Abundance of planktonic microbes and inorganic contamination levels along the lowland hydrological continuum of the River Tevere (Rome, Italy)

Urban rivers at lowland areas experience a direct disturbance owing to increasing water resource exploitation and urbanization, with important implications for ecosystem functioning and services (Gurnell et al., 2007). Despite decreasing water quality and altered hydrological regime may dramatically affect the natural ecosystem status as a whole, the microbiological contamination is considered inattentive when the occurrence of selected indicator bacteria is below a standard concentration, estimated as the most probable number [MPN colony-forming units (CFU)/100 mL] or CFU (CFU/100 mL). The water quality assessments are based exclusively on the detection of coliforms and enterobacteria as indicators of fecal contamination (Decreto Legislativo 152/06, APHA, 1995; WHO, 2001). There remains a lack of knowledge concerning the effects of the anthropic impact on the structural and functional dynamics of the aquatic microbial communities. In particular, little is known on the quantitative patterns of the planktonic microbial biomass, including those not-cultivable heterotrophic and photoautotrophic microorganisms that are passively transported by the flowing waters (Read et al., 2015). Moreover, most field studies on freshwater ecology reported the relations among microbial decomposition activities and dissolved organic matter characteristics, while the levels of inorganic contamination by trace elements (e.g., metal ions) were marginally discussed in view of an integrated ecological assessment (Lair et al., 2009). The objective of this study was to explore (i) the abundance patterns of dominant planktonic microbial populations, by an in situ molecular approach overcoming cultivation techniques, and (ii) the occurrence of inorganic trace elements along the hydrological continuum at the lowland reach of the Tevere river catchment.