Reinterpretation of the dinoflagellate Glenodinium sanguineum in the reddening of Lake Tovel, Italian Alps

Lake Tovel in the Italian Alps is famous for its blood-red water during summer, caused by a dinoflagellate named Glenodinium sanguineum. The red colour has been largely absent since 1964 and a project aimed at understanding the underlying cause of the colour change was begun in 2000. It appears that there are three dinoflagellates in the lake that morphologically somewhat resemble ‘G. sanguineum’. One of these agrees with the ‘red form’ of G. sanguineum studied in the detailed work of Baldi (1941), and is now very scarce in the plankton. The other agrees with Baldi's ‘green form’ and now dominates the plankton. Transmission electron microscopy has demonstrated that the third taxon is identical to what Dodge et al. (1987) identified as G. sanguineum from Lake Tovel. It did not develop a red colour under any of the growth conditions used in our experiments. The red form is very similar to Woloszynskia coronata, but differs in cyst morphology. It is shown that the red and green forms of G. sanguineum sensu Baldi are clearly distinct species. Reduction in nutrient loads entering the lake subsequent to changes in animal husbandry practices in the lake's catchment occurred around 1964. This apparently tipped the balance between the three species of dinoflagellates, resulting in the near disappearance of the red form from the plankton and concomitant disappearance of the red colour from the lake.