Meromixis as a Pattern

In limnology (the physics, chemistry, and biology of inland waters), the circulation patterns of lakes are classified by the extent to which the water mixes vertically, from top to bottom, during the course of a year (Table 1).

The scheme is merely a guide because lake circulation patterns, like most natural phenomena, do not fall easily into discrete categories. A more accurate, but certainly more arduous, approach would be to characterize lakes only after long term data are available.

Another term, oligomixis, is used sometimes to refer to lakes with limited episodes of complete mixing (perhaps 1-4 times in a decade), whereas lakes that do not mix over decades, centuries, and millennia are unequivocal examples of meromixis. Oligo-mictic lakes are not distinguished in Table 1 because the distinction is arbitrary, but they underscore the point that circulation patterns form a continuum rather than a series of discrete types. The boundaries between types are also pliable in the sense that lakes may be meromictic (or some other condition) for many years and then, through some natural event, or human influence, the pattern of mixing may change.

Table 1 Categories of annual vertical mixing in lakes

Category

Definition

A

Amixis

A (no) mixing

B

Meromixis

Mero (partial) mixing; incomplete mixing

C

Holomixis

Holo (complete) mixing at least once per year

Monomixis

Mono (one) period of mixing

Dimixis

Di (two) periods of mixing

Polymixis

Poly (multiple) periods of mixing

Adjectives are formed by appending '-ictic', thus amictic, meromictic etc.

Adjectives are formed by appending '-ictic', thus amictic, meromictic etc.

'Spring meromixis' is another phenomenon that blurs the boundaries between discrete types. In some cold, Temperate Zone lakes, stratification formed under ice in winter persists into spring and summer. The surface water may warm rapidly after melting of the ice, reinforcing the winter stratification, or the winds in spring may be unusually mild, so that the stratification is not destroyed by mixing. As a consequence, there is no spring 'overturn,' as in a dimictic lake. 'Spring meromixis' effectively converts a dimictic lake to a monomictic lake for that year.

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