Fall Turnover -- A Case Study

Across seasons one yearly aquatic event, fall turnover, continues to puzzle many anglers. While this process remains mysterious to some, turnover is a relatively predictable phenomenon.

by Cory Schmidt*

*Cory Schmidt, Brainerd, Minnesota, is an avid multispecies angler and a frequent contributor to In-Fisherman publications.

A Glance at Yearly Lake Stratification

In-Fisherman editor Matt Straw.

Just before ice-out in most temperate-zone lakes and reservoirs, water temperature near the bottom hovers around 39 degrees Fahrenheit (4 degrees Centigrade), while just under the remaining sheet of ice the temperature approaches 32 degrees Fahrenheit. As ice melts and surface water warms, it also decreases in density and begins sinking. Once surface temperature matches that of the bottom — around 39 degrees Fahrenheit — little wind energy is needed to mix the entire water column. This event is “spring turnover.” Fish begin returning to shallow water as new weedgrowth emerges. Sun-driven photosynthesis in aquatic plants adds oxygen to the shallows.
As surface temperatures continue rising, the upper layer of water becomes progressively lighter than the cooler water below. Because of differences in water temperature and density from top to bottom, wind can no longer mix the entire water column.
At this point, all but shallow lakes and reservoirs begin to stratify into threedistinct water layers as dictated by temperature and density. The upper layer (epilimnion) contains the warmest most buoyant water. Generally, larger, deeper lakes have a thicker epilimnion than shallower lakes.
Descending in the water column, a narrower band of water that drops rapidly in temperature contains the middle layer (metalimnion). The metalimnion also encompasses the thermocline, the area of the most precipitous change in water temperature.
Finally, the lower layer (hypolimnion) extends from the lower level of the thermocline to the bottom. Throughout summer, water temperature in the hypolimnion remains relatively constant. Oxygen levels in this layer may decrease during stratification, due to the wind’s inability to mix the water column. Subsequently, in some lakes, anglers rarely find fish dwelling below the thermocline by the end of summer.
Lakes remain temperature-stratified into fall. As cooler weather and strong winds persist, surface temperatures continue to decrease, eventually approaching temperatures in the metalimnion. Eventually this layer fades and mixes with the metalimnion. The thermocline remains, but descend in the water column as the heavier upper water layer forces it deeper. Soon, though, the temperature in this mixed upper layer equals that of the hypolimnion; wind easily mixes the entire water column because similar temperature and density exist throughout. This event is known as fall turnover, which signals equilibrium in water temperature and a restoration of dissolved oxygen to all depths.
Exactly what happens during the month surrounding the annual fall turnover? Does every part of a lake turn over at the same time? Does turnover occur within hours, or does it take days? At what temperatures do lakes turn over? How deep is the thermocline just prior to turnover? How much do wind and weather bear on turnover? Is turnover always accompanied by a pungent smell and large mats of floating weeds and debris? And do fish quit feeding?

About Gull Lake
In an attempt to answer these questions, I conduct water temperature profiles on Gull Lake in Cass County, Central Minnesota. Gull Lake is a “classic” natural walleye lake. It has an average depth of 30 feet and a maximum depth of about 100 feet. The shoreline is heavily developed. The lake is fed primarily by the Gull River via an inlet on the north end of this multilake reservoir.
Walleye and largemouth bass are important, along with pike and panfish. Primary forage species include yellow perch, cisco, bullhead, sucker, panfish, and crayfish. This diverse fishery also hosts a variety of aquatic vegetation; emergent plants, such as bulrush and wild rice grow primarily in Wilson Bay and in smaller lakes on the upper end of the chain. Submergent species such as coontail, chara, and several types of pondweed (cabbage) grow as deep as 30 feet, although the true weedline ceases at approximately 16 feet.
In summer, Gull Lake stratifies. By early July, the thermocline ranges between 20 and 36 feet. Below about 30 feet, dissolved oxygen levels dip to deficient levels in most of the chain. July secchi disk readings average 10 to 11 feet.
Using Carlson’s Trophic State Index, Gull has been classified as a 47. This index uses three main factors -- transparency (based on secchi disk readings), chlorophyll (suspended algae) and total phosphorus levels -- to determine a lake’s nutrient levels and plant growth (trophic status). In the case of Gull Lake, transparency, chlorophyll and phosphorus levels converge within the mid to late mesotrophic range. An index of 20 to 35 refers to an oligotrophic condition, 35 to 55 mesotrophic, 55 to 70 eutrophic, and 70+ hypereutrophic. By way of comparison, nearby Pelican Lake measures 38 (early to mid mesotrophic), Whitefish Lake 42 (early to mid mesotrophic) and North Long 45.
These distinctions are important for this study because each lake, and indeed each separate basin within a given lake operates somewhat independently. Factors such as water clarity -- somewhat dictated by algae and phosphorous levels -- differ between lake basins, bearing on thermocline depth and turnover.
Following stratification, many mesotrophic lakes eventually contain inadequate dissolved oxygen levels below the thermocline to support most species of freshwater fish. There are, however, lakes in which high levels of oxygen do permeate into the depths. Deeper, clearer and less fertile (oligotrophic to early mesotrophic) lakes sometimes offer this condition. These lakes, often referred to as “two story” environments, host both shallow and deep-water habitat. While most of water in Gull Lake’s hypolimnion contains inadequate oxygen for summertime fish use, exceptions do exist.

