Shifts in diatom assemblages in Lake Maggiore: insights from 20 years monitoring data

Lake Maggiore is a deep oligotrophic subalpine lake in Northern Italy which undergone an eutrophication phenomena from the 1960s to mid-1980s and a recovery process from mesotrophy that began in the early 1980s. But over the last decades, climate-driven events became more prominent that overlapping on the long-term oligotrophication pattern. However, recent studies have demonstrated that shifts in diatoms assemblages can be considered robust indicators both of climate and trophic changes. Here, we report the results of our study on variations in diatom community structure and diversity as possible response to hydrometeorological and chemical conditions in Lake Maggiore over the last 20 years. We used phytoplankton monthly and fortnightly data for the period 1999- 2019 collected within the framework of the research program of International Commission for the Protection of Swiss-Italian Waters (www.cipais.org). The results suggest clear shift in diatom assemblages species. Pennate diatoms such as Synedra spp. declined during the oligotrophication period but recently have increased as well as Fragilaria crotonensis that shows an increase in the relative abundances during the last years. In fact, recently F. crotonensis has always been accounted in the number of significant and dominant species as competes to form minimum 80% of the total community biovolume at least once per year. An important change is also the complete disappearance of Tabellaria flocculosa. Further analyses are performed to suggest the role of hydrodynamics and nutrient concentrations as driving factors for the observed shifts in diatom assemblages in Lake Maggiore.