THE 4 SEASONS OF LEMAN

Lake Geneva in winter

Winter mixing and end of algal development

In winter, the campaigns of measurement of the quality of the water of the lake become monthly again because the development of the algae in suspension in water slows down. In fact, it is at this time of the year that the surface waters are the most transparent.

Winter mixing adds oxygen to the lake by mixing the oxygen-rich surface water with the deep water, which contains less oxygen because of decomposing algae. In the case of a complete winter mixing, the oxygen deficit is completely filled. In the case of a repeat of incomplete winter stirring, as has been the case since the winter of 2012-2013, the oxygen deficit at the bottom of the lake increases. Currently, the lake lacks oxygen at the bottom, resulting in adverse consequences for aquatic life and the release (salting out) of phosphorus or certain metals, which were trapped in the sediments.

Winter mixing also leads to the remobilization towards the surface of nutrients necessary for the development of algae (mainly nitrogen and phosphorus). But the cold weather, combined with the low solar radiation, does not immediately allow the algae suspended in the water to develop.