PAUL NELSON FISHING COLUMN: Behind the times when it comes to weather
October arrives this weekend with no frost yet in most of Minnesota. Surface water temperatures in the local lakes remained in the 60s for most of September, which is very unusual for the Bemidji area.
The thermocline is the thin layer of water between the warm water on the surface and the colder water on the bottom of the lakes.
Once the water temperatures above the thermocline equalize with water temperatures immediately below the thermocline, the thermocline in the lakes will disappear. This usually happens as the water temperatures in the lakes dip into the 50s.
Turnover happens when the surface water on the lakes becomes significantly colder than the water on the bottom. Water becomes more dense as it cools, so the colder surface water eventually begins to sink in plumes that are visible on sonar.
Water is most dense at 40 degrees, which is a good thing because the lakes would freeze solid during the winter if water was most dense at 32 degrees and ice didn't float.
The denser water on the surface of the lakes usually begins to sink at night, when winds are calm and temperatures are at the lowest point of the day.
When the denser surface water begins to sink, it eventually displaces the less dense water on the bottom of the lake and causes it to rise to the surface.
The water on the bottom of most lakes becomes stagnant and oxygen depleted during the summer. When the lakes begin to cool in the fall, the algae begins to die and eventually settles to the bottom. The dead algae collects on the bottom will come to the surface of the lakes during turnover as the water column in the lakes flips from the bottom to the surface.
This leaves a thick slimy layer of dead algae on the surface of the lakes in the mornings, until the wind mixes it back into the lake, where it finishes breaking down.
Turnover is nature's way of getting the entire water column in the lakes fully oxygenated and ready for winter.
Fish and other living things use more oxygen during the winter than is added to the lakes. The wave action stops, the plant growth slows down, so the lakes need to have enough oxygen in reserve during the winter to keep everything alive until spring.
All of the lakes keep getting clearer as the water cools down and more of the algae dies. The fish are able to move even deeper as the thermocline disappears, which is when most fall fishing patterns become established in the lakes.
The lakes are just dipping into the 50s, so the thermocline will begin to disappear in most of the deep lakes. Anglers should be able to see when the thermocline is gone in the lakes on sonar when they travel over the deeper parts of the lakes.
The walleyes have been going back and forth between the weedline and the top edge of the thermocline while the temperatures have been going back and forth from cool to unseasonably warm during most of September.
Shallow lakes like Upper Red Lake do not have a thermocline because the shallow water periodically turns over during storms with high winds. The wave action in the lakes can flip the water column down to between 30 and 40 feet, so that is where the thermocline is located in most deep lakes.
Fish act differently in shallow lakes that do not have a thermocline than they do in the deep lakes. Instead of going deep, many fish move shallow in the fall to feed.
There can be good bites for walleyes along shoreline structure late in the season in lakes like Upper Red, Lake of the Woods, the main lake portion of Leech Lake, Winnibigoshish and any other body of water where there is very little water deeper than 40 feet.
Food is still the key to fish location in virtually all lakes, so anglers need to use their electronics and do some searching before they drop their lines into the water and start to fish.
Paul A. Nelson runs the “Bemidji Area Lakes Guide Service.” He can be contacted at firstname.lastname@example.org.