sentences:  If a lake is eutrophic and has a high sediment oxygen demand, the hypolimnion in dimictic lakes can become hypoxic during summer stratification, as often seen in Lake Erie.During summer stratification, most lakes are observed to experience internal waves due to energy input from winds.If the lake is small (less than 5 km in length), then the period of the internal seiche is well predicted by the Merian formulae.