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Thermal Stratification of Wisconsin Lakes

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Thermal Stratification of Wisconsin Lakes THERMAL STRATIFICATION OF WISCONSIN LAKES RICHARD C. LATHROP AND RICHARD A. LILLIE Bureau of Research Wisconsin Department of Natural Resources Abstract A model predicting summer temperature stratification in lakes utilizing lake surface area and maximum depth information was developed from vertical profile temperature and dissolved oxygen data collected on approximately 500 Wisconsin lakes. From the model, the number of stratified versus non-stratified lakes (natural and impoundments) was estimated for the 3,000 plus Wisconsin lakes with surface areas 25 acres (10 hectares) or greater. Statewide, about one-half of the lakes are predicted to be non-stratified. Impoundments, which represent about 16 per­ cent of the state's lakes, are about 86 percent non-stratified. Potential uses for the lake stratification model are noted. INTRODUCTION sity differences between surface and bottom Thermal stratification in moderately deep waters become too great for the wind to temperate latitude lakes is a well docu­ maintain complete homoiothermy. Thermal mented phenomenon. Hutchinson (1957) stratification results with the establishment provides a thorough discussion of the con­ of an epilimnion (upper warm water, freely tributions of earlier researchers. Thermal circulating), hypolimnion (deep, cold, rela­ stratification results from density differences tively undisturbed water), and a zone of in lake water of varying temperatures (Birge, steep thermal gradient called the metalim­ 1916). After the winter ice melts, water nion (or thermocline). These regions exist temperatures increase above the point of throughout the summer months until fall, maximum density of 4 °C until maximum when the lake surface water cools sufficiently Wisconsin lake surface temperatures, gen­ to again equalize water density differences erally between 21 °-27°C (Wisconsin DNR, between top and bottom, thereby initiating Bureau of Research lake data files), are fall overturn. reached by mid-summer. The wind provides Shallow lakes exhibit complete mixing energy during the spring to circulate the regularly throughout the summer as the wind warming surface waters throughout the entire provides enough energy to destabilize the water column (spring overturn) maintaining minor density differences that develop be­ homoiothermal (uniform) lake tempera­ tween the surface and bottom as a result of tures. As water temperatures increase above surface warming on hot, calm summer days. 4°C, water density decreases, with each suc­ Certain lakes have sufficient depth to allow cessive degree of rising water temperature for temporary thermal stratification, which resulting in a greater decrease in water den­ persists until major weather systems with sity. Consequently, more wind energy is re­ high winds again cause complete mixing. quired to completely circulate the warmer These weather systems occur frequently lake surface waters with the cooler, more enough during the summer months in Wis­ dense bottom waters. consin (Stauffer, 1974) that these weakly In deeper lakes, as surface temperatures stratified lakes can be considered as non­ increase on calm, warm spring days, the den- stratified. Stratified lakes do not exhibit 90 1980] Thermal Stratification of Wisconsin Lakes 91 complete miXlng during the summer, al­ southern Wisconsin, they are also greatly though metalimnetic deepening, as a result affected by thermal stratification (Lillie and of these strong weather fronts, does occur Mason, in press). In general, southern (Stauffer, 1974). Wisconsin lakes are more fertile, and those Rigorous mathematical expressions have that stratify usually exhibit dissolved oxygen been developed to describe the heat flux pro­ depletion throughout the hypolimnion as a cesses of lakes that ultimately result in ther­ result of respiration and bacterial decompo­ mal stratification (see Hutchinson, 1957). sition of organic matter. The lack of oxygen Calculations based on various physical lake in the colder hypolimnion precludes the sur­ characteristics can describe the stability of vival of cold-water-adapted fish such as trout a lake, or the amount of work needed to since surface water temperatures are high cause a lake to destratify to a uniform tem­ where dissolved oxygen concentrations are perature. Lake depth is an important vari­ adequate. Other aquatic life such as bottom able in the calculation. However, the lake feeding insects and zooplankton are re­ depth required before thermal stratification stricted from the anoxic hypolimnion except develops varies greatly between individual for brief periods when certain species mi­ lakes as a function of lake surface area, ba­ grate into the hypolimnion. Northern Wis­ sin orientation relative -to prevailing winds, consin lakes are generally less fertile and lake depth-volume relations, protection by therefore in many cases do not undergo com­ surrounding topography and vegetation, and plete hypolimnetic oxygen depletion. Cold­ otherfactors (Wetzel, 1975). water-adapted fi sh do well in the hypolim­ Few generalizations about stratification nion of these lakes during the summer have been attempted for diverse groups of months when surface waters are too warm. lakes. Hutchinson (1957) noted that the The lack of oxygen in the hypolimnion of eddy diffusivity (related to the process of fertile lakes causes the hypolimnetic lake turbulent mixing) is greatest in the wind­ sediments to release such dissolved constitu­ swept epilimnion of large, exposed lakes. ents as inorganic phosphorus, ammonia, and Consequently, lakes of similar maximum hydrogen sulfide into the overlying water depths may be either stratified or non-strati­ throughout the summer stratification period fied, depending on their surface area. (Mortimer, 1941-1942). In shallow, fertile Ragotzkie (1978), using data from Wis­ lakes a significant amount of dissolved nu­ consin and central Canadian lakes, developed trients released from the lake sediments dur­ one of the first simple lake stratification ing periods of brief stratification can be models. Lake fetch (F) was used to predict transported by subsequent mixing to the sur­ the depth of the summer thermocline (Dth) face waters where high levels of algal pro­ for lakes having fetches from 0.1 to over 20 duction are maintained. km: Resuspension of sediments is another im­ portant effect of lake mixing. Shallow lakes continually resuspend nutrient rich sediments Summer stratification of a lake has a tre­ that contribute to increased nutrient concen­ mendous impact on the chemical constituent trations for algal growth. concentrations of each lake and a great in­ The combined result of sediment resus­ fluence on the lake's biological community pension and frequent stratification followed structure. Although Wisconsin lakes are very by lake mixing in shallow lakes results in diverse in their geochemical characteristics potentially high rates of internal nutrient (Poff, 1961 ) and watershed nutrient load­ recycling during the summer months. As a ings, particularly between northern and result, surface waters of non-stratified lakes 92 Wisconsin Academy of Sciences, Arts and Letters [Vol. 68 in Wisconsin generally show a net increase limited data could provide useful informa­ in total phosphorus concentration from tion for the classification process. spring to summer, while deep stratified lakes usually exhibit a net decrease in total phos­ METHODS phorus concentration (Lillie and Mason, in Data used in this report came from two press . Thermal stratification effectively sources: (1) vertical profile temperature creates a temporary nutrient barrier between and dissolved oxygen data on approximately the epilimnion and the hypolimnion, while 500 lakes 25 acres (1 0 hectares) or greater nutrients are being removed from the epilim­ in surface area, collected by the Wisconsin nion by sedimenting algae. The importance DNR, Bureau of Research; and (2) lake of this barrier varies between lakes as a func­ surface area and maximum depth informa­ tion of lake basin morphometry. tion on Wisconsin lakes 25 acres or greater The classification and inventory of lakes (data compiled by DNR Bureau of Fish in relation to their trophic status has been Management). The lake inventory data was emphasized increasingly in recent years by subdivided into natural lakes and impound­ state and federal agencies. Since thermal ments. stratification can significantly affect lake Decisions about the establishment of water quality and concomitant recreational thermal stratification are based on inspection potential of a lake, a model capable of pre­ of the temperature and dissolved oxygen dicting stratification in Wisconsin lakes from vertical profiles. Three main types of tern- I I STRATIFIED WEAKLY STRATIFIED NON-STRATIFIED I TEMPERATURE (°C) 0 10 20 0 10 20 30 0 10 20 0 +-"----1--'--,-<>'-~· o+--,__--£_ _.___...._____,~:;;;0 0 -llf--'--"---<O.i............L ,..j July cf I 5 : I t 0 ...... 5 I 5 .. 0 0 5 10 ~ .. _oI .. I ~ E 10 -- I I 0 -0'~---.---"T"•----, KEY :J: -- Temperature 1- 15 0 +--~__,__.....__~.-/-, 0 Q.. w Aug. 3 / 0--0 Dissolved 0 / Oxygen / 0 20 5 I 0 I 0 I I ,.....0 251 1 10 -1---o..... -....., 0 5 10 0 5 10 15 DISSOLVED OXYGEN (mg/1} Fig. 1. Temperature stratification patterns found in Wisconsin lakes. (Stratified = Lake Monona, Dane Co., Aug. 1, 1978; Weakly Stratified= Lake Waubesa, Dane Co., July 7 and Aug. 3, 1976; Non-stratified = Round Lake, Chippewa Co., July 15, 1975). 1980] Thermal Stratification of Wisconsin Lakes 93 perature profiles are found in
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