Measurements and sampling carried out in the first months of 2021 show a further deterioration in the state of the lake. The content of phosphorus, the crucial nutrient for algal growth, has increased to a value of 17 µg/l, a level never reached before; the concentration of oxygen in the deep layers of the lake has decreased, and in December 2020 an oxygen-free zone at the bottom of the lake more than 9 metres thick was measured. Oxygen deficiency has dire consequences: 1) it prevents phosphorus in the water from being fixed in the sediment; 2) it can release previously fixed phosphorus thus accelerating general deterioration. For a detailed analysis, see this post from the Osservatorio Lago di Bolsena.

The graph summarises the level of phosphorus concentration over the last 20 years based on measurements made by the Lake Bolsena Association. Fortunately, and exceptionally, in the winters from 2016 to 2019 there was a total mixing of the water layers with complete oxygenation of the lakebed, which kept the level of phosphorus stable. In the last two winters, however, the Tramontana winds have not been strong enough to stir the waters, and the phosphorus rate has started to increase again.

We cannot control the winds, but we can and must stop the degradation of the lake by repairing the sewage system (see here) and by reducing the amount of phosphorus from agriculture (see here). The road to the rehabilitation of Lake Bolsena will be long.