lakes

Holomictic

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Dimictic lakes commonly have turnover in spring and fall with stratification in summer and winter (Nature Times).

Dimictic lakes commonly have turnover in spring and fall with stratification in summer and winter (Nature Times).

Holomictic references the most common type of lake which turns over at least once per year (as opposed to meromictic lakes which are constantly stratified).  This mixing is an important process for maintaining fish and aquatic communities by distributing nutrients and oxygen throughout the lake before stratification occurs again.  There are four categories of holomictic lakes:

  • Oligomictic: mixing is irregular,
  • Monomictic: turnover occurs once a year (most common in polar areas)
  • Dimictic: turnover occurs twice a year (most common in temperate areas), and
  • Polymictic: frequent turnover (most common in tropical areas).

Meromictic

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The Black Sea is the largest meromictic lake (NASA).

The Black Sea is the largest meromictic lake (NASA).

Meromictic references a type of lake which is constantly stratified.  The surface and bottom waters do not ever mix.  In most cases, the bottom layer has very low oxygen levels, where few fish and other organisms can live, restricting them to the surface layer.  Meromictic lakes are uncommon (most lakes are holomictic and turnover at least once per year) and may be formed because they are:

  1. small and unusually deep, or
  2. the lower layer is denser and more saline than the surface layer.