A modelling approach to evaluate the present and future effectiveness of hypolimnetic withdrawal for the restoration of eutrophic Lake Varese (Northern Italy).

Hypolimnetic withdrawal has been applied as a restoration measure in lakes subject to eutrophication together with external load reduction, to decrease internal load by removing limiting nutrient phosphorus (P) from anoxic deep waters and contributing to the unloading of bottom sediments from previously deposited nutrients and organic matter. The aim of this study is to evaluate the effect of hypolimnetic withdrawal on Lake Varese, a 24 m-deep and 14.8 km2-large subalpine lake in North-Western Italy. The lake suffered from extended eutrophication in the second half of the 20th century due to uncontrolled delivery of untreated urban sewage. Several restoration measures have been implemented during the years, including hypolimnetic withdrawal. In 2019, a cooperative programme for the protection and management of the lake and its surroundings was launched, establishing a systematic annual hypolimnetic withdrawal in the stratified season since 2020. In this research, we calibrated a one-dimensional (1D) coupled ecological-hydrodynamic model (General Lake Model/Aquatic EcoDynamics - GLM/AED2) of Lake Varese with data surveyed in the lake in 2019-2021. Model simulations of the period 2020-2021 with and without the performed withdrawal proved the effectiveness of this measure on hypolimnetic P concentration reduction. Then, future simulations of 2023-2085 were carried out to predict the future efficiency of hypolimnetic withdrawal and of reductions in external nutrient loads under climate change scenarios. Results show that the prescribed withdrawal increases hypolimnetic temperatures. This effect, coupled with thermocline deepening due to global warming, will possibly lead to decreasing water mass stability in autumn and shorter stratification in the moderately deep Lake Varese, with an eventual decrease of P concentrations in the water column. The future effectiveness of hypolimnetic withdrawal is further discussed considering the possible role of dry periods.

Metagenomics Analysis to Investigate the Microbial Communities and Their Functional Profile During Cyanobacterial Blooms in Lake Varese.

Toxic cyanobacterial blooms represent a natural phenomenon caused by a mass proliferation of photosynthetic prokaryotic microorganisms in water environments. Bloom events have been increasingly reported worldwide and their occurrence can pose serious threats to aquatic organisms and human health. In this study, we assessed the microbial composition, with a focus on Cyanobacteria, in Lake Varese, a eutrophic lake located in northern Italy. Water samples were collected and used for obtaining a 16S-based taxonomic profile and performing a shotgun sequencing analysis. The phyla found to exhibit the greatest relative abundance in the lake included Proteobacteria, Cyanobacteria, Actinobacteriota and Bacteroidota. In the epilimnion and at 2.5 × Secchi depth, Cyanobacteria were found to be more abundant compared to the low levels detected at greater depths. The blooms appear to be dominated mainly by the species Lyngbya robusta, and a specific functional profile was identified, suggesting that distinct metabolic processes characterized the bacterial population along the water column. Finally, analysis of the shotgun data also indicated the presence of a large and diverse phage population.

Anthropogenic pollution drives the bacterial resistome in a complex freshwater ecosystem.

Aquatic ecosystems in anthropogenically impacted areas are important reservoirs of antibiotic resistance genes (ARGs) of allochthonous origin. However, the dynamics of the different ARGs within the bacterial communities of lakes and rivers, as well as the factors that drive their selection, are not completely understood. In this study, we analysed the fate of the bacterial resistome (total content of ARGs and of metal resistance genes, MRGs) for a period of six months (summer-winter) in a continuum lake-river-lake system (Lake Varese, River Bardello, Lake Maggiore) in Northern Italy, by shotgun metagenomics. The metagenomic data were then compared with chemical, physical and microbiological data, to infer the role of anthropogenic pressure in the different sampling stations. ARGs and MRGs were more abundant and diverse in the River Bardello, characterised by the highest anthropogenic pollution. The date of sampling influenced ARGs and MRGs, with higher abundances in summer (August) than in fall or in winter, when the impact of the treated wastewater discharge in the river was limited by a higher water flow from Lake Varese. ARG and MRG abundances were significantly correlated and they co-occurred in the main network analysis modules with potential pathogenic bacteria. Different levels of anthropogenic impact selectively promoted specific ARGs while others, generally abundant in waters, were not affected by anthropogenic pressure. Reducing the level of anthropogenic pressure resulted in a rapid decrease of most ARGs. From our results, the role of anthropogenic pressure in promoting the spread of specific antibiotic resistances and of potential pathogens in aquatic ecosystem becomes clear. Finally we highlight the strict correlation between ARGs and MRGs suggesting their potential co-selection in stressed aquatic bacterial communities.