Temperature Characteristics . 



A majority of lakes in coastal Maine are too shallow to exhibit summer 

 stratification with a deep-water hypolimnion. In summer, waters of shallow 

 lakes mix well from the surface to the bottom, because of wind and current 

 action. Consequently, temperatures and oxygen levels are nearly uniform at 

 all depths. In moderately deep lakes (many small natural lakes in Maine fit 

 into this category) the surface and bottom waters do not mix well and the 

 water stratifies into a warmer layer near the surface and a cooler one near 

 the bottom. In deep lakes, thermal stratification is well developed. Warmer 

 water temperatures persist near the surface (the epilimnion) , a metalimnion 

 (strata where temperatures decline rapidly with depth) forms in mid-water, and 

 colder water temperatures persist to the bottom (hypolimnion). Stratified 

 lakes have little vertical movement of water in their metalimnia and 

 hjrpoliraina during summer, which often results in oxygen depletion near the 

 bottom and limitation in biological activity and productivity in the 

 epilimnion. Graphic descriptions of thermal stratification and its effects 

 are given in figure 7-2. 



Thermal stratification, or its absence, in lakes is largely determined by 

 depth, surface area, exposure to wind, and climate. Temperature profiles are 

 available for 181 of the named lakes in the coastal zone of Maine, as 

 determined from the data in appendix table 1. Of the 181 lakes, 121 (67%) 

 have some thermal stratification during summer. The duration of 

 stratification in lakes with no hypolimnion may be short and may vary 

 considerably with the weather from year to year. 



Only 68 (38%) of the named coastal zone lakes (appendix table 1), possess a 

 hypolimnion. Temperature data are not available for most of the small natural 

 lakes, which are shallow and would not be expected to be stratified. Because 

 most of the small, shallow unnamed lakes are not included in the data for 

 stratified and nonstratif ied lakes in table 7-2, the true percentage of 

 stratified lakes with hypolimnia is considerably less than is indicated in the 

 table. 



The spatial and temporal patterns of water temperatures in lakes have major 

 effects on free oxygen levels, nutrient recycling, biological productivity, 

 and species composition of fishes and other biota. Many of the nutrients in 

 lakes are released into the aquatic system by the decomposition of organic 

 materials on or near lake bottoms. As a result, nutrients become available 

 throughout the aquatic system of shallow, well-mixed lakes, making them more 

 productive than deep stratified lakes (Richardson 1975). In well stratified 

 lakes much of the nutrient release from decomposition may accumulate near the 

 bottom and is largely unavailable to the trophogenic zone. A comparison of 

 total phosphorus in the epilimnion (surface layer) and the hypolimnion (bottom 

 layer) of eight thermally stratified lakes in the coastal zone of Maine during 

 summer illustrates this point. A mean value of 8 ppb (range: trace to 12 

 ppb) near the surface and of 33 ppb (range 10 to 95 ppb) near the bottom is 

 indicative of how thermal stratification potentially can affect biological 

 productivity. In addition, high rates of decomposition in these waters may 

 result in severe oxygen depletion during mid- and late summer. 



7-11 



10-80 



