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Recent studies have highlighted an increase of chloride in many lakes worldwide, with negative effects on chemical and physical properties of inland waters and freshwater biota. In this study, we assessed the long-term trend (1993–2017) of chloride in Lake Iseo, located in the midlatitudes, and analyzed its relationship with discharge data of the inflows and hydrological and hydroclimatic changes. Additionally, we performed a specific annual survey, collecting water samples at a biweekly frequency in tributaries, calculating chloride load, and comparing the result with the chloride increase observed in lacustrine water. The combined use of Geographic Information System (GIS) and multivariate analyses allowed identification of possible sources of chloride and their seasonal dynamic within the mixed-land-use watershed: impervious surface and deicing salt, population and sewage effluents, livestock, and rainwater. Their potential contribution to total mass balance was assessed, cross-checking the results between chloride source evaluation and load estimation. Chloride load from wastewater treatment plants played an important role, albeit we highlighted that the main source was the road deicing salt, with a primarily winter contribution. The increased anthropization in the watershed was likely the main cause of the chloride enhancement. This study demonstrates that the problem of salinization can also affect lakes located in the midlatitude areas and provides a reproducible method for the identification and quantification of the different sources. Our results will help to better understand the potential sources of salinity in rivers that may reveal processes controlling the salinization of freshwater systems, with implications for future management practices.

A combination of field observations and 3-D hydrodynamic simulations were used to identify the phytoplankton species and to estimate the various time scales of the dominant physical and biological processes in Lake Iseo, a deep subalpine lake located in northern Italy, during a stratified period (July 2010). By ordering the rate processes time scales, we derive a phytoplankton patch categorization and growth interpretation that provides a general framework for the spatial distribution of phytoplankton concentration in Lake Iseo and illuminates the characteristics of their ecological niches. The results show that the diurnal surface layer was well mixed, received strong diurnal radiation, had low phosphorus concentrations and the phytoplankton biomass was sustained by the green alga Sphaerocystis schroeterii. The vertical mixing time scales were much shorter than horizontal mixing time scales causing a depth-uniform chlorophyll a concentration. The horizontal patch scale was determined by horizontal dispersion balancing the phytoplankton growth time scale, dictating the success of the observed green algae. The strongly stratified nutrient-rich metalimnion had mild light conditions and Diatoma elongatum and Planktothrix rubescens made up the largest proportions of the total phytoplankton biomass at the intermediate and deeper metalimnetic layers. The vertical transport time scales were much shorter than horizontal transport and vertical dispersion leading to growth niche for the observed phytoplankton. The study showed that time-scale hierarchy mandates the essential phytoplankton attributes or traits for success in a particular section of the water column and/or water body.