+ Greek límnē
"standing water, pool, marshy lake" + -ion,
noun suffix — more at limnetic
The term was introduced, along with epilimnion, by the American limnologist Edward A. Birge (1851-1950) in "On the evidence for temperature seiches," Transactions of the Wisconsin Academy of Sciences, Arts, and Letters, vol. 16, part 2 (1910), p. 1005: "I employ two new words in this paper, which seem convenient in writing of the temperature and other phenomena of lakes. These terms are epilimnion, for the upper warm layer of water which develops in the lake in summer, and hypolimnion, for the older colder water." The formative -ion could be read as a Greek diminutive suffix, but this meaning is not specified by Birge (who says nothing of the etymology) and does not fit the context. Possibly Birge took -ion as the Greek equivalent of the Latin compounding suffix -ium.