Development of temperature and respiratory gases during summer stratification in two lakes in southern Finland

Hypolimnetic density currents and oxygen consumption were studied in Lakes Paajarvi and Ormajarvi in southern Finland during the summer stratification period of the year 2004. Water temperature, dissolved oxygen and dissolved inorganic carbon data were collected from both lakes to detect the possible density currents from changes in temperature and respiratory gases. Additional data of air and sediment temperature, rainfall and chlorophyll a concentration was also used in the interpretation of the results. Periodical warming of hypolimnion indicated, that internal seiches have strong influence on heating and distribution of respiratory gases in the upper hypolimnion in both studied lakes. Although no clear signs of hypolimnetic density currents near sediment water surfaces were found during the stratification period, some features of density currents occurring during early stratification were seen especially in Lake Ormajarvi. The results suggest, that during stratification hypolimnion is affected by currents with various origins, element exchange between water and sediment and organic matter fed from the epilimnion. All these contribute to the development of the concentrations of respiratory gases and consequently may influence, for example, the leakage of nutrients from the sediment.