Hypolimnion - Summer Stratification
This zone forms the bottom layer which is deep, cold and non- circulating. The hypolimnion is generally rich in nutrients though its oxygen content is low due to its utilisation by decomposition process which as you know generally occurs here and uses up oxygen. Temperature fall here is gentle.
Stratification of certain temperate lakes is not limited to the summer season as they undergo stagnation or stratification in winter also which is called winter stratification or stagnation.