Fall turnover evens lake temperature weeks before deep oxygen recovers

STEP 1 · Density cap

Cooling lets wind reach the bottom of a stratified lake

Fresh water weighs the most at 4°C (39°F). The Water on the Web lake ecology primer builds its account of temperate lakes on that density peak. Once spring mixing ends the surface warms and rides lighter than the water under it. Wind may still turn the basin for a while. Once the top is warm enough it stops mixing with the heavier bottom.

That primer calls the warm mixed surface the epilimnion. The metalimnion is where temperature falls fast and the thermocline is the steepest slice of that band. The hypolimnion is the cold bottom sealed off for months.

Through autumn the surface cools and the density difference narrows. Wind works deeper and the thermocline drops. The primer says wind is able to stir the whole basin once surface water and bottom water get close in temperature and density. Dimictic lakes mix once in spring and a second time in fall. Ponds and lakes shallower than 4 m (13 ft) fall in the polymictic group and can remix many times in a year.

For northern temperate lakes of 50 to 1,000 acres the primer treats 3 to 7 m (10 to 23 ft) as a fair summer mixed depth.

STEP 2 · Oxygen lag

Ice Lake temperature matched weeks ahead of the deep oxygen rebound

The dissolved oxygen page says spring and fall lean toward uniform well mixed water in fertile lakes and in infertile lakes alike. Through summer the surface stays oxygenated by plants and by contact with the air. A fertile lake can go anoxic on the bottom because that water is sealed away while respiration keeps going. A clear infertile lake can keep high oxygen under the thermocline. Those summer contrasts tend to fade once fall turnover arrives.

The same oxygen page says phosphorus often leaves the mud through chemical processes once dissolved oxygen over the sediments drops below 1 mg/L. That spring mixing can lift phosphorus and ammonium into the lighted layer.

Ice Lake near Grand Rapids in Minnesota supplies the profile. The stratification primer gives Ice Lake as 41 acres and its greatest depth as 16 m (52 ft). From May through September in 1998 the thermocline remained at 4 to 5 m (13 to 16 ft). By Halloween it sat at 9 m (30 ft). The lake turned over from late October into the middle of November and kept mixing until it froze in the middle of December.

On 10 October 1998 the Ice Lake table still shows a split column. Saturation was 57 percent and temperature was 12.9°C (55°F) at 5 m (16 ft). At 8 m (26 ft) saturation was 36 percent and temperature was 8.4°C (47°F). At 11 m (36 ft) saturation was 5 percent and temperature was 6.0°C (43°F).

By 5 November the same table shows a temperature match. The column read 7.3°C at 5 m, 7.2°C at 8 m and 7.1°C at 11 m. The spread is 0.2°C (0.4°F) around 45°F. Saturation at those depths was 48 percent, 47 percent and 46 percent. Oxygen was still near half of saturation.

On 2 December the same table shows 3.3°C (38°F) at every listed depth. At 5 m saturation was 88 percent. At 8 m and at 11 m saturation was 89 percent.

The Ice Lake record shows spring mixing left the bottom short of a full reset. The stratification primer says a small sheltered lake that is deep for its size probably completes a full mix only once each year in the fall before ice.

In both 1998 and 1999 the column stayed nearly isothermal for weeks after the ice went out. In those same years oxygen below about 8 m remained under 3 mg/L until turnover arrived in November 1998. The Ice Lake record flags chronic oxygen under 3 mg/L as major stress for fish in this lake and names coolwater species and coldwater species.

On 30 June 1999 the Ice Lake record shows the lake still stratified with dissolved oxygen of 0.2 mg/L at 23 ft (7 m) and 0.0 mg/L at both 30 ft (9 m) and 39 ft (12 m). Routine surface chlorophyll-a usually stayed under 3 µg/L and the highest values came during fall turnover. Spring Secchi depth was 2 to 3 m (7 to 10 ft) and later reached about 4 m (13 ft).

STEP 3 · Walleye depths

Erie fish sat near 16 ft as fall return overlapped turnover

Kraus and colleagues published walleye depth and temperature work in the journal Movement Ecology. On 5 April 2021 they tagged 48 walleye in Lake Erie. Two fish were females and the rest were males. Total length spanned 440 to 628 mm (17 to 25 in) with a mean of 522 mm (21 in).

Under summer stratification these tagged walleye mainly used epilimnetic water. Kraus and colleagues place the center of use for depth and temperature at about 13 m (43 ft) once spawning ended and the layers had formed. Mean depth from the 14 tags that were recovered was 12 to 17 m (39 to 56 ft) in July, August and September. Some observations went to 23 m (75 ft) and were deeper than the metalimnion.

Kraus and colleagues treat October and November as fall turnover plus the return from deeper summer water in the central basin and the eastern basin. Depth and temperature still covered a spread like the stratified months. The distribution center sat at about 5 m (16 ft) and about 11°C (52°F). That 11°C reading is the 25th percentile among temperatures available to the fish. The same center stayed at 6 m (20 ft) from December through the middle of March.

The recovered tag logs show these 14 fish almost never used water shallower than the 4 m (13 ft) mark.

The fish were mostly males and were tagged in April 2021 on a single spawning reef in Lake Erie. Because that shallower center arrived on the return migration the 16 ft figure folds turnover together with travel. That figure is a weak guide on a modest inland lake that keeps deep oxygen low through most of summer.

STEP 4 · Bass and panfish

Depth follows where oxygen remains

Our read for largemouth bass, smallmouth bass and bluegill follows the oxygen profile. A walleye map from Lake Erie does not stand in for those fish.

On 30 June 1999 the Ice Lake reading at 23 ft was already 0.2 mg/L. The dissolved oxygen page treats levels above 5 mg/L as optimal and says most fish cannot last long below 3 mg/L. With the bottom near zero the water that still holds oxygen is the upper layer. By 5 November 1998 temperature was already uniform while saturation between 16 and 36 ft sat at only 46 to 48 percent. That recharge stayed well short of full until the 2 December reading.

The primer classes polymictic ponds under 4 m (13 ft) as waters that remix often through the warm season and sit outside this deep lake sequence. A creek arm that stays mixed does not follow the deep hole calendar.

STEP 5 · Field check

Measure the mixed layer and the bottom before you move

On the deep hole you fish measure temperature and dissolved oxygen one time in the upper mixed water and one time near the bottom.

The 10 October 1998 split means mixing had not finished. At 16 ft Ice Lake read 55°F and 57 percent saturation. At 36 ft it read 43°F and 5 percent saturation. Between those depths the difference was 6.9°C (12°F). Our read puts fish that need oxygen above that bottom water.

The 5 November 1998 match means the density cap has lifted while air has yet to finish the oxygen recharge. From 16 ft to 36 ft the temperature span was 0.2°C. Those depths sat at 46 to 48 percent saturation.

Our read says a temperature meter reads the 5 November lake and the 2 December lake the same way because each column is even through the full depth. Oxygen is what separates 46 to 48 percent from 88 to 89 percent.

The 2 December 1998 profile means temperature and oxygen have both evened at 38°F with saturation of 88 to 89 percent.

In October and November the Lake Erie walleye center of use sat near 16 ft and about 52°F while the return migration was underway and the depth spread stayed wide. Choose depth from the profile on the lake you are fishing.