page_title: Lake ecosystem
section_title: Temperature
hierarchical_section_title: Lake ecosystem / Important abiotic factors / Temperature
caption_attribution_description: English: created by author
context_page_description: A lake ecosystem includes biotic plants, animals and micro-organisms, as well as abiotic physical and chemical interactions.
  Lake ecosystems are a prime example of lentic ecosystems. Lentic refers to stationary or relatively still water, from the Latin lentus, which means sluggish. Lentic waters range from ponds to lakes to wetlands, and much of this article applies to lentic ecosystems in general. Lentic ecosystems can be compared with lotic ecosystems, which involve flowing terrestrial waters such as rivers and streams. Together, these two fields form the more general study area of freshwater or aquatic ecology.
  Lentic systems are diverse, ranging from a small, temporary rainwater pool a few inches deep to Lake Baikal, which has a maximum depth of 1642 m. The general distinction between pools/ponds and lakes is vague, but Brown states that ponds and pools have their entire bottom surfaces exposed to light, while lakes do not. In addition, some lakes become seasonally stratified Ponds and pools have two regions: the pelagic open water zone, and the benthic zone, which comprises the bottom and shore regions.
context_section_description: Temperature is an important abiotic factor in lentic ecosystems because most of the biota are poikilothermic, where internal body temperatures are defined by the surrounding system.  Water can be heated or cooled through radiation at the surface and conduction to or from the air and surrounding substrate. Shallow ponds often have a continuous temperature gradient from warmer waters at the surface to cooler waters at the bottom.  In addition, temperature fluctuations can vary greatly in these systems, both diurnally and seasonally.
  Temperature regimes are very different in large lakes (Fig. 2).  In temperate regions, for example, as air temperatures increase, the icy layer formed on the surface of the lake breaks up, leaving the water at approximately 4 °C.  This is the temperature at which water has the highest density.  As the season progresses, the warmer air temperatures heat the surface waters, making them less dense.  The deeper waters remain cool and dense due to reduced light penetration.  As the summer begins, two distinct layers become established, with such a large temperature difference between them that they remain stratified.  The lowest zone in the lake is the coldest and is called the hypolimnion.  The upper warm zone is called the epilimnion.  Between these zones is a band of rapid temperature change called the thermocline.  During the colder fall season, heat is lost at the surface and the epilimnion cools.  When the temperatures of the two zones are close enough, the waters begin to mix again to create a uniform temperature, an event termed lake turnover.   In the winter, inverse stratification occurs as water near the surface cools freezes, while warmer, but denser water remains near the bottom.  A thermocline is established, and the cycle repeats.
caption: Fig. 2 Seasonal stratification in temperate lakes
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