Endogenous origin of foams in lakes: a long-term analysis for Lake Maggiore (northern Italy)

The formation of foams on lakes is a complex phenomenon whose origin is often hardly identifiable. Recently (2007, 2008, and 2010) foam episodes started to occur in Lake Maggiore, northern Italy. The present work aimed to verify the hypothesis of an endogenous-natural origin of these foams, driven by trophic or climatic changes. To this purpose, a long-term (2000-2013) analysis of phytoplankton biovolumes, meteorological, and hydrological data has been performed together with the chemical characterization of foams. Foams resulted of endogenous origin and likely related to phytoplankton biomass degradation. Data analysis highlighted atypical warm temperature and residual lake stratification in winter in two of the three years of foam events, coupled with exceptional Bacyllariophyceae blooms in spring. Tabellaria flocculosa mostly contributed in terms of biomass in 2007 and 2008, but not in 2010; thus overall algal biomass seemed a better predictor of the risk of foam formation. Foam events occurred from July to December, driven by atypically windy conditions, and congruently with the time needed to degrade biomass into surface-active compounds. A co-occurrence of different factors resulted essential to generate foams, and climate changes likely contribute to enhance their occurrence in Lake Maggiore.