Garda Lake’s Health Assessed as Stable by Laboratory Monitoring
Chiara De Francesco, the manager responsible for the laboratory that has been monitoring Garda’s health for about ten years at Forte San Nicolò, assured the members of the Lyons of Arco-Riva and Tione that the largest Italian lake is doing quite well; she promises it will thrive for many more years to come. She easily dismisses the discharges from the Adige Garda as entirely insignificant for its balance; it does not seem to suffer from the whims of an unpredictable climate.
The algal bloom and its causes
Even the exceptional algal bloom of last summer — known to laypeople as the horrible and disgusting series of yellowish foam patches — was explained through basin dynamics and classified among phenomena that, while ugly, are harmless.
Preliminary note: a lake dies when too much phosphorus appears in its waters. More phosphorus means more algae; more algae consume more oxygen.
When oxygen is depleted, the lake ‘fades away’ asphyxiated. The San Nicolò laboratory, among other data (dissolved oxygen, temperature), collects and measures the amount of nutrients available for algae.
Mountain lakes contain very few nutrients; they are oligotrophic, clear, oxygen-rich, transparent, and potable. Eutrophic lakes, on the other hand, have excessive nutrients, leading to more algae, and the layers near the bottom lack oxygen.
Garda oscillates between the upper limit of oligotrophy and the lower limit of mesotrophy. In the summer of 2000, as Dr. De Francesco explained, the extraordinary bloom of Anabaena was caused by an unexpected second overturn of the lake’s waters.
Under normal conditions, the surface layers of water, warmer than the 8.2 degrees Celsius at the bottom, ‘float’ above the denser, colder, and heavier underlying masses. In these conditions, nutrients discharged into the lake sink to the bottom, where they are deposited and eventually processed.
If the surface temperature drops to match that at the bottom, the barrier between layers breaks, and the water mass begins circulating. This means nutrients are lifted to the upper layers where, encountering sunlight, algae develop.
This is confirmed by the fact that the phosphorus concentration detected at the bottom sharply decreased after months of slight, steady increase.
Considering the climate trend of 2001, Dr. De Francesco believes that this summer the phenomenon is unlikely to recur. The characteristic yellow-green streaks, which are ugly and foul-smelling, should no longer appear in the middle of the season, threatening Garda’s blue waters.