About this Study
On ten separate dates (approximately once every 3 days), temperature profiles were taken from several locations (gauge sites). From Gull Lake, I chose two primary sites offering divergent basin characteristics.
Site #1, the southern basin of Wilson Bay -- Water is 77 to 79 feet deep, the deepest water in Wilson Bay, a deep clear basin previously separated from the Gull Lake chain prior to construction of the Army Corps of Engineers dam in the late 1800s. Although the bay is now attached to Gull proper, it may in fact be considered a two-story environment. In order to return easily to this exact site for each temperature profile measurement, coordinates were logged into a GPS.
Site #2, just south of Bowtie Bars -- This location in central Gull Lake was chosen because it reflects the approximate average depth of the lake’s main basin (a few feet plus or minus 60). Its coordinates were also logged into GPS.
In addition to these gauging sites, I took measurements at two other “reference” locations on certain days. Site #3, in the basin west of Polks Flat, is 62 feet deep. Site #4, in Pike Bay basin due southeast of Government Point is 42 feet deep. Finally, I also took readings on several other lakes in the area, including Little Lake X and Lower Hay. These lakes served as supplemental points of reference, to compare to the main survey lake.

Process and Logistics
To gather the data, I used a prototype Nature Vision Aqua-Vu ZT-60 underwater video camera with temperature display. At each gauging site, I lowered the camera cable in one-foot increments, as marked on the cable. Also, I observed the depth of the camera housing with a Lowrance LMS-350A liquid crystal display unit. In other words, I could confirm that the temperature probe was reading in 24 feet by watching for a single continuous black line on the sonar screen at that depth. During the course of the study, I used the LCD as a source of supplemental observation. I recorded water temperature every two feet to the limit of the cable (54 feet).
To keep this study relevant to anglers, on most of these days, I spent at least an hour fishing and also viewing with the camera on adjacent productive structural elements. For many hours, I worked the breaklines near Mission Point and Crane Island in Wilson Bay, as well as the south side of Bowtie Bars. The purpose was to determine the location and activity level of fish like walleyes, pike, and bass relative to changing water temperature and thermocline conditions. Additionally, I spent many hours conversing with fishing friends, local guides and anglers at boat ramps, and the owners of S' Bait and Koep’s Bait, both near Gull Lake. From personal fishing time and the observations and reports of other individuals, I was able to determine fish activity during the study period.
Other observations included air temperature, wind velocity and direction, sky and prevailing weather conditions, bird presence and activity, shoreline foliage coloration, air aromas, and other natural factors. As bass anglers commonly recount, the bass spawn peaks as the dogwoods reach full spring bloom. Perhaps I could determine fall turnover with a similar predictable and observable environmental event, such as the final fall peak of color in deciduous trees.

Notes About Thermoclines
Strictly speaking, although many individuals use the terms metalimnion and thermocline interchangeably, they actually refer to two different layers of water, one within the other. The metalimnion refers to the thinner middle layer of water where temperature drops rapidly. The thermocline, which lies within the metalimnion, is defined as the area where water temperatures drop by at least one-half degree per foot. Some lakes exhibit several thermoclines within the metalimnion. This did not appear on Gull Lake.