Lake Garda's homeothermy reached

06 February 2024

These days the waters of Lake Garda are approaching homeothermia, i.e. the moment when the temperature of the surface layers equals that of the deeper layers, where it remains constant at around 7.5°.
These are among the ideal conditions for a more or less complete mixing of the Lake Garda water columns to occur. This is a very important phenomenon for the oxygenation of deep waters and beyond.
Then in spring, as temperatures warm up, the waters will begin to stratify, dividing, in the upper lake above all, virtually into three different columns at different temperatures.
Below is an in-depth study by the Vice President of the Garda Community, Filippo Gavazzoni, dubbed "the long breath of Lake Garda" for those interested in understanding this phenomenon.

"What happens in the lake in spring as temperatures rise?
What happens is that as the water columns warm up, they begin to stratify, dividing into defined thermal zones, from the surface towards the deeper layers.
Stratification is slowly 'triggered' with the first warmth of spring, normally towards the end of March, when the lake, after winter homeothermia (i.e. when it registers almost the same temperature at the surface as at the deepest depths), begins to warm up by stratifying, so much so that it can divide into 3 different thermal zones in late summer.
Difficult to visualise? I will try to give a 'nice' example that I think will help to make the idea clear.
So let us think of Lake Garda as a big 3-layer cake.
Let's cut a slice and we can visually appreciate them. The most superficial layer, the epilimnion, extends from the surface to a depth of about 20 metres in summer, with a temperature ranging from 26° to 16°.
Below the epilimnion there is then an intermediate layer called metalimnion.
The metalimnion is nothing more than a 'buffer' layer between the warm surface and the deep, cold part, which is around 7.5°, like, thinking back to the slice of cake, the layer of sponge cake that divides the cream on top (epilimnion) and the chocolate underneath (hypolimnion).
In the metalimnion, indicated more by 'convention', temperatures drop sharply.
Finally, below the metalimnion, we have the hypolimnion, the chocolate layer of the slice of cake to understand us, which is the deepest and coldest part, reaching the greatest depths of Lake Garda, where the temperature never drops below 7.5°.
The attached graph, from the book "L'Ichthyofauna del Lago di Garda" by Ivano Confortini, shows in August 1993, a constant drop in temperature down to a depth of about 40mt, with the end of the epilimnion at about 25mt, then a further reduction down to about 50mt, the metalimnion.
From about 60m down to the seabed it drops below 8°, then the hypolimnion, which you see practically constant in temperature, over 250m down.
To visualise how a liquid layer can float on the bottom, think of oil poured into a glass of water. The heat of the sun in fact heats the water, which becomes lighter and floats on top of the colder water, which is heavier and denser.
As the surface cools, that water then increases in weight as a result of the increase in density and sinks more and more, stirring up the water volumes like a slow centrifuge.
This summer stratification is then followed by autumn/winter stirring.
This is the silent dance that the waters of Lake Garda have always performed as the seasons change, as temperatures rise and fall.
It is a slow dance, or better yet, a long breath, which the water performs to oxygenate its entire volume... just as we breathe to oxygenate the blood, which in turn permeates every tissue and organ of the human body through blood circulation.
There is not much difference when you think about it...the concept is this.
This stirring, which begins in autumn and reaches its maximum intensity in March, does not, however, always succeed in affecting the deep layers.
Global warming, with increasingly mild winters, has a negative influence on this 'breathing', on this vital dance and biological cycle for Lake Garda and beyond.
So there you have it...sometimes we hear news on TV about global warming that always seems 'distant', that does not concern us...in reality it is not like that at all'. 

Read the article on: https://www.filippogavazzoni.it/2024/01/31/raggiunta-lomeotermia-del-lago-di-garda/