Mechanisms of long‐term variations in the thermal structure of a warm lake

Abstract

Analysis of a 39-yr record of temperature profiles from Lake Kinneret, Israel, revealed that average epilimnion thickness decreased by , 1.2 m (, 3 cm yr 21 ), metalimnion thickness decreased by between , 1.0 m (December) and , 2.0 m (April), and average temperature of the epilimnion increased by , 1uC (, 0.028uC yr 21 ). Average hypolimnetic temperature remained constant (, 15uC), so that the thermal gradient across the metalimnion increased. The average duration of the stratification period (, 286 d) remained unchanged. We show that the stratification changes were the result of a decrease in lake level, a slight increase in air temperature during the spring and summer, and a reduction of inflows to the lake. These changes in time affected the stratification pattern in the same direction, i.e., an increase in the lake epilimnetic temperature and an increase in the thermal gradient across the metalimnion. Analysis of the data indicates that the effect of overpumping, which leads to a reduction in lake level, is larger than the effect of changes due to air-temperature increase and the reduction in inflow volumes.Many subtropical and temperate lakes are thermally stratified over the course of the year, resulting in a wellmixed warm surface layer (epilimnion), a transition layer (metalimnion) with a steep temperature gradient, and colder and relatively isothermal bottom waters (hypolimnion). The onset, strength, and duration of vertical stratification depend on several internal lake-specific factors and the external influence of weather. Water depth, water transparency, lake size, and the length of fetch along the direction of the prevailing wind are all lake-specific factors that influence the characteristics and strength of vertical stratification. Similarly, radiation, air temperature, wind speeds, and wind shadowing by hills or mountains surrounding the lake are weather-driven factors affecting lake stratification.In thermally stratified eutrophic lakes, there is a hydrochemical differentiation of the water column into an oxic epilimnion and an anoxic hypolimnion caused by the interplay between physical forcing and a succession of microbiological processes (Wetzel 1983). With the onset of thermal stratification, hypolimnetic dissolved oxygen is gradually depleted, while the epilimnion often becomes nutrient depleted (Nishri et al. 2000;Eckert et al. 2002). The result is a water column that consists of a gradual vertical gradient in chemical and biological characteristics, which has an effect on the replenishment of nutrients and on the biological activity in the lake and is greatly affected by vertical mixing processes (Adrian et al. 2009).Long-term changes in the thermal stratification are therefore expected to be the trigger for considerable changes in the biology of a lake. These stratification changes can be caused by (1) Lake Kinneret, in northern Israel (Fig. 1a), is an example of a thermally stratified lake that has exhibited stratification changes over the recent past (Hambright et al. 1994). ...

Atmospheric stilling leads to prolonged thermal stratification in a large shallow polymictic lake

“…Other factors are also known to influence stratification in some lakes. For example, water level has previously been shown to be an important factor influencing thermal stratification in Lake Kinneret, Israel (Rimmer et al 2011). Moreover, lake water level has been described as one of the most influential factors affecting the physics (Nõges & Järvet 1995), chemistry (Nõges & Nõges 1999), and biology (Nõges et al 2003;Järvalt et al 2005) of Võrtsjärv.…”

“…Polymictic lakes can be categorised further into continuous and discontinuous polymictic, depending on whether stratification occurs at most on a daily basis or for periods of several days to weeks, but with irregular interruption by mixing events (Lewis 1983). The mixing class a lake belongs to is mostly determined by lake morphometry, although the strength and extent of stratification are also influenced by extrinsic features of a lake, such as altitude (Woolway et al 2015a), inflows (Rimmer et al 2011), and meteorological conditions (Churchill and Kerfoot 2007), and intrinsic factors such as water clarity (Heiskanen et al 2015).…”

Lake heat content and stability variation due to climate change: coupled regional climate model (REMO)-lake model (DYRESM) analysis

“…The resulting heating of the vertical water column in turn leads to substantial changes in the physical properties of the lake (Gaiser et al, 2009;Ambrosetti et al, 2010;Braig et al 2010;Rempfer et al, 2010), more precisely in terms of mixing processes, stratification characteristics (Danis et al, 2004;Ambrosetti and Barbanti, 2005;Austin and Colman, 2008;MacIntyre et al, 2009;MacKay et al, 2009;Rimmer et al, 2011) and heat content (Hondzo and Stefan, 1993;Dokulil et al, 2006;Vetter and Sousa, 2012). Investigations dealing with these limno-physical changes are crucial, as the latter directly influence nearly all biological and chemical processes.…”

Spatial heterogeneity and seasonal succession of phytoplankton functional groups along the vertical gradient in a mesotrophic reservoir

“…Their (over-)exploitation during summer when precipitation is scarce, reduce water level, which in turns, alter the physical structure of lakes, disrupting stratification patterns (Rimmer et al, 2011). Ikizcetepeler reservoir is a warm monomictic Mediterranean lake, used for irrigation and providing drinking water to Balkesir city.…”