Lake Garda Microalgae Bloom Explained by Water Temperatures, Not Pollution

Dr. Chiara De Francesco, head of the Provincial Laboratory in Porto San Nicolò, shares the same belief—as does the hydrobiologist Alvise Vittori, whom we contacted yesterday—that the unusual and concerning proliferation of anabaena (the microalga that has been accumulating in Lake Garda for about ten days) is a temporary phenomenon caused by water temperatures.

“The most recent measurement,” she explains, “was taken in the center of the Riva Gulf on Wednesday. The water samples collected and analyzed in the laboratory showed a cell density—those that are giving the lake its greenish coloration, as you know—more or less the same as previous measurements. Therefore, the situation is stable. At the beginning of next week, we will conduct additional samplings to further monitor the situation, which from a scientific standpoint, certainly warrants close observation.”

Causes of the proliferation and seasonal trend

Regarding the causes that have led to the proliferation of millions of anabaena, resulting in extensive yellow-greenish patches and in the ports of Riva and Torbole, turbid masses that are unsightly and even more unpleasant to smell—Dr. De Francesco reiterates and expands upon Vittori’s explanations.

The unusual season, with colder than usual water, has caused the overturning of water layers. This has brought the deeper waters (even those at 350 meters below the surface) to the surface, which are notoriously richer in so-called «nutrients», phosphates and nitrates, that are essentially a very tasty food source for microalgae—thus enabling their multiplication.

“What’s strange,” adds the researcher, “is that these water overturnings are typically cyclical. They occur every seven to ten years. However, after the last event in 1992, we observed them in 1999 and again this year. But this cannot be classified as pollution, because, on average, considering the entire water column rising from the bottom to the surface, the percentage of dissolved nutrients remains the same. There have been no worsening conditions; that’s the key point to consider.”