No abstract

The results from a winter field study in 2009–2010 in Lake Vanajanselkä are presented. This lake is shallow, eutrophic, and annually ice-covered on average for 5 months. The ice sheet was 41–55 cm thick at the annual maximum, consisting of snow-ice and congelation ice. The e-folding depth of light intensity was 50–100 cm for congelation ice and 5–10 cm for snow. The water body had a 4-m thick upper mixed layer and lower continuously stratified layer. Fall cooling process was crucial to determine the temperature of the lower layer at freeze-up, anything within 0–4 °C. Oxygen concentration decreased in winter, especially close to the bottom sediments, and carbon dioxide concentration increased due to respiration activity. Phytoplankton production and biomass level were low or very low and, therefore, heterotrophic and mixotrophic species were abundant. Oxygen depletion in the hypolimnium had several chemical and ecological consequences, such as release of phosphorus from the bottom sediments. In spring, just before the ice-out, photosynthesis was at a high level beneath the ice due to improved light conditions and started to elevate the oxygen concentration in the topmost water layer.