Lake Garda Water Quality & Algae Growth: Sesse Phenomenon Explained
“I have read recent news reports mentioning a potential decline in the water quality of Lake Garda due to the abnormal presence of Anabaena algae, which, by its massive proliferation, reduces water transparency and creates an unappealing aesthetic view.”
Precisely, Dr. Chiara De Francesco confirms what Dr. Alvise Vittori stated: algae are caused by the mixing of the water column due to temperature differences between surface and deep waters, leading to the emergence of nutrients (phosphorus and nitrogen compounds), which are the primary nourishment for algae.”
However, this overturning is not climate-driven but is a physical phenomenon.
The Sesse Phenomenon and Lake Garda
The French term “seiche” has been adopted since 1870 in Italian as “sessa” to describe a curious phenomenon observed in Lake Geneva.
It involves, succinctly, the oscillation of the entire water mass, which lifts from one side while lowering on the opposite, followed by a reversal of positions.
After the impulse, and following several oscillations, the lake’s surface returns to a horizontal state.
Periodically, lake Garda experiences internal sesse events, which, when particularly intense due to unusual weather conditions, affect the thermocline, the point where temperature varies between surface and deep layers, resulting in different densities.
These sesse cause mixing between two water masses of different temperatures and induce significant physical and chemical variations at various depths of the lake, as nutrients deposited on the bottom are dragged upward, providing nourishment for algae and enabling their proliferation.
The Garda sesse have definitely impacted the central area of the Rivano Gulf, where recent sampling has confirmed the non-toxicity of these algae.
Remarks on Water Quality
Ultimately, there is no worsening of water quality, as algae are a natural and cyclical phenomenon; from a scientific perspective, assessments should not rely on subjective impressions, which are often fallacious, but on continuous measurements that over time provide a real picture of the lake’s health.
The analysis of the most common trophic parameters—total phosphorus, chlorophyll-a, and transparency—classifies Lake Garda as oligomesotrophic.
This classification is attributable to the low phosphorus concentrations in the epilimnion and the consequent low productivity, as evidenced by chlorophyll-a levels, phytoplankton counts, and high transparency.
There is little new information compared to previous years, confirming that the large water mass is unlikely to undergo drastic changes.
The phosphorus concentration data in the deeper layers could be significantly influenced by anthropic and tourist loads along the basin’s shoreline, making this an important metric to monitor.
If climatic conditions allow for occasional full circulation of the waters, nutrients currently confined to the deep layers would disperse throughout the water column and become available for algal development in the euphotic zone.
Likely, the negative effects of such an event, indicated by increased algae production, would resolve within a single reproductive season, restoring oligomictic conditions.
Environmental attention towards Lake Garda cannot be underestimated, and every measure aimed at nutrient containment should be adopted across the entire basin.