There is something extraordinary happening in the depths of Lake Garda. A rare phenomenon, almost invisible to the eyes of tourists walking along the banks, but fundamental for the health of the largest Italian lake. After about twenty years, Garda has completed a natural process of total mixing of the water, a sort of gigantic “breathing” capable of regenerating the entire lake ecosystem. The last similar episode occurred in the winter of 2006. Since then, the lake had no longer been able to completely mix the surface and deep layers. This time, however, the perfect weather conditions put everything back in motion.
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The decisive role of cold and wind
What triggered this exceptional event was a very precise combination: cold temperatures, strong currents from the north and a phenomenon called homothermia. In practice, the water temperature has become almost identical from the surface to the greatest depths. When this happens, the differences in density between the layers of the lake disappear. And this is where the wind comes into play, which pushed the water masses to mix completely, even beyond 270 meters of depth. In large deep lakes such as Garda, where in some points the depths reach 350 metres, this process is very rare. Normally only partial recirculations occur, but they are insufficient to truly regenerate the seabed.
Deep oxygen and nutrients that resurface
The data collected by the APPA multiparametric probe, positioned in Riva del Garda between Punta Lido and Sabbioni beach, tell precisely what happened. On April 1st, 9.45 degrees were recorded almost uniformly between surface and depth. But the most important data concerns oxygen: at 270 meters it increased by about a third, going from 6.61 mg/l to 8.80 mg/l in a few days.
Translated into ecological terms it means a real natural recharge. The surface waters, rich in oxygen, sank towards the seabed, while the deep waters rose, dragging with them nutrients and microelements accumulated over the years. An essential process to avoid stagnation, keep biodiversity alive and guarantee the survival of the species that inhabit the deepest areas of the lake.
A positive signal, but also a climate bell
Experts speak of very positive news for Garda, because the lake once again demonstrates a surprising ability to self-regenerate. The mixing could also favor an increase in phytoplankton and algal blooms in the coming months, natural phenomena linked to the new availability of nutrients.
At the same time, however, some sobering data emerge. In recent decades the surface temperatures of Garda have increased constantly due to climate change. Today the deep waters are significantly warmer than in the 1990s. And this is precisely the crucial point: events like the one that just happened are becoming increasingly rare. Because to make the lake “breathe” you need really cold winters, strong winds and a climate balance that is increasingly difficult to achieve.
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