mixolimnion
/ˌmɪksə(ʊ)ˈlɪmnɪən/noun
The freely circulating upper layer of a meromictic lake.
Origin
1930s; earliest use found in George Hutchinson (b. 1903). From mixo- + -limnion, after epilimnion and hypolimnion. Compare slightly earlier metalimnion.