The curious case of the red lake that isn’t …

From Venice, I crossed to the mainland to meet a colleague before driving two hours inland, first west across the flat lands of Veneto province, then into the increasingly mountainous terrain of Trentino and finally to the dramatic landscapes of the Dolomites. The last four or five kilometres were on a gravel track that brought us, eventually, to Lago di Toval, set in a beautiful location, 1178 metres above sea level, amidst wooded slopes, with rocky alpine peaks visible all around us.


Lago di Toval, Trentino Province, Italy, September 2014

We were here for a paper-writing workshop at a small limnological research station beside the lake, eating and sleeping at a small albergo a few hundred metres away.   Confusingly, for a hotel situated beside a perfectly blue lake, its name was “Albergo Lago Rosso” but there was a story behind this name as my Italian colleague, Marco Cantonati, later explained.

The name of the albergo becomes clear when you see photographs of the lake taken in the 1950s and early 1960s when the water in some parts was a bright red colour due to growths of a an alga called (at the time) Glenodinium sanguineum.   The species epithet comes from the Latin sanguis, meaning blood, an allusion to the red colour of the cells which lends the lake its distinctive colour.   This alga belongs to a group called the “dinoflagellates”, which we have not previously encountered on this blog. The red colour comes from the same pigment that we encountered in Haematococcus (see “An encounter with a green alga that is red”).


Albergo Lago Rosso, overlooking Lago di Toval, September 2014

For a long time, this alga was thought to exist in two forms, one of which was red, the other green.   However, the latest evidence suggests that there are at least three different forms, sufficiently different from one another to be assigned to separate genera. Two of these are only ever green whilst the third gives the lake its distinctive red colour.   This latter form was placed in new genus named after the lake where it was found, Tovellia.

Whilst the lake was famous for the distinctive red colouration that Tovellia sanguinea gave to it during late summer, this phenomenon has not been observed since 1964.   The other dinoflagellate species are still abundant, but T. sanguinea is now very rare.


A postcard of Lago di Toval probably dating from the 1950s or early 1960s, showing the red coloration.

There is no definitive explanation for this change in lake colour but it is thought that changes in land use and, in particular, the way cattle were housed in the catchment, may have reduced the already small quantities of phosphorus entering the lake and tipped the scales in favour of the two green dinoflagellates rather than T. sanguinea.   There are other hypotheses and, as ever, it is difficult to untangle causation and correlation from the available evidence.   There is, however, also evidence that T. sanguinea was also rare before the 1860s, which does lend weight to the suggestion that the reddening of the lake was a response to human factors.   If this is the case, then Lago di Toval represents a relatively rare case of a lake that is returning to a more natural state.   That, of course, poses another fraught question: what exactly do we mean by “natural”? But that is a topic for another day.


Borghi, B., Borsato, A., Cantonati, M., Corradini, F. & Flaim, G. (2006). Il fenomeno del mancato arrossamento del Lago di Tovel alla luce dei risultati emersi dal Progetto SALTO. Studi trentini di scienze naturali – Acta biologica 81 Supplement 2: 471-472.

Cavalca, L., Ferrari, P. & Andreoni, V. (2001). Glenodinium sanguineum March. and the reddening phenomenon of Lake Tovel: biological and environmental aspects. Annals of Microbiology 51: 159-177.

Flaim, G., Moestrup, Ø, Hansen, G. & d’andrea, M. (2006).   Da Glenodinium a Tovellia.   Studi trentini di scienze naturali – Acta biologica 81 Supplement 2: 447-457.

Hansen, G., Daugbjerg, N., Flaim, G. & D’andrea, M. (2006). Studies on woloszynskioid dinoflagellates II: On Tovellia sanguinea sp. nov., the dinoflagellate responsible for the reddening of Lake Tovel, N. Italy. European Journal of Phycology 41: 47-65.


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