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Weather-induced episodic mixing events in boreal lakes studied

Weather-induced episodic mixing events in boreal lakes studied

FMI measurement station, lake Pallasjärvi, has been part of a study examining weather-induced episodic mixing events in boreal lakes and their impacts on lake ecosystems.

Short-lived impacts such as changes in water temperature and stratification are typically detected, but long-last­ing effects on the lake's biology may also occur. In this study automated water quality monitoring data from 8 boreal lakes were used.  to examine how the episodic weather-induced mixing events influenced thermal structure, hypolimnetic dissolved oxygen, fluorometric chlorophyll estimates (Chl-a), and lake metabolism and how these events varied in frequency and magnitude in lakes with different characteristics. Rise in wind speed alone had an effect on the lakes with the weakest thermal stability, but a decrease in air temperature together with strong wind induced mixing events in all lakes. The return period of these mixing events varied widely (from 20 to 92 d) and was dependent on the magnitude of change in weather. In lakes with strong stability, thermal structure and hypolimnetic oxygen concentra­tion were only slightly affected. Weather-induced mixing in the upper water column diluted the surface water Chl-a repeatedly, whereas seasonal maximum occurred in late summer on each lake. Although Finnish lakes have been char­acterized with stable stratification during summer, we observed many substantial mixing events of relatively short return periods relevant to both chemical and biological properties of the lakes.

More information:

Researcher Annalea Lohila, tel. +358 50 366 3242,

Kuha J., Arvola L., Hanson P.C., Huotari J., Huttula T., Juntunen J., Järvinen M., Kallio K., Ketola M., Kuoppamäki K., Lepistö A., Lohila A., Paavola R., Vuorenmaa J., Winslow L. and Karjalainen J. 2016. Response of boreal lakes to episodic weather-induced events. Inland waters 6, 523–534, doi: 10.5268/IW-6.4.886


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