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In the last decade, many new records for Thorea hispida (Thore) Desvaux 1898 emend. Sheath, Vis et Cole 1993 (Rhodophyta) have been collected from Europe as a probable result of the enactment of the Water Framework Directive which has fueled a renewed interest in the characterization of the macroscopic primary producers of river systems. Despite this, the species remained poorly documented, especially regarding habitat requirements and related physical and chemical drivers. To further add to the knowledge of these parameters, a three-year survey (2009-2011) was conducted along the southern reach of the Oglio River, a mid-size tributary of the Po River in Northern Italy that hosts three newly recorded populations of T. hispida. In parallel, a comprehensive review of the literature was performed. In this work, we present the first records for T. hispida from Italy, and a first detailed physical, chemical and hydromorphological characterization of its habitat. We confirm the predilection of T. hispida for turbid waters (> 80 mg L -1 of total suspended solids) with high nutrient (up to 9.4 mg L -1 for nitrates and up to 173 μg L -1 for soluble reactive phosphorous) and high conductivity levels (up to 660 µS cm -1 for conductivity). In addition, our data extended the range of tolerance of the species for temperature (5.1–26.2 °C) and pH (7.1–8.6). In general, our results and previously published data corroborate with the idea that T. hispida can not be considered a sensitive species (i.e., a taxon scarcely adapted to increasing levels of pollution), showing a preference for rivers characterized by high nutrients availability. Moreover, its rarity must be traced to the low detectability of the thallus due to species life cycle and the very limited accessibility of colonized habitats.
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