover is that turnover happens when the surface water temperatures drop into the 50s. That is incorrect. I have been observing the process of turnover for the past 5 weeks or more, starting when the surface temperatures were still in the 70s. It isn’t the surface temperatures that signal that turnover is happening, it is what is happening below the surface near the thermocline that signals turnover. In fact, it doesn’t signal turnover… it IS turnover.
The reason I have been able to make these observations is because I have a water temperature sensor that allows me to get water temperatures from the surface all the way to the bottom. Some people try to get the same data by lowering their transducer temperature sensors to the bottom. I am skeptical of results from such a technique because I am not convinced that transducer sensors are fast/responsive enough to give quality temperature data. For example, my transducer takes a minute or so to come to equilibrium when I first turn on my graphs after putting my boat in the water. It takes time to get the true water temperature on the surface when the air temperature the transducer was measuring is significantly different than the water temperature. I think you if you want good temperature data, you need a sensor that is designed for the task.
I take temperature profile measurements regularly during my trips out in the late summer and early fall to note the turnover process. I have been meticulous in taking water temperature profiles for three lakes that I fish. One is a shallower stained lake. Its maximum depth is about 35 feet, and its water clarity is about 6 feet or so in mid-summer. The second lake is a mid/deep lake. This second lake has lots of water in the 40-foot range with a maximum depth of about 80 feet. Its water clarity is about 13 feet. The third lake is a deep lake. It has lots of water in the 60-foot range with a maximum depth of 110 feet. Its water clarity is about 12feet.
If you want to fully understand the details of what I write in the following description of lake conditions in central Minnesota, please be sure to read my Musky 360 article on how water temperatures affect muskies . If you don’t want to read that whole article, just look at the section where I describe what the thermocline is. For the sake of brevity, I’m going to leave descriptions of those concepts out of this article.
The shallower stained lake I described above had its thermocline descend all the way to the bottom long ago in mid-summer. In technical terms, this lake did not stratify this year (although it has in the past). The reason, I believe, is because of the sustained strong winds and warm air temperatures we experienced in the early summer period which caused extensive thermal mixing. An observation that may surprise you, though, is that this lake had uniform water temperatures from top to bottom beginning in the first week of September. This lake has therefore already “turned over”. Its water temperatures have been descending now for weeks, dropping from 69 degrees in early September, experiencing a slight bump up to 73 during the warm weather in late September, but now settling down into the mid-60s (it was 64 degrees on 3 October). The water temperature profile has remained almost completely uniform during this time: when it was 71 on the surface on 21 September, it was about 70 on the bottom of the lake in 35 feet.
In contrast, the mid/deep lake did stratify. Currently its thermocline is descending from its stable, mid-summer depth of 25 feet (a typical occurrence in late summer/early fall). The depth of that thermocline was 58 feet in mid-September. 58 feet! Remember that this lake has lots of water that is 40 feet deep and less. When I took temperature profiles of water 50 feet deep and less, those profiles showed all depths being nearly uniform at 69 degrees. All areas except the deepest hole in the lake have “turned over”: their temperatures are already uniform. Like the shallow lake described above, those uniform temperature profiles have slowly been shifting to lower temperatures, with a bump upward in temperature into the low-70s during the unusually warm weather at the end of September. Only the sections of the lake that are deeper than 58 feet or so are still in the process of mixing to achieve uniform temperatures. In shallower areas of the lake (i.e. 45 feet deep and less), the water temperature profile has remained close to uniform with temperatures slowly dropping from day to day. Since all my “spots” on this lake are less than 45 feet deep, I am not worried about turnover disrupting my fishing patterns. The jump in surface temperatures due to unseasonably warm air temperatures has disrupted my patterns a bit, but I have confidence that new patterns will emerge and stabilize as the weather stabilizes more toward normal.
The third lake (the deep one) is about 100 miles further north than the other two, so it is further along in the cooling process. When I took its temperature profile on the last day of September, I found the thermocline to be at 75 feet below the surface. Like the other lakes, the temperature profile of the water above the thermocline was almost completely uniform, dropping about 1 degree from the surface down to about 70 feet deep. Again, this means that all sections of the lake that are less than about 70 feet deep have already “turned over” and will slowly cool down as long as stable cooler weather persists.
This “turnover” process that has been happening over the last month or so has had noticeable effects! The slow mixing of the bottom water layers with surface layers has brought nutrients from the bottom up to the surface layers of water where sunlight can reach. Any algae in this layer are going to respond to these added nutrients by going into a short period of explosive growth. This is why algae blooms are so prominent in the late summer and early fall period: the process of turnover brings new nutrients from the bottom up to the surface layer where algae can use them for photosynthesis. The result, of course, is that the water becomes “dirty” during this time. If you’ve noticed that water clarity has diminished during the past month compared to what it was in the mid-summer, you’re noticing the effects of turnover. Once these nutrients are exhausted and the levels of sunlight drop (due to the noon altitude and the hours of daylight both decreasing), the water will clear up significantly. Lower water temperatures in the surface layer will also contribute to this clearing up of the water because the rate of photosynthesis will also go down, inhibiting algae growth.
As long as weather patterns remain relatively stable, we can expect water temperatures (up and down the water column) to remain stable as well: a steady cool-down into the 50s and 40s. If there is an extended period of cold air temperatures (eg. 5-7 days of daytime highs that are 10-15 degrees less than surface water temperatures), expect some disruption in fishing. The reason is that surface temperatures will drop significantly below the temperatures in the lower layers, necessitating significant thermal mixing for the layers to come to equilibrium again. This effect is what people normally call “turnover”, but the main effects of turnover are already in play right now (dissipation of the thermocline, algae blooms, etc…) and diminishing in importance as water temperatures become more uniform. Likewise, if there is an extended period of warm temperatures (eg. 5-7 days of daytime highs 10-15 degrees above surface water temperatures), expect the same sort of disruption, but in reverse. So, in my experience, the next important stage in muskies’ seasonal progression isn’t turnover. Rather, it is muskies following the migration of forage fish as the water temperatures fall from the 60s through the 50s into the 40s. If you know how the muskies’ primary forage responds to these water temperature guideposts, you’ll know where your active muskies should be. Best of luck on the water!
Dr. Bob
Musky 360