Lake Garda’s Anabaena Blooms Explained by Physics and Upwelling Dynamics

Alvise Vittori, a rivano, ichthyologist, and for many reasons, a “historical” memory of Garda, explains the phenomenon of the anabaena algae by resorting to physics. Cold water weighs more than warm water, so when the temperature of the surface layers decreases, they sink and continue to “fall” toward the bottom until they stop where the density, that is, the temperature, is the same as theirs.

Sedimentation Mechanism and Upswelling

As the surface layers sink, other layers from two or three hundred meters depth are pushed to the surface: just like in a pot when boiling begins. Phosphates and nitrates, the nutrients of algae, tend naturally to concentrate at the bottom.

Due to a noticeable upwelling of the lake resulting from the sudden drop in air temperature and the subsequent sinking of the highest portion of the large water mass, nutrients from depths where they normally reside were brought to the surface, causing an extraordinary proliferation of algae. The presence of yellow-green streaks is thus explained.

History of the Phenomenon and Current Conditions

That the lake’s water could overturn was only a suspicion until about fifteen years ago, after the launch of the center at Forte San Nicolò, when the phenomenon was monitored and controlled. It was thought to have a certain periodicity; however, over the last three years, it has repeatedly occurred.

However, the amount of nutrients does not increase, so the health conditions of Garda remain stable. The yellow streaks will disappear once the water temperature reaches around 18 degrees Celsius: since this type of algae prefers cold water, as soon as the sun raises the water temperatures to more seasonally appropriate levels, the algae will sink again.

Role of Anabaena and the Lake’s Health Status

The anabaena is “eaten” by zooplankton. In these amounts, according to Vittori, there is no reason to worry: thanks to the low nutrients flowing into it, Lake Garda’s health continues to remain decidedly good